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Fast-Track Concrete Pavements

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Airport authorities and road agencies face major challenges from increasing traffic volumes on existing airports, roadways and urban streets. Agencies must repair or replace deteriorated aging pavements while maintaining traffic on these structures. Traditional pavement construction, repair or replacement solu­tions are no longer acceptable due to increasing public impatience with traffic interruption. Traditional solu­tions are especially inappropriate in urban areas where congestion is severe. Fast-track portland cement concrete (PCC) pavement construction resolves these problems by providing high-quality, long-lasting pave­ments with quick public access. Fast-track tech­niques are suitable for new construction, reconstruc­tion or resurfacing projects.

Fast-track concrete pavement construction entails many methods for accelerating construction. Traditional acceleration methods include time incen­tives or disincentives for project completion. Agencies have been using these completion-date incentives for many years, and often contractors will meet these requirements by lengthening the work day or increas­ing the size of construction crews. Using fast-track concrete construction techniques a contractor can often complete a project without increasing crew size or changing normal labor schedules.

To build a fast-track project, both the contractor and agency must make some changes to traditional con­struction specifications and processes. Often this entails high-early-strength (fast-track) concrete, but also can include revising: opening criteria, construction stag­ing, joint construction and worker responsibilities. Table 1 suggests changes to project components that can decrease construction time. This publication discusses background information and details for these changes.

PROJECT APPLICATIONS

Fast-track construction techniques allow engineers to consider concrete for projects thought unfeasible because of lengthy concrete cure-times. Some speci­fications require cure intervals from five to fourteen days for conventional concrete mixes [1]. With fast-track techniques concrete can meet opening strengths in less than 12 hours [2][3][4]. The following four sec­tions describe the use and potential benefit of fast-track for various road and airport applications.

Highways and Tollways

Some highway agencies are using public relation cam­paigns to inform highway users of major work on urban expressways and arterial streets. Many highway agencies also are using fast-track concrete pavement techniques to expedite construction and ease work zone congestion. Major fast-track projects in Chicago and Denver are good examples of how fast-track meets the need to decrease construction time for urban and suburban roadways [5][6].

Tollway authorities lose revenue as a result of lane clo­sures because traffic delays cause many drivers to find alternate routes. Fast-track concrete minimizes rev­enue loss by allowing earlier access at high-congestion areas like toll-booths and interchanges.

The need for fast-track techniques on rural highway or road construction is more limited. However, a contrac­tor may use fast-track techniques to accelerate con­struction on portions of a project to allow construction equipment on the pavement sooner than usual. The contractor also may use fast-track for the last portion of a project to speed final opening to public vehicles. The Federal Highway Administration (FHWA) is encour­aging all highway agencies to use fast-track concrete to meet special construction needs [7].

Table 1. Changes to project components useful to shorten concrete pavement construction time [8].

Project Component Possible Changes
Planning Implement partnering-based project management.
Implement of lane rental charges
Allow night construction
Allow contractor to use innovative equipment or procedures to expedite construction (for example:minimum clearance machines, dowel inserters, ultra-light saws)
Specify more than one concrete mix for varied strength development
Provide options to contractors not step-by-step procedures
Use of time-of-completion incentives and disincentives
Concrete Materials Try different cement types (particularly III)
Use helpful admixtures
Use a uniform aggregate grading
Keep water-cement-plus-pozzolan ratio below 0.43
Jointing & Sealing Allow green sawing with ultra-light saws
Use dry-sawing blades
Use step-cut blades for single pass joint sawing
Use a sealant that is unaffected by moisture or reservoir cleanliness
Concrete Curing & Temperature Specify blanket curing to aid strength gain when beneficial
Monitor concrete temperature and understand relationship of ambient, subgrade and mix temperature on strength gain
Elevate concrete temperature before placement
Strength Testing Use non-destructive methods to replace or supplement cylinders and beans for strength testing
Use concrete maturity or pulse-vector testing to predict strength.
Traffic Opening Criteria Revise from time to strength criteria
Channel early loads away from slab edges
Restrict use to automobile traffic during early age period.

Streets

Fast-track technology also provides solutions for pub­lic access on residential and urban streets. Residents along suburban streets can gain access to their drive­ways within twenty-four hours. In Denver, Colorado, fast-track reconstruction of an urban arterial cut 75 days from the 200-day schedule (Figure 1) [9].

Figure 1. Denver, Colorado, completed arterial reconstruction project [10].

Intersections

Intersections pose major construction staging and traf­fic interruption challenges because they encompass two or more streets. As a result, agencies will often resurface intersections to cover-up rutting, raveling, corrugation and other safety problems instead of replacing the worn pavements. However, a unique project by the Iowa Department of Transportation involved the replacement of nine intersections using fast-track concrete [11][12]. Using two concrete mixes and night construction, the contractor finished each intersection without disrupting daily rush-hour traffic [13].

Reconstructing intersections one quadrant at a time allows traffic to continue to use the roadways. With fast-track techniques and quadrant construction, a contractor can pave the intersection in less than one week. Where it is feasible to close the entire intersec­tion for a short time, a contractor can use fast-track techniques to complete reconstruction over a week­ end.

Airports

On airport aprons, runways and taxiways, fast-track concrete speeds sequential paving pours. Fast-track concrete gains strength quickly and allows contractors to operate slipform equipment on the initial paving lanes sooner than normal. This reduces the construc­tion schedule by shortening the wait before paving interior lanes (Figure 2). Fast-track also can speed reconstruction of cross runway intersections, runway extensions, and runway keel sections. This may be necessary to maintain traffic at commercial airports or for the national defense at military air bases.

Figure 2. Paving interior lanes on an airport facility (note the use of initial lanes for construction platform).


PLANNING

Developing a traffic-handling plan before construction is essential for projects with high traffic volumes. The goal is to reduce the construction period and minimize traffic disruption. An agency will benefit because meeting this goal will curtail public complaints, busi­ness impacts, user delay costs and traffic control costs. The contractor will benefit by reducing workers' exposure to accidents and reducing the time equip­ment is needed at a project. Fast-track techniques provide an option for minimizing traffic disruption by shortening lane closure time [14].

Planners should include fast-track techniques in assessment of project feasibility or in development of construction staging plans. Table 2 lists other issues that also should be considered when planning a fast-track project.

Table 2. Important considerations for planning fast-track projects.

Important Planning Considerations
Access for local traffic
Local business disruption
Utility work
Construction equipment access and operation
Pavement edge drop-off requirements
Crossovers that disrupt both directions of traffic
Detour routes can suffer damage & congestion from prolonged construction zone detours
Using fast-track concrete near the end of one day's paving can facilitate next-day startup

One common method specifiers use to assure project completion by a certain date is through a completion-time contract that offers monetary incentives and disin­centives to the contractor. With time incentives or dis­incentives, the agency specifies the completion date and the daily incentive or disincentive value. The con­tractor earns the incentive for completing the project before the deadline or pays the disincentive for finish­ing late. These arrangements are easily understood and usually assure timely construction. However, cer­tain new lane rental contracting techniques may be more useful for fast-track concrete construction because they encourage more contractor flexibility and innovation than a completion-time contract.

Lane Rental

Lane rental is an innovative contracting practice that encourages contractors to lessen the construction impact on road users [15][16]. There are three basic lane-rental methods, cost-plus-time bidding, continu­ous site rental and lane-by-lane rental. For each method the agency must determine a rental charge for use of all or part of the roadway by the contractor. The rental charge usually coincides with the user cost esti­mate for delays during project construction. The user costs vary in each project and consequently so should rental charges. Computer programs, such as QUEWZ, can be helpful to determine work zone user costs [17].

Not all projects warrant lane rental assessments. A lane rental contract requires special contracting terms and is most suitable for large projects where construc­tion congestion management is critical. To reduce congestion on smaller projects an agency can modify concrete materials and construction specifications to decrease road or lane closure time. Contract manage­ment and record-keeping on lane rental projects can be difficult. There can be confusion in determining how to account for partial completion of portions of a project. Therefore, it is important for contract lan­guage to cover these situations.

Cost-plus-time bidding (also called "A+B bidding") divides each contractor's bid into two parts, the con­struction cost and the time cost [18][19]. Along with construction costs, the contractor must include an estimate of the number of days necessary to complete the project in the bid. The agency multiplies the time estimate by a daily rental charge to determine a time cost, and then adds the time cost to the construction cost to determine each contractor's total bid value. The contractor with the lowest combined cost receives the contract for construction. To encourage maximum production, cost-plus-time bidding should also include a completion-time incentive and disincentive.

With lane-by-lane rental, the contractor pays for the lanes or combination of lanes that his crew occupies during construction. The agency can vary the lane rental rates depending on the lane in use (outside, inside, shoulder) or upon the time of day or week (Table 3). This encourages the contractor to occupy lanes in off-peak hours and stage construction thoughtfully. This contracting arrangement may not be suitable for certain reconstruction projects with limited staging options.

