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23 This chapter explores the various facets of TWT and UTW overlay construction. A synthesis of best practices has been summarized for the various topics listed herein. Overall, the best practices for constructing whitetopping overlays are nearly identical to those for constructing any concrete pave- ments. However, some of the guidance provided here is more applicable to whitetopping overlays. PREOVERLAY PREPARATION Before placement of the whitetopping overlay, existing dis- tresses in the existing HMA pavement should be repaired according to the design specifications. For more information on this, refer to the âPreoverlay Repairâ section in chapter four of this synthesis. After all repair activities have been completed, the exist- ing HMA surface should be cleaned to remove any signifi- cant dirt or other debris that may be detrimental to the bond between the existing HMA layer and the new PCC surface. Sweeping is the preferred method. Compressed air or water has also been used, but it is not cost-effective or necessary for whitetopping applications (1). Under extreme conditions, solar effects can significantly heat and dry the existing HMA before placement. To miti- gate this situation, measures could be considered to cool the surface before placement. Light water fogging is the recom- mended practice, and it can be used when the surface tem- perature of the existing HMA is uncomfortable to an open palm (1). Just before PCC placement, the HMA surface should be lightly moistened to prevent water from being drawn from the fresh concrete (1). A loss of water in the concrete mix at this critical location could reduce the level of bonding between the two layers. However, care should be taken to ensure that pools of water are not present on the HMA sur- face before placement. Should that happen, debonding can occur, because a weak layer may be formed within the con- crete at the bond. Although detrimental to the bond between the HMA layer and the new PCC overlay, whitewashing has been recom- mended when the temperature of the HMA surface is expected to exceed 45°C (110°F) (1). Note that this practice is not rec- ommended for UTW and most TWT projects. It should be used only on TWT overlays that are not designed to bond to the underlying HMA pavement. If used, whitewashing should be made of lime slurry (83). Lime slurry is made using water and hydrated lime. Chloride salts should not be added to the mixture, even if the directions suggest doing so. It can be particularly harmful to reinforcing steel (e.g., tie bars or dowels) and to the PCC itself. Curing compound has also been used for a whitewash, but it is even more detrimental to the bond between the HMA and white- topping layers and is therefore not recommended. In short, the preferred surface condition of the HMA before whitetopping is a clean and milled surface with no standing water. CONCRETE BATCHING AND TRANSPORTATION Batching operations for whitetopping concrete are the same as for conventional paving concrete. The mix design selected for the project should be adhered to as closely as possible. Additional recommendations on the concrete mixture can be found in chapter four of this synthesis. From the survey responses, the only notable difference may be that UTW overlays are commonly constructed with ready- mix concrete. As a result, control over the batching process may not lie with the paving contractor. It is therefore recom- mended that the batch tickets be collected from the trucks before placement and compared with the approved mix design. Additions of mix water not approved by the engineer should be avoided, because they may adversely affect the strength and durability of the pavement. If fiber reinforcement is used in the concrete, measures should be taken to minimize the possibility of fiber balling. AASHTO recently published the findings of a task force addressing the use of fibers in paving concrete. The reader is encouraged to follow the guidelines outlined in the AASHTO document (94). Among the survey respondents, 68% reported that they add fibers to the wet mixture by means of bags or bundles, 16% add fibers to the wet mixture in loose form, and 8% add fibers to the dry constituents before mixing. Special care should be used in transporting concrete from the batching facility to the project site. It is during trans- portation, handling, and placement that segregation in con- crete can occur. If the concrete is placed by truck in front of a slipform paver, the concrete should be placed in small, overlapping CHAPTER FIVE CONSTRUCTION PRACTICES
