Diamond Grinding and Concrete Pavement Restoration

Diamond grinding is a procedure used to restore or improve pavement ride quality and surface texture. Although diamond grinding has been an available restoration procedure since the 1960's, recent develop­ments and increased experience have made diamond grinding and concrete pavement restoration the best first rehabilitation option for concrete pavements. In particu­lar, the cost competitiveness of diamond grinding has been enhanced with new, high-production equipment, and improved synthetic diamonds for saw blades. In addition, long-term performance data of diamond ground pavements has become available demonstrating excellent performance. Documented performance of diamond ground pave­ments has shown that 90 percent last at least 9.5 years and 50 percent last at least 13.5 years. After reaching its useful life, a diamond ground pavement may be reground to further extend its service life. Regrinding a pavement up to three times is possible without signifi­cantly compromising its fatigue life. Diamond grinding helps concrete pavements last far longer than their initial design lives.

Introduction

Diamond grinding is a procedure used to restore or improve pavement ride quality and surface texture. Although diamond grinding has been an available restoration procedure since the 1960's, recent develop­ments and increased experience have made diamond grinding and concrete pavement restoration the best first rehabilitation option for concrete pavements. In particu­lar, the cost competitiveness of diamond grinding has been enhanced with new, high-production equipment, and improved synthetic diamonds for saw blades. In addition, long-term performance data of diamond ground pavements has become available demonstrating excellent performance.

Figure 1. Diamond-ground surface.

Documented performance of diamond ground pave­ments has shown that 90 percent last at least 9.5 years and 50 percent last at least 13.5 years. After reaching its useful life, a diamond ground pavement may be reground to further extend its service life. Regrinding a pavement up to three times is possible without signifi­cantly compromising its fatigue life. Diamond grinding helps concrete pavements last far longer than their initial design lives.

Need

Diamond grinding is a widely adaptable procedure. It can remove bumps from new pavements, reprofile rough lanes, and smooth a pavement in conjunction with a complete restoration or preventative maintenance package. Although the size of the equipment is smaller for bump grinding on new pavements, many of the same principles used for restorative grinding apply. Diamond grinding is used to address the following concrete pavement problems:

Faulting at joints and cracks— The steady in­crease in the number and weights of trucks has resulted in faulted joints, particularly in truck lanes of undoweled concrete pavements. Poor draining soils, bathtub designs, and inadequate base materials have contrib­uted to joint faulting. These factors have not always been adequately considered in the design of existing pave­ments, and therefore have contributed to pumping and void development (Figure 2). Removal of roughness caused by excessive faulting has been the most com­mon need for surface restoration.

Figure 2. Joint faulting.

Built-in construction or rehabilitation rough­ness— Initial smoothness of concrete pavements depend on a number of factors including concrete mixture, steadiness of delivery, uniformity of construction platform (base layer), grading and staking, presence of construction joints, capabilities of the paving crew, and finishing operations. If the pavement surface does not meet smoothness specifications after initial construction, diamond grinding can be used to eliminate construction roughness. Depending on the specification requirements and cost-effectiveness, either full-lane or spot grinding can be performed.

Rehabilitation techniques such as partial-depth repairs and full-depth repairs may also result in increased roughness if not finished properly. This is typically because of differences in elevation between the repair areas and the existing pavement. The best method to blend repairs into a concrete pavement is with diamond grinding. The smooth surface results in an increased service life after rehabilitation.

Wheelpath rutting caused by studded tires— In northern and mountainous areas, the use of studded tires and tire chains can wear the surface of a concrete pavement as well as an asphalt pavement. This rutting increases water trapped in the wheelpaths during rainy weather, thereby creating hazardous conditions that involve decreased visibility due to spray and a greater possibility of hydroplaning. The severity of studded tire wear depends on the hardness of the mortar and the coarse aggregate used in the concrete. Diamond grinding removes wheelpath ruts caused by studded tires, improves drainage in wet weather by eliminating pooling of water in the ruts, and reduces the possibility of hydroplaning.

Polished concrete surface exhibiting inad­equate macrotexture— Diamond grinding also restores a polished surface to provide increased skid resistance. Although surface polishing is uncommon, it does occur on some concrete pavements subject to extremely heavy traffic volumes. Surface polishing reduces skid resistance and results in poor friction characteristics. In addition to increasing skid resistance by increasing the macrotexture of the concrete surface, diamond grinding improves cornering friction numbers, providing directional stability by tire tread pavement-groove interlock.

Unacceptable noise level— The noise level on pavements increases with increasing roughness. Transversely tined pavements can also produce a whistling and whining sound (discrete frequencies) that can be annoying to the driver and nearby residents. Diamond grinding retextures worn and tined surfaces with a longitudinal texture and provides a quieter ride. Diamond grinding also removes faults by leveling the pavement surface, thus eliminating the thumping and slapping sound created by the faulted joints. Grinding also significantly reduces whistling and whining sounds, resulting in a more pleasant ride.

Permanent upward slab warping caused by moisture gradient or curling— In very dry climates, slabs can get permanently warped at the joints. Long joint spacing and stiff base support may result in curled slabs that are higher at the joints than at midpanel, resulting in a bumpy ride. Diamond grinding can be used to restore smoothness and level off the surface of warped slabs.

Inadequate transverse slope— Good surface drainage is required to minimize the infiltration of surface water into the pavement. Minor changes in the pavement cross slope are also attained by diamond grinding. Proper cross slope helps transverse drainage and reduces the potential for hydroplaning, especially if studded tire wear has produced ruts in the pavement.

Diamond Grinding and Milling

Diamond grinding should not be confused with milling or scarifying. Milling is an impact process that chips small pieces of concrete from the pavement surface. Milling causes spalling at the joints and leaves an extremely rough and noisy surface. Diamond grinding is a cutting process and does not damage joints. Figure 3 shows the difference between the surface left after each procedure. Unless surface unevenness, aggregate fracturing, and joint spalling are tolerable, milling should not be allowed as a final surface.

Advantages

Diamond grinding is used to restore or improve pave­ment ride quality. Restoring ride through grinding improves traffic carrying capacity and adds value to an in-place pavement. Diamond grinding provides numer­ous benefits over other rehabilitation alternatives, including the following:

1. Diamond grinding provides a smooth surface which is often as good as a new pavement. As trucks travel across bumps and dips, they bounce verti­cally on their suspension resulting in dynamic loading (Figure 4). The increased load due to dynamic impact results in higher stresses in the pavement materials and consequently lower road life. By providing an extremely smooth surface, diamond grinding limits dynamic loading.

Figure 3. The difference between a diamond ground surface and a milled surface are great (grinding top/ milling bottom). Note that a milling machine can cause damage to transverse and longitudinal joints.

2. Diamond grinding reduces road noise. Diamond grinding's longitudinal texture provides a quieter surface than most transverse textures. A multi-State study on noise and texture on PCC pavements concluded that longitudinal texture concrete pave­ments are among the quietest pavements for interior and exterior noise. Diamond grinding also removes faults by leveling the pavement surface, thus eliminating the thumping and slapping sound created by faulted joints.

Figure 4. Trucks bouncing vertically from bumps cause dynamic (impact) loads as the truck mass (m) accelerates (a) downward on the suspension. An accelerating mass causes a higher force (F) than a static mass.

3. Diamond grinding enhances surface texture and skid resistance. The corrugated surface increases surface macrotexture and provides ample channels for water to displace beneath vehicle tires, reducing hydroplaning potential. Diamond grinding also improves cornering friction, providing directional stability by tire tread-pavement groove interlock.

4. Diamond grinding has been found to reduce acci­dent rates. The increased macrotexture provides for improved drainage of water at the tire-pavement interface, thus improving wet-weather friction, particularly for vehicles with balding tires. The longitudinal nature of a diamond-ground texture also provides directional stability and reduces hydroplan­ing, thus contributing to the safety of diamond-ground surfaces.

5. Diamond grinding does not significantly affect fatigue life. The reduction in thickness due to diamond grinding is highest at the faulted joint and lower at the interior. The small reduction in slab thickness caused by diamond grinding has negli­gible effect on service life. A typical concrete pavement may be ground up to three times and still add traffic carrying capacity.

6. Diamond grinding does not affect material durability. The fact that the diamond-ground surface is nearly always dry (except during storms) reduces any freeze-thaw problems. Diamond grinding does not introduce any unusual condition that would lead to poor surface durability.

7. Diamond grinding does not raise the pavement surface elevation. Grinding does not affect overhead clearances underneath bridges and eliminates the need for tapers at highway entrances, exits, and at side streets. Grinding does not affect the hydraulic capacities of curbs and gutters on municipal streets. On the other hand, bituminous overlays fill curb and gutter, reducing drainage capabilities (Figure 5).