Table 3. Sample proportional hourly lane-by-lane rental charge for a project length [20].

Closure or Obstruction Peak Time Periods
6:00-9:00 a.m.
3:00-6:00 p.m.
All Other Hours
One Lane $X 0.25*($X)
One Shoulder 0.25*($X) 0.0625*($X)
One lane and shoulder 1.25*($X) 0.3125*($X)
Two Lanes 2.25*($X) 0.6250*($X)
Two lanes and shoulder 2.50*($X) 0.6875*($X)

Partnering

The agency's goal is usually clear for fast-track pro­jects — perform the work with minimal traffic disrup­tion. Many agencies and contractors are now using partnering arrangements to focus on project goals and to maintain open communication. The result is timely decision making that keeps construction moving, saves money, and reduces the chance a problem will grow into a dispute.

Specifications

Small specification changes that expand the contrac­tor's construction and equipment choices often result in significant time savings. Examples include: mini­mum-clearance slipform paving machines, dowel-bar inserters, and ultra-light saws. Specifying more than one concrete mix will also allow a contractor to meet different construction needs within a project.

End-result specifications provide the most freedom to the contractor. With end-result specifications the con­tractor must provide a pavement meeting material, thickness and smoothness criteria. The agency does not closely control proportioning of the concrete mix or the method of paving. Fast-track concrete construc­tion automatically becomes a contractor option with end-result specifications [21].

Providing a choice of concrete mixes is a simple way of expanding contractor flexibility. Fast-track project specifications might include a mix for normal, moder­ate and high-early strength concrete. The contractor can choose from the different concrete mixes to suit different construction situations. For the majority of a large project the choice would probably be the normal mix. The contractor might decide to use the fast-track mix for the final several batches each work day to ensure that sawing could be done before nightfall. The fast-track mix also will ensure that the concrete at the construction joint (header) is strong enough for startup the following day. The moderate strength-gain mix might be useful for areas where construction traffic enters and leaves the new slabs.

Innovative Equipment

Recent improvements in paving equipment enhance the versatility of fast-track concrete. Minimum-clear­ance slipform paving machines allow placement of concrete pavement adjacent to traffic lanes or other appurtenances (Figure 3a). This allows single-lane reconstruction or resurfacing next to traffic on adjacent lanes or shoulders.

With dowel bar inserters, dowel bar supporting bas­kets are not necessary. The dowel insertion equip­ment mounts to a slipform paving machine and frees the construction lane(s) for concrete haul trucks and other construction vehicles. Tests of the modern dowel bar inserters show that their placement accura­cy is as good as or better than that with traditional dowel baskets [22]. Advancements in large-diameter [up to 1270-mm (50­in)] coring equipment may reduce urban construction time. The new equipment can cut concrete around existing or planned manholes and eliminate the need to place utility box-outs before paving new streets (Figure 3b). The coring equipment is also useful to cut around a manhole so it can be raised for an overlay.

Figure 3. a) Minimum-clearance slipform paving machine, b) large-diameter coring equipment for utility cuts.

CONCRETE MATERIALS

One of the primary ways to decrease facility closure time is to use a concrete mix that develops strength rapidly. Rapid strength gain does not require special blended-cements or sophisticated construction methods. It is possible to proportion a mix using locally available cements, additives, admixtures and aggregates.

When proportioning fast-track concrete mixes, materi­als engineers also should consider the additional influ­ence of heat of hydration, aggregate size distribution, entrained air, water temperature, curing provisions and, ambient and subbase temperature. These factors may influence early and long-term concrete strength.

There is no specific proportioning necessary for a fast-track mix. Many different combinations of materials will result in rapid strength gain. Table 4 shows typical fast-track mix proportions and components for American Society of Testing Materials* (ASTM) C 150, Type I and Type III cements [23][24]. Certain propri­etary blended cements and other admixtures also may produce acceptable results.

Table 4. Typical fast-track mix components and proportions [25][26][27][28][29].

Material Type Quantity1
Cement ASTM C 150 Type I
ASTM C 150 Type III
415-475 kg/m3 (700-800 lb/yd3)
415-475 kg/m3 (700-800 lb/yd3)
Fly ash ASTM C 618 Class C 0-48 kg/m3 (0-80 lb/yd3)
Aggregate ratio Coarse/Fine 1:1 - 1.5:1
Water-cement-ratio-pozzolon ratio 0.37-0.48
Air-entraining admixture ASTM C 260 As necessary
Water-reducing admixture ASTM C 494 As necessary


For specific mix proportions on actual projects see Table 11 (page 21). A thorough laboratory analysis is important before specifying a fast-track mix. The lab work should determine plastic and hardened concrete properties using project materials and should verify the compati­bility of all chemically active ingredients in the mix. Table 5 shows some factors that influence mix properties and may aid mix proportioning.

Generally, fast-track concrete will provide good dura­bility This is because most fast-track mixes have entrained air and a relatively low water content that both improve strength and decrease chloride perme­ability [30]. Freeze-thaw deterioration can occur if water freezes and expands within a concrete binder with a poor air-void distribution, or if the concrete contains poor quality aggregates. However, fast-track concrete with an adequate air-void distribution resists water penetration and relieves pressures that develop in the binder [31]. Air-entrained fast-track concrete is resistant to freeze-thaw deterioration even in the presence of deicing chemicals.

*Equivalent Canadian standards for all ASTM standards and tests addressed in this publication are found in appendix B.

Table 5. Some factors that influence fresh and hardened mix properties [32][33][34].

Fresh or Hardened Mix Property Mix Proportioning or Placement Factor
Long-Term Strength Water-cement-plus-pozzolan ratio
Cement (composition and fineness)
Aggregate type
Entrained air content
Presence and tupe of admixtures
Concrete temperature
Curing method and duration
Time
Early Strength Gain Rate Cement Type
Total water content
Concrete texture
Mix materials temperature
Presence and type of admixtures
Curing method
Freeze-Thaw Durability Aggregate quality and grading
Entrained aire (bubble size and spacing)
Total water content
Water-cement-plus-pozzolan ratio
Curing method and duration
Workability Aggregate particle shape
Combined aggregate grading
Total water content
Presence and type of admixtures
Presence of pozzolans
Abrasion Resistance Aggregate hardness
Percent of entrained air
Compressive Strength
Curing Method and duration

Cement

ASTM C 150 Types I, II, or III, portland cement can produce successful fast-track mixes [35]. There also are several proprietary cements that develop high-early strengths useful for fast-track applications [36]. However, not every portland cement will gain strength rapidly and testing is necessary to confirm the applica­bility of each cement [37][38][39].

The speed of strength development is a result of the hydration and heat generation characteristics of a par­ticular combination of cement, pozzolan and admix­ture(s). Cements play a major role in both strength and heat development, and these properties depend on the interaction of the individual compounds that constitute the cement. High levels of tricalcium silicate (C3S) and finely ground cement particles will usually generate strength quickly [40][41]. Tricalcium aluminate (C3A) also can be a catalyst to enhance the rate of hydration of C3S by releasing heat early during cement hydration. However, C3A does not contribute much to long-term strength, and in general, C3S is the major chemical contributor to both early and long-term strengths (Figure 4) [42][43][44].


Figure 4. Contribution of cement compounds to strength development [45].

Finely ground cement increases surface area and allows more cement contact with mix water and consequently faster hydration. Type III cement, which is much finer than other types of portland cement, usual­ly develops strength quickly. Blaine fineness values for Type III cement range from about 500 to 600 m2/kg. Type I cement Blaine fineness values range only from 300 to 400 m2/kg [46][47]. Although the fineness of Type III cement provides a much greater surface area for the hydration reaction, it also may require a little more water to coat the particles.

However, because Type III cement is ground finer than other cements, there is more potential for problems that may result from overheating the cement during the grinding phase of manufacturing. These include false set, and excessive water or air entraining agent demand. False set is a rapid stiffening of the concrete shortly after mixing. This is not a major problem and it is possible to restore workability without damaging the normal set of the concrete through further mixing in a transit mixer [48]. The materials engineer and contrac­tor should be aware of these phenomena when testing materials, and proportioning mixes and trial batches. It is advisable to prepare tests using the same cement that the contractor will use in construction.

A low water-cement-plus-pozzolan ratio contributes to low permeability and good durability [49]. A water-cement-plus-pozzolan ratio between 0.40 and 0.50 provides moderate chloride permeability for concrete made from conventional materials. A water-cement-plus-pozzolan ratio below 0.40 typically provides low chloride permeability [50]. Most fast-track mixes have a ratio less than 0.43 and consequently provide mod­erate to low permeability.

