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96 Introduction Although the construction of the PCC/PCC composite pavement requires an additional paver and careful planning to ensure the production and placement of two PCC mixes, the process of construction itself does not use technologies and equipment that are not already available in the United States. The key steps in the construction include ⢠Prepare the sublayers (including subgrade, subbase, and base courses). ⢠Place the lower PCC layer and tied shoulders (if specified). 4 For PCC/CRC construction, the steel reinforcement needs to be securely placed on chairs on top of the base course before paving the lower PCC layer. The reinforcement depth for CRC should be at middepth or higher of the total PCC thickness. 4 For PCC/JPC, dowels may be placed in dowel baskets that are securely attached to the base course before paving the PCC layer. Alternately, dowel bar inserters (DBIs) may be used. The dowels should be located at middepth of the total PCC thickness. ⢠Place the upper PCC layer within 15 to 90 minutes (ideally within 60 minutes) of the lower PCC. ⢠For conventional textures (tining, burlap drag, turf drag, and so forth), texture the PCC surface as per specifications. ⢠Apply curing compound to the surface PCC layer. If EAC texture is specified, the curing compound should also include a surface retardant. ⢠For EAC texture, brush concrete surface with a rotating wire brush (within 5 to 24 hours of placement) to create EAC surface texture. Several brushings may be required. ⢠Saw cut and seal (if specified) joints in the PCC layer(s) (for PCC/JPC composite pavements). ⢠Place the shoulders (if not paved monolithically with the mainline PCC). PCC shoulder must be tied to the traffic lanes regardless of when they are placed. Guidelines for each of the steps mentioned are included below. Sample specifications are included in Appendix W for PCC construction, texturing, curing, saw cutting, and sealing. Construction Details Prepare the Sublayers The uniformity of the support conditions beneath a PCC/PCC pavement is critical for the long-term performance of the pavement, such as is in the case of conventional PCC pave- ments. For the purposes of preparing the sublayers, there is no difference between a PCC/PCC composite pavement and conventional PCC pavements such as JPCP or CRCP. Fig- ure 5.1 shows an example of grading and compacting the subgrade and base in preparation for PCC placement. The same procedures and specifications that have been used by an agency to prepare the sublayers for PCC construction should be used and specified for the construction of PCC/PCC com- posite pavements. An agency may choose to incorporate one or more of the following to prepare the sublayers: ⢠Cement or lime treatment of the subgrade soils; ⢠Asphalt- or cement-treated base course; ⢠Permeable base courses with drainage features (such as edge drains); and ⢠Recycled pavement materials (such as RCA or RAP) that are not used in the PCC mix. Reinforcing steel (for PCC/CRC composite pavements) and dowel baskets (if used for PCC/JPC composite pavements) can be placed directly on the base course following normal agency practices. Dowels should be placed middepth of the total PCC thickness (and not middepth of the lower PCC layer). They need to be securely fastened to the base course to ensure that they are not pushed by the paver. CRCP longitudinal steel must be placed on chairs and securely fastened to the base course. C h a p t e r 5 PCC/PCC Construction Guidelines
97 Place the Lower PCC Layer The lower PCC layer can be paved following the same pro- cedures and guidelines for PCC/PCC composite pavements as for conventional JPCP or CRCP (Figure 5.2). Because the lower layer serves an entirely structural function in the pave- ment system, concerns about ride quality or surface texture durability are not applicable. The only concerns in the con- struction of the PCC/PCC lower layer are meeting structural and durability criteria (such as compressive strength, flexural strength, air content, consolidation around dowel bars), as is the case for conventional JPCP and CRCP. It is recommended that the locations of random dowels across transverse joints be measured using a probe just after completion of paving oper- ations to ensure they are being placed in the proper location. The same goes for depth of CRC reinforcement, for which having the proper depth is even more critical. Agency QA/QC practices for testing materials and monitoring construction activities for JPCP or CRCP should be followed for the lower PCC placement of PCC/PCC composite pavements. This includes testing for slump, mix temperature, entrained air, and so forth (Figure 5.3). The lower PCC layer is placed such that the paving width is 1.5 to 2 in. less than the final desired paving width. This allows for a clearance of 0.75 to 1 in. on both sides for paving the top lift PCC. The lower PCC is a stiffer âdryerâ mix with low slump (~1 in.) to support the weight of the upper PCC (Figures 5.2 and 5.4). The mix typically is stiff enough to support the weight of an average person without the person sinking (see footprints and member of paving crew walk- ing on the lower PCC in Figure 5.4) but still has enough moisture to form a good wet-on-wet bond with the upper PCC layer. If DBI is used to place dowel bars in JPC, vibration of the lower PCC is not necessary (as is the practice in Europe). However, if dowel baskets are used for the JPC or CRC is used for the lower PCC, vibration of lower PCC may be necessary for adequate consolidation of the stiff mix around the dowel Figure 5.1. Grading and compacting the subgrade and base in preparation for PCC placement. Figure 5.2. Paving the lower PCC layer.
