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Concrete Inlays

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The following guidelines cover many available options for inlaying both concrete and asphalt pavements with concrete. Engineers often refer to concrete inlays as overlays or reconstruction. Inlays options do encompass features of these strategies. However, it is the optimization available through inlay strategies that warrants their specific explanation.

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Advantages

Concrete pavement inlays allow engineering optimization of pavement reconstruction. Inlays can replace all deteriorated travel lanes, or only one deteriorated lane. Unlike overlays and full reconstruction, inlays can be lane-specific. The difference between inlays and structural overlays is that inlays do not significantly alter the roadway elevation. Inlays strategies include removal of existing travel lane(s) rather than building atop the existing lane(s). Table 1 lists the four basic types of inlays.

A multi-lane inlay provides a strategy to reconstruct a roadway while keeping the existing shoulders. Shoulders receive comparably little traffic and often remain in good condition except for surface distress [1]. Existing shoulders create an excellent haul road for batching operations during construction. Shoulders also provide a stable trackline for slipform paving equipment.

Lane-specific inlays are effective for highway pavements with unequal deterioration between lanes. Often, asphalt roads deteriorate (rut) rapidly in the truck lane while the shoulders and the inner lane re­main in acceptable condition. The difference in deterioration is usually attributable to overdesign of the inner lane. However, performance differences are also attributable to poor design, increased loads, in­creased tire pressure and inadequate materials [2].

Inlays are also practical for deteriorated concrete pavements[3][4][5][6]. Multi-lane inlays are common for concrete reconstruction. However, lane-specific inlays are also feasible for old concrete pavement. One agency even employed a partial-depth bonded inlay to address a surface problem [7].

Figure 1 (a) Overlays require spreading the funds available for rehabilitation, (b) Inlays concentrate funding where needed.

Lane-specific inlays concentrate pavement strength and rehabilitation funding where needed. The outer lane usually carries from 80 to 90 percent of the design loads on a pavement and deteriorates before other lanes [8][9]. An outer lane inlay places the new pavement where the majority of loading occurs. Overlays require spreading the repair across all lanes, despite the need (Figure 1).

Existing materials can be recycled in an inlay. Asphalt materials removed before placing the inlay make excellent backfill for shoulder fillets. In some cases agencies reuse the materials in a base for the inlay. Concrete removed from an existing pavement can also be crushed and reused as aggregate for base and surface concrete [10][11][12].

Inlays do not require major highway appurtenance adjustments. Many non-pavement improvements are necessary with the significant increase in roadway elevation after overlay. Non-pavement factors that elevation changes affect include: guardrail, median barrier, signs, cross-slopes, vegetation, right-of-way, ramps, and overhead and at-grade structures [13]. The cost of these safety factors can approach 33 percent of a structural overlay strategy per mile [14]. Because inlays do not significantly alter roadway elevation, they do not require appurtenance adjust­ments or associated costs.

Traffic disruption is minimal - especially for lane-specific inlays. Adjacent lanes and shoulders can handle traffic during inlay construction. Hauling operations are constrained to the trench or to the shoulder adjacent to the lane being replaced. Multi­lane inlays require building cross-overs and placing traffic in opposing lanes on four-lane divided facilities.

Table 1. Four basic types of inlays

Type Use
Lane Specific Roadway/Street
Multi-lane Roadway/Street
Partial-Depth Roadway/Street/Airport
Keel Replacement Airport Runway

Design Considerations

Figure 2 Inlay concepts for existing concrete roadway pavements.
Figure 3 Inlay concepts for existing asphalt roadway pavements.
Figure 4 Inlay concepts for existing municipal concrete street pavements.
Figure 5 Keel section replacement illustration.

Inlays are effective for both existing concrete and bituminous pavements. For either pavement type there are many possible scenarios. Figures 2 and 3 show some of the most common for highway ap­plications. Figure 4 shows municipal inlay ideas.

Figure 5 shows an airport runway keel replacement. The keel of a runway is generally the center 50-75 ft (15-23 m). This portion of a runway receives the majority of loading and often wears before the outer portions.

Although generally distinctions are necessary be­tween runway and highway design, many inlay design and construction factors are similar. The ma­jor difference is that no surface elevation adjustment is acceptable for keel replacement. Keel replacements strengthen the pavement by increasing structure and depth of pavement. Surface elevation changes on airfield require significant adjustment to lighting, sensing equipment and drainage patterns. Usually airfields cannot tolerate the down time necessary to accomplish these changes.

Inlay design requires special consideration of several important features: structure, support, drainage, joint­ing and surface elevation.

Structural

Structural or thickness determination can vary from the traditional approach. Three distinct methodologies are available to the designer: traffic-based, constrained and balanced. Evaluation of foundation support is a key element in all three approaches. Support should be as uniform as possible [15].

Most inlay designs start by considering where the in­lay/existing layer interface will occur. The interface is usually within an existing base layer. A portion of the base layer and each underlying layer become the foundation for the inlay.