It is important to remember that durability is not a function of early strength, but is a function of long-term strength, water-cement-plus-pozzolan ratio, perme­ability and proper air-void system. Some available cements and pozzolans will contribute to early con­crete strength, but may not continue to influence long-term strength. Mixes using these materials may appear to meet the quick strength development nec­essary for fast-track concrete paving, but may not pro­vide adequate durability. Because of this inconsisten­cy, concrete technicians should evaluate a mix at sev­eral phases of hydration to ensure it meets both early strength and long-term durability requirements.

Type III cement has been primarily used for the manu­facture of precast concrete products. Before using a Type III cement in paving, it may be advisable for agency and contractor material technicians to confer with local precast concrete manufacturers to learn of any peculiarities. At least one state uses a minimum hydraulic-mortar cube-specimen strength (ASTM C 109) to test Type III cement [51][52]. The cement must reach 9.0 MPa (1300 psi), at 12 hours to qualify for use in fast-track concrete paving.

Concretes using Type I and Type II portland cement also can produce adequate characteristics for fast-track construction. However, to develop adequate early and long-term strength, concrete made from these cements will usually require chemical admix­tures.


Supplementary Cementing Materials

It is possible to use fly ash or ground-granulated blast­furnace slag in addition to portland cement in fast-track concrete. During cement hydration, these sup­plementary cementing materials react with the chemi­cal products of portland cement to extend strength gain. They also act as fine particle fillers in the binder to aid concrete workability and finishability [53][54].

Fly Ash — There are two fly ash classifications, ASTM C 618, Class C and Class F [55]. Class C fly ash has some cementitious properties that allow it to hydrate like cement. Therefore, adding small quantities of Class C fly ash to a fast-track mix will usually not impede early strength development. When compatible with portland cement, Class C fly ash will also lower water demand, improve workability and increase long-term strength [56].

Although experience on most fast-track projects is with concrete employing Class C fly ash, Class F also may produce acceptable results. Class F fly ash is generally not cementitious and can only react with the chemical products of portland cement hydration. Therefore, Class F fly ashes do not contribute much to early strength of concrete. However, Class F fly ash can extend long-term strength, reduce permeability, and combat the deleterious effects of sulfates or alkalis [57].

It is important to evaluate fast-track concrete contain­ing fly ash. Include both fly ash and cement to deter­mine the water-cement-plus-pozzolan ratio for mixture proportioning [58]. Strength tests should be made through the range of probable mix temperatures to indicate how temperature influences hydration. As the overall mix temperature drops, fly ash can slow hydra­tion and significantly delay final set. Knowledge of this temperature sensitivity will be useful to the inspector and contractor during construction under field condi­tions. Accelerating admixtures will probably be neces­sary should the laboratory study show unacceptable strength retardation with fly ash.

Normally substituting about 10% Class C fly ash for cement in fast-track concrete should provide accept­able results. At this rate a fast-track concrete mix should not be overly sensitive to fly ash inconsistencies and should achieve adequate early strength gain.

Ground-Granulated Blast-Furnace Slag — Ground-granulated blast-furnace slag is another cementitious material that might be acceptable for fast-track concrete. In concrete, ground-granulated blast-furnace slag can increase long-term strength and improve the finishability [59]. However, because its effects are temperature sensitive, laboratory studies are necessary to determine the optimal dosage rate and the effects of temperature on strength develop­ment. Strength development should be similar to nor­mal concrete at temperatures around 21°C (70°F) [60]. For cooler temperatures it may be necessary to extend the curing and insulating period, or impose tempera­ture and seasonal limitations for use in fast-track paving.

Air-Entraining Admixtures

Air-entraining admixtures meeting ASTM C 260 requirements are used to entrain microscopic air bubbles in concrete [61][62]. Entrained air improves con­crete durability by reducing the adverse effects of freezing and thawing [63][64][65][66]. The volume of entrained air necessary for good durability varies by the severity of the environment and the concrete's maximum coarse aggregate size. Normal concrete mixes have 4.5 to 7.5% entrained air.

Air-entrainment is necessary for fast-track concrete. During field mixing it is important to use the appropri­ate air-entraining admixture dosage rate so that the air content after placement is adequate (4.5%-7.5%). Higher percentages of entrained air can reduce the early and long-term strength of the mix, while lower percentages will reduce the concrete durability.

Water-Reducing Admixtures

Water-reducing admixtures reduce the quantity of water necessary in a concrete mix or improve worka­bility at a given water content [67]. Many highway agencies only use water reducers to improve the work­ability of normal concrete. However, water-reducing admixtures also can increase early strength in fast-track concrete by lowering the quantity of water nec­essary for cement hydration. This is because water reducers lower the number of cement particle agglomerations and disperse cement particles [68][69]. Water reducers can be used to increase early concrete strength with any cement, but are especially useful when using Type I cement in a fast-track mix.

Table 6 lists five water-reducing admixtures covered by ASTM C 494 [70]. Water-reducing admixture Types A, E and F generally provide the necessary properties for fast-track concrete. ASTM C 1017 also classifies cer­tain high-range water-reducing admixtures as super-plasticizers [71]. Many available high-range water-reducing admixtures meet both ASTM C 494 and C 1017 requirements. While most water-reducing admixtures will work well with different portland cements, laboratory testing is essential to determine if a concrete containing the admixture will develop the desirable properties.

ASTM C 494, Type A admixtures are common in fast-track concrete. Generally a concrete containing a Type A water-reducing admixture will require from 5 to 10% less water, than a similar mix without the admixture. A Type D water-reducing (set-retarding) admixture may be desirable when very high mix temperatures induce early set that preempts placing and finishing opera­tions. Type D water-reducers slightly retard the initial set to extend the period of good workability for placing and finishing. The retardation period usually lasts for about the first 12 hours. Admixtures meeting Type E, F or G requirements require thorough laboratory evalu­ation to determine if the concrete properties are acceptable for anticipated environmental conditions and placement methods. These materials may be more appropriate for high slump mixes.


Table 6. Water-reducing admixtures specified in ASTM C 494 [72].

Type & Classification Effect
Water-Reducer (Type A) Reduces water demand minimum 5 percent
Water-Reducer & Retarder (Type D) Reduces water demand minimum 5 percent
Retards set
Water-Reducer & Accelerator (Type E) Reduce water demand minimum 5 percent
Accelerates set
High-Range Water-Reducer (Type F) Reduces water demand minimum 12 percent
High-Range Water-Reducer & Retarder (Type G) Reduces water demand minimum 12 percent
Retards set

Accelerating Admixtures

Accelerating admixtures aid strength development and reduce initial set times by increasing the reaction rate of C3A [73]. Accelerating admixtures generally consist of soluble inorganic salts or soluble organic com­pounds and should meet ASTM C 494, Type C requirements [74].

The most common accelerator is calcium chloride salt (CaCl2). Many agencies use CaCl2 for full-depth and partial-depth concrete pavement patching for quick curing and opening to traffic. The optimum dose is about 2 percent by weight of cement and will approxi­mately double the 1 -day strength of normal concrete [75]. However, it is very important to test both fresh and hardened concrete properties before specifying a mix containing an accelerating admixture. With some aggregates, concrete will be susceptible to early freeze-thaw damage and scaling in the presence of CaCl2- Another drawback of CaCl2 is its corrosive effects on reinforcing steel. If the pavement requires any steel, it is advisable to select a non-chloride accel­erator or alternate method of achieving early strength.

Aggregate

Aggregates that comply with standard ASTM C 33 specifications are acceptable for use in fast-track con­crete (see reference 28[76]). Existing fast-track projects made with concrete containing these aggregates have met their early strength requirements and are providing good service. However, consideration of grading uni­formity and aggregate particle shape may further opti­mize early and long-term concrete strength. These factors also can have a significant influence on the plastic and hardened mix properties and may warrant consideration for fast-track applications.

Typical procedures consider the proportions of coarse and fine aggregates without significant concern to the combined or total grading. Consequently, concrete producers draw aggregate from two stockpiles at the plant site, one for coarse and one for fine material. To improve grading uniformity may require additional inter­mediate size material (blend sizes) at the plant site during project construction.

Grading — Grading data indicates the relative quan­tity of aggregate by particle size. Sieve analyses of source stockpiles is necessary to characterize the materials. However, the best use of such data is to calculate the combined aggregate grading based upon the proportions of aggregate in the mixture. Well-graded mixtures generally have a uniform distribution of aggregates on each sieve. Gap-graded mixtures have a deficiency of particles retained on the 2.36 mm through 600 µm (Nos. 8 through 30) sieves.

The optimum combined aggregate grading is the one that most efficiently uses locally available materials to fill the major voids in the volume of concrete so as to reduce the need for mortar. However, particle shape and texture, especially in the intermediate sizes, are important to the response of the concrete to vibration. A concrete with an optimum aggregate grading and good consolidation will produce dense and durable concrete without edge slump.