8. Diamond grinding need be applied only to the portion of the pavement where restoration is needed. For example, a highway agency can require grinding only the truck lanes of a four-lane divided highway. This presents a significant cost advantage. Whereas, bituminous resurfacing requires all lanes, shoulders, and ramps to be resurfaced to meet the new mainline elevation.

Figure 5. Bituminous overlays reduce curb height and gutter storage for rain and water runoff.

9. Diamond grinding can be accomplished during off-peak hours with short lane closures, without having to close adjacent lanes, and can be used on all road classes, from interstates to city streets.

Cost Effectiveness

Diamond grinding costs on average between 2.50 and 7 $/sq. m. (2 and 6 $/sq. yd.) although some very hard river gravels can cost up to 12 $/sq. m (10 $/sq. yd). The cost depends on many factors including aggregate and PCC mix properties, average depth of removal, and smooth­ness specifications.

Diamond grinding is a cost-effective treatment, whether used alone or as part of an overall concrete pavement restoration (CPR) program. In most cases, the cost of diamond grinding is only about half the cost of bitumi­nous overlays. This cost competitiveness, in conjunc­tion with eliminating bituminous overlay problems (rutting, corrugation, poor skid resistance, drainage reduction, vertical clearance reduction) makes diamond grinding an appealing alternative for many rehabilitation projects. Diamond grinding should be part of any preventive maintenance program for concrete pavements.

Table 1. Cost comparisons for comparable CPR with grinding and AC overlay rehabilitation alternatives.

Location	Rehabilitation	Project Size	Cost / Lane KM*
North Carolina I-2610	CPR with grinding Crack/Seat and AC Overlay	106.2km 4.2 km	$77,640 $232,920
Florida I-1010	CPR with grinding Crack/Seat and 100 mm AC Overlay	106.2 km 51.5 km	$38,820 $117,190
Washington I-9012	Retrofit dowel bars with diamond grinding in truck land Tied PCC shoulders with diamond grinding in truck lane 110 mm AC Overlay	53.1 km 53.1 km 53.1 km	$73,800 $69,100 $118,300

Several states have compared the cost of diamond grinding with other appropriate treatments. Georgia DOT has used CPR with grinding for more than 20 years, and they find CPR with grinding to be 3 to 4 times more cost-effective than a 150-mm (6-in) AC overlay. Much of the cost savings can be attributed to the fact that the grinding options can be applied only where needed (the truck lane), whereas the AC overlay must be placed on all lanes. Table 1 compares the costs for several compa­rable CPR techniques with grinding and asphalt overlay alternatives in other states. An example of cost compari­son of diamond grinding to bituminous overlay for a typical 4-lane divided highway is shown in Appendix A.

Diamond Grinding Applications

Long-Term Application— For concrete pavements with roughness problems, diamond grinding is a highly effective and economical rehabilitation alternative. Diamond grinding of a pavement in fair to good structural condition increases the service life of the pavement by 8 to 10 years or more with a high degree of reliability.

Short-Term Application— Highway, county, and city engineers confront the problem of how to restore their pavement network. The engineers must attempt to optimize the condition of the network by repairing as many miles of the network as possible. Diamond grind­ing, when used alone can be a reliable short-term (less than 5 years) solution for pavements with structural problems. Structurally deficient pavements can be diamond ground to address roughness problems until a more comprehensive rehabilitation method can be applied. Diamond grinding provides excellent use of funds for short-term life extension and is a less costly procedure than bituminous overlays. It can also be accomplished more rapidly.

Spot Grinding— Spot grinding is performed after initial construction to meet agency specification requirements for initial ride quality. Spot grinding after construction does not degrade pavement performance. The smoother pavement can be expected to last longer due to re­duced dynamic loading on the pavement.

Feasibility and Limitations

The presence of progressive transverse slab cracking and corner breaks indicates a structural deficiency in the pavement. Slab cracking - and the faulting of these cracks - will continue after grinding and will reduce the life of the restoration project. The need for significant slab replace­ment and repairs may indicate continuing progressive deterioration that grinding would not permanently remedy.

Even in such cases, however, it may be appropriate to consider diamond grinding as an economical short-term solution to a roughness problem until the pavement section can be overlaid or reconstructed. For example, CALTRANS has used diamond grinding to extend their service life of highly cracked pavements before complete reconstruction.

The presence of widespread distress related to concrete durability, such as D-cracking, reactive aggregate or freeze-thaw damage, may indicate that diamond grinding is not a suitable restoration technique. However, if the extent and severity of durability distresses are very low, then significant and cost-effective life extension can be attained by full-depth replacement of deteriorated areas, followed by grinding to restore rideability. Diamond grinding does not increase the rate of deterioration of concrete durability distresses.

Feasibility of diamond grinding should be assessed based on the collection and analysis of pavement condition and roughness data. If the pavement needs more load carrying capacity or has deteriorated to poorer conditions, other procedures—such as bonded concrete overlay, unbonded concrete overlay, or PCC reconstruc­tion—may be better alternatives.

Design

Concurrent Work

In early applications, diamond grinding was sometimes applied to structurally deteriorated pavements without repairing major distress. Highway engineers learned that concurrent repair was necessary to realize the full benefit of diamond grinding, which led to the inclusion of diamond grinding in an overall CPR and preventive maintenance program.

Diamond grinding alone does not add or restore structural capacity to a pavement. However, it does add traffic carrying capacity by increasing the serviceability (Figure 6). Diamond grinding is often combined with at least one other CPR procedure. Typically, the other procedure restores structural problems. However, if performance data indicate skid or rideability problems only, grinding is effective when used alone. When significant structural distresses are present, other CPR techniques are also necessary for long-term life extension.

Full-Depth Repairs— Full-depth repairs (FDRs) fix cracked slabs and joint deterioration by removing at least a portion of the existing slab and replacing it with new concrete. This maintains the structural integrity of the existing slab and pavement. FDRs also repair shattered slabs, corner breaks, punchouts in CRCP, and some low-severity durability problems.

Figure 6. Grinding adds traffic carrying capacity by increasing the serviceability of the pavement.

Figure 7. Placing concrete during full-depth repair.

FDR involves marking the distressed concrete, saw cutting around the perimeter, removing the old concrete, providing load transfer, and placing new concrete (Figure 7). Each repair must be large enough to replace all significant distress and resist rocking under traffic, yet small enough to minimize the repair material. Typically, repair areas that are a full-lane wide and at least a half-lane long meet this requirement. For more information on full-depth repairs, see ACPA publication Guidelines for Full-Depth Repair (TB002P).

Partial-Depth Repairs— Partial-depth repairs (PDRs) correct surface distress and joint/crack deterioration in the upper third of the concrete slab. When the deteriora­tion is greater than one-third the slab depth or contacts embedded steel, a full-depth repair must be used instead. Placing a PDR involves removing the deterio­rated concrete, cleaning the patch area, placing new concrete, and reforming the joint system. For more information on partial-depth repairs, see ACPA publica­tion Guidelines for Partial-Depth Repair (TB003P).

Dowel-Bar Retrofit— Dowel-bar retrofit increases the load transfer efficiency at transverse cracks and joints in JPCP and JRCP by linking the slabs together so the load is distributed across the joint. Improving the load transfer increases the pavement's structural capacity and greatly reduces the potential for faulting by decreasing the stresses and deflections in the pavement. Dowel-bar retrofit consists of cutting slots in the pavement across the joint or crack, removing the concrete, cleaning the slot, placing the dowel bars, and backfilling the slots with new concrete (Figure 8). For more information on dowel-bar retrofit, see ACPA publications Concrete Pavement Rehabilitation: Guide for Load Transfer Restoration (JP001) and Joint and Crack Sealing and Repair for Concrete Pavements (TB012P).

Figure 8. Repairing crack with dowel-bar retrofit.

Slab Stabilization— Slab stabilization restores support to concrete slabs by filling small voids that develop underneath the concrete slab at joints, cracks, or the pavement edge (Figure 9). The voids, often not much deeper than 3 mm (1/8 in.), are caused by subgrade pumping or consolidation due to high corner deflections. Without proper support, the pavement may develop faulting, corner breaks, and extensive cracking. This procedure is sensitive to construction practices, so care must be taken when performing slab stabilization. For more information on slab stabilization, see ACPA publication Slab Stabilization Guidelines for Concrete Pavements (TB018P).

Figure 9. Slab stabilization.

Joint and Crack Resealing— Joint and crack resealing minimizes the infiltration of surface water and incompressible material into the joint system. Minimizing water infiltration reduces subgrade softening; slows pumping and erosion of subgrade or subbase fines; and may limit dowel-bar corrosion caused by deicing chemi­cals. Minimizing incompressibles reduces the potential for spalling and blow-ups. Joint sealing also can main­tain small sliver spalls that can develop into larger spalls if left alone. For more information on joint and crack resealing, see ACPA publication Joint and Crack Sealing and Repair for Concrete Pavements (TB012P).