24 If floating is used, it should be kept to a minimum (see Figure 11). If the concrete surface cannot be finished without the continuous use of floats, changes should be made to the concrete mix or to the finishing machines (1). Slipform Most slipform pavers spread, consolidate, screed, and float finish fresh concrete in one operation, without the use of fixed forms. Concrete is placed in front of, or fed from the side of, the paver by ready-mix trucks, pumping machines, or other piles that minimize lateral movement of the material by the paverâs auger. Care should also be taken not to track paste or dirt onto the surface of the HMA in advance of the paver. Doing so can lead to debonding if the overlay is placed after the paste has dried (119). For the same reason, any wash water residue from adjacent sawing operations should be cleaned before placement. PLACEMENT TECHNIQUES Equipment For whitetopping construction, several choices are available with respect to equipment and placement techniques. Accord- ing to the user survey, where whitetopping sometimes differs from conventional concrete placement is the use of more com- pact and less costly techniques. For example, whitetopping inlays have been successfully constructed with either hand placement or slipform paving. For overlays with exposed edges, hand placement with fixed forms or slipform paving is available. The following sections expand on these methods. Fixed Forms For fixed forms and hand placement, the forms must be rigidly secured to support the concrete and construction equip- ment so that settling or other moving does not occur. The forms must also support the lateral pressure of the concrete as it is placed from a ready-mix truck, pumper, or other means. An example of using fixed forms is shown in Figure 8. Forms should be cleaned and lightly oiled before each use, and cleaned again after each use. They should be removed carefully after the concrete has gained sufficient strength. Care must be taken to prevent damage to the concrete edges and corners (120). Concrete should be placed on the existing HMA pave- ment as evenly as possible to avoid segregation and to mini- mize the lateral force necessary in additional spreading as the work progresses. A mechanical spreader or strike-off screed that rides on the forms can be used (see Figures 9 and 10). Handheld vibrators (âstingersâ) should also be used to ensure adequate consolidation along the forms and at other discon- tinuities (e.g., manholes or drains). If a finishing machine is not used, a vibrating screed or roller screed can be used to strike off and consolidate the con- crete. In addition, handheld stinger vibrators should be used, because screeds alone may not provide adequate consolidation at the bottom of the overlay. Mechanical spreaders and strike- off screeds can also be used for inlays, where the equipment can ride on the edges of the existing pavement, which describe the finished elevation. FIGURE 8 Use of fixed forms. FIGURE 9 Vibrating screed.
25 means (Figure 12). Slipform pavers usually require string- lines for horizontal and vertical grade control (Figure 13). Where possible, dual stringlines should be used to enhance grade control and smoothness. Because many slipform pavers employ automated floats, additional floating is commonly specified, although often not required behind the paver. Equipment Inspection and Placement Uniformity If batching and mixing equipment is used on site, it is impor- tant to maintain and inspect the equipment at regular inter- vals. Batch size, constituent quantities, mixing time, mixer speed, blade wear, and other aspects of on-site batching and mixing should be watched carefully. Also, as the concrete is placed on the existing HMA sur- face, it should be placed evenly across the width of the paving area. Doing so will minimize the extra effort of the paver, vibrating or rolling screed, and finishing labor required. If care is not taken, poor consolidation can result, as shown in Figure 14. Dowels When dowel baskets are used on TWT projects (see Figure 15), their placement must be coordinated with the movements of the paving train. If there is adequate space to the side of the paving lane where the dowel baskets are placed, baskets can be positioned well in advance of the paving train. Where there is not adequate space, baskets can be placed and secured after trucks have delivered the concrete to the area. As the paving operation moves over the dowels and baskets, care must be taken not to move the baskets with the force of the concrete or paving equipment. It can sometimes be difficult to securely anchor the dowel baskets to the underlying HMA. Misalignment in the dowel basket can result in a locked joint or premature cracking in the two slabs adjacent to the joint (121). A potential problem with the use of baskets is voids beneath the dowels and the basket steel. The contractor must ensure that the concrete is consolidated adequately to eliminate voids that may form beneath the dowels held in a basket, but that it not be overvibrated, which would cause segregation or exces- sive loss of air. FIGURE 10 Rolling screed. FIGURE 11 Floating. FIGURE 12 Use of a slipform paver. FIGURE 13 Stringline for guiding paver.