Figure 5. Grading plot showing gap graded mixture and mixture with adequate intermediate particles.

One approach to evaluate the combined aggregate grading is to assess the percentage of aggregates retained on each sieve [77]. A grading that approach­es the shape of a "bell-curve" on a standard grading chart indicates an optimal distribution (Figure 5). Blends that leave a deficiency in the intermediate ­particles are partially gap graded and can produce highly variable concrete. There is a definite relationship between aggregate grading uniformity and concrete strength, workability and long-term durability [78][79][80]. Intermediate size aggregates fill voids typically occupied by less dense cement paste and thereby optimize concrete density (Figure 6). Increasing concrete density in this manner will result in:

  • Reduced mix water demand and consequently improved strength because less mortar is necessary to fill space between aggregates.
  • Increased concrete durability through reduced avenues for water penetration in the hardened mix.
  • Better workability and mobility because large aggregate particles do not bind in contact with other large particles under the dynamics of fin­ishing and vibration.
  • Less edge slump because of increased particle-to-particle contact.
  • Reduced wear on concrete mixers, drums and equipment.


Gradation uniformity also influences workability or the ease of placing, consolidating and finishing concrete. While engineers traditionally look at the slump test as a measure of workability, it does not reflect that characteristic of concrete. Slump evaluates only the consistency of a single concrete batch and provides a relative measure of consistency between separate concrete batches of the same mix proportions [81].

Concrete with a well graded combined aggregate will often be much more workable at a low slump than a poorly graded mixture having a higher slump. A uni­form grading may change slump by 89 mm (3.5 in) over a similar gap-graded mix. This is because about 320-480 kg/m3 (20-30 lb/yd3) less water is necessary to maintain mix consistency than is necessary with a gap grading [82].

Particle Shape and Texture — The shape and tex­ture of aggregate particles impact concrete properties [83]. Sharp and rough particles generally produce less workable mixes than rounded and smooth particles at the same water-cement-plus-pozzolan ratio [84][85]. However, the bond strength between aggregate and cement mortar improves as aggregate texture increas­es. The improved bond will improve concrete flexural strength [86].

Cube-shaped crushed or natural coarse aggregates and natural sands are very mobile under vibration. These shapes are ideal for reinforced pavements that contain dowel baskets or continuous steel. Good mobility allows the concrete to flow easily around the baskets, chairs and reinforcing bars.

Flat or elongated intermediate and large aggregates can cause mix problems [87][88]. These shapes gener­ally require more mix water and/or fine aggregate for workability, and consequently result in lower concrete flexural strength. It is advisable to allow no more than 15 percent flat or elongated aggregate by weight of total aggregate [89]. Use standard test method ASTM D 4791 to determine the quantity of flat or elongated particles [90].

Water

Cement hydration is exothermic, consequently the sooner the temperature of a mix rises, the faster the mix will develop strength. One way to raise the tem­perature of plastic concrete is to heat the mix water. However, this is more practical for small projects that do not require a large quantity of concrete, such as intersection reconstruction.

Several factors influence the water temperature need­ed to produce a desirable mix temperature at place­ment. The critical factors are: ambient air temperature, aggregate temperatures, aggregate free moisture con­tent and cement type. When necessary, ready-mix concrete producers heat water to 60-66°C (140-150°F) to elevate mix temperature sufficiently for cool-weather construction. To avoid a flash set of the cement, it is important to combine the hot water and aggregates before adding the cement when mixing batches [91].

Hot water is only a catalyst that facilitates early hydra­tion and its benefits are generally short-lived. Several hours of heat containment through insulation may be necessary for rapid strength gain to continue particu­larly when cool conditions prevail.

CONSTRUCTION

No special equipment is necessary for a contractor to place fast-track concrete pavement. However, because the time for placement can be shorter than with conventional paving, fast-track paving requires well-planned construction sequencing. Contractors and specifying agencies should be aware that opera­tion adjustments will be necessary while the paving crew becomes accustomed to mix characteristics. It will take time for workers to become comfortable with accelerating their duties. Constructing test slabs will familiarize an inexperienced crew with the plastic prop­erties of the fast-track concrete before starting full-scale operations.

Contractors have built successful fast-track concrete pavements using both slipform and standard form construction techniques (Figure 7). There are no reports indicating unusual problems with mixing, plac­ing and finishing fast-track concrete. However, the contractor and agency should carefully consider con­crete haul distances on large projects.

Figure 7. Slipform paver placing fast-track concrete on a res­idential route.

It may be necessary to adjust mechanical vibration on slipform pavers for mixes with a high cement content and large proportion of fine aggregate. These mixes can have low mobility and require adjustment for good consolidation and ease of finishing. Before paving, the slipform crew should check that vibrators are function­ing properly and that each is at the correct location, depth and spacing.

The adjustments that accompany construction start­up on fast-track projects normally will not interfere with the ride quality. Contractors have built fast-track pro­jects to meet conventional ride specifications, and agencies should not modify their smoothness specifi­cations for fast-track concrete pavements.

Curing & Temperature Management

Curing provisions are necessary to maintain a satisfac­tory moisture and temperature condition in concrete [92]. Internal concrete temperature and moisture directly influence both early and ultimate concrete properties, and therefore it is important to apply curing provisions immediately after placing and finishing activities [93][94]. More than standard concrete, curing is critical to fast-track concrete for the moisture and heat retention nec­essary to fuel hydration during the early strength gain period. Fast-track pavements require thorough curing protection in difficult environmental conditions.

Air temperature, wind, relative humidity and sunlight, influence concrete hydration and shrinkage. These factors may heat or cool concrete or draw moisture from exposed concrete surfaces. The subbase can be a heat sink that draws energy from the concrete in cold weather, or a heat source that adds heat to the bottom of the slab during hot, sunny weather.

Monitoring heat development in the concrete enables the contractor to adjust curing measures to influence the rate of strength development, the sawing window, and the potential for cracking. It is particularly impor­tant to monitor temperature when environmental or curing conditions are unusual or weather changes are imminent [95]. Maturity testing allows field measure­ment of concrete temperature and correlation to con­crete strength. The section "Non-destructive Testing" describes maturity testing in more detail.

Curing Compounds - All liquid-membrane forming curing compounds should meet ASTM C 309 material requirements [96]. Typically white-pigmented compound (Type 2, Class A) is applied to the surface and exposed edges of the concrete pavement. Most paving specifications require an appli­cation rate around 5.0 m2/l (200 ft2/gal). The materials create a seal that limits evaporation of mix water and contributes to thorough cement hydration. The white color also reflects solar radiation during bright days to prevent excessive heat build up in the concrete surface. Class A liquid curing compounds are sufficient for fast track concrete under moderate placement conditions when the application rate is sufficient.

Agencies that build concrete pavements in mountain­ous and arid climates often specify a slightly heavier dosage rate of resin based curing compound meeting ASTM C 309, Type 2, Class B requirements. The harsher climate causes dramatic daily temperature changes often at low humidity levels. As a result, con­crete is often more susceptible to plastic shrinkage cracking and a shorter window for joint sawing.

Fast-track concrete rapidly consumes mix water during early hydration and may lead to a larger potential for plastic shrinkage at the surface. Therefore, it is advis­able to increase the application of curing compound for fast-track projects to about 3.75 m2/l (150 ft2/gal). Because deep tining increases surface area, the higher application rate also is important where surface texture tine depth exceeds about 3 mm (1/8 in). Bonded overlays less than 150 mm (6 in) thick require an appli­cation rate of 2.5 m2/l (100 ft2/gal). The thin overlay slabs have a large ratio of surface area to concrete vol­ume so evaporation consumes proportionately more mix water than with typical slabs [97].

The first few hours, while the concrete is still plastic, are the most critical for good curing. Therefore, the con­tractor should apply the curing compound as soon as possible after final finishing. Construction and public vehicle tires may wear some of the compound off the surface after opening, but this does not pose a problem because the concrete should have reasonable strength and durability by that time.

Blanket Insulation — Insulating blankets provide a uniform temperature environment for the concrete. Insulating blankets reduce heat loss and dampen the effect of both air temperature and solar radiation on the pavement, but do not negate the need for curing com­pound [98]. The purpose of blanket insulation is to aid early strength gain in cool ambient temperatures. Table 7 indicates when insulation is recommended [99].

Table 7. Blanket use recommendations [100].

Ambient Air Temperature Opening Time, Hr
8 16 24 36 48
<10°C (50°F) Yes Yes Yes Yes No
10-18°C (50-65°F) Yes Yes Yes No No
18-27°C (65-80°F) Yes No No No No
>27°C (>80°F) No No No No No

Contractors will usually place blankets soon after applying curing compound. However, if conditions are warm, it may be acceptable to wait several hours and instead place the blankets as the joint sawing pro­gresses (see photo below). In any case, it is inadvis­able to wait until after finishing all joint sawing to start placing insulating blankets.