Cross-stitching— Cross-stitching repairs longitudinal cracks that are in fair (low-severity) condition. It in­creases load transfer at the crack by adding steel reinforcement to hold the crack together tightly. This limits the crack's horizontal and vertical movement and prevents it from widening.

Cross-stitching is not an alternative for cracks that are severely deteriorated or functioning as a joint. At se­verely deteriorated cracks, there is too much deteriora­tion to reestablish effective load transfer. Cross-stitching transverse cracks that are functioning as joints may restrain the pavement and may cause a new transverse crack to form. In these cases, dowel-bar retrofit, which does not restrain axial movement, is a better CPR technique. For more information on cross-stitching, see ACPA publication Joint and Crack Sealing and Repair for Concrete Pavements (TB012P).

Grooving— Grooving is the process used to cut slots into a concrete pavement surface. Grooving restores skid resistance to concrete pavements, particularly on bald tires, it increases the surface friction and surface drainage capabilities of a pavement by creating small longitudinal or transverse channels that drain water from underneath the tire, reducing the hydroplaning potential and consequently reducing wet weather accident rates.

Retrofitting Concrete Shoulders— Retrofitting concrete shoulders adds a tied concrete shoulder to an existing pavement. It decreases the critical edge stresses and corner deflections and reduces the poten­tial for transverse cracking, pumping, and faulting. For retrofit concrete shoulders to be effective, good design and construction practices are essential.

Retrofitting Edge Drains— Adding a longitudinal drainage system to a pavement aids in the rapid removal of water and may prevent pumping, faulting, and dura­bility distress from developing. Despite these potential advantages, the placement of retrofit edge drains must be considered carefully. For some pavements built on fine grained subgrade soils, the addition of edge drains has accelerated the loss of fines from the underneath the pavement. Retrofitting edge drains may not be suitable on distressed pavements that have been in service for more than 10 years and a subgrade soil consisting of silt or clay.

Sequence of Work

A full CPR job can improve structural capacity to levels near those of new construction. Full-depth repair, load transfer restoration, addition of edge drains, and slab stabilization combine to restore structural integrity to the pavement. Partial-depth repair helps restore rideability by removing spalled areas at joints and cracks. Diamond grinding restores rideability by blending the old slabs with new repairs to a uniform profile. It is often necessary to reseal the pavement joints after diamond grinding.

The sequence of work is very important in a total restora­tion project including concurrent CPR techniques. Slab stabilization and retrofit edge drains should pre­cede partial- and full-depth repairs. Partial- and full-depth repairs, dowel-bar retrofit, retrofit concrete shoul­ders, and cross-stitching must precede diamond grinding. Grooving and resealing joints follow grinding to ensure proper groove and sealant depth. Figure 10 shows the sequence of operations.

CPR - Area Management Contracts— A multiyear contract between a highway or airport agency and a contractor for repairing and managing pavement deterio­ration is a CPR area management contract. The contract stipulates broad CPR quantities, traffic-control windows, and distress survey information. The contractor commits to specific unit prices based on the broad quantities, without specific areas or items marked on the pavement or plans.

Figure 10. Sequence of CPR techniques. Not all projects will require every procedure, but the sequence should be maintained. The most common procedures are in double boxes.

After awarding the contract, the agency and winning contractor jointly conduct a detailed distress survey and agree on the specific repairs for one year. After deter­mining the amount of work, the contractor develops a site-specific work plan and traffic-control scheme. Finally, the agency issues a work order to begin work. The sequence is repeated in successive years.

There are several advantages of the CPR - Area Manage­ment contract for both the agency and the contractor.

For the agency, these are:

• Fewer contracts

• Shorter delay's between design and construction

• More contractor input on constructability issues

• Fewer claims for overruns and unforeseen conditions

• Reduced need to develop and maintain in-house maintenance expertise

• Benefits of volume pricing even for repairs per­

formed on isolated areas.

For the contractor, the advantages include:

• Cooperative working relationship with specifying agency

• Up-front understanding of the distress survey and repair decisions, lowering risks associated with unknown job-site conditions

• Flexibility in determining exact construction times

• Ability to expand or use other professional expertise by contracting with engineering firms for distress surveys.

Pavement Evaluation

Before construction, detailed design and performance information should be gathered to assess the feasibility of diamond grinding and CPR. Factors that influence the type and quantity of concurrent work required must be considered. Pavement evaluation is a multi-step process, as shown in Table 2.

As-built design information provides insight during pavement evaluation and is necessary for a contractor to prepare bids. Accurate as-built design information may also contribute to lower prices. As the amount of as-built information increases, and the number of assump­tions decreases, so does the perceived risk assumed by the contractor.

The most important part of the pavement evaluation process is to use condition information to evaluate the cost-effectiveness of repair strategies. The distresses found on the pavement govern the repair (CPR) procedures needed to restore the pavement. Detailed distress surveys conducted by the highway department should indicate and rate all distress types. Structural distresses such as pumping, loss of support, corner breaks, and working transverse cracks and joints should be clearly marked on distress survey data sheets. These areas require repair.

The severity of material distresses, such as D-cracking and alkali-silica reactivity, will influence the feasibility of CPR and grinding. If the severity and extent of distress is low, significant pavement life extension can be attained by full-depth replacement of deteriorated areas, followed by grinding to restore rideability. If a pavement exhibits high-severity D-cracking or alkali aggregate reactivity, it should not be rehabilitated with CPR. These pave­ments are best rehabilitated using an unbonded overlay.

Windows of Opportunity

The period in a pavement life during which diamond grinding is an economical preventive maintenance or rehabilitation option is defined by trigger and limit values on key distresses.

Table 2. Guidelines for Collection of Pavement Data.

Task	Description/Objective
1. Collect design data	Collect original design information: Construction date(s) Slab thickness Joint design (dowels, diameter) Steel reinforcing Joint spacing Shoulder design Concrete materials (aggregate source and type) Base materials, subgrade soil Drainage provisions traffic loadings
2. Conduct distress survey	Collect performance information: Present serviceability index profile index Crack survey (type, severity, amount) Measure Faulting Durability distresses (ASR, AAR, D-cracking) Drainage Skid resistance Non-destructive deflection testing*
3. Evaluate data	Identify causes and extent of deterioration Analyze void development Asses need for additional data take cores if needed to supplement data Assess drainage needs
4. Lab testing	Determine material properties and extent of deterioration
5. Final evaluation	Conclude the cause and extent of deterioration Develop restoration alternatives that repair existing condition and prevent future development

*Deflection testing recommended, but not required for proper evaluation.

Trigger values define the point when diamond grinding (or any CPR procedure) is viable and appropriate. Limit values define the point when the pavement has deterio­rated so much that CPR is not likely to be effective. Between the trigger and limit values, the window of opportunity is open and diamond grinding and CPR is likely to be effective and economical.

Table 3. Trigger Values for Diamond Grinding

Measurement	Traffic Volume
	High*	Medium**	Low***
IRI(m/km)	1.25	1.50	1.75
PSR	3.6	3.3	3.0
California Profiograph+	15	18	21

*High-(ADT>10,000) **Medium-(3,000<ADT<10,000) ***Low-(ADT<3000) ⁺0.2 in lanking band 25mm = 1.0 in, 1 km = 0.62 mi, 1 m = 3.1 ft

Table 4. Faulting Ratings

Average Fault, mm (in)	Comments
0.8 (1/32")	No roughness
1.6 (1/16")	Minor Faulting
2.4 (3/32")	Consider grinding project
3.2 (1/8")	Expedite Project
4.0 (5/32") 4.8 (3/16") 5.6 (7/32")	Discomfort begins
6.4 (1/4")	Grind immediately

Trigger Values—Trigger values indicate when a highway agency should consider diamond grinding or a CPR program to restore a pavement's rideability. The trigger values are based on serviceability, roughness measurements, or faulting measurements.

The present serviceability index/rating (PSI/PSR) of a pavement, used in the American Association of State Highway and Transportation Officials (AASHTO) design approach, can also be used as a trigger for restoration work. If the PSR drops within a range of 3.4 to 3.8, a thorough evaluation of the cause of this loss in service­ability should be conducted. After addressing any functional deficiencies, diamond grinding should be conducted to restore the serviceability to a higher level. Roughness measurements using pavement profile data can also be used to set trigger values for diamond grinding projects. The trigger values that can be used to determine the need for grinding are shown in table 3.