26 An alternative to dowel baskets is the use of a dowel bar inserter (DBI) (Figure 16). In some cases, this alternative can save time and money compared with placing and aligning dowel bar baskets. However, the contractor must take extra care to ensure that the DBI inserts the dowel to meet the spec- ified tolerances and does not leave marks on the overlay sur- face, even after final floating and finishing. When a DBI is used, the concrete around the dowels must be well con- solidated after insertion. Additional finishing may also be required to repair the surface of the concrete after the dowels have been inserted. Other Considerations Rain Protection An important practice when paving is to maintain a contin- gency plan in case of inclement weather. If more than a light rain is expected, construction should be stopped, and fresh concrete already placed should be covered for protection (122). Failure to do so can result in the concrete surface losing sig- nificant strength and prematurely spalling and/or abrading. If at any time fresh concrete is found to have standing surface water, the finishing operations should be delayed until the water has evaporated or otherwise been removed from the surface. Floating or troweling the excess surface water into the fresh concrete will change the waterâcement ratio at the surface, adversely affecting the durability of the surface and leading to spalling or other surface defects. Thickness When placing whitetopping concrete on uneven, rutted, or shoved HMA, the concrete should be placed so that the thinnest part is equal to the design thickness (see Figure 17) (82). The surface elevation of the new whitetopping sur- face should be constructed to meet the requirements of the design and plans. FIGURE 14 Poor consolidation. FIGURE 15 Concrete being placed over a dowel basket. FIGURE 16 DBI submerging dowels. FIGURE 17 With an uneven HMA, err on the thick side (82).
27 Smoothness Almost all states now require the pavement surface to meet specific smoothness criteria (123). An important method for producing smooth whitetopping overlay surfaces is to main- tain consistency of the concrete throughout the duration of the project (124). The use of dual stringlines with a slipform paver can also improve the smoothness of the whitetopping surface (124). Dual stringlines can act as a redundant system, minimizing impact if one string is inadvertently moved or bumped. Dual stringlines also provide excellent grade control. The forward motion of the paving operations is also a key to improving smoothness. Any interruptions in the concrete delivery or forward motion of the screed or slipform paver can lead to a bump in the surface of the overlay. Owner-agencies are encouraged to specify end-result requirements for smooth- ness. Doing so will allow innovative grade controls that can improve smoothness, including those employing lasers, ultra- sound, or global positioning system technologies. TEXTURING AND CURING Surface Texturing Timing Concrete overlay surface texturing operations should be per- formed at just the right time, to not disturb the curing or set- ting of the concrete. Texturing should be performed just after the water sheen has disappeared, but before the concrete becomes nonplastic. The water sheen can be seen in Figure 18. Texturing Methods Depending on the facility type and traffic speeds, several dif- ferent methods of concrete texturing can be used. For exam- ple, during common wet-weather conditions, and for high- way speeds, vehicles require the additional tireâpavement interaction provided by increased texture in the overlay sur- face. For lower-speed facilities, alternate texturing methods may be used. The following sections describe texturing meth- ods that have been used on UTW and TWT projects. Burlap or Turf Drag For lower-speed facilities such as city streets, residential areas, or other urban or municipal appli- cations, a lesser degree of texturization is required (125). One potential exception would be whitetopping at intersections where transverse texture may be desirable to improve stop- ping skid resistance. Texture applications such as burlap drag (Figure 19), turf drag, or a coarse broom (Figure 20) may be sufficient for these projects. With a burlap or turf drag, the material is often dragged from the paving train across the entire paving width. With a coarse broom, as shown in Figure 20, the broom is often pulled transversely one broom width at a time, along the entire length of the freshly placed overlay. Uniformity should be emphasized when using this type of texture. Tining For high-speed facilities, such as highways, free- ways, and higher-speed urban streets, increased texturization is often recommended. This type of texture is created by the use of a tining comb (Figure 21). It is important, when using a tining comb, to make shal- low striations [typically 3 mm (1/8 in.] in the transverse or lon- gitudinal direction. The tining comb should not overlap its previous passes. Some states have or are considering discontinuing the use of tining owing to possible adverse effects on curing, noise, FIGURE 18 Water sheen on a concrete surface. FIGURE 19 Application of texture with a burlap drag. FIGURE 20 Application of texture with a coarse broom.