Joint sawing just ahead of blanket placement on fast-track bond­ed overlay project.

Experience indicates that an insulating blanket with a minimum thermal resistance (R) rating of 0.035 m2oK/W (0.5 hr ft2°F/Btu) is adequate for most condi­tions [101][102][103][104][105]. The blanket should consist of a layer of closed-cell polystyrene foam with another pro­tective layer of plastic film. Additional blankets may be necessary for temperatures below about 4°C (40°F). Figure 8 shows how effective insulating blankets are in maintaining the temperature of concrete compared to an exposed surface of the same mix.

Sawing Window — The sawing window is a short period after placement when the concrete can be cut successfully before it cracks. The window begins when concrete strength is acceptable for joint cutting without excessive raveling along the cut. The window ends when significant concrete shrinkage occurs and induces uncontrolled cracking.


Figure 8. Time-temperature plot from fast-track project showing effectiveness of insulating blankets.

Uncontrolled cracking has not been a problem on fast-track concrete pavements because sawing can usually be done while the concrete temperature is still high from hydration and insulation. However, contractors and inspectors should be aware of the factors that influence the sawing window, and in particular, differ­ential shrinkage and thermal shock that may bring about rapid shrinkage.

Internal concrete temperature and moisture also influ­ence the time available for joint sawing. Concrete tem­perature directly relates to the strength of concrete, which controls the ability to commence sawing. Under warm sunny summer conditions, the maximum con­crete temperature will vary depending on when the concrete is placed during the day. Concrete placed in early morning will often reach higher maximum tem­peratures than late morning or afternoon concrete because it receives more radiant heat (Figure 9). As a result, the morning concrete will generally have a shorter sawing window.

The sawing must be complete before significant con­crete shrinkage. For fast-track concrete it is preferable to complete sawing before the temperature begins to moderate after initial set. Drying shrinkage partially occurs from moisture loss through hydration and mois­ture loss to the environment [106]. Thermal contraction begins when the concrete temperature falls.

Figure 9. Surface temperatures of slabs poured at different times of the day; Type I cement; no blankets [107].

After the concrete sets, uncontrolled cracking might occur when conditions induce differential concrete shrinkage [108]. Differential shrinkage is a result of tem­perature differences throughout the pavement depth. Normally, the concrete surface temperature drops before the temperature at mid-depth or bottom (Figure 10). The temperature at mid-depth usually remains warm for the longest period.

Research indicates that a drop from maximum surface temperature more than 9.5°C (15°F) can result in excessive surface shrinkage and induce cracking [109]. This is critical in most regions during the spring and fall because air temperature often drops significantly from day to night. Differential shrinkage also occurs from rainshowers that cool the slab surface. Therefore, it is important for the contractor to monitor the weather and saw control joints, as soon as possible, when conditions change from that during placement.

Thermal shock also may occur within a few hours after removing curing blankets from a new slab. It may be necessary to remove only the blankets needed to allow joint sawing. Blankets should not be completely removed until after completion of all sawing to elimi­nate uncontrolled cracking from thermal shock.

Plastic Shrinkage — The temperatures of fast-track mixes often exceed air temperature and require special attention to avoid plastic shrinkage cracking. Plastic shrinkage cracks can form after concrete placement when certain prevailing environmental con­ditions exist. The principal cause of plastic shrinkage cracking is rapid evaporation of water from the slab surface [110]. When this occurs while concrete is in a plastic or semi-plastic state, it will result in shrinkage at the surface. Air temperature, relative humidity, wind velocity and concrete temperature influence the rate of evaporation. The tendency for rapid evaporation increases when concrete temperature exceeds air temperature [111].

Several ways to moderate the environment and cool concrete components to slow evaporation are:

  • to pave during the evening or nighttime.
  • to water mist aggregate stockpiles and sub­base before paving.
  • to use an evaporative retardant (monomolecu­lar compound) on the surface.


Using Figure 11 it is possible to estimate evaporation [112]. When the evaporation rate exceeds 1.0 kg/m2/hr (0.2 lb/ft2/hr) plastic shrinkage cracking is likely. As a precaution it is advisable to closely monitor and adjust field curing practice if the evaporation rate exceeds 0.5 kg/m2/hr(0.1 lb/ft2/hr).

Jointing & Sealing

The typical time sequence for joint sawing and sealing is not compatible with rapid strength gain and early opening to traffic. Rapid strength gain reduces the time for sawing (sawing window). The contractor must be conscious that sawing is necessary much sooner after paving then with normal concrete. To meet pub­lic traffic opening requirements, it also may be neces­sary to seal the reservoir sooner and require special consideration of sealant materials.

Sawing — Light saws which handle easily and are more versatile will generally be more effective for fast-track projects. Often the curing blankets are in place before sawing and the saw crew must move the blan­kets aside at the location of each joint.

To decide when to begin sawing any concrete pave­ment requires some experience and judgment; sawing too late could lead to uncontrolled cracking in some cases. The quality of saw cut will vary with concrete strength. Excessive spalling and raveling along the joint face will result if the sawing is too soon. Slight raveling is acceptable if a second saw cut will be made to form a sealant reservoir. Weather (tempera­ture, wind, humidity and direct sunlight) has a large influence on concrete strength gain and the optimal time to begin sawing.

Some design factors also influence the optimal time to begin sawing. Subbase or subgrade friction will restrain shrinkage as the concrete cools after final set. The high-friction surface of asphalt or cement-stabi­lized subbases decrease the time allowable before sawing is necessary. In some extreme cases, bond between the surface and subbase have induced cracking before sawing was possible without unac­ceptable raveling. Fill-in lanes for airport pavements and parking areas also tend to have a shorter time for joint sawing due to edge restraint. Granular subbases and subgrade soils provide the least frictional restraint and the longest sawing time.

Mixes with softer limestone aggregates require less strength for sawing than do mixes with harder coarse aggregates. Table 8 shows compressive strengths necessary to begin sawing different mixes for accept­able and excellent results [113].

Contractors have successfully cut joints in fast-track construction using wet-sawing, dry-sawing and ultra­light sawing equipment [114]. It is usually possible to dry-saw concrete slightly earlier than to wet-saw. Dry-sawing also does not require a water flushing for slurry removal and may shorten the drying time necessary before sealing.

A contractor should choose blade type depending on the hardness of the aggregate in the concrete. Silicon carbide or Carborundum (dry-sawing) blades are only effective for softer aggregates like limestone. Wet-saw diamond blades are acceptable for all types of aggre­gates, and are most advantageous for concrete con­taining hard aggregates. A contractor also may saw through most aggregates without water using certain diamond blades mounted on saws powered by less than 26-kW (35-hp) engines.

Table 8. Required compressive strengths necessary to begin sawing using conventional saw equipment [115]. Note that the rounded soft condition was not measured in the lab study and was developed using a regression analysis.

Coarse Aggregate Shape Coarse Aggregate Hardness Cement Content
KG/m3 (lb/yd3)
Acceptable Cut (Some Raveling)1
MPa (psi)
Excellent Cut (Almost No Raveling)2
MPa (psi)
Crushed Soft 300 (500)
385 (650)
4753(800)
2.5 (370)
2.2 (320)
1.9 (270)
3.9 (560)
3.7 (530)
3.4 (500)
Crushed Hard 300 (500)
385 (650)
4753(800)
4.9 (715)
4.8 (700)
4.7 (685)
7.0 (1010)
6.8 (980)
6.6 (950)
Rounded Soft 300 (500)
385 (650)
4753(800)
1.4 (210)
1.0 (150)
1.0 (150)
2.5 (360)
2.1 (310)
1.8 (260)
Rounded Hard 300 (500)
385 (650)
4753(800)
3.3 (480)
3.1 (450)
2.9 (420)
4.9 (710)
4.8 (690)
4.6 (670)

1. Some raveling present on cut [540 mm2 (0.84 in2) per 7.3 m (24 ft) of cut], acceptable if another saw cut will be made for a sealant reservoir. 2. Almost no raveling present on cut [80 mm2 (0.12 in2) per 7.3 m (24 ft) of cut]. 3. Compressive strength criteria extrapolated from data at 300 and 385 kg/m3 (500 and 650 lb/yd3).


Ultra-light saws allow cutting very early during the initial concrete set stage. Cutting is feasible after com­pressive strengths reach about 1.0 MPa (150 psi) usu­ally about an hour or two after paving. All cutting should be done before the final set of the concrete. Most currently available ultra-light saws provide only a shallow initial cut at about 25 to 33 mm deep (1 to 1.5 in) and require a second cut using a standard saw for a sealant reservoir or to meet typical D/3 or D/4 cut depth specifications. However, using ultra-light equip­ment can allow cutting before curing blanket place­ment and can be effective for fast-track projects.