Table 5. Faulting Measurement Frequency

Joint Spacing m (ft)	Measure Cracks?	Measurement Interval	NO. of Fault Measurements, per lane km [mile]
<3.65 (<12)	No	Every 9th joint	>32 (>50)
3.65-4.57 (12-15)	No	Every 7th joint	32-40 (50-63)
4.57-6.10 (15-20)	No	Every 5th Joint	33-44 (53-70)
6.10-9.14 (20-30)	Yes	Every 4th Joint	28-42 (44-66)*
>9.14 (>30)	Yes	Every 4th joint	>19 (>30)*

Include transverse cracks with joint fault measurements

Grinding should be conducted before faulting reaches critical levels. Such levels are dependent upon many factors. For example, less faulting is tolerable for pave­ments with short joint spacing than for pavements with long joint spacing. Highway agencies should establish threshold values of faulting for various pavement con­figurations. Diamond grinding should start being consid­ered when the average fault measurement is between 2.0 to 3.0 mm (0.08 to 0.12 in). At this faulting level, some joints may have already faulted to the critical level of 6.4 mm (1/4 in). Average joint faulting data can be useful in establishing threshold values for diamond grinding as shown in table 4.

A representative number of joints are needed to accu­rately characterize the degree of faulting. Table 5 provides recommendations on the number of faulting measurements needed for different slab lengths.

Limit Values— The limit values for grinding are based on the practical amount of diamond grinding that can be performed in a single pass of the diamond grinding equipment. Typical values that can be used to determine the practical limit of grinding on a single pass is shown in table 6. However, it is possible to perform multiple passes (up to 3) of the diamond grinder thus increasing the limit values shown in table 6 by a factor of 3.

It is important that these values be adjusted for local conditions. Limit values should be based on overall cost vs. the benefit of the rehabilitation procedure. New high productivity grinding equipment and high quality syn­thetic diamonds that can hold under severe heat can allow much higher limit values.

Table 6. Typical Single Pass Limit Values for Diamond Grinding

Measurement	Traffic Volume
	High*	Medium**	Low***
Faulting, mm (in)+	12 (0.5)	15 (0.6)	18 (0.7)
IRI, m/km	2.5	3.0	3.5
PSR	3.0	2.5	2.0
California Profilograph (0.2)	60	80	100

⁺Mean Joint Faulting *High-(ADT>10,000) **Medium-(3,000<ADT<10,000) ***Low-(ADT<3000) 25mm = 1.0 in, 1 km = 0.62 mi, 1 m = 3.1 ft

If the pavement is beyond the window of opportunity for cost-effective ride quality restoration, other procedures— such as unbonded concrete overlay, or PCC reconstruc­tion— may be better alternatives.

Plans and Specifications

To receive accurate bids for CPR and grinding projects, the agency must collect and provide certain information. Many factors affect the mechanical effort required to diamond grind a pavement. Table 7 lists information that contractors find useful for preparing bids. Contractors use this information to estimate grinding head life, grinding machine speed (productivity), and repair quantities. The more information provided by the agency, the more accurate a contractor will estimate the bid price, gener­ally resulting in lower costs to the agency.

Plans should show which CPR techniques are required and provide contingency recommendations for changes in conditions since the plan development survey. This information helps the contractor schedule an efficient operation with a minimum number of lane closures.

Aggregate Data

The coarse aggregate properties (hardness, size, and amount of exposure) significantly affect grinding produc­tivity. For example, large (50- to 75-mm [2- to 3-in]) aggregate of medium or greater hardness will decrease grinding productivity if the pavement surface is weath­ered. A weathered pavement has higher exposure of aggregates at the surface. Also, fine aggregate properties may affect grinding efficiency. Sand wears away the "bond" that holds diamonds in the matrix and affects diamond grinder head life.

Coverage

Specifications should require grinding across the entire lane surface. Spot grinding is not recommended for rehabilitation projects. Continuous grinding typically develops profiles as good as, or better than, new pave­ment. However, the restorative grinding specifications should allow for some isolated low areas that may exist in a concrete pavement surface. When more than 95 percent of a 0.9 m by 30.5 m (3 ft by 100 ft) area receives the corduroy texture, coverage is adequate. Isolated low spots less than 0.6 sq. m. (2 sq. ft.) are acceptable and do not require retexturing. This would require lowering the grinding head, which could cause damage to the pave­ment surface and reduce grinding efficiency.

Table 7. Pavement Information Required for Preparing an Accurate Bid and Effective Grinding Plan.

Type	Pavement Factor	Necessary Information	Additional Information
Design	Year the pavement was built		X
	Pavement type (plain, reinforced)	X
	Transverse joint spacing	X
Aggregate	Aggregate sources		X
	Aggregate hardness	X
	Aggregate/sand quantity and abrasiveness		X
	Aggregate size and amount of exposure
Distress	Average faulting	X
	Existing pavement profile (roughness)	X
	Studded tire rut depth	X*
	Amount of warping	X**
Other information	Partial- and full-depth repair quantitites and locations		X
	Average depth of removal	X
	Slurry deposit regulations	X
	Traffic control options	X

*For removal of rutting due to studded tire wear. **For restoration of smoothness to warped pavements.

Typically, three or four passes of one or more grinding machines are made to cover the entire width of a lane. The maximum overlap between passes should be 50 mm (2 in). The machine operator should feather the grind depth to meet the surface of an unground lane beginning approximately 0.9 m (3 ft) from the lane.

Smoothness Requirements

Diamond grinding and CPR projects should be evaluated for rideability before and after grinding. Profile traces taken before grinding help the contractor estimate removal quantities , whereas traces taken after grinding allow the agency to determine the quality of ride and improvement over pre-grind conditions.

Although several different devices are available for profile evaluation, the California profilograph is most widely used and is currently recommended by the American Concrete Pavement Association and the International Grooving and Grinding Association. More information on measuring pavement smoothness are available in ACPA technical bulletin TB-006.P, "Constructing Smooth Concrete Pavements."

Recent advances in light-weight profiling equipment and availability of high-speed profilers for pavement manage­ment make this equipment useful for pre- and post-grind evaluation. If chosen for evaluating CPR and grinding quality, the agency must arrange for availability while the project is underway. Scheduling conflicts and tie-up of the equipment can make use for this purpose difficult. Regardless of the choice of profiling equipment or index, the requirements for grinding should be the same as those for new pavement in the State.

On high-speed roadways, CPR projects should always be followed by diamond grinding to restore the ride quality of the pavement to that of a new pavement and to negate any roughness caused by the repairs. In short, grinding after CPR will greatly increase the performance and life of the new repair.

Figure 11. Basic components of a grinding machine.

Equipment

Diamond grinding equipment works like a wood plane. The front wheels pass over a fault or bump. The blade assembly, set at a predetermined level across the pavement surface, produces closely spaced longitudinal saw-cut grooves. The rear wheel follows in the path left by the grinding head. The uncut concrete between each saw cut breaks off more or less at a constant level above the saw-cut grooves, leaving a level surface with a longitudinal texture. Figure 11 shows the basic compo­nents of a grinding machine. The three most important aspects of a grinding machine are:

• the weight of the machine,

• the horsepower available to the grinding head, and

• the grinding head.

Grinding Head

Diamond grinding equipment uses diamond saw blades that are gang mounted on a cutting head. The grinding head that cuts the concrete is about 1 or 1.25 m (3 or 4 ft) wide (Figure 12) and consists of many diamond saw blades on a shaft or arbor. Most concrete pavements will require around 18 diamond blades per 100 mm (55 blades per ft) on the grinding head. The blades should not be installed in a uniform pattern on the grinding head. Lining up the blade segments may produce an uneven grind due to vibration. Randomly installed blades are preferable.

Saw blade selection has an impact on grinding produc­tivity, cost, and quality. The three factors that the contrac­tor should consider in selecting saw blades are diamond concentration, diamond size, and bond hardness.

Diamond Concentration— Diamond concentration is the overriding factor affecting the quality of diamond grinding. Essentially, more diamonds make a harder grinding head and allow for more efficient cutting.

Figure 12. Gang-mounted diamond blades (top). Close up view of diamond blades (middle). Cutting head removing about 6 mm (0.25 in) of concrete surface (bottom).

Diamond Size— Diamond size also affects the life, cutting speed, and costs of the grinding head. When grinding soft aggregates choose large diamond par­ticles. For harder aggregates, use smaller diamonds. This is similar to the teeth sizes used to cut wood or metal on standard saws. Wood teeth are much larger than those used to cut metal.

Figure 13. Effect of bond hardness on wear of diamond blade.

Bond Hardness— Bond hardness provides support (bond strength) to each diamond in a cutting segment on the blade edge and refers to the composition of the metal matrix that holds the diamond crystals. Ideally, the segments (diamonds and metal) must wear at a similar rate. As the metal matrix wears, worn diamonds are released and new ones are exposed. If the bond hard­ness is too low, the diamonds will break free before they wear and become dull. This is inefficient and costly. If the bond hardness is too high for a hard aggregate, the diamonds will wear faster than the metal that holds them in place. The blades will lose their abrasiveness, which will result in poor cutting speed and shortened grinding head life. This concept is illustrated in Figure 13.