28 and long-term durability, instead relying on other techniques such as a drag finish (126,127). Detrimental effects observed by some DOTs as a result of tining include a weakened plane in the concrete surface, which may cause delamina- tion, spalling, and popouts. Tining may also cause undue delay in the curing operation. With the advancement of vehicle tire technology in the past 40 years, the interaction between tires and the overlay surface in wet-weather conditions has dra- matically improved (128). Curing The major objective of concrete curing applications is to pre- vent the rapid loss of water from the concrete. Proper curing of UTW and TWT is particularly important, because these overlays are thin with large surface areas compared with the volume of concrete. As concrete loses water as a result of evaporation from the top surface, differential drying shrink- age can occur (39). This is a major contributor to shrinkage cracking, especially in whitetopping overlays where the sec- tions are thinner than in conventional concrete paving. The application of curing reduces the loss of water from the sur- face of the concrete. It also permits more complete hydration of cement in the concrete itself (39). Minimizing evaporation also helps to control the temperature of the concrete during its early-age stage. Curing operations should begin immediately after the water sheen disappears from the surface and immediately after any texturing operations have been completed. In the case of a curing compound, the membrane formed by the compound should not be disturbed after it is placed. The HIPERPAV computer program (High Performance Concrete Paving Software), developed by the FHWA, is a useful tool for predicting the effect of various curing tech- niques (39,86). It can model the design, materials, construc- tion, and environmental conditions affecting the concrete dur- ing the early age. Curing Methods Various concrete curing methods are available and each pro- vides different levels of protection. A single coat of liquid cur- ing compound provides the least protection, but additional coats can improve its performance. According to responses to the synthesis survey, numerous agencies specified a âdouble applicationâ of liquid curing compound. Polyethylene sheets, cotton mats, and wet burlap can provide additional protection. Liquid Curing Compound White-pigmented, liquid mem- brane curing compound is used most often because of its low cost and ease of application. It requires neither substantial labor nor expensive and bulky material, such as cotton mats. Its disadvantages are that it provides a minimal amount of protection and the membrane can be ruptured inadvertently. The liquid curing compound should be white to avoid excess heat absorption from the sun (see Figure 22) (39). In addition, the white color enables construction workers to check more easily for coverage uniformity. The liquid com- pound must be constantly agitated during application to ensure that the mixture is applied correctly. The curing compound spraying operation should be shielded from the wind through- out the process. The compound must cover all exposed surfaces, including the sides of the overlay. If joints are formed before curing, care should be taken not to apply the compound into them, for doing so can prevent joint sealant from adhering. For UTW, curing compound should be applied at twice the normal appli- cation rate, because of its increased sensitivity to drying shrinkage (101). This practice is confirmed by the responses to the user survey. Plastic or Waterproof Paper Plastic (typically polyethyl- ene) sheeting provides good protection to the concrete from water evaporation from the surface (see Figure 23) (39). It requires more labor than liquid curing compound, yet it is not as bulky as cotton mats or burlap. Waterproof paper may also be used in the same manner described here for plastic sheet- ing, but it is not as common. FIGURE 21 Tining comb texturization on freshly placed concrete. FIGURE 22 White-pigmented, liquid membrane curing compound.
29 The plastic sheeting must not have any rips or tears through which water can escape. The sheets should overlap to pro- vide full coverage for the concrete surface. Just as with cur- ing compound, the sheeting should cover all exposed con- crete surfaces, including the edges of the overlay. To prevent removal by the wind, weight must often be added to the edges to hold the sheeting in place. Cotton Mats or Burlap Cotton mats represent a great increase in evaporation protection, both by providing additional mois- ture, if needed, and by protecting the concrete from ambient conditions such as low humidity, high wind speeds, and high temperatures. Cotton mats and wet burlap must be kept con- tinually moist. When the mats get dry, they can become more harmful than proceeding without them as a result of the âwickingâ action that draws moisture from the concrete into the mat. Weather Considerations In rapid drying conditions, a light water fog may be neces- sary to maintain moist surface conditions before the applica- tion of curing methods (92). Light water fogging can be accomplished during a brief period of time when the concrete surface begins to dry but before the curing operations can begin, such as before the completion of texturing operations. Ambient weather conditions, such as wind speed, relative humidity, and air temperature, can interact with the temper- ature of the concrete to cause excessive water evaporation from the concrete surface (68). Because different curing meth- ods