Step cut blades also are available to allow sawing the joint seal reservoir and depth-cut at the same time. This eliminates the time necessary for a second cut to form the joint seal reservoir.

Sealing — Joint sealing should begin when practica­ble after sawing is complete. Normally liquid sealant manufacturers recommend delaying installation for a considerable moisture-free period. However, most sealant manufacturers also provide recommendations for use of their product in fast-track construction. The rapid strength gain and low water-cement-plus-poz-zolan ratio of fast-track concrete reduce excess mois­ture on the side walls of the joint reservoirs. This allows sealing earlier than with standard concrete.

Therefore it is important to always consult the sealant manufacturer's particular product recommendations.

Cleaning is the most important aspect of joint sealing for a liquid sealant [116]. Every liquid sealant manufac­turer requires essentially the same cleaning proce­dures. Likewise the performance claims of any liquid sealant product is predicated on those cleaning proce­dures. Cleaning is not as critical for compression seals.

Cleaning operations will vary depending on the saw blade type. Reservoir faces require a thorough clean­ing to be sure of good sealant adhesion and long-term performance. Proper cleaning after wet sawing requires mechanical action and pure water flushing to remove contaminants. Dry sawing requires only an air blowing operation to remove particulate residue from the joint reservoir. This can produce considerable dust and may be inadvisable in urban areas.

Preformed seals are not sensitive to dirt or moisture on side walls and may allow sealing earlier than any liquid sealant. However, on one fast-track project a low-modulus rubber sealant sufficiently adhered to the reservoir faces within eight hours after paving [117]. Silicone sealants also have been used for fast-track projects. Reference 43[118] provides more information on joint sealants and sealing procedures.

Table 9. Non-destructive test methods for concrete [119][120].

Test Method Standard Basic Description Testing Precision to Baseline Cylinder Strength2
Surface Hardness (Swiss Hammer) ASTM C 805 (see ref. 47) Rebound of hammer correlates to surface hardness & compressive strength ±40%
Penetration Resistance (Windsor Probe) ASTM C 803 (see ref. 48) Penetration depth of gun-fired probe correlate to surface hardness & compressive strength ±20%
Pullout1 ASTM C 900 (see ref. 49) Force to remove cast-in metal probe correlate to surface compressive strength ±15%
Break-off ASTM C 1150 (see ref. 50) Force necessary to break a circular core or cut partially into slab correlates to flexural strength ±15%
Maturity ASTM C 1074 (see ref. 46) Internal temperature of concrete relates directly to concrete strength ±5%
Pulse Velocity ASTM C 597 (see ref. 51) Velocity of sound wave from transducer to receiver through concrete relates to concrete strength ±10%
  1. Cap and pull-out (CAPO) variation of pull-out test not approved by ASTM.
  2. These are estimates of precision based on cylinder strength tests made using recommended ASTM procedures. Inaccurate concrete strength characterization by destructive cylinder testing is a common problem.

NON-DESTRUCTIVE TESTING

Some agencies, consultants and contractors use non­destructive testing to adequately determine strength at early ages. Table 9 describes six non-destructive test methods for concrete. Maturity and pulse velocity testing are common for predicting strengths on fast-track concrete pavement projects.

Maturity — Maturity testing provides strength evalu­ation through monitoring of internal concrete tempera­ture in the field. The basis of maturity is that each con­crete mix has a unique strength-time relationship [121][122][123][124]. Therefore, a mix will have the same strength at a given maturity no matter what conditions (time or temperature) occur before measurement.

There are two methods for computing maturity. The first method is the Nurse-Saul method that calculates the time-temperature factor using the following equation:

M(t) = Σ(Ta-T0)Δt

M(t) = temperature-time factor, degree-days or degree-hours,

Δt = time interval, days or hours

Ta = average concrete temperature during time interval, °C

TQ = datum temperature, °C [typically -10°C (14°F)]

The second method uses the Arrhenius maturity equa­tion and is less common for concrete pavement work in the United States [125]. More information is available in ASTM C 1074 (reference 46[126]) and references 32[127] and 45[128].

Thorough laboratory testing is necessary before a technician can accurately analyze concrete in the field. Laboratory testing requires preparation of trial batches using the actual field mix materials. Technicians must monitor the batch temperature and break cylinders to develop a relationship between the strength criterion and the temperature-time factor (Figure 12). This relationship becomes the calibration curve for evaluating the field concrete strength.

Figure 12. Typical plot from maturity data per ASTM [129].

Field maturity evaluation begins with embedment of thermocouples or temperature probes in the concrete when practicable after finishing and curing. Positioning the temperature probes along the project requires forethought to ensure they are in areas of critical importance for joint sawing and opening to traffic. The probes must connect to either commercially available maturity meters or temperature recorders with an accuracy to 1°C (2°F) [130]. Technicians take readings at regular intervals then estimate strength using the temperature-time relationship from the laboratory study

Pulse-Velocity — Pulse-velocity is another available non-destructive test for determining concrete strength at early ages. It is a true non-destructive test that measures the time required for an ultrasonic wave to pass through concrete from one transducer to another. The velocity of the wave correlates to concrete strength or stiffness [131][132].

Like maturity testing, pulse-velocity testing requires laboratory calibration to produce meaningful field infor­mation. Pulse-velocity readings are sensitive to aggre­gate, water-cement-plus-pozzolan ratio, moisture con­tent and concrete consolidation. Therefore trial batch­es must contain the same mix materials at similar pro­portions as the project mix. In the laboratory, techni­cians take pulse-velocity measurements through a rep­resentative number of cast concrete specimens, test the specimens for strength, and plot the results against the pulse-velocity readings to create a calibra­tion curve (Figure 13).

Field measurement of pulse velocity is relatively simple. Technicians hold the sending and receiving transduc­ers flush to the pavement surface. Sometimes it may be necessary to grind a rough surface, but usually a layer of grease or jelly will sufficiently fill surface voids and provide full transducer contact. Optimal readings occur with the transducers held axially for direct mea­surement, but this arrangement usually requires a cast-in box out in the slab. An acceptable alternative is to hold the transducers in a perpendicular arrange­ment providing a semi-direct measurement (Figure 14).

Figure 14. Semi-direct pulse-velocity testing.

Comparing field pulse-velocity readings to the calibra­tion curve provides an early-age estimate of concrete strength. However, it is necessary to study the manu­facturer's equipment instructions for specific recom­mendations and to make reading corrections neces­sary for concrete temperature and moisture content [133][134]. To avoid inaccurate measurements, take readings away from any embedded steel that will dis­rupt travel of the ultrasonic pulses.

TRAFFIC OPENING

The ultimate factor in fast-track construction is deter­mining when traffic can begin to use the new pavement. The basis for this decision should be made on the con­crete strength and not arbitrarily on the time from place­ment [135]. Strength directly relates to load carrying capacity and provides certainty that the pavement is ready to accept loads by construction or public traffic.

For most concrete pavement applications, flexural strength is the most appropriate structural strength cri­terion to evaluate load capacity. Flexural strength val­ues provide an assessment of the tensile strength at the bottom of the slab where wheel loads induce ten­sile stresses. For that reason, this document lists opening criteria in third-point flexural strengths. However, flexural strength tests from ASTM C 78 are very sensitive to the test beams and testing proce­dures [136]. Many agencies realize this shortcoming and use the more consistent compressive strength test (ASTM C 39) to evaluate concrete for acceptance and opening [137].

To use the flexural strength opening criteria in this pub­lication, it may be necessary to develop a correlation between compressive strength and flexural strength in the laboratory for each unique mix. The following equation converts compressive strength to third-point flexural strength [138].

fr = C•(f'cr)0.5

Where:

fr = flexural strength (modulus of rupture) in third-point loading, MPa (psi).

f'cr = required average compressive strength, MPa (psi).

C= A constant between 8 and 10 for normal mixtures [for high-strength concrete C ranges from 7.5 to 12 (11.7 recommended)].

Note: It also may be necessary to convert strengths from maturity or other non-destructive tests to use the opening criteria in this publication.

The strength necessary to allow vehicles onto a new pavement will depend on the following factors [139]:

  • Type, weight and number of anticipated loads during early-age period
  • Location of loads on slab
  • Concrete Modulus of Elasticity
  • Pavement design (new construction, unbonded overlay bonded overlay or overlay on asphalt)
  • Slab thickness
  • Foundation support (Modulus of Subgrade Reaction, k)
  • Edge support condition (widened lane or tied curb & gutter or tied concrete shoulder)


As slab support or pavement thickness increase, stress in the concrete will decrease for a given load. This relationship allows different opening strength crite­ria for different pavement designs and early traffic loads [140][141]. An opening strength as low as 1.0 MPa (150 psi) in third-point loading is acceptable if the pavement will carry only automobiles [142]. If the pave­ment will carry trucks, a strength of up to 4.5 MPa (650 psi) may be necessary for thin slabs [143][144].