Blade spacing— The texture and friction developed by a ground surface varies with the blade spacing on the grinding head. Proper spacing improves the life and friction of the surface. The number of grooves per meter of pavement ranges between 164 and 197 (50 and 60 per foot). For example, skid resistance of the corduroy texture can be prolonged for concrete containing softer aggregates by spacing blades farther apart. A harder aggregate may require tighter spacing so the break-off of the fins will occur. Figure 14 shows grinding texture and gives recommended dimensions for hard and soft aggregates.

Monitoring the Equipment

The grinding machine should be checked for blade wear on the grinding head after each day of operation. Particular attention should be paid for coning in the blade segments - the uneven wear, which will alter the desired spacing between fins on the concrete surface. The blades should also be checked for wear with a pie tape. The circumference of the grinding head should be measured in several locations to judge evenness of wear and remaining life of the blades (and grinding head).

Figure 14. Dimensions of grinding texture.

Construction

A grinding operator must know the effects of the weight, horsepower, and head blade setup to properly control the machine and produce an acceptable ground surface. The weight of the grinding machine is the ballast against the force (down pressure) that keeps the grinding head from riding up on the bumps in the pavement. If the down-pressure is too low, the machine will merely trace a bump profile and not cut through the bump. The operator must control the forward speed of the grinder and set the grinding head depth and down pressure to keep the machine cutting through the bumps (Figure 15).

An operator or inspector should also check for variance in the longitudinal cut line to see if the machine is cutting through the bumps. If the variance is nearly uniform, most likely the down pressure is not set properly and the operator should lower the grinding head. When the cut depth varies it is a good indication that the operator has the machine cutting through bumps properly.

The grinder operator should try to maintain a constant down pressure on the grinding head. This will help the machine cut through bumps with similar depth from pass to pass. Unnecessarily altering the down-pressure of the grinding head will likely result in a poor vertical match between passes (Figure 16). The pass-to-pass vertical match should be checked using a 3 m (10 ft) straight edge. A vertical overlap requirement of less than 3 mm per 3 m (0.12 in per 10 ft) is typical.

Figure 15. Illustration of grinding machine tracing profile and cutting bumps.

Traffic Control

Diamond grinding or CPR projects do not require shutting down adjacent traffic lanes (Figure 17). Traffic can be maintained on adjacent lanes with no detrimental effects. When given flexibility, a contractor can se­quence grinding and CPR work to enable the pavement to be fully opened for peak traffic (rush hours). Of course under all work conditions, the contractor and agency must consider work zone safety.

Productivity

Diamond grinding should begin and end at lines normal to the pavement centerline. The direction of grinding does not affect smoothness or quality, so specifications for grinding projects should allow the contractor to grind in either direction. This gives a contractor the flexibility to sequence grinding and CPR work in the most efficient manner. Several machines can be used together to allow a lane to be completed in one pass, improving produc­tivity, and minimizing lane closures on large projects (Figure 17). On smaller projects, it may be more cost-effective to use a single machine. The contractor would then need to make several passes to cover the entire width of the lane. The maximum overlap between passes should be 50 mm (2 in).

Figure 16. Checking vertical match of passes.

Figure 17. Several machines working together. Traffic is still maintained on adjacent lanes.

Finished Surface

The finished surface just after grinding will have thin fins remaining from the area between saw blades (Figure 18). These fins will quickly break free with one or two passes of a roller, or under normal traffic. If the fins do not break free easily, the grinding head may be exces­sively worn, or the blade spacing on the grinding head may need to be reduced.

Figure 18. Thin fins remaining after diamond grinding quickly break free after a few traffic passes.

Figure 19. Dip in surface due to wheels dropping into an expansion joint (top).

Figure 20. Weight of grinding machine deflecting slabs with voids beneath the joints (bottom).

Common Construction Problems

Dogtails— Dogtails are the portion of the pavement surface that is not ground because of a lack of horizontal overlap between two consecutive passes. Weaving is the primary cause of dogtails. With some practice, an opera­tor should develop the skill to steadily steer the machine without weaving.

Holidays— Unground areas resulting from isolated low spots are termed "holidays." The grinding specifications should be read carefully to determine how much cover­age is required. Most specifications require 95 percent coverage with the grinding texture and allow for 5 percent unground isolated low areas. It is not productive to lower the grinding head for isolated low spots that are less than 0.25 m² (3 ft²). If the grinding operation leaves an unacceptable amount of holidays it will be necessary to lower the grinding head and make another pass.

Expansion joints— Expansion joints or other wide gaps in the pavement surface may make the cutting head dip into the surface, as shown in Figure 19. These should be watched for by the grinding operator or filled with a cementitious grout prior to grinding. The remain­ing grout must be removed prior to opening to traffic.

Deflecting slabs— If there is not much change between the pre- and post-grind profile index values, the slab(s) may be deflecting too much. As shown in figure 20, the weight of the grinding head deflects the slab and does not allow much surface removal by the grinding head. After the machine passes, the slab rebounds and faulted joints return. Adjusting the down pressure on the grinding head may help, but a better alternative is to stabilize these slabs or retrofit with dowel bars.

Slurry Removal

The grinding equipment uses water to cool the cutting head. Before starting, the operator should check to see that the water supply functions properly. Inadequate or faulty water supply will result in the grinding head burning up quickly, which is very costly. All grinding machines contain on-board wet vacuums to ensure continual removal of slurry or residue from the grinding area (Figure 21). The slurry should not flow across lanes occupied by public traffic or into gutters and other drainage facilities. The slurry pickup system leaves a damp, but relatively clean surface. A pavement surface that remains wet with thick, stained water after the grinder passes, is a sign that a slurry pickup system may be clogged or malfunctioning. The vacuum hoses should be checked first to locate the problem.

Grinding slurry is inert and poses no chemical threat to vegetation (Table 8). A grinding machine can deposit slurry on the side of the roads, where possible, without environmental problems. Highway agencies have allowed this along rural highways across the country without a problem (Figure 22). In an urban environment, the slurry should be deposited into a truck equipped to handle the liquid material as shown in figure 23. After collection, the material can be deposited off-site.

Payment

Payment for grinding is on a square yard or square meter basis. The price includes all labor, materials, supplies, tools, equipment, and incidentals. The cost also includes clean-up.

Smoothness Incentives

Diamond grinding lends itself to the use of an incentive-type specification for pavement smoothness. Modern equipment and experienced contractors have enabled smoother grinding projects to be obtained in recent years. The level of smoothness required has a great effect on the cost of the grinding operation. The setting of unrealistic levels that require extensive removal or additional grind­ing will cause a large increase in cost with limited returns. An incentive approach to smoothness specifications will help assure that good results will be achieved for a given set of conditions. The thrust of the contractors concern will not be just to meet the minimum specification require­ment but to fine-tune grinding operation and equipment to earn the incentive payments that are available. Since smoother pavements are expected to have a longer life, there is a direct payoff in performance to be realized by the incentive payment. However, all sudden changes in elevation (e.g., joint faulting, crack faulting, full-depth and partial-depth repairs) should be eliminated to comply with the smoothness specifications.

Figure 21. Vacuum pickup on grinding machine.

Figure 22. Grinding machine depositing slurry on roadside ditches.

Figure 23. Under urban conditions, slurry is deposited into a tank.

Table 8. Chemical Analysis of Slurry Samples Taken From Grinding Projects in Delaware, Pennsylvania, and South Carolina. The Results Show Grinding Slurry to be Non-Hazardous.