provide different levels of protection, knowing the amount of protection required is important in determining the method to use. To determine the level of protection required, the ambi- ent conditions and concrete temperature must be known. A portable weather station that records the ambient con- ditions and automatically predicts evaporative water from the concrete surface can be an invaluable tool for controlling water loss from the concrete surface (129). Such a tool can also provide advanced warning when conditions approach predefined limits of evaporation. In the absence of such a tool, the plastic shrinkage prediction nomograph, found in the ACI guidelines for hot-weather concreting, can be used to identify adverse ambient conditions (92). Other Curing Considerations If even further protection is needed from evaporation, cot- ton mats can be covered with plastic sheeting to slow the evaporation of water from the mats, thereby maximizing the protection of the concrete surface. Where paving operations can be completed at night, that measure should also be con- sidered (39). The lack of sunlight and the associated heat, com- bined with a higher ambient humidity, can greatly improve the conditions for lower potential evaporation. The temperature of the concrete itself has a large impact on the amount of water lost to evaporation. Cooling aggre- gates before batching with a light mist of water can help keep the concrete temperature slightly lower (92). An evaporative retardant, such as a monomolecular com- pound, similar to a liquid curing compound, can resist the tendency of water to evaporate when conditions approach an evaporative condition. JOINT CONSTRUCTION Sawcut Timing and Tools Some practitioners involved in whitetopping have reported distresses related to the timing of the sawcutting operation (10,12,48,109). The timing of this construction procedure is critical in preventing early-age distress. Sawing joints too early can cause the weak concrete to ravel excessively at the joint. Conversely, sawing too late may allow stresses to build and lead to uncontrolled, random cracking in the slabs (130). The amount of time required after placement before the concrete is ready for sawcutting without joint raveling depends on the mix design, the climatic conditions, and the type of coarse aggregate (131). Raveling can be caused when the saw blade catches a particle of coarse aggregate and the sur- rounding cement matrix is not strong enough to hold it in place while the blade cuts it. Concrete with soft coarse aggre- gate can usually be cut earlier (at a lower strength) than can the same mix with hard coarse aggregate (131). Concrete joints are usually sawcut between 4 and 12 h after placement. However, in hot weather conditions or with a fast-track mix, this period may be shorter. In cold weather, the opposite is true, with perhaps up to 24 h required. The appropriate time can be determined by several meth- ods. A common method is the âscratch test,â in which a nail is run along the concrete surface to determine if the concrete has hardened (132). Comparing the concreteâs maturity with laboratory data is another method that can help determine the FIGURE 23 Polyethylene sheeting used as a curing method.
30 appropriate time for sawcutting (106). A third method, the pulse velocity meter, also requires correlation to laboratory data (133). Because the maturity method and the pulse veloc- ity meter are also nondestructive, they can be used several times at the same location to determine the ability of the con- crete to withstand a joint sawcutting blade. The FHWA HIPERPAV computer program may also be useful in helping to predict the appropriate time for joint saw- ing based on the concrete mix design, construction times, and environmental conditions (39,86). As a planning tool, it can help in predicting the window in which sawcutting is effective in preventing random cracking, especially if similar placements have been done with known sawing windows. The timing of joint sawcutting is important, because at early ages large climatic temperature fluctuations and the slab restraint at the PCCâHMA interface can have critical effects on the slab (81). Large temperature fluctuations can cause high curling stresses, as a result of temperature differentials through the thickness of the whitetopping slab. Such temperature vari- ations can also affect the axial displacement and stress in the slab. When the concrete temperature decreases dramatically, the high restraint at the bonded PCCâHMA interface can cause large axial stresses. The result is a high probability for uncon- trolled cracking (86). Another impact of the existing HMA layer is that it is stiffer than normal base materials; therefore, it can help induce higher curling and warping stresses by decreas- ing the support beneath the slab as it deforms vertically (109). Longitudinal joints should be sawed as soon as possible after the transverse joints. TWT and UWT overlays are espe- cially vulnerable with the associated short joint spacing, and they should be sawed at least down the slab centerline to relieve the stresses that may build in the transverse direction. Sawcut Method and Depth Early entry or âgreenâ sawcutting is a technique that can help to ensure that the joints are cut in a timelier manner (see Fig- ure 24). According to the user survey, this method is more commonly used than is conventional sawing. Early entry saws are readily available and have been developed specifically for early-age sawcutting, minimizing the raveling of the con- crete at the sawcut. To