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Roller-Compacted Concrete (RCC) Materials Selection
Roller-compacted concrete (RCC) pavements use the same materials as conventional concrete. The essential coarse and fine aggregates, cementitious materials (cement, fly ash, ground granulated blast furnace slag, etc.), and water are all used, but they are used in different proportions compared to mixes for conventional concrete pavements. As with any concrete mix, the selection of materials is very important. Knowledge of the mixture ingredients and experience with construction specifications and requirements for specific applications is important to ensure a well-designed and proper performing RCC mixture. RCC mixture proportioning is not the same as conventional concrete mixture proportioning and more information can be found here.
Aggregates account for up to 85% of RCC mixtures by volume. They are influential in achieving desired properties including workability, specified density, compressive and flexural strengths, thermal properties, durability, and long-term performance. These aggregates can be the same as those used in conventional concrete but the same considerations are of concern such as abrasion resistance, alkali-silica reactivity (ASR), and durability of the aggregates themselves.
While the aggregates can be the same as conventional concrete, the gradation requirements are different. The aggregate skeleton must allow for consolidation under rollers while still maintaining its form and not segregating. The aggregate gradation, along with the aggregate properties, can reduce the potential for segregation and improve strength and durability. Depending on the local availability, local practices, and specifications; crushed or uncrushed aggregates can be used with different blending practices. Both coarse and fine aggregates should be hard and durable. RCC gradations are typically dense and well-graded to supply the necessary aggregate interlock for the mixture. This usually requires a mixture of fine and coarse aggregate with the fine faction being higher than in conventional paving concrete. Intermediate aggregates (i.e. those with a size distribution between coarse and fine aggregates) can also be used in order to create the dense, well-graded gradation common to RCC. Total aggregate fines are typically in the range of 2-8%.
Coarse aggregates should meet ASTM C33/AASHTO M6/M80 standards and should typically be held to a nominal maximum sized aggregate (NMSA) of 3/4 in. (19 mm) for unsurfaced RCC pavements. If a wearing course is being used or if the quality of the surface is not of importance, a higher NMSA can be utilized. Smaller NMSA coarse aggregates lower the potential for segregation, enhance cohesiveness and improve ride quality, but can also increase the cement consumption. Additional tests for abrasion, soundness, alkali-silica reactivity, durability index, and coefficient of thermal expansion should be performed where required.
Fine aggregates should also meet ASTM C33/AASHTO M6/M80 standards.
RCC mixtures can utilize basic cement types, blended cements, and pozzolans. The selection of cementitious materials should take into consideration the potential for sulfate attack, ASR, and abrasion. Type I and II cements are the most common. Early strength gain is typically inherent to RCC and thus Type III is not overly common but can be used. Type V can also be used when necessary. Supplementary cementitious materials (SCM) can be used to provide additional fine material and ensure adequate compaction and may also increase workability and reduce potential for ASR. When SCMs are used, the availability should be considered in addition to any extra specifications pertaining to the SCM being used. When SCMs are utilized, potential drawbacks and limitations should be noted. Fly ash should not be used in excess of 25% of the total volume to prevent scaling. The use of fly ash when paving RCC in low ambient temperatures has been shown to be problematic. Ground granulated blast furnace slag or silica fume can be used to increase strength and freeze-thaw durability but can also drive up costs.
Water in RCC comes from two sources. The first is the water that is contained in the aggregates and the second being the water added during mixing. This is similar to conventional paving concrete and as such the water quality should meet the requirements of ASTM C1602. If possible, mixture designs should be done with site-specific water and any limitations should be noted early in the project.
Due to the dryness of RCC mixes, admixtures typically need to be added in higher dosages to be effective. Water reducing and retarding admixtures can enhance cohesiveness and extend the workability of RCC, thus aiding in compaction. This can also improve bond between adjacent lanes or successive layers. Water reducing admixtures are affected somewhat by the amount and type of aggregate finer than the No. 200 (75 μm) sieve. Superplasticizers are occasionally used in dry batch plant production to reduce mix and offload times, in addition to improving workability and production rates. When opening early to traffic is of concern, set accelerating admixtures can also