Sample	1	2	3	4	5	6	7	Limits
								EPA	NC
Milligrams Per Kilogram (Part Per Million)
Arsenic	<.05	<.05	<.05	<.05	<.05	<.05	<.05	<0.50	<0.50
Barium	.80	1.10	.96	2.10	2.00	1.65	1.80	<10.00	<10.00
Cadmium	<.05	<.05	<.05	<.05	<.05	<.05	<.05	<0.10	<0.10
Chromium	<.05	<.05	<.05	<.05	<.05	<.05	<.05	<0.50	<0.50
Lead	<.05	<.05	<.05	<.05	<.05	<.05	<.05	<0.50	<0.50
Mercury	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.02	<.02
Selenium	<.05	<.05	<.05	<.05	<.05	<.05	<.05	<0.1	<0.1
Silver	<.05	<.05	<.05	<.05	<.05	<.05	<.05	<.05	<.05
Copper	3.10	1.60	1.70	2.60	3.15	2.10	1.85	NA	NA
Zinc	2.60	2.90	1.65	2.65	2.80	1.76	1.90	NA	NA
Aluminum	6570.00	6900.00	8210.00	7420.00	6840.00	7250.00	9130.00	NA	NA
Benzene	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.05	<.01
Toluene	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.06	<.01
Ethyl Benzene	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.08	<.01
Xylene	<.01	<.01	<.01	<.01	<.01	<.01	<.01	<.08	<.01
Gasoline	<.10	<.10	<.10	<.10	<.10	<.10	<.10	<1.00	<.10
Fuel Oil	<.10	<.10	<.10	<.10	<.10	<.10	<.10	<1.00	<.10
Diesel Fuel	<.10	<.10	<.10	<.10	<.10	<.10	<.10	<1.00	<.10
Lube Oil	<.10	<.10	<.10	<.10	<.10	<.10	<.10	<1.00	<.10
Other Solvents	<.10	<.10	<.10	<.10	<.10	<.10	<.10	NA	NA
Silica (SiO2)%	15.60	12.95	13.10	16.90	18.10	19.10	16.20	NA	NA
Iron Oxide (Fe2O3)%	1.40	1.60	1.95	1.65	1.40	1.31	1.29	NA	NA
Alumina (Al203)%	1.80	1.70	1.25	1.65	1.28	1.16	1.40	NA	NA
Lime (CaO)%	25.60	24.10	20.90	26.50	30.70	27.10	29.60	N/A	NA
Magnesia (MgO)%	.85	.96	1.10	.87	.97	1.20	.89	N/A	NA

Contract incentives for diamond grinding provide a means to account for contractor performance. Adjustment of the contract unit price depends on the ride quality achieved by the contractor. Where a contractor achieves high ride standards, added value to the public is attained and incentives are appropriate. Likewise, low ride stan­dards do not provide desired results, and the contractor should be required to regrind the affected area.

Monetary incentives have been very successful in increasing the quality of pavements. Incentives encour­age quality and develop a sensitivity toward teamwork. They reward excellence and encourage contractors to go beyond typical specification requirements during pavement construction. The Texas Transportation Institute has found that incentives lead to less costly pavements. Incentives help the quality-sensitive contractor by providing a competitive edge. The quality contractor can, and frequently does, lower the bid value to help win the contract, with the aim of receiving the incentive to offset costs. Overall, incentives result in improved pavements that can carry more loads and provide better service to the public.

Based on this success, several states have investigated using incentives for CPR. Most often, the incentive has been based on a percentage of the diamond grinding bid price—similar to new construction. For new construction, where typical costs are $24 - $35 sq m (20 - 30 sq yd), this is reasonable. However, for diamond grinding, where prices are in the range $2.40 -3.5 sq m (2 - 3 sq yd), the same percentage does not cover the additional effort necessary to produce the improved result. For this reason, an additive incentive is preferable to a percentage-based incentive. An additive incentive is an addition to the bid price when the results surpass the specified standard.

Equations 1 and 2* give recommended additive incen­tive values for a typical retrofit smoothness specification using the 5 mm (0.2 in.) blanking band.** Equation 1 is for pavements with a posted speed greater than 70 km/hr (45 mph). Equation 2 is for pavements with posted speeds less than 70 km/hr (45 mph).

1. Inc. = 0.25* (4 -PI)

2. Inc. = 0.15* (7 -PI)

Where:

Inc. = Incentive in dollars

PI = Profile Index obtained after grinding each section (in./mile)

For agencies using the zero blanking band, Mays Meter, Rainhart Profilograph, or some other smoothness evaluation equipment, different values are necessary. The incentive should be set so that it is obtainable and covers the extra effort expended by the contractor. As a rule, there should be one set of incentive values for pavements with a posted speed greater than 70 km/h (45 mph) and another set of values for pavements with posted speeds less than 70 km/h (45 mph). Isolated sections with a pre-existing settlement problem, such as a roadway over a culvert or utility, or a bridge approach may inhibit a correction to the specified profile. These areas should require that the final profile have at least a 70% reduction of PI, without any effect on the incentive. If any of the completed work does not meet the minimum specified profile index (allowing for the pre-existing settlement problems), the work is unacceptable and the contractor must re-grind.

Performance of Diamond-Ground Pavements

The history of continuous diamond grinding for pavement restoration dates back to 1965, to a then 19-year-old section of the San Bernardino Freeway (I-10) in California that was diamond-ground to eliminate excessive faulting. Since then, diamond grinding has become a major element of CPR projects. Many states in different climatic regions, such as California, Georgia, Pennsylvania, and Minnesota, have had significant success in increasing concrete pavement life through diamond grinding.

In general, the performance of diamond-ground pave­ments is excellent. A concrete pavement may be ground up to three times without significantly compromising its fatigue life, and there is no evidence that diamond grinding has any deleterious effects. The long-term effectiveness of a diamond-ground pavement depends on numerous factors, but the most significant factors are the condition of the existing pavement structure and level of CPR applied.

Life Extension

The AASHTO design equations can be used to estimate the added value of grinding by adjusting the serviceabil­ity of the pavement. Serviceability increases after grind­ing; the amount depends on how well the contractor grinds the pavement, and how rough the pavement was before grinding. Under normal operations, diamond grinding can develop a profile index between 0.11 and 0.19 m/km (7 and 12 in/mile) (by California profilograph). This corresponds to a PSI between 4.3 to 4.5. In some instances, the contractor can grind to 0.05 to 0.06 m/km (3 to 4 in/mile), or over a PSI of 4.8.

Table 9 shows the life extension from increased smooth­ness (serviceability). A typical 225-mm (9-inch) jointed plain concrete pavement was analyzed to determine the figures. Service life extension depends on the ride developed by the grinding procedure. The analysis assumes that the grinding removed 6 mm (0.25 in) of the existing surface. Table 9 confirms that smoothing the profile in a CPR process adds traffic carrying capacity to the pavement.

Table 9. Life Extension with Differing Levels of Post-Grind Smoothness

Profile Index After Grinding, m/km (in/mile)	Serviceability, PSI	Percent Increase in ESALs with Increased Smoothness
0.16 (10)	4.3	Specified Goal
0.11 (7)	4.5	+10.3%
0.06 (4)	4.8	+25.9%

Developed using the 1993 AASHTO Guide for Design of Pavement Sturctures ²²

Concurrent CPR is usually necessary to realize the full benefit of diamond grinding. In some instances in the past, diamond grinding and rehabilitation techniques have been misapplied. Some highway departments ground structurally deteriorated pavements without repairing major distresses, resulting in inadequate performance. Diamond grinding does not address the structural deficiencies or the causes of the service­ability loss. CPR techniques attack the causes of ser­viceability loss and add to the long-term effectiveness of grinding. Existing problems are repaired and/or their recurrence is prevented through CPR. A full CPR job can restore structural capacity to levels near those of new construction.

Smoothness

The immediate effect of diamond grinding is a significant improvement in smoothness. Figure 24 shows a com­parison of International Roughness Index (IRI) values before and shortly after rehabilitation of the Long-Term Pavement Performance (LTPP) Specific Pavement Studies, SPS-6 sections. Min Prep and Max Prep refer to the following levels of CPR:

• Min Prep—includes some full-depth, doweled concrete repairs of severely distressed areas, sealing of transverse and longitudinal joints, sealing of midslab breaks, bituminous shoulder removal and replacement, and diamond grinding (in most cases).

• Max Prep—similar to Min Prep, but all working cracks are full-depth repaired and retrofit edgedrains are added. Again, diamond grinding was performed in most cases.

The Indiana sections were the only sections that were not diamond ground, and the CPR activities actually caused a slight increase in roughness. Other sections show a significant drop in IRI after diamond grinding. The level of smoothness that can be achieved through diamond grinding is comparable to that of a new pave­ment or an AC overlay. Figure 25 shows the IRI is slightly better for the diamond ground surface than for the AC-overlaid sections, after 4 years of service. It is important to recognize that the cause of roughness should be treated prior to grinding otherwise redevelopment of roughness over the long term is likely.

Surface Texture

Although the primary reason for diamond grinding is to reduce roughness, grinding also improves skid resis­tance by increasing surface macrotexture. A conse-quence of this increased macrotexture is improved friction characteristics of the pavement surface. Immedi­ately after grinding, the locked-wheel longitudinal friction number (skid number) of a pavement measured using ASTM E274 increases dramatically. One study showed an increase in average friction number from 42 before grinding to 80 after the grinding on 5 projects in the United States. Tyner observed that although these values decrease over the first few years, an adequate macrotexture is normally maintained for many years.

The surface macrotexture wear depends on age since grinding and the climatic region (freeze vs. non­freeze). Although soft aggregates are more suscep­tible to wear than hard aggregates, the effect of aggre­gate hardness on surface texture wear is often compen­sated by blade spacing. Wider blade spacing is typically used on soft aggregates for increased grinding effi­ciency. The wider spacing results in greater land area, which wears down slowly. Closer blade spacing is used on harder aggregates so that break-off of the fins will occur. The closer spacing results in smaller land area that would wear down faster.