ensure that the joint sawcutting oper- ations can be completed, backup saws are recommended in the event of equipment failure. The depth of sawcut for standard joint saws should be between one-fourth and one-third the thickness of the slab (96). Where possible, the depth should be at least one-third of the thickness. If early-age saws are used, the sawcut depth may be less, but it still should not be less than 1 in. (see Fig- ure 25) (116). It is also important to maintain some intact concrete for adequate aggregate interlock between adjoining slabs, which is obtained by an adequate crack face rather than by sawcut. The sawcut depth must be adjusted to account for thick- ened sections to maintain the appropriate depth (1). If the slab thickness varies more than 25 mm (1 in.) from the mean thickness, the sawcut should be made deeper into the slab. Such thickened areas may occur at pavement edges, the crown of cross slopes, transitions between typical sections, or sig- nificant distortions in the existing surface. Of particular con- sideration with whitetopping is where concrete is used to fill ruts in the existing HMA layer. An alternative to sawcutting longitudinal joints that has been successfully demonstrated on numerous projects in Iowa employs a vibrating knife that is passed through the concrete mix immediately behind the profile pan (134). This tech- nique creates a separation in the coarse aggregate skeleton that is replaced with mortar. Natural cracking then occurs along the resulting weakened plane. Joint Sealing The purpose of joint sealants is to keep water and incom- pressibles (e.g., sand) out of the joint and the crack face. On some TWT overlay construction, joint sealant is used, as it is in standard PCC pavement construction (1). UTW joints are usually not sealed (111). That is, UTW joints are usually placed at a short spacing and do not open as much as joints FIGURE 24 Sawcutting on a fast-tracked UTW. FIGURE 25 Sawcut depth using an early-age saw.
31 associated with longer slabs. For this reason, these joints are typically not sealed, a situation shown in Figure 26. QUALITY CONTROL AND QUALITY ASSURANCE Quality Control and Quality Assurance Program For many projects, a formal quality control/quality assurance (QC/QA) program can be beneficial to the quality of the final product. By measuring and tracking the various mea- sures of quality, changes and anomalies can be quickly iden- tified and corrected before the quality falls below a critical level. Although a QC/QA program is typically employed on projects placed over weeks or months, some of the elements described herein can be used for smaller placements as well, such as for a short weekend project. In most cases, the QC/QA elements used on UTW and TWT projects are identical to those of other PCC pavement projects. Typical components of a successful QC/QA pro- gram include the following (135): ⢠The contractor performs QC and acceptance testing. ⢠The owner (state highway agency, city, etc.) performs assurance and verification testing. ⢠Statistical comparisons are used to determine data con- formity between contractor and owner test results. ⢠Incentives and disincentives are used to encourage the contractor to achieve higher quality. ⢠Both the contractorâs and the ownerâs technicians are required to be trained in the same manner, necessitating that both meet the same standards. The contractor should also be required to implement con- trol charts to track the processes involved in its concrete pro- duction and placement operations. Such control charts should be part of an overall QC plan developed by the contractor (with guidance from the owner) and subsequently approved by the owner. A QC plan describes all the activities that will be performed by the contractor to ensure quality in the final product, the whitetopping overlay (136,137). Recommended QC Tests In controlling the processes of whitetopping concrete pro- duction and placement, the contractor must conduct tests to quantify the level of quality being achieved. Recommended QC (or process control) tests include the following: ⢠Mixture temperature as it arrives at the site; ⢠Concrete slump; ⢠Final waterâcement ratio as it is placed; ⢠Cement factor; ⢠Admixture dosage; ⢠Aggregate moisture content, both coarse and fine aggregates; ⢠Aggregate gradation; ⢠Fine aggregate fineness modulus or sand equivalent; and ⢠Unit weight or air content of fresh concrete. These QC tests should be conducted by the contractor and plotted on control charts whether or not they are required by the owner, simply because doing so provides a clear picture of the state of the process. When there is a graphical repre- sentation of the process, adverse trends can be detected early and corrected before becoming critical. Recommended Acceptance Tests In contrast to QC tests, acceptance tests measure the finished product; either at the time of placement or after the concrete has hardened (137). Recommended acceptance tests include the following: ⢠Strength â Compressive â Flexural â Splitting tensile ⢠Thickness ⢠Unit weight or air content ⢠Smoothness. Note that not all of these tests are required for every whitetopping project. In addition, newer tests such as bond strength between the PCC and HMA could be used for QC or acceptance, but they should not be adopted until the specify- ing agency is comfortable with the techniques and test vari- ability involved. Payment By using QC/QA-type specifications, the owner is able to pay for the final product based on the level of qualityFIGURE 26 Joints without joint sealant.
