Table 11-4 While most of this discussion has focused on RCA concrete mixes for use in the surface course, it applies to the use of RCA in lean concrete base courses as well. RCA was used in lean concrete bases before it was used in concrete surfaces. Table 11-4 summarizes the properties of hardened concrete using RCA. Design of RCA for Asphalt Mixtures RCA can also be used in the development of asphalt surface or base course mixes, as the typical RCA roughness and angularity contributes to overall stability and surface friction (ACPA 2009). Moreover, the presence of the mortar fraction on the RCA can reduce the occurrence of stripping in the asphalt. However, RCA tends to absorb more asphalt binder than conventional aggregates because of higher porosity and absorption capacity of RCA, which often makes its use Table 11-. Properties of hardened concrete using RCA (Snyder et al. 1994). Property RCA Mixes Compressive Strength Generally lower due to the reduction of the percent of natural aggregates. Flexural Strength Generally slightly lower, but it varies depending on the quality of the original aggregate and the amount of recycled fine aggregate used. Modulus of Elasticity Generally 20 to 40 percent lower than mixes with natural aggregates due to the lower elastic modulus of the RCA particles. Durability Freeze-Thaw Superior to virgin aggregate mixes. D-Cracking Reduced potential due to the crushing of original aggregate to smaller sizes. ASR Increased potential due to increased surface areas of the aggregates that have been crushed. Bond Strength with Reinforcement to virgin aggregate mixes, but reduced if recycled fines are used. Creep Higher due to the higher cement fraction. Drying Shrinkage Higher due to the higher cement paste fraction. Structural Design Considerations Conventional concrete pavement design procedures can be followed for concrete pavements utilizing RCA concrete. However, special consideration must be given to the low abrasion resistance of RCA and its typically smaller top size (used to reduce durability problems) to obtain good long-term pavement performance. These properties of RCA tend to cause poor load transfer conditions at undoweled joints or at transverse cracks, a condition that can lead to pumping, faulting, and corner breaks. Consequently, the following recommendations are provided to improve the structural performance of concrete pavements constructed with RCA (Snyder 1994; ACPA 2009): JPCP: Use shorter joint spacing (to reduce crack opening).
Cracking Fatigue cracking Deep patching of fatigued areas and structural overlay Block cracking (low to moderate) Thin cold treatment, chip seal, thin asphalt overlay Edge cracking treatment Longitudinal cracking Crack treatment Reflection cracking at joints Crack treatment Transverse cracking Crack treatment Patching and potholes Patch/patch deterioration Extensively patched pavements are candidates for structural repairs, including deep patching and thick overlays Potholes Same as for patched pavements above Surface defects Rutting – densification of pavement Fill rut with microsurfacing or strip chip seal, then thin cold treatment, chip seal, thin overlay; or mill and replace. Rutting – unstable asphalt concrete Mill and replace. Shoving Mill unstable material and replace. Bleeding Sand seal, chip seal, microsurfacing Polished aggregate Thin cold treatment, chip seal, thin asphalt overlay Raveling Fog seal, thin cold treatment, chip seal, thin asphalt overlay Table 11-. Candidate repair and preventive methods for distresses in concrete pavements (adapted from © ACPA 1997). Distress Type Repair Techniques Preventive Techniques Transverse Cracking repair Dowel bar retrofit and crack sealing Dowel bar retrofit Longitudinal Cracking Full-depth repair Cross stitching Dowel bar retrofit Joint and crack sealing stitching Dowel bar retrofit Slab stabilization Corner Cracking Full-depth repair Edge joint resealing Slab stabilization Intersecting Cracking Full-depth repair Cross stitching Dowel bar retrofit Slab stabilization Spalling Partial-depth repair Joint and crack resealing repair Pumping Slab stabilization Full-depth repair bar retrofit Retrofitted concrete shoulder Joint and crack resealing bar retrofit Retrofitted concrete shoulder Blowups Full-depth repairs Joint and crack resealing Punchouts Full-depth repairs Slab stabilization Retrofitted concrete shoulder
The overall rating of each treatment is calculated by multiplying the weights for each rating by the assigned in the previous step. The total score for each treatment is the sum of the individual scores. The alternatives are ranked in order, from the highest score to the lowest. If the process is followed exactly, the alternative with the highest score is the one that is selected. A generic example that follows these steps is illustrated in Table 11-10. After developing ten alternatives, the six decision factors shown in the second row are selected and a relative weight is assigned to each factor so that their total adds up to 100 percent. Then each alternative is scored against each decision factor, and the result is entered in the upper triangle in the appropriate cell. The score is multiplied by the weighting at the top of the column in order to obtain the value entered in the lower triangle. Then all of the values in the lower triangle are summed across the row for each alternative to arrive at a total score. The alternative with the highest score is the one that best addresses all of the selected decision factors. In the case of ties or when two alternatives are very close, a more in-depth evaluation of each alternative is warranted, including a review of what factors are most critical for the specific rehabilitation project. Table 11-. Example worksheet of a selection process incorporating multiple selected decision factors and assigned weightings. Decision Factors Initial Cost Life Cycle Expected Life Ease of Repairing/ Maintaining Traffic Control Proven Design in Agency Total Rank Weightings  25 15 20 15 10 15 Alternative 1 60 15 60 9 100 20 80 12 90 9 100 15 80 1 Alternative 2 60 15 60 9 100 20 80 12 90 9 100 15 80 1 Alternative 3 60 15 60 9 70 14 50 7.5 60 6 40 6 57.5 6 Alternative 4 60 15 60 9 70 14 50 7.5 60 6 40 6 57.5 6 Alternative 5 60 15 40 6 100 20 80 12 100 10 90 13.5 76.5 3 Alternative 6 60 15 80 12 40 8 20 3 40 4 20 3 45 9 Alternative 7 40 10 60 9 40 8 50 7.5 50 5 30 4.5 44 10 Alternative 8 70 17.5 80 12 60 12 50 7.5 80 8 40 6 63 5 Alternative 9 100 25 100 15 20 4 20 3 40 4 40 6 57 8
Wu, C. L., S. M. Tarr, T. M. Refai, M. A. Nagi, and M. J. Sheehan. 1997. Development of Ultra-Thin Whitetopping Design Procedure. PCA Research and Development Serial No. 2124. Portland Cement Association, Skokie, IL. Yrjanson, W. A. 1989. Recycling of Portland Cement Concrete Pavements. NCHRP Synthesis of Highway Practice 154. Transportation Research Board, Washington, DC.