98 baskets or reinforcing steel. The vibrators should be higher than the top of the dowel basket assembly (or reinforcing steel) to prevent dragging or moving the dowel baskets (or steel rebars). Delivering the appropriate concrete mix to the appropriate paver is a detail that needs to be addressed before construction. At the I-70 construction in Kansas, both concrete mixes were batched and mixed at the same central mix batch plant. Batch plant personnel would identify to the driver which mixture they were transporting through the use of a green card for the lower lift or a red card for the top lift. Truck drivers then displayed a card of the appropriate color in the windshield of their truck. The belt placers used for supplying concrete to the pavers were also identified with green and red paint. During the PCC/PCC construction at MnROAD, the con- tractor had personnel specifically responsible for this task. Because both mixes were produced at the same ready-mix plant, the contractor requested, via radio communication with the plant, the desired mix, and a paving foreman directed the trucks to the appropriate pavers once the truck arrived at the job site based on the mix ticket. Although important for all slipform paving operations, consistent delivery of uniform concrete is even more crucial for PCC/PCC construction, as was detailed in Chapter 2. In the placement of both layers, it is important that construction logistics in the delivery of PCC mixes for the layers be under- stood by all parties involved. This allows the paving train to move at a regular pace and place both layers within the required time frame. Location of the dowels and tie bar steel in the lower lift must be checked constantly. Place the Upper PCC Layer The upper PCC layer must be placed between 15 and 90 min- utes (ideally less than 60 minutes) after the placement of the lower PCC layer. Placing the second lift soon after the place- ment of the first is important to ensure that the two PCCs in the layers bond well at their interface. Typically, because of Figure 5.3. QA testing of the PCC layer for entrained air and slump. Figure 5.4. Paving the upper PCC layer.
99 logistical constraints, the upper PCC is placed over the lower PCC using a belt placer (Figure 5.4). During the Kansas construction, the belt placer for the top lift was modified slightly with the addition of a grid fabricated from square steel tubing that was placed under the discharge end of the belt to reduce the potential for deformation of the bottom PCC while concrete was being unloaded from the belt onto the bottom PCC. During both the Kansas construction and the construction at MnROAD, the vibrators were raised above the elevation of the extrusion pan to prevent comingling of mixes in the upper and lower PCC (Figure 5.5). As an added precaution, at MnROAD, the vibrations were reduced to 4,000 vpm. Width and alignment of the two pavers also require special attention. Even a slight drift to one side or another on the part of either paver can result in edge problems because the upper lift PCC edge is less than 1 in. from the edge of the lower lift PCC (Figure 5.5). Texture the Upper PCC Layer Conventional Texture Typical agency guidelines for texturing the PCC surface (for conventional JPCP or CRCP) through tining, turf drag, burlap drag, and so forth (Figure 5.6) can be followed, and no modifications specific to PCC/PCC composite pavements are necessary. To control rapid moisture loss from the surface of the wet PCC to prevent rapid surface drying and early-age cracking, the upper layer PCC should be cured adequately. Agency guidelines and specifications to control moisture loss, such as application of curing compound (or other practices, such as wet burlap) should be specified (see Figure 5.6). EAC Texture The upper PCC should be finished smooth and then sprayed with an adequate retarding/curing compound. The PCC/PCC Figure 5.5. Vibrating the upper lift PCC and edge of upper lift paver relative to the lower PCC layer. Figure 5.6. Longitudinal tining texture (left) to create adequate surface texture on top of the upper lift PCC and application of curing compound to control surface moisture loss (right).
100 should be sprayed immediately after finishing, the compound application should be uniform, and the compound application should be sufficiently thick. If the finishing is uneven, the later texturing efforts can be compromised: areas of lighter com- pound application will result in less surface paste removed than will areas with the heavier application. Once the compound is sufficiently thick, a slight increase in its thickness does not affect the depth of etch. The compound can be applied using an automated spray bar, with spray nozzles tuned to apply the compound at a regular rate (Figure 5.7). Spray nozzle overlap should be accounted for, and dripping nozzles should be repaired quickly. The most critical aspect of the compound application is that enough of the compound is applied. Only in the event that not enough of the compound is used does nonuniformity in application become a serious problem. The spray nozzles may need to be protected from wind gusts by installing protective shields (Figure 5.7). In Kansas, a combined curing/retarding compound was not used, as was done at MnROAD. During the trial construc- tion, curing compound applied over the surface retardant appeared to interfere with the effectiveness of the brushing operations. As a result, during final paving, the initial curing was accomplished using polyethylene sheeting placed after the surface retardant was sprayed, instead of a liquid membrane curing compound. The timing of the EAC brushing requires a great deal of âhands onâ experience that only comes with constructing the texture in the field. The timing is determined by the ease of dislodging an aggregate embedded in the concrete. If the aggregate moves easily, the concrete should continue to cure. Once aggregates resist dislodging somewhat, a push broom can be used to gauge the readiness of the surface paste for brush- ing at approximately 20-ft intervals (Figure 5.8). The surface requires more curing time before brushing if (1) the surface paste and curing/retarder compound clumps together when Figure 5.7. Spraying of the curing/retarding compound over the upper PCC at MnROAD. Figure 5.8. Gauging the brushing readiness of the EAC surface with a push broom.