32 achieved by the contractor (135). For example, when using a percent within limits (PWL) payment methodology, the owner can apply rational pay factors to the unit bid price for the concrete. PWLs and pay factors are described by AASHTO (83). Alternative methods for calculating payment, and for PWLs in general, have also been developed by the ACPA and FAA (101,138). Temperature Management Cold Weather Concreting There are several methods for protecting concrete in cold weather. The major consideration is to keep the concrete warm enough for the chemical reactions to take place (106). The temperature of the concrete is critical during the early- age period. A method for managing the concrete temperature is to compensate for the cold weather-induced heat loss by producing and delivering the concrete at a higher temperature. General guidelines for ambient temperature include main- taining a minimum temperature of 10°C (50°F) during the first 3 to 7 days and 13°C (55°F) for thinner slabs (i.e., UTW). It is important that the temperature of any surfaces that come in contact with the concrete be above the freezing point of water (106). Maintaining the temperature of the concrete above a min- imum level begins with the concrete constituents. The fol- lowing methods and actions can be beneficial to concrete in cold weather situations (106): ⢠Reduce or eliminate SCMs in the mix. Although SCMs are beneficial under many circumstances, their use in cold weather may delay set and lead to other detrimen- tal effects. ⢠Increase the cement content in the mix. ⢠Heat the mixture water and the aggregates before mixing. ⢠Change the cement type to Type III, if not already specified. Other methods include using curing blankets or insulated forms. Under extreme cold weather conditions, a heated cov- ering for the whitetopping surface could be provided. Provid- ing an external heat source to concrete slab coverings can slow the escape of heat from the concrete into the surroundings. Hot Weather Concreting There are several methods available to protect the overlay in hot weather. The major consideration for the concrete is to maintain the temperature in an appropriate range (92). A method for managing the concrete temperature is to com- pensate for the hot weather-induced heat gain by producing and delivering the concrete at a lower temperature. Guidelines for hot weather concreting include limiting whitetopping placement to times when the ambient tempera- ture is less than 32°C (90°F). The temperature of the concrete surface should also be limited to 32°C (90°F) during the curing period. This can be accomplished by any of several methods, including shading the concrete surface or paving at night. These are only general recommendations. However, a number of spe- cific actions can be taken to ensure proper concrete temperature control when ambient temperatures exceed 32°C (90°F) (92). As with cold weather concreting, the process of managing the concrete temperature throughout the early-age period begins with the constituents. The following guidelines can help manage the concreteâs temperature during the early-age period (39,92): ⢠Type III cement may not be appropriate for hot weather concreting owing to its high heat of hydration. ⢠It is advisable to use SCMs such as fly ash or ground- granulated blast furnace slag. In addition to controlling early-age temperatures, SCMs usually improve long-term strength and durability. ⢠Aggregates should be kept cool by storing them in a shaded area or by misting them with water. ⢠Cool or cold mixing water should be used. In extreme cases, ice has been usedâand it is helpful as long as it has been completely melted by the end of the mixing process. ⢠Water-reducing admixtures can be used to slow the hydra- tion of the cement, thus decreasing the heat generated by the curing concrete. Other methods include those that protect or affect the con- crete either during placement or once it has been placed and are noted here. ⢠Nighttime paving can help coordinate the maximum heat generation with the coolest time of the day, thus mini- mizing the potential for internal and external heat to com- bine and cause a critical situation. ⢠Where fast-track whitetopping placement is used, paving should be avoided between 10 a.m. and 4 p.m., for the same reasons cited previously. ⢠The amount of wind blowing across the surface of the whitetopping overlay should be minimized. This can be accomplished by wind screens. Although wind can aid in evaporation, resulting in the cooling of the concrete, it can be even more detrimental when causing the evap- oration of needed water, creating a compounded effect of high temperatures and low moisture in the concrete near the surface. The result can be drying, shrinkage, cracking, and other distresses over both the short and long term. ⢠Application of additional curing measures can keep mois- ture in the concrete while the hot weather is causing it to evaporate. A double or triple application of curing com- pound, or even more extensive measures, as described in