Diamond Grinding and Accident Rates— The increased macrotexture of a diamond-ground pavement surface provides for improved drainage of water at the tire-pavement interface (especially for worn tires), thus reducing the potential for wet weather hydroplaning-related accidents.

The longitudinal texture of grinding also helps to provide directional stability and reduce hydroplaning. This may be a very important factor for controlling the accident rates, particularly two or more years after grinding. A Wisconsin study found that the overall accident rate for diamond-ground surfaces was 60 percent of the rate for the unground surfaces. The diamond-ground pave­ments provided significantly reduced accident rates up to 6 years after grinding. This suggests that, in addition to macrotexture depth, the direction of texture may be a significant factor reducing accident rates on diamond-ground pavements.

Noise

Pavement roughness caused by abrasion (e.g., "rutting" caused by studded tires or chains) results in increased tire-pavement interaction noise. The faulting of joints and cracks add to the noise generated by the tire-pavement interaction. Transversely tined pavements can also pro­duce a whistling and whining sound (discrete frequencies) that can be annoying to the driver and nearby residents. A multi-State study on noise and texture on PCC pavements, sponsored by Wisconsin DOT and the FHWA, concluded that longitudinally textured concrete pavements are among the quietest pavements for interior and exterior noise.

Diamond grinding retextures worn surfaces with a longitudinal texture and provides a quieter surface. Diamond grinding also removes faults by leveling the pavement surface, thus eliminating the thumping and slapping sound created by faulted joints. Michigan DOT measured noise generated from a road surface before and after a tined pavement had been diamond ground. The noise level measured inside a vehicle showed that there was a considerable reduction in noise at the peak frequencies of 500 Hz and the first harmonic of 1000 Hz. Grinding of PCC pavements reduces noise peaks or spikes in the noise spectrum, thus providing significant reduction in objectionable noise.

Faulting

Excessive faulting of joints and transverse cracks is perhaps the most common reason for grinding jointed concrete pavements. Faulting is the single most important factor that affects ride quality. Faulting is noticeable when the average faulting in the pavement section reaches about 2.5 mm (0.1 in). When the average faulting reaches 3.8 mm (0.15 in), diamond grinding or other rehabilitation measures should be considered.

Figure 26. Undoweled JCP faulting after diamond grinding.

Immediately after grinding, the faulting on a pavement is zero. Average faulting on a diamond-ground pavement then increases with time under the influence of traffic loading and erosion of the base. Faulting redevelops at a relatively high rate initially on undoweled diamond-ground pavements, but the rate of faulting levels off to a much slower rate after about 2 million ESALs as shown in figure 26.

Factors Affecting Faulting Performance of Diamond-Ground Pavements— Diamond ground pavements are typically those pavements that have developed significant faulting prior to grinding, with accompanying void and loss of support problems at slab corners. The extent and severity of voids and loss of support results in variability in faulting performance after grinding.

The amount, type, and performance of rehabilitation performed concurrent with diamond grinding can also affect the faulting performance. Full-depth repairs are typically doweled and reduce average faulting by replacing existing original faulted joints. Slab stabilization and retrofit edgedrains also reduce faulting. However, variability in the performance of stabilized slabs and retrofitted edgedrains can result in variability in faulting performance after grinding.

The amount of precipitation is also a significant factor affecting undoweled faulting. In wet climates, faulting on diamond-ground pavements (undoweled) becomes excessive after 8 to 11 million ESALs, whereas a similar pavement in a dry climate may carry up to 20 million ESALs before reaching the same level of faulting. Other factors that affect faulting performance include presence of dowels, joint spacing, subgrade support (k-value), and slab thickness.

Diamond Grinding and Dowel Bar Retrofitting— The undoweled concrete pavements constructed in the U.S. during the 1960s and 1970s have served well, carrying significantly more traffic than for which they were designed. Unfortunately, excessive faulting has been a problem for many of those pavements. Inad­equate load transfer capacity of the undoweled joints is the main reason for the faulting problem. Since diamond grinding has no effect on load transfer efficiency, a diamond-ground pavement will eventually redevelop faulting unless the load transfer efficiency is improved. Dowel bar retrofitting offers a practical solution to the poor load transfer efficiency problem in existing undoweled pavements.

Dowel bar retrofitting followed by diamond grinding results in a smooth pavement with good load transfer capabilities and very low joint or crack faulting. As with diamond grinding, the placement of retrofit dowels in one lane does not require retrofitting the adjacent lane, offering a signifi­cant cost advantage over an overlay option.

Pierce studied the effectiveness of retrofit dowel bars in the State of Washington. Different rehabilitation strategies were performed on four pavement sections on I-90 (230­mm [9-in] JPCP). Prior to rehabilitation, the average faulting in the four sections was 8 mm (0.30 in). Seven months and approximately 700,000 ESALs after the rehabilitation, the diamond-ground section (without any other concurrent CPR) had an average faulting of 1.8 mm (0.07 in). A section with retrofitted tied shoulders and grinding had an average faulting of 0.6 mm (0.02 in). Two sections that were retrofit­ted with dowel bars and ground had an average faulting of 0.1 mm (<0.01 in). Thus, the retrofitted dowel bars were highly effective in minimizing redevelopment of faulting on this project as it has been on other projects.

Cracking

Since slab thickness is one of the most sensitive factors affecting theoretical cracking performance of concrete pavements, any reduction in slab thickness (e.g., through diamond grinding) can be a concern. However, analytical results have shown that the removal of a thin layer from the pavement surface does not adversely affect cracking performance of concrete pavements because concrete strength increases with age and offsets the slight loss of thickness.

Field observations are consistent with the analytical evaluation results. The results of the cracking evalua­tion clearly show that diamond grinding is not expected to cause increased slab cracking. These findings are consistent with the findings from a California study. Some pavements have been ground three times with no obvious increase in cracking.

Service Life

The service life of a ground pavement is the time be­tween project completion and development of roughness or other distresses to a degree that requires additional treatment. The service life depends upon many factors:

• Rate of heavy traffic loading.

• Existing pavement design (presence of dowels, slab thickness, joint spacing, type of base, type of subgrade soil, and subdrainage capability).

• Climate (freezing index, precipitation).

• Condition of pavement at the time of restoration.

• CPR (additional concurrent work).

• Performance of the existing load transfer system.

Engineers must often optimize limited funds available to restore their pavement network. Some projects may be restored for long-term life (10 to 20 years or more), whereas others may only be addressed with short-term alternatives (4 to 6 years of anticipated service life).

Rao et. al., performed a survival analysis of diamond ground pavements that were surveyed in 1986 and again in 1997. Table 10 shows the expected life of a diamond-ground surface (the time from grinding to regrinding, overlaying, or reconstruction) at different levels of reliability.

Based on actual field data of over 75 projects, 90 percent of the diamond-ground surfaces provided 9.5 or more years of service, and 50 percent provided 13.5 years. At this time, the pavement may be reground to provide further extension to service life. Pavements in dryer climatic regions had longer lives than pavements in wet climatic regions.

Table 10. Expected Life of Diamond Ground Surface (Time From Grinding to Regrinding, Overlaying, or Reconstruction).

Survivability	Age (Years)	Traffic (Million ESALs - Outer Lane)
50%	13.5	12
75%	11	8
85%	10.5	7
90%	9.5	6.5

Pavement Age

The addition of service life to a concrete pavement results in an increase in total pavement age since initial construction. The average age of diamond ground pavements based on survival analysis is 37 years or 35 million ESALs. Some of these pavements were diamond ground two or more times, and many sections had survived 40 or more years.

The probability that a diamond-ground pavement will not have to be overlaid or reconstructed by age 30 years (since initial construction) is less than 15 percent as shown in figure 27. Similarly, the probability that a dia­mond-ground pavement will be overlaid before carrying at least 15 million ESALs since initial construction is less than 10 percent. Thus, diamond-ground pavements contribute significantly to extension of both pavement age and traffic-carrying capacity.

Figure 28 shows a comparison of predicted traffic based on the AASHTO design procedure, actual cumulative traffic from construction to CPR (with grinding), and total traffic since initial construction. The difference between the total traffic since initial construction and cumulative traffic from construction to CPR represents the extension in service life due to diamond grinding.

Figure 27. Diamond grinding with CPR results in more than 85 percent of concrete pavements lasting more than 30 years since initial construction and more than 50 percent of concrete pavements lasting in excess of 37 years since initial construction.

Figure 28. Predicted traffic based on AASHTO design procedure and actual traffic since initial construction on diamond-ground projects.