101 brushed with the hand broom instead of coming off as indi- vidual particles or (2) the shearing of the sweeping action of the broom dislodges aggregates. If the brush tests adequately remove paste from the surface without dislodging aggregates, the next step is to create the EAC texture. This is done with a wire brush mounted to a grader, as shown in Figure 5.9. The brush bristles should be stiff and sufficiently durable to remove the mortar but not overly stiff such that they damage the PCC surface or dislodge the aggregates. The stiffness of the brush is one of the factors that affect the number of passes needed (typically two to five) to obtain the desired EAC surface texture. A properly brushed EAC surface is shown in Figure 5.10. Much of the brushing will be a trial-and-error process involving passes with the wire brush and quick assessments using the sand patch test. The brush should be raised or lowered to provide the pressure necessary to etch the desired texture depth (at MnROAD this was 0.8 to 1.2 mm). The sand patch test should follow ASTM E965 (Standard Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique) and be conducted on interspersed locations across the PCC/PCC pavement. It is important to note that attempting an EAC finish is not advisable if there is any threat of precipitation. Approximately 250 ft of MnROAD R21 EAC finish appeared marbled and lacked a uniform etch depth because the surface, covered with curing/retarder compound, was saturated with rain. The contractor attempted to cover the 250 ft length of pavement with plastic sheeting, but the rain came too fast and was too heavy to avoid damage. The water mixed with the curing/retarder compound and cement paste at the surface. As a result, the slabs had to be saw cut before the surface was ready to be brushedâapproximately 15 hours after the curing/retarder compound had been applied. Furthermore, the water from the saw cutting removed some of the curing/retarder compound at the joints. Figure 5.9. Rotating sweeper brush with mechanical arm extended to relieve pressure on the brush (left) and close-up of the brush bristles (right). Figure 5.10. Properly brushed EAC surface.
102 Diamond Grinding Texture If specified, the upper lift PCC can be diamond ground after it has cured sufficiently. Agency specifications for diamond grinding of conventional JPCP or CRCP can be used, and no changes are necessary. Diamond grinding options include conventional diamond grinding (with closely spaced diamond blades and a deeper texture) or next-generation diamond grinding (a combination of shallow surficial grind and deep grooves), as shown in Figure 5.11. Saw Cut Joints in the PCC/JPC Joints in PCC/JPC should be saw cut and sealed (if specified) following conventional agency practices. Typical specifications for JPCP require a minimum saw cut depth of 1â3 the PCC thickness. For PCC/PCC composite pavements, a minimum saw cut depth of the greater of 1â3 the total PCC thickness (of both layers) or thickness of the upper PCC layer plus 0.5 in. should be specified to ensure proper opening of the joints. Table 5.1 shows the minimum recommended saw cut depths for PCC/PCC composite pavements. Table 5.1. Minimum Recommended Saw Cut Depths for PCC/PCC Pavements Upper PCC Thickness (in.) 1.5 2.0 2.5 3.0 3.5 Lower PCC Thickness (in.) 6.0 2.50 2.67 3.00 3.50 4.00 6.5 2.67 2.83 3.00 3.50 4.00 7.0 2.83 3.00 3.17 3.50 4.00 7.5 3.00 3.17 3.33 3.50 4.00 8.0 3.17 3.33 3.50 3.67 4.00 8.5 3.33 3.50 3.67 3.83 4.00 9.0 3.50 3.67 3.83 4.00 4.17 9.5 3.67 3.83 4.00 4.17 4.33 10.0 3.83 4.00 4.17 4.33 4.50 10.5 4.00 4.17 4.33 4.50 4.67 11.0 4.17 4.33 4.50 4.67 4.83 11.5 4.33 4.50 4.67 4.83 5.00 Note: Cut depths are measured in inches. Figure 5.11. Next-generation diamond grinding texture (left lane) and conventional diamond grinding texture (right lane) at MnROAD.