Using any of the three design approaches will likely result in an inlay thickness that meets or exceeds the thickness of an existing concrete pavement. Inlays thinner than an existing concrete pavement are not recommended. Consider the existing concrete slab thickness the minimum for an inlay in a concrete pavement. No special thickness limits are applicable to an inlay slab for an existing asphalt pavement.

Generally there is no performance problems when the inlay is thicker than an existing concrete lane. However, consideration of differential frost heave potential is necessary in areas prone to frost heave and formation of ice lenses. Differential frost heave can occur on adjacent sections of unequal thickness. Water that may collect in the dam created at the thickness transition may enhance ice lense formation. Ice lenses may move the thinner slabs more than ad­jacent thicker slabs. One simple solution is to con­sider using a tapered design. With a taper, the inlay thickness matches the existing pavement at the lane transition and tapers toward the outer shoulder. The tapered design also eases drainage because there is no damming effects from an abrupt thickness change.

Inlay terminus design depends on the type of adjoin­ing pavement. Adjoining asphalt pavement requires only a straight butt joint. An adjoining concrete sec­tion requires a doweled joint. Continuously reinforced inlays require either a wide-flange or lug-anchor ter­minal restraining system. The adjoining pavement butts against the restraining system.

Figure 6 Design flowchart for traffic-based inlay design.

Traffic-Based — Thickness is designed to handle future traffic for thirty years or longer. This is the most common design practice. The strength and thickness of layers that remain after existing pave­ment removal influences the foundation support used in design (Figure 6).

Constraine d — Thickness is derived by constraints on surface elevation and existing layer requirements. Where constraints limit slab thickness, traffic becomes a design output rather than design input.

Figure 7 Design flowchart for constrained inlay design.

This approach would only be likely for very thin ex­isting sections, or for municipal pavements. To deter­mine the traffic-carrying capacity of a constrained design, the designer must work the thickness design procedure in reverse (Figure 7). To improve the capacity, the designer may also choose to increase concrete strength or improve load transfer.

Balance d — For lane-specific inlays, the designer may balance inlay structural capacity with the re­maining structural capacity of the existing lanes. This requires thorough evaluation of the existing lane(s).

Figure 8 Design flowchart for balanced inlay design.

The existing lane's remaining structural capacity becomes the future traffic input for inlay thickness design (Figure 8).

Matching inlay load carrying capacity to that of ex­isting lanes will likely provide the thinnest inlay sec­tion. This approach assumes that the entire roadway will require reconstruction at the end of the inlay life. The inlay thickness should never be thinner than an adjacent concrete traffic lane.

Characterizing Support

Figure 9 Plot of core thickness measurements helps locate changes in existing pavement layers and areas of potentially thin base for the inlay.

Often the existing thickness of pavement layers will vary from that shown on the original plans. Cores provide accurate thickness measurements and iden­tify changes in the existing pavement design. Coring locations are extremely important. Enough cores are needed to provide reasonable assurance of accurate layer thickness along the entire project length. A plot of core thickness measurements locates potential thin base areas (stations) (Figure 9). It is important to note these locations and adjust the design profile to meet minimum base thickness requirements.

For example, the design may require several inches of an existing stabilized layer. If milling removes the entire stabilized layer during construction, support will vary from that expected in design. Areas show­ing significant differences in support should be corrected.

Figure 10 Falling weight deflectometer.

Non-destructive testing provides a tool to characterize support from existing pavement layers [16][17]. A falling weight deflectometer (FWD) measures the deflection of the pavement system from a fallen load (Figure 10). The stiffness (modulus of elasticity) of each layer is derived from the deflec­tion measurements and the actual thickness of each layer. Using the modulus values, an engineer can employ a back-calculation technique to quantify the support. The result is a composite modulus of subgrade reaction (Keff).

Unfortunately deflection testing may be too expen­sive for some municipal agencies. Instead of testing, procedures outlined in references 7[18] and 13[19] provide a means to estimate the foundation support. A foun­dation support estimate is also acceptable to characterize support for a constrained design.

Lane Dimensions

Lane dimensions are important in inlay design, par­ticularly for lane-specific inlays. Free-edge loads only occur at the longitudinal shoulder joints on typical concrete pavements (without concrete shoulders). Lane-specific inlays have two free edges subject to edge loads. Edge loads produce high slab stresses. Extending the lane width will reduce the number of vehicles running directly on the slab edges. This is because trucks tend to track based on the lane stripe and not the slab edge [20].

Figure 11 Edge support provided by 2-foot extended lane reduces edge load stresses within a slab [21].

Most agencies provide a widened lane wherever possible for both lane-specific and multi-lane inlays. A 12-18 in (30-46 cm) extension of the lane ade­quately reduces edge stresses from loads traveling within the travel lane [22]. Figure 11 shows the stress reduction from a 2-ft (60-cm) lane extension. The designer should consider moving the shoulder joint even if a tied concrete shoulder is desired with the inlay. This takes advantage of the added support of shoulder and reduced stresses from the lane extension.