33 the curing section, can help in protecting the concrete during periods of hot weather. Fast-Track Construction Many whitetopping projects are constructed under accelerated (fast-track) schedules (39,139). This means that the facility is closed to public traffic for only a short period and that the con- crete is not very old when it is opened to traffic. Because of these constraints, fast-track mixes are specially prepared to withstand traffic within 1 to 2 days. In addition to high-early- strength mixes, the construction schedule is usually acceler- ated to maximize the use of the equipment. Fast-track mixes usually contain increased amounts of Type I or Type II cements or normal amounts of Type III cement to increase the early-age strength gain (139). Because an appropriate heat of hydration of the cement is crucial to the concrete strength development, some projects use poly- styrene foam insulating blankets to retain heat, where needed. Other methods for increasing the heat development at the appropriate time are used. Fast-track mixes must be carefully designed to generate the right amount of heat at the right time. Maturity methods are one of the best techniques for con- trolling strength and temperatures during fast-track construc- tion. Because there are a number of inexpensive systems com- mercially available, such techniques should be considered on all whitetopping applications (106). When maturity is the basis, the FHWA HIPERPAV computer program can also be used to predict the early-age behavior of concrete during the first 72 h after placement (86). It can predict the effects of early opening to traffic, as well as help identify the appro- priate time for the concrete to be opened to construction or public traffic. OTHER CONSIDERATIONS Overhead Structures When overhead clearance is a problem, the pavement struc- ture may need to be tapered to a full-depth reconstruction and then transitioned again to the typical whitetopping section (1). Another option is to raise any overhead structures such as bridges or utilities; however, this is not conducive to the cost- saving and fast-tracking that many whitetopping projects attempt to achieve (1). Such issues should be addressed dur- ing the project planning stages. Inline Bridges The approaches to inline bridges must maintain the same ele- vation as for the bridge, because the bridge will not be over- laid with whitetopping and because it most likely will not be raised. When the overlay is adding thickness to the pavement, the milling operation should be increased to include a taper of approximately 40 ft per inch of increased depth (1). This addi- tion to the milling operation will help to provide a smooth transition when one is approaching and leaving the bridge. Expansion joints at the bridge approaches must be included in the whitetopping overlay. Pavement Shoulders The continuity between the whitetopping overlay and pave- ment shoulders, or curbs and gutters if in an urban setting, must be maintained (140). Shoulders must be increased in thickness as necessary (see Figure 27). Curbs and gutters may also need to be replaced or elevated to meet the new ele- vation of the pavement edge. Opening to Traffic Opening to traffic, either construction or the public, must be coordinated with the strength of the concrete and the saw- cutting operation, especially in conditions where the facility must be opened to traffic as soon as possible (139). Consid- erations for UTW and TWT in opening to traffic are usually identical to those for other PCC pavement projects. When either the early-strength testing or the maturity method is used, the strength at which the concrete can sustain vehicu- lar traffic can be determined (106). Once this strength has been attained, the entire sawcutting operation is complete and the facility may be opened to traffic. Another require- ment before opening to traffic includes the resolution of all elevation issues at shoulders or curbsâby permanent solu- tion or by temporary signage and warnings. When a whitetopping overlay lane must be opened to traf- fic temporarily, such as for the night or for the weekend, a lon- gitudinal taper must be constructed to allow traffic to drive off the whitetopping overlay onto the existing HMA pavement. When construction resumes, the temporary HMA transition is removed. In addition, it is important that the concrete has gained adequate strength and that the joints have been saw- cut before opening to traffic. FIGURE 27 New shoulders meeting a new whitetopping elevation.