The significant difference between total traffic since initial construction and predicted traffic based on the AASHTO design procedure indicates the significant benefits of grinding with respect to increase in traffic-carrying capacity of a pavement. The figure also shows that diamond grinding has provided a significant in­crease in service life for pavements located in all four climatic regions.

Case Studies

California completed the first significant grinding project in the fall of 1965 on a section of the San Bernardino Freeway, which was originally built in 1946 as part of the historic Route 66. State ride specifications required 7 inches/mile as measured by the profilograph (0.2-in blanking band). The contractor met the specifications using primitive, but effective, grinding equipment. Diamond grinding was the only remedial step taken on the project. That first-ever grinding project provided 18 years of service.

In 1983, the California DOT awarded a contract that called for complete rehabilitation of the freeway, includ­ing diamond grinding. The 200 mm (8 in), 51-year-old concrete pavement was ground for a third time in 1997. The pavement has carried more than 43 million ESALs between 1946 and 1997, and is still in service today with ADT more than 200,000 vehicles.

Since 1965, many other States have achieved excellent results with diamond grinding. Some Georgia highways have exceeded five times their design life through diamond grinding and CPR. A portion of Interstate 17 in downtown Phoenix, Arizona, has carried approximately 60 million ESALs and has had a service life of more than 36 years, which has been attained by CPR and diamond grinding. Portions of a low-volume rural highway (Trunk Highway 10) in Sherburne County, Minnesota, have lasted more than 50 years since construction and more than 15 years since rehabilitation with diamond grinding.

Additional Information

Additional information on diamond grinding is available from the American Concrete Pavement Association or International Grooving and Grinding Association. Information is also available regarding other CPR procedures (full-depth repair, partial-depth repair, joint repair, dowel bar retrofit, and slab stabilization).

Summary

Diamond grinding and CPR are cost-effective procedures and efficient use of restoration funds. The following are major points in considering and implementing grinding and CPR projects.

• Restoring ride improves traffic carrying capacity and thus adds value to an in-place pavement.

• Diamond grinding extends concrete pavement life by providing a smooth surface. It removes surface defects that develop over many years under traffic and weather.

• A smooth, level road lasts longer under repeated traffic loads because dynamic or impact loads are reduced.

• Diamond grinding is effective on all classes of pavements, from airports to Interstates and city streets.

• Diamond grinding is about half the cost of a bituminous overlay and eliminates bituminous overlay problems like rutting and reflection cracking.

• Diamond grinding does not require adjustment to any adjacent pavement structure and requires application only in the lane(s) where needed.

• Diamond grinding improves pavement rideability without interfering with drainage conditions.

• Diamond grinding is not milling or scarifying. Milling of concrete pavements causes spalling at the joints and leaves a rough and noisy surface. Diamond grinding is free of impact and does not damage joints.

• A full CPR job can restore structural capacity to levels near those of new construction. Unlike a bituminous overlay that merely covers problem areas on old concrete pavements, CPR techniques repair existing problems and/or prevent their recurrence.

• If a pavement exhibits excessive high severity "D" cracking or alkali aggregate reactivity, it should not be rehabilitated through CPR.

• Smoothness requirements for grinding and CPR projects should be equivalent to those required for new construction. Incentives and disincentives should be used to encourage superior contractor performance.

• The residual slurry from grinding is inert. The slurry poses no chemical threat to vegetation and is characterized as non-hazardous by the Environmental Protection Agency.

• The direction of diamond grinding does not influence the smoothness of the resulting profile. The best direction for grinding depends on sequencing operations and work zone limitations. Grinding direction should be determined by the contractor.

• Field performance shows that a concrete pavement may be ground up to three times without significantly compromising its fatigue life. Diamond grinding does not introduce any unusual conditions that would lead to poorer surface durability.

• Documented performance of diamond ground pavements has shown that 90 percent last at least 9.5 years and 50 percent last at least 13.5 years. The pavement can then be reground to further extend the service life.

Appendix A

Example

Cost Comparison of Diamond Grinding to Bituminous Overlay for a typical 4-lane divided highway. Not consid­ered are bituminous pavement problems (rutting, reflection cracking, corrugation, poor skid resistance), oil-driven price fluctuation, and reduced drainage capabilities.

COST FOR DIAMOND GRINDING: (Assume average cost diamond grinding $2.50 per square-yard)

• One 12 foot lane:

7040 square-yard/lane-mile x $2.50/square-yard = $17,600.00 per mile

• Both 12 foot lanes:

$35,200.00 per mile

COST FOR ASPHALT RESURFACING: (Assume average cost bituminous resurfacing mix $25.00 per ton)

Required coverage 38 feet (24 foot mainline & 10 foot outside & 4 foot inside shoulders.)

• Apply typical 3-inch overlay thickness.

38 ft x 5280 ft x 3/12 ft x 145 pounds/cubic feet x 1/2000 ton/pound = 3637 ton

3637 ton x $25.00 per ton = $90,915 per mile

GRINDING IS FIVE TIMES AS EFFECTIVE:

Metric Conversion Factors

The following table provides metric conversion factors for common English units used in pavement engineering, as well as concrete pavement design and construction. Where possible the values given reflect standard conversions provided by ASTM E 380.

For	If you know	Multiply by	To get metric unit
Angle	degree of curvature	0.0175	radians (rad)
Area	square inch (in2)	645.16	square millimeter (mm2)
Area	square feet (ft2)	0.093	square meter (m2)
Area	square yard (yd2)	0.836	square m (m2)
Area	square mile (mi2)	2.59	square kilometer (km2)
Area (land)	Acre (acre)	4046.9	square meter (m2)
Density (material)	pounds/cubic foot, (lb/ft3)	16.02	kilograms/cubic meter (kg/m3
Flow	cubic feet/second, (fts)	0.028	cubic meter per second (m3)
Flow (liquid)	gallon/minute (gal/min)	6.31x10-5	cubing meter per second (m3)
Flow (liquid)	gallon/minute (gal/min)	0.063	liter/second (L/s)
Force	kips (1000 lb) (KIP)	448.24	newtons (N)
Horizontal curvature	feet (f)	.3048	meters (m) round to 5 m
Length	inches (in)	25.4	millimeters (mm)
Length	feet (ft)	.3048	meters (m)
Length	Mile (U.S.) (mi)	1609.35	meter (m)
Length	yard (yd)	.914	meter (m)
Mass	pounds (lb)	0.454	kilograms (kg)
Mass	ton (U.S. 2000 lb) (ton)	907.2	kilogram (kg)
Power (engine)	horsepower (hp)	0.7457	kilowatt (kW)
Rideability	inches/mile (in/mile)	15.783	millimeter/kilometer (mm/km)
Stress (pressure)	pounds/quare inch (psi)	0.00689	megapascals (MPa)
Subgrade support, k	pounds/square inch/inch (psi/in)	0.27	megapascals/meter (MPa/m)
Temperature	degree Fahrenheit (°F)	use t°C = (t°F - 32)/1.8	degree Celsius (°C)
Velocity (speed)	miles/hour (mph)	1.61	kilometer/hour (km/h)
Viscosity	poise	0.10	Pascal second (Pa*s)
Volume	ounces (oz)	29.57	milliliters (mL)
Volume (liquid)	gallon (gal)	3.785	liter (L)
Volume	cubic feet (ft3)	0.028	cubic meter (m3)
Volume (admixture)	ounces/cubic yard (oz/yd3)	22.61	milliliter/cubic meter (mL/m3)
Volume (admixture)	ounce/100 weight (lbs) cement (oz/100 cwt)	0.015	milliliter/100 kilograms cement (mL/100 kgc)

The table below provides equivalent metric factors for common U.S. factors in concrete pavement engineering.

Current Factor	Metric Factor
Survey station 100 ft	Survey station 1 km
Bag of Cement 94 lb	Bag of Cement (Canadian) 40 kg

IGGA: An Overview The International Grooving and Grinding Association: The voice of the ACPA's Pavement Restoration Division Concrete pavement restoration, or CPR, is a series of engineered techniques developed over the past 30 years to manage the rate of pavement deterioration in concrete streets, highways and airports. The Interna­tional Grooving and Grinding Association (IGGA) is a non-profit trade association founded in 1972 by a group of dedicated industry professionals committed to the development of the CPR process. In 1994, the IGGA joined in affiliation with the American Concrete Pavement Association (ACPA) to represent it's newly formed Concrete Pavement Restoration Division. The IGGA/ ACPA Pavement Restoration Division serves as the technical resource and industry representative in the marketing of CPR to DOT's, municipalities and engineers around the world. The IGGA/ACPA Pavement Restora­tion Division is managed by Executive Director, John H. Roberts, with his office located at 49 Reed Street, Coxsackie, NY, 12051.

American Concrete Pavement Association 5420 Old Orchard Road, Suite A100 Skokie, IL 60077-1059 Printed in U.S.A. TB008.01P