The inlay/existing-lane joint will be subject to edge load conditions also. Major width extension of the in­lay is not possible without significantly encroaching on the adjacent lane. However, one agency moved the longitudinal joint 6 in (15 cm) to relieve some edge stress [23]. No documentation of performance is available to show the effect of this extension.

Drainage

A drainage system is desirable in all but a few in­stances. This is because inlays are built within an ex­isting system. The designer has little control over the drainage of the existing materials. However, con­sideration of drainage in municipal application is not as critical as highway or airport work. Generally the curb and gutter and storm sewer systems are ade­quate for controlling municipal pavement drainage.

A longitudinal drain and trench allows water to escape from beneath an inlay. In most cases, new pavements on relatively free-draining subgrade soils do not require a drainage system [24]. Water that enters from the pavement surface runs through and along pavement layers to the ditches. The natural soil then carries water from the pavement. When im­permeable stabilized materials surround an inlay, a drain system is needed even where in situ soils are relatively free-draining.

Typical drainage systems for concrete inlays consist of a permeable base layer and one longitudinal edge drain and outlet system. Most agencies employ a standard 3-4 in (8-10 cm) slotted drain [25]. It is im­portant that the base be cut to a uniform slope toward the drain. The spacing of outlet pipes depends on the terrain, rainfall and roadway slopes. Typically, 300-500 ft (92-152 m) between outlets is sufficient. Figure 12 details a typical drain, trench and backfill. Reference 16[26] provides further informa­tion on drainage.

Pavements with an existing drainage system that is functioning well do not require further drainage con­sideration. An existing system in good condition will be sufficient to drain the inlay. However, it is impor­tant to thoroughly inspect the existing system before deciding if it will continue to function properly. Replace portions of the system which do not function after cleaning and rodding.

Some agencies "stabilize" a subgrade by placing an edge drain system as much as one year before rehabilitation. This allows the pavement layers to drain and the water table to lower [27]. The benefit of this practice depends on the soil, the drain effec­tiveness and the amount of moisture present.

Joints

Transverse and longitudinal joint design should follow the same recommendations made for standard con­crete pavement. Saw transverse joints to one third of the slab depth (D/3). Longitudinal joints in multi-lane inlays also require sawing to D/3 [28]. For more in­formation see ACPA technical bulletins, "Design and Construction of Joints for Concrete Highways," "Design and Construction of Joints for Concrete Streets," and "Joint and Crack Sealing and Repair for Concrete Pavements".

The joint spacing on lane-specific inlays of existing concrete pavements should match the existing lane joint spacing. Sympathy cracks through the inlay will often develop from mismatched spacings. The sym­pathy cracks are a result of tiebar restraint of thermal movements.

The need for load transfer devices in transverse con­traction joints depends on support conditions, traffic and slab design. Routes which will carry heavy truck traffic require doweled joints. Smooth dowel diameters should not be less than 1-1/4 in. (32 mm). The diameter of the bar will depend on inlay thickness as outlined in Table 2. Dowels need not be greater than 12-16 in (30-41 cm) in length with a spacing of 12 in (30 cm). To avoid corrosion and future load transfer problems in areas using deicing chemicals, use epoxy coating or other suitable protection.

Figure 13 Tiebars and keyway form in place in the inlay trench. The keyway form is nailed to the asphalt trench edge.

Some agencies provide for possible future replace­ment of asphalt lane(s) next to lane-specific inlays [29]. They require a keyway and tiebars along the inlay longitudinal joint for the full project length (Figure 13). Tiebars anchored in the inlay slabs are ready if the adjacent asphalt lane is eventually reconstructed. After removing the adjacent lane, the contractor straightens the bars which are immediately ready to tie a new concrete lane. Since a considerable amount of surface water can enter the longitudinal joint, consider epoxy-coated tiebars for this purpose.

Another option is to provide a butt face edge. In the future tiebars can be epoxied into holes drilled in the slab edge just before paving the lane.

Transverse and longitudinal joints in inlays require sealing with the same procedures as for new pave­ment. Minimizing water entry into concrete pavement built into existing stabilized layers is important. Seal­ing minimizes surface water intrusion that might undermine support of the inlay. Sealant also blocks the entry of incompressibles which might spall the concrete and ravel adjacent asphalt.

Care is needed in sealing the longitudinal joint be­tween an inlay and adjacent asphalt lane(s). In stan­dard pavement, poorly sealed longitudinal shoulder joints may permit entry of 80 percent of the surface water [30]. A sealant used along the longitudinal in­lay joint separating concrete and asphalt must be capable of adhering to both materials. Both silicone and low-modulus asphalt sealants have been effec­tive [31].

The Oregon Department of Transportation has ex­perimented with the use of longitudinal joint seal along their lane-specific inlays. After eight years of service it reports no significant difference between sealed and unsealed sections. However, it has not made a firm conclusion on sealing practice and in­tends to seal all joints during maintenance activities. Oregon also provides an open-graded drainage layer and piping system under all inlay projects.

Table 2. Recommended dowel sizes for concrete inlay pavements.

Pavement Thickness in (cm) Dowel Diameter in (mm) Length of Bar in (cm) Spacing on Centers in (cm)
5 (13) -- -- --
6 (15) -- -- --
7 (18) -- -- --
8 (20) 1.25 (32) 15 (38) 12 (30)
9 (23) 1.25 (32) 15 (38) 12 (30)
10 (25) 1.5 (38) 16 (41) 12 (30)
11 (28) 1.5 (38) 16 (41) 12 (30)
12 (30) 1.5 (38) 16 (41) 12 (30)

Surface Elevation

Surface elevation requirements can be critical in inlay design. A nominal elevation increase of about 2-5 in (5-13 cm) from existing is typical for highways. After completing the inlay, the contractor resurfaces the shoulders even with the inlay surface. Municipal street and airport inlays usually meet existing surface elevations.

It may be necessary to consider support uniformity when making decisions on geometric adjustments [32]. Major adjustments may interfere with the planned depth of slab/base interface or desired surface eleva­tion. This is particularly true in making design adjustments to vertical curves. It can also arise in ad­justing roadway cross-slope or superelevation on horizontal curves. The designer can provide for these types of adjustments by allowing some flexibility in the elevation selection.

Figure 14 Adjusting vertical alignment for inlays requires care to ensure base thickness that meets design expectations.

Figure 14[33] provides an example of flattening a vertical curve. The designer must weigh the consequences of cutting through base layers versus raising the new surface elevation. If possible, it is helpful to maintain the desired base thickness at the low point of the curve. A deep cut will introduce a location of non­uniform support. Some removal and replacement of the base may be necessary to establish uniform sup­port in these areas.


Construction Considerations

Inlay construction requires special consideration of drainage, existing pavement removal and steps necessary to develop a smooth profile. Sequencing of construction operations may differ between inlays in existing asphalt and inlays in existing concrete pavement.

Edge Drain

Figure 15 Drain orientation.

The most beneficial time to install edge drains depends on the edge drain location, and the existing shoulder and mainline material. Some agencies place the drain directly below the inlay slab. Other agen­cies place the drain beneath the shoulder. Figure 15 provides a schematic of drain orientation. The preferable drain orientation is beneath the shoulder. This avoids disruption of base support along the critical slab edges. For inlays that maintain existing surface elevation, orientation beneath the shoulder will be less disruptive to the existing shoulder.

Trenching for drains beneath the mainline slab must occur after removal of the existing pavement. The contractor cuts a drain trench through remaining asphalt and base layers. A crew installs the pipe, fabric and backfill before further construction activities.

Trenching through concrete pavement can be dif­ficult. Therefore drain installation usually follows pave­ment removal for existing concrete pavements. Another reason is that breaking and removal ac­tivities may damage a new subsurface drain installation.

It is best to construct drains oriented under the shoulder after inlay paving. This delays cutting and backfilling through the trackline area. Trenching and backfilling might otherwise hinder slipform paver stability and resulting pavement profile.

Existing Asphalt

If the new inlay will meet the existing surface eleva­tion, the first step is to establish inlay boundaries [34]. This occurs before removal begins. A diamond or carbide saw blade provides an excellent boundary cut which accurately establishes inlay dimensions. Where possible a full-depth cut eases removal. The smooth vertical face of the diamond or carbide cut acts as a side form during concrete placement.

A relief cut near the full-depth boundary cut may also help protect surrounding material [35]. A 4-in (10-cm) wide wheel saw cut provides an adequate buffer for the removal operation. However, it is important to maintain a true corner on the shoulder cut especially where the inlay will maintain the existing grade.

No special boundary cuts are necessary if the inlay surface is to be higher than the existing surface. A cold-milling machine provides an adequate edge. Any raveling of the trench edge is covered with an overlay of the adjacent lanes or shoulders.

Figure 16 Cold-milling asphalt pavement for inlay trench.

Removal - Cold milling is the most common pro­cedure for asphalt pavement removal (Figure 16). It has been used extensively for inlay projects. The speed and grade-accuracy of cold milling meets several key needs of concrete inlay construction.

Cold milling equipment uses carbide teeth mounted on a rotary drum. The teeth chip away existing asphalt as the drum rotates. The size of broken material is largely dependent on the tooth configura­tion, drum rotation, machine speed and removal depth [36]. Particle size also varies with temperature, quality, condition and asphalt content of existing material. The ability to control particle size is helpful when the asphalt millings are reused in the base course or shoulder fillets.

Profiling the slipform paver tracklines is an essential part of the removal process [37][38]. Milling or grinding equipment delivers effective profiling. The area about 3 ft (1 m) along both edges of the inlay require this operation. The profiled tracklines establish the reference plane for the project. This operation affects mill-depth, concrete yield, and pavement smoothness.