Wheel load location also influences the magnitude of stress. Critical flexural stresses occur from wheels that ride directly on the pavement edge away from a slab corner. Wheel loads that ride near the center of the slab induce considerably lower stresses.

Two traffic categories exist for early opening assess­ment: construction and public traffic. In most cases the construction contractor's vehicles use the pave­ment before any public traffic, however, this may not be typical for fast-track projects. It is important to keep traffic off the pavement until after joint sawing so not to over-stress the concrete and induce uncon­trolled cracking.

Construction Traffic

Typical construction vehicles include span saws, haul trucks and water trucks. Except for slabs less than 175 mm (7.0 in) thick, span saws do not induce con­crete fatigue even during very early ages. The 80 kN (18,000 lb) single-axles and 151 kN (34,000 lb) tan­dem axles (TAL) on the construction trucks induce much higher stresses and can fatigue the concrete.


Table 10. Flexural strength requirements for opening concrete pavements to use by construction traffic. Span saws criteria allows 0.5 percent fatigue consumption. Truck axle criteria allows 1.0 percent fatigue consumption [145].

Required Flexural Strength For Opening, MPa (psi)
Slab Thickness
mm (in)
Foundation Support, k
MPa/m (psi/in)
To Support Span Saw Loadsa To Support Legal 151 kN (34,000 lb) Tandem Axle Poles
MPa (psi)
10 loads 50 loads
150 (6.0) 27.2 (100)
54.3 (200)
135 (500)
1.5 (210)
1.3 (190)
0.8 (100)
2.8 (410)
2.5 (360)
2.1 (300)
3.2 (460)
2.7 (390)
2.0 (300)
165 (6.5) 27.2 (100)
54.3 (200)
135 (500)
1.3 (190)
1.1 (160)
1.0 (150)
2.5 (360)
2.1 (300)
2.1 (300)
2.7 (390)
2.4 (350)
2.1 (300)
175 (7.0) 27.2 (100)
54.3 (200)
135 (500)
1.0 (150)
1.0 (150)
1.0 (150)
2.1 (300)
2.1 (300)
2.1 (300)
2.3 (340)
2.1 (300)
2.1 (300)

a For concrete pavements more than 150 mm (6.0 in) thick, span saws cause no fatigue when the modulus of rupture exceeds .0 MPa (150 psi), the practical minimum for sawing operations (40,52).

Fortunately, operators tend to drive these vehicles within the center of new slabs to avoid drop-offs that exist before shoulder placement or final grading. Table 10 provides opening criteria for span saw and truck loads and assumes that these loads will occur at least 0.6 m (2.0 ft) from the edge of the slab.

Public Traffic

Public traffic includes many different vehicles. To deter­mine the acceptable opening strength for public traffic requires an estimate of the number of loads before the concrete reaches design strength [146].

A table of public traffic opening criterion for municipal and highway pavements is found in Appendix A. To use the table requires estimates of traffic volume, slab thickness and foundation support. The table assumes a 0.6 m (2.0 ft) offset of traffic from the lane edge. Wide truck lanes, tied concrete shoulders and curb and gutter all serve to reduce load stresses to levels equivalent of a 0.6 m (2.0 ft) traffic offset. If the pave­ment design does not include these features, the con­tractor can place barricades to prevent edge loads. Normally after the concrete flexural strength reaches 3.0 MPa (450 psi) the contractor may remove the bar­ricades. However, it may be necessary to wait for concrete to gain full design strength on thin municipal pavements that require more than 4.5 MPa (650 psi) flexural strength for opening. Appendix A provides an example calculation.


Aircraft Traffic

No studies have been made to determine early-age opening criteria for aircraft traffic. The Federal Aviation Administration's current specifications allow opening to traffic at 3.8 MPa (550 psi) flexural strength with no time limitation [147].

ACTUAL PROJECT MIXES

There is no limit to the combination of materials possi­ble for producing fast-track concrete. Table 11 indi­cates the concrete mix proportions from fourteen fast-track projects. Figure 15 shows the flexural strength of these mixes for the first 24 hours and through 28 days after placement. The development of the mixes found in the table was with locally available materials at the time of construction -it is important to use a prop­er series of tests to evaluate and qualify mixes for all new projects.

Table 11 also provides construction and environmental information on each listed project. The projects used a variety of placement methods, sawing equipment and curing measures. These methods were considered successful in construction of the projects. The time that each mix met the opening strength criterion is found in the last column of the table; many projects actually met the specified opening strength in less than 12 hours.

Table 11. Mix proportions, construction factors and environmental conditions from fast track projects [148][149][150][151][152][153][154][155][156][157].

Location & Description Year Cement Type Cement Content kg/m3 (lb/yd3) Water/Cement Ratio Fly Ash kg/m3 (lb/yd3) Coarse Aggregate kg/m3 (lb/yd3) Fine Aggregate kg/m3 (lb/yd3) Admixtures Type & Quantity ml/m3 (oz/yd3) Method to place Curing/Insulation Air Temp. Range First 24 hours Maximum Concrete Temp. Sawing/Sealing Opening Strength Specified MPa (psi) Time to Meet Specified Strength, Hours
1 Us-71 Bonded Overlay Storm Lake, IA 1986 III 380 (640) 0.45 42 (70) Type C 1006 (1696) 670 (1130) Air-226 (10) WR-Type A-1018 (45) Slipform Wax-Based Compound/R=0.5 31-33°C (87-92°F) 47°C (116°F) Dry-Abrasive/Hot-Pour Flexural5 2.4 (350) 7.5
2 Runway Keel Reconstruction Barksdale, AFB (LA) 1992 Special Blended 418 (705) 0.27 None 1020 (1720) 819 (1380) None Form-rider Wax-Based Compound/None 0°C (32°F min.) 14°C 58°F Wet Diamond/Hot-Pour 4 Hr. Flex. 3.1 (450) 4
31 Highway 100 Intersection Replacements Ceder Rapid, IA 1988 III 440 (742) 0.380 47 (80) Type C 774 (1305) 772 (1302) Air-249 (11) WR-Type A-565 (25) Form-Rider & Hand Wax-Based Compound/R=0.5 Blankets 16-22°C (61-85°F) 34°C (93°F) Dry-Abrasive/Hot-Pour & Compression 12 Hr. Flex.5 7.5
4 SR-81 Arterial Reconstruction Manhattan, KS 1990 III 427 (719) 0.44 None 869.42 (1465) 422.52 Air-296 (13) WR-Type A-848 (38) Slipform Wax-Based Compound/R=0.5 Blankets 19-22°C (66-72°F) 52°C (126°F) Wet Diamond/Hot-Pour Flexural 3.1 (450) 24
5 Lane Addition to I-496 1989 III 418 (705) 0.45 Non3 1127 (1900) 736 (1240) AIR-475 (21) WR-Type A-791 (35) Slipform Wax-Based Compound/R=0.5 Blankets 9-15°C (48-59°F) 41°C (105°F) Tooled & Wet-Diamond/Silicone 244 Hr. Flex 3.8 (550) 19
6 I-25 to I-70 Interchange Ramp Reconstruction Denver, CO 1992 I 446 (752) 0.32 None 534 (900)3 593 (1000) Air-430 (19) WR-Type A-2035 (90) 2% CaCl2 Hand-form Wax-Based Compound & Plastic Sheets/None ±16°C (±60°F) 54°C (129°F) Wet Diamond/Hot-Pour 12 Hr. Comp. 20.7 (3000) 205
7 Single-Route Access Road Reconstruction Dallas Count, IA 1987 III 380 (640) 0.425 None 1054 (1777) 658 (1109) Air-226 (10) WR-Type A-678 (30) Slipform Wax-Based Compound/None N.A. N.A. Dry Abrasive/Hot Pour Flexural5 2.4 (350) 9
8 Interstate 80 Widening Rawlins, NY 1992 III 390 (658) 0.47 None 1054 (1777) 658 (1109) AIR-226 (10) WR-Type A-678 (30) Slipform Wax-Based Compound/None 16-32°C (60-90°F) 29°C (85°F) Wet Diamond/Compression 24 Hr. Comp. 20.7 (3000) 205
9 SR 832 & I-90 Interchange Reconstruction Erie County, PA 1991 I 446 (751) 0.37 None 1023 (1725) 583 (983) AIR-1018 (45) SRA-Type D-136 (6) WR-Type F-2374 (105) Hand-form Monomolecular Compound & Plastic Sheets/R=2.5 Blankets 21-32°C (70-90°F) 23°C (73°F) Wet Diamond/Compression 24 Hr. Comp. 20.7 (3000) 13
10 I-70 Bonded Overlay Cooper County, MO 1991 III 421 (710) 0.40 None 961 (1620) 777 (1310) AIR-317 (14) WR-Type A-497 (22) Slipform Polyethylene Sheets/None ±32°C (±90°F) N.A. Wet Diamond/Hot Pour 18 Hr.Comp. 24.1 (3500) 10
11 Runway 18/36 Extension Reconstruction Dane County, WI 1992 III 392 (660) 0.455 None 524 (884)4 700 (1180)4 AIR-656 (25) WR-Type A-3935 (174) Slipform Wax-Based Compound/R=0.5 Blankets 23-33°C (73-91°F) N.A. Wet Diamond/Silicone 12 Hr. Comp. 24.1 (3500) 116
12 SR 13 Bonded Overlay North Hampton, VA 1990 II 445 (750) 0.420 None 1114 (1877) 620 (1045) AIR-452 (20) WR-Type D-565 (25) Slipform Wax-Based Compound/R=0.5 Blankets 18-33°C (65-91°F) 38°C (100°F) Wet Diamond/Hot-Pour & Silicone 24 Hr. Comp. 24.1 (3500) 36
13 US-81 Reconstruction Menominee, NE 1992 III 363 (611) 0.423 None 534 (900) 1241 (2092) AIR-271 (12) WR-Type F-950 (42) Slipform Wax-Based Compound/R=0.5 Blankets 13-17°C (55-62°F) N.A. Dry Abrasive/Compression 24 Hr. Comp. 24.1 (3500) 36
14 US-70A Inlay of Aphalt Intersection Approaches Smithfield, NC 1990 I 424 (715) 0.35 None 1127 (1990) 644 (1085) AIR-136 (6) SRA-Type D-317 (14) WR-Type F-927 Handform None/R=0.5 Blankets ±18°C (±65°F) 26°C (78°F) Wet Diamond/Hot-Pour 48 Hr. Flex. 3.1 (450) 18