Removal equipment must be capable of referencing the mill depth from the profiled tracklines. A 25-30 ft (8-9 m) averaging ski mounted on the equipment will provide an acceptable milled trench tolerance.

Attaining the desired milling depth may require several passes. It is the contractor's decision based on his equipment and schedule. Although particle size consistency increases for shallow removal, the project expediency also impacts the decision. Com­mon machines are capable of removing material down to 6 in (15 cm) below the surface. Larger equipment can remove an entire 12-ft (3.75-m) width in one pass. Most projects document single-pass removal of about 3-4 in (7-10 cm)[39][40][41].

Ripping/scarifying procedures common in removing thin lifts of existing asphalt are not recommended for most inlay projects. Ripping is acceptable where removal will go below the depth of all asphalt layers.

Some depth or base profile control is necessary on subsequent removal procedures. Ripping involves motor-graders or bulldozers with mounted scarifying equipment. Although it is an inexpensive operation, it does not provide the grade control possible with cold-milling.

Existing Concrete

Boundary establishment before removal is also necessary for existing concrete pavements. Essen­tially the same procedures apply as described for existing asphalt. A full-depth cut by a diamond or carbide blade saw is necessary to sever reinforcing bars. Sawing along existing joints cuts through ties between shoulders or lanes that will remain in-place.

Additional relief cuts may be necessary to protect any surface concrete that is to remain in the pave­ment system. The relief cut provides a buffer to the pounding and vibration imparted by breakup equip­ment. Micro cracking and damage to adjacent con­crete might result if left without this protection. Relief cuts are not necessary to protect asphalt shoulders.

Some degree of concrete recycling is usually part of removing an old concrete pavement. The major use for the old concrete is as aggregate for base and surface courses. Phases in concrete pavement recycling include concrete breakup, removal, rein­forcement separation and concrete crushing/sizing. ACPA technical bulletin "Recycling Concrete Pave­ment" discusses each phase in detail [42]. Discussion in this bulletin is only on breakup and removal of ex­isting concrete. There are three basic methods for removing existing concrete in an inlay project: breakup, milling and lift­out.

Breakup Removal - Breakup of an existing con­crete pavement eases removal and recycling. The advantage of breakup is that it can size the material for ease of handling, transport and crushing [43]. Many types of equipment will break the concrete adequately. The two basic breaking machine classifications are impact and resonant. Table 3 characterizes some of the available breaking equip­ment [44].

Removal of any asphalt overlay or patching materials should precede breaking operations [45][46][47][48]. This may not be critical if the agency intends to simply discard the material. However, for recycling this will limit the contaminating material in recycled ag­gregate. The contractor uses a front-end loader after milling or scraping the asphalt. The loader can dislodge any material still adhering to the old con­crete surface. Brooming removes most loose material and provides the final preparation.

The desirable size of broken pieces is less than 24 in (60 cm) [49][50][51]. Each breaker type will have unique characteristics and require different operational pat­terns. The forward speed of the breaking equipment and spacing of breaker passes can control the break size. It is up to the contractor to develop a breaking pattern that produces desirable results. Impact equip­ment provides broken pieces from 12-24 in (30-60 cm) while resonant breakers provide pieces less than 6 in (15 cm).

Where steel is present in the existing slabs, breakup equipment must be capable to some degree of separating reinforcement from the concrete. Some separation is important for removal productivity and crushing.

Figure 17 Rhino horn removing continuous reinforcement.

Furthermore, steel separation may be necessary before loading and transporting the broken concrete. Many contractors use a rhino horn to penetrate the broken concrete and snag steel strands (Figure 17)[52][53][54]. The rhino horn is a 30-in (75-cm), curved and pointed, hard-steel pick. The rhino horn mounts on front-end loaders, backhoes or bulldozers and can load snagged steel on to trucks for disposal. Some handwork to cut the steel usually accompanies the rhino horn operation. In some cases, the breakers provide enough steel separation to enable contractors to remove steel with loaders [55]. Final steel removal occurs at the crushing plant.

The force of some breakup equipment, particularly diesel hammers, may damage underlying base layers. Special care is needed with diesel hammer equipment. The large impact from this equipment can easily depress broken concrete into a soft base course.

Contractors use front-end loaders to pick up broken concrete and load dump trucks after initial steel removal. The degree of removal depends on the in­tended use for the old concrete. The loader operators use more care if the salvaged concrete is being recycled for surface concrete. They are careful not to scrape too much base/subgrade material.

Earth and cohesive soils can adhere to the under­side of the broken concrete during wet weather [56]. In these circumstances, the contractor may limit his removal operation to dry weather only. This is not usually a problem for granular or stabilized base materials.

Table 3. Characteristics of available breaking equipment

Type Typical Break Pattern in (cm) Approx. Blows Per Minute
Diesel Pile Driver Impact 18 (46) 80-90
Diesel Hammer Impact 24 (61) 90
Falling Weight Impact 24 (61) --
Swing Hammer Impact 24 (61) --
Wrecking Ball Impact 24 (61) --
Vibrating Resonant 6 (15) 2640

Milling Removal - Cold milling also works to remove existing concrete pavement. It is an excellent method for plain pavement without mesh, dowels or continuous reinforcement. Embedded steel can break carbide milling teeth and wrap around the milling drum. Except in partial-depth removal for bonded in­lays, milling is not likely to be effective for other than plain pavements. However, a boundary cut that severs any tie steel between lanes is usually necessary even on plain pavements.

Milling concrete provides depth control and little removal damage. It will not damage an existing base, even if the material has bonded to the con­crete slabs. Milled concrete can be recycled into granular aggregate for shoulders and fill material [57].

Milling concrete requires heavy-duty equipment and experienced operators. One contractor reports removal up to 2.5 in (6 cm) in one pass [58]. Asphalt milling depth-control measures apply as well for con­crete milling.

The productivity of a milling operation will depend on the aggregate hardness, bit configuration and removal depth. In some regions the cost of milling hard aggregate may be prohibitive. It is important to evaluate milling feasibility before specifying milling removal. Table 4 provides a simple list of aggregates and their relative hardness.

Embedded steel mesh was not a problem on one bonded inlay project. The one-lane inlay was placed to correct a problem of reinforcing steel situated in the upper 2 in (5 cm) of the slabs. Steel corrosion began spalling the surface. The contractor sawed the slab into 24-in (60-cm) transverse intervals [59]. The 2 in (5 cm) deep cuts severed the steel so no strand was greater than 24 in (60 cm) long. Milling pro­ceeded at 950 square yards per hour (794 square meters per hour) without problems after this treatment.

Table 4. Aggregate Hardness

Type Hardness
Limestone Soft
Dolomite Soft
Coral Soft
River Gravel Medium
Trap Rock Medium
Granite Hard
Flint Hard
Chert Hard
Quartz Hard

Lift-out Removal - Lift-out is a gentle removal method that essentially does no damage to most underlying base layers [60]. It also does very little damage to adjacent shoulders. Unlike breakup or milling, lift-out does not reduce the concrete to small pieces. Concrete removal is in manageable panels. Contractors originally developed lift-out to speed pro­duction in patching operations [61].

Figure 18. Front-end loader lifting the sawed pavement segments from the base. Note how the base is not damaged.

Effective lift-out first requires sawing the pavement in­to small panels (slabs). Determining the practical panel size must include consideration of the lift and transport equipment as well as panel weight. Typi­cally, front-end loaders equipped with a forklift at­tachment lift the slabs out of place (Figure 18) [62]. Flatbed or other suitable trucks transport the panels.

Segmenting plain concrete pavements may be as simple as lifting each slab, while continuously rein­forced pavements (CRCP) require severing the steel. One contractor found panels 8 x 10 ft (2.5 x 3 m) to be safe and effective [63]. His removal crew made longitudinal saw cuts 8 ft (2.5 m) from each shoulder on a 24 ft (7.5 m) CRCP pavement. Then the crew made transverse cuts at 10 ft (3 m) intervals.

There are many uses for the old concrete panels. Contractors report using the panels as fill in large ex­cavation, or for erosion control along ocean or lake shores [64]. Of course, the contractor can send the panels to a crushing plant for recycling. However, using breakup equipment in a recycling project is probably more cost-effective.

Lift-out may damage base materials which bond to concrete slabs. As lifting begins, a bonded base may tear or shear internally leaving an uneven surface. In comparison to other methods this might make lift-out unattractive for projects requiring gentle base treatment.

A cost comparison of removal methods will help the contractor and engineer assess feasibility. The com­parison will also clarify the secondary goals of removal process. It is important that the comparison weigh the trade-off between cost, productivity and damage. For example, the sawing operation before lift-out removal may be expensive on concrete con­taining hard aggregates. This expense may over­shadow the benefit of gentle base treatment.

Placement

Most placing operations are typical of concrete pavement construction. However certain project conditions unique to inlays may require special consideration. These factors are base preparation, dowel bar place­ment, haul road access, traffic control and foul weather measures.

Granular base disturbance from loader operations after breakup and removal of concrete necessitates some repair. Rolling and compacting the base is typical.

Figure 19 Permeable base material being dumped and spread in the trench.

New base materials are placed on the reworked material (Figure 19). Autograding or trimming equipment can then effectively shape the new material.

The contractor usually uses the shoulder as a haul road for concrete batching vehicles. This allows the contractor freedom to set dowel baskets or con­tinuous reinforcement in the inlay trench. In most cases the existing shoulder is stable and structurally adequate to handle batch trucks. Where rutting oc­curs in the shoulder surface, subsequent resurfacing will alleviate the problem.

Agencies may consider allowing the contractor to place hauling operations in the trench on the existing base course. This option is useful in tight working areas that do not have clearance for shoulder bat­ching. Municipal and urban pavements with many bridges often pose clearance problems, it is impor­tant to evaluate the base to ensure it can support the haul trucks.

Figure 20 Timber ramp for batch trucks to enter the inlay trench without damaging the trench edges.

The option of hauling on the existing base course is very effective for plain undoweled pavement. It is also possible in doweled pavement if the contractor can employ dowel insertion equipment. Base course hauling requires a ramping system ahead of the pav­ing operation. Batch trucks back up to the paver after ramping into the trench (Figure 20). Contractors typically fabricate ramps from railroad ties or other heavy timber.

Lane-specific inlays usually have clearance to allow traffic to run in adjacent lanes. There is no need to construct cross-overs except for heavy traffic routes. This reduces the cost of traffic control and eases traf­fic congestion. The contractor requires at least 3 ft (1 m) of encroachment for the paver trackline. An addi­tional 2 ft (0.6 m) provides room for a barrier wall for work zone safety. Traffic rides on both the open lane and shoulder. Multi-lane inlays provide no traffic control advantages over other reconstruction/overlay options.

Figure 21 Drainage relief cut through the existing shoulder.

Rain can flood an inlay trench. Drying periods are extensive without some means for the water to escape. Most contractors provide a narrow transverse opening from the inlay trench through the shoulder (Figure 21). Water can drain through the opening and into roadway ditches. The agency should allow the contractor to make transverse open­ings where needed to allow runoff.

Smoothness

Figure 22 Inlay paving operation. Note the milled tracklines for the slipform paver.

Milled tracklines improve pavement smoothness [65]. Because these surfaces are usually overlaid after in­lay placement, engineers do not always consider shoulder profiling. The profile plane developed within the trench during removal by milling only helps con­trol pavement thickness and material yield. It does not aid smoothness. In most instances a better pave­ment will result if the contractor also controls the pro­file beneath the tracklines (Figure 22). As previously discussed, this operation also impacts removal operations.

Dual stringlines also provide excellent grade control and help increase pavement smoothness [66]. In some cases the outer and inner trackline elevations are substantially different. Placing construction stringlines along both sides of the roadway maintains consistent orientation of the slipform paver. Otherwise abrupt changes of the profiling pan may result in bumps in the concrete pavement.

Surface feathering may be necessary on projects where the inlay elevation matches the existing eleva­tion. Surface feathering creates a smooth transition from the inlay surface to the adjacent shoulder or lane [67]. Diamond grinding or milling machines (for asphalt) are capable of planing and feathering. A smooth transition is desirable from inlay edge toward the adjacent shoulder/lane. The operation should not leave a ridge or lip in the adjacent shoulder/lane.

Surface Texture

Surface texture requirements for multi-lane or full-width inlays follow the same guidelines as for new pavement. Transverse or longitudinal tine texturing for Interstate and primary routes provides excellent long-term skid resistance. For low-speed municipal or urban projects, a burlap or turf drag texture is sufficient.

There are no known safety implications of using con­crete and asphalt in adjacent lanes. Some engineers concerned with the effects of different adjacent sur­face materials, question the effects on drivers [68]. An Oregon Department of Transportation study revealed no statistical evidence of a safety problem on lane-specific inlays [69].


Payment

Payment for concrete inlays should be by the cubic yard for supplying the material and by the square yard for placing, finishing, texturing, curing and joint­ing the inlay. Generally the actual inlay thickness will vary from the nominal thickness shown on the plans. This is especially true for inlay designs which meet existing grades or where base preparation omits trim­ming. Variations are due to vertical displacements in the existing surface and changes in cross-slope or grade. Because of this, the most equitable and economical basis of payment is to separate the cost of supplying the concrete and placing the inlay.

Cubic Yard Payment - The cost of concrete mix materials, mixing operations and transporting opera­tions are included in the unit price per cubic yard for supplying the concrete. To determine the volume of concrete requires that each concrete batch quantity be recorded at the point of placement. The engineer and contractor can then agree on the actual volume from the sum of all batches delivered and placed.

Square Yard Payment - The placement cost of the inlay includes concrete placing, finishing, curing, reinforcing, and sawing and sealing joints.

A divided payment reduces the initial cost of con­struction. The contractor does not have to estimate unknown quantities. Payment is made for only the volume of concrete actually used. Improved rideabil­ity may also result by reducing the number of equip­ment adjustments to regulate thickness requirements and yield controls.

Performance

Figure 23 The present serviceability history of the first concrete inlay in the state of Iowa.

Concrete pavement inlays have performed well in many different areas of the United States. Some in­itial projects brought attention to the requirements of drainage. Figure 23 shows the present serviceability history of the first concrete inlay in the state of Iowa. Table 5 lists some concrete inlays and shows the wide variety of design and construction techniques.

Additional Information

Additional information on concrete inlay strategies is available through the American Concrete Pavement Association.