1. Contractor had two fast track mix choices on the project depending on desiered set speed - details are for faster set mix and intersection work. 2. Third aggregate size also in mix [383.9 KG/m3 (846 lb/yd3)]. Ten to ninety percent retained on #4-#16 sieves. 3.AASHTO No.4 stone gradation. Third aggregate size also in mix [489.9 KG/m3 (1100 lb/yd3)] No.57 stone. 4. Third aggregate also in mix [401.4 KG/m3 (885 lb/yd3)] 19 mm (0.75 in) maximum size. 5. Centerpoint flexural strength (flexural strength for all other projects in table are 3rd point). 6.Interpreted from available data.

Figure 15. Flexural strengths of mixes shown in Table 10 during first 24 hours and through 672 hours (28 days) after placement.

SUMMARY

Fast-track is being done successfully for all types of concrete pavement. To use fast-track techniques will require some changes by the agency and contractor to traditional methods and materials. However, the changes are not significant and the materials are readi­ly available.

Planning is a key ingredient to make fast-track con­crete pavement techniques successful. The agency and contractor will find that high-quality work is possi­ble with minimal traffic disruption. Partnering arrange­ments can enable both contractor and agency workers to focus on project goals and make timely decisions that help keep construction moving.

Specification changes that expand contractor con­struction and equipment choices will also result in sig­nificant time savings during construction. Modern equipment is available to consolidate construction operations. End-result specifications provide freedom to the contractor to employ this equipment, and to use more than one concrete mix to meet different con­struction needs within a project.

There are many options for mix proportioning and material usage that will produce concrete that gains strength rapidly and decreases pavement closure time. It is possible to proportion fast-track mixes using local­ly available cements, additives, admixtures and aggre­gates.

Under certain conditions curing and sawing of fast-track concrete can require special attention. In lower air temperatures, curing blankets are necessary to pro­vide the moisture and heat retention necessary to fuel hydration during the early strength gain period.

It is preferable to saw joints in fast-track concrete before the concrete temperature begins to fall. Light saws which handle easily and are more versatile will generally be more useful to maneuver around in-place curing blankets.

Non-destructive testing provides information to field engineers that they can use to adjust curing measures in order to influence the rate of strength development, and to determine when concrete is ready for sawing or opening to traffic. This is particularly useful on fast-track projects where opening to traffic is the ultimate goal. Available opening strength criteria directly relates to concrete pavement load carrying capacity and should provide certainty that a pavement is ready to accept loads.

Maturity and pulse velocity testing are common for predicting strengths on fast-track concrete pavement projects. Field measurements using either method are relatively simple, but both require laboratory calibration to produce meaningful field information.

Fast-track concrete pavements are proven to:

  • Allow engineers to consider concrete for projects thought unfeasible because of lengthy concrete cure-times.
  • Perform under many different traffic and applica­tion conditions.
  • Expedite construction and ease work zone con­gestion during major highway restoration, resur­facing or reconstruction.
  • Shorten the time before residents and business­people can gain normal access to their homes and businesses.
  • Allow agencies to rebuild intersections instead of resurfacing them to cover-up rutting, raveling, corrugation and other safety problems.
  • Shorten the time before a contractor can begin interior lane paving pours on airport aprons, run­ways and taxiways.


Fast-track concrete pavement provides a valuable alternative that meets the public's demand for solu­tions to traffic congestion during construction.

ADDITIONAL INFORMATION

For additional information on fast-track concrete pave­ment design and construction contact the American Concrete Pavement Association. Technical publica­tions are also available on many other concrete pave­ment topics.

ACKNOWLEDGMENT

The American Concrete Pavement Association spe­cially thanks the Federal Highway Administration office of technology transfer whose outstanding work has advanced fast-track technology. Many of the improve­ments to this publication are a result of their work.

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APPENDIX A

The simplest way to estimate traffic is from the one-way average daily design truck traffic (ADTT). First, assume all trucks are multi-unit vehicles and contain full legal loads. Then convert the number of trucks to 80 kN (18,000 lb) equiv­alent single axle loads (ESAL's) using a truck factor. The American Association of Highway and Transportation Officials (AASHTO) pavement design procedure bases concrete pavement thickness design on ESAL's and a design modulus of rupture (usually the 28-day average flexural strength) (55). Finally, it is necessary to either estimate the time for the con­crete to reach the design flexural strength in the field, or use data from non-destructive testing to trigger opening to traf­fic. Table A-1 provides the flexural strength requirements for opening concrete pavements to use by public traffic.

Table A1. Flexural strength requirements for opening concrete pavements to use by public traffic (52). Traffic is estimate of the total one-way ESAL's that will use the pavement truck lane between time of opening and time concrete reaches design strength (usually 28-day strength).

EXAMPLE FOR DETERMINING OPENING STRENGTH FOR PUBLIC TRAFFIC:

A 200 mm (8.0 in) municipal fast-track pavement will carry an average daily traffic ADT = 5000 (one way). Trucks make up ten percent of the traffic stream. The pavement is plain-doweled with curb and gutter and resting on a foundation with an equivalent subgrade modulus of 27.2 MPa/m (100 psi/in). The designer based the thickness design of the fast-track pavement on a third-point flexural strength of 4.8 MPa (700 psi).


OPENING REQUIREMENT ESTIMATE METHOD:

In the lab the concrete mix took just 24 hours to reach 4.8 MPa (700 psi). The pavement is being built in the fall, so the engineers feel it may take slightly longer (36 hours) to reach 4.8 MPa (700 psi) than under warmer conditions.

5000 ADT x 0.10 trucks = 500 trucks (assume all are fully loaded multi-unit trucks)

500 trucks/day x 2.0 ESAL's/truck = 1000 ESAL's/day

1000 ESAL's/day x 1.5 days = 1500 ESAL's to specified design strength

From Table A1, the required opening flexural strength is 2.4 MPa (350 psi).


FIELD EVALUATION WITH MATURITY DATA:

The daily air temperature ranged between 11 -19°C (52-67°F) during paving. In the lab it was determined that a maturity value of 540°C•Hr (1004°F•Hr) was equivalent to the opening requirement of 2.4 MPa (350 psi). The field maturity was monitored every hour after placement - and the opening requirement value was met at 30 hours after placement.

HOUR MATURITY 6 - 53°C•Hr(128°F•Hr) 12 - 131°C•Hr(267°F•Hr) 18 - 276°C•Hr (529°F•Hr) 24 - 404°C•Hr (759°F•Hr) 30 - 542°C•Hr (1008°F•Hr) 36 - 823°C•Hr (1513°F•Hr)


APPENDIX B

APPENDIX C

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

The following table provides equivalent metric factors for common U.S. factors used in pavement engineering and concrete pavement.


© American Concrete Pavement Association, 1994 TB004.02P