Table 5. Selected inlay projects n the United States

State Nearby City Route Name Year Built Inlay Type Inlay Pavement Type Thickness
in. cm
Existing Pavement Type Removal Method Elevation Above Existing Pavenemt
in. cm
Remarks
FL Jacksonville Airport 1983 Keel JPCP 14-21 36-53 Plain Break-up 0.0 0 Tapered cross-section Keel replacement 50 ft.
IA Adair I-80 1979 Multi-Lane JPCP 10 25 HMA Milling 1.0 3 1st in the state
IA Adair I-80 1981 Lane Specific JPCP 10 25 HMA Milling 0.0 0 Surface matched to original
IA Atlantic I-80 1988 Multi-Lane JPCP 11.5 29 JPCP Break-up 1.5 4 10-in. JPCP recycled for drainable granular base
IA Ames I-35 1984 Multi-Lane JPCP 10 25 CRCP Break-up 1.0 3 Replaced existing 8 in. CRCP
ID Boise I-84 1984 Multi-Lane JPCP 8 20 HMA Milling 3.0 8 Preserved existing CTB
KS Kansas City I-435 1989 Lane-Specific JPCP 2 5 JRCP Milling 0.0 0 Bonded inlay Removal also used sawing of top 2-inches.
MA Lowell Main Street 1990 Lane-Specific CRCP 10 25 JRCP Break-up 0.0 0 Center lane of 3 lane facility
OK Oklahoma I-40 1983 Multi-lane Plain 10 25 Plain Break-up 1.0 3 Recycled Concrete/Econocrete Shoulders
OR Goshen Grove I-5 1984 Lane Specific CRCP 13 33 HMA Milling 2.0 5 1st in state
OR Meacham I-84 Lane-Specific CRCP 8 20 HMA Milling 3.0 8 Contains Experimental Section
OR Meachem I-85 Multi-Lane CRCP 11 28 HMA Milling 3.0 8 Inner lane only 8-in. CRCP
OR Baker I-84 Lane-Specific CRCP 11 28 HMA Milling 2.0 5
VA Emporia I-95 1989 Multi-Lane JPCP 8 20 Plain Milling 0.0 0 Milling done in four passes. Shoulders replaced.
VA Charlottesville I-64 1988 Multi-Lane CRCP 8 20 CRCP Lift-out 0.0 0 CRCP sawed full depth at 10 ft. to facilitate lift-out
WY Point of Rocks I-80 EB 1989 Lane-Specific JPCP 10 25 HMA Milling 2.0 5 No drainage improvement used. Replacement deeply rutted truck lane.
WY Rawlins I-80 EB 1989 Lane-Specific JPCP 10 25 HMA Milling 2.0 5 Longitudinal drains employed. Widened outer lane to 14 ft.
WY Arlington I-80 EB 1990 Lane-Specific JPCP 10 25 HMA Milling 2.0 5 Longitudinal drains employed. Widened outer lane to 14 ft.

Summary

There are many ways to apply inlays for specific pro­ject needs. Inlays optimize project materials and funds by directing the improvement only where it is needed. The following are important factors for con­sidering concrete inlays as a rehabilitation strategy:

  • Lane-specific inlays solve the difficult problem of differing performance between passing lanes, truck lanes and shoulders.
  • Multi-lane inlays provide a strategy to reconstruct a roadway while keeping the ex­isting shoulders.
  • Runway keel replacement allows strength upgrade in channelized-traffic areas without altering surface elevation.
  • Inlays do not require significant adjustment to elevation or non-pavement roadway features.


The following are important considerations in the pro­posal, design and construction of concrete inlays:

  • Inlays are effective for both existing concrete and asphalt pavements.
  • Inlay design can be based on design traffic, constrained by vertical clearances or balanced to the traffic capacity remaining in existing lanes.
  • A drainage system is desirable in all but a few instances for high-volume heavy traffic routes.
  • Inlay construction requires special consideration of existing pavement removal.
  • Inlays allow recycling of existing pavement materials.


References

  1. "Concrete Inlays, An Alternative Overlay," Federal Highway Administration, Pavement Division, Washington D.C., March 1991.
  2. "Concrete Inlays, An Alternative Overlay," Federal Highway Administration, Pavement Division, Washington D.C., March 1991.
  3. Calvert, G., "Portland Cement Concrete Inlay Work in Iowa," Transportation Research Record 924, Transportation Research Board, National Research Council, Washington D.C., 1983, pp. 15-18.
  4. "Virginia Interstate Reconstruction Gets Gentle Subbase Treatment," Concrete Pavement Progress, Vol 26, No. 3, American Concrete Pavement Association, May 1990.
  5. Lane, J., Chase, G., "Rehabilitation of a Portion of Interstate 35 with Pavement Inlay using Recycled Concrete for Subbase," Proceedings, 4th International Conference on Concrete Pavement Design and Rehabilitation, Purdue University, April 18-20, 1989, pp. 649-655.
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  10. "Recycling Concrete Pavement," TB-014.0, American Concrete Pavement Association, Arlington Heights, IL, 1993.
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  12. "Portland Cement Concrete Recycling," Pavement Rehabilitation Manual, Chapter 1, Federal Highway Administration, Rev. 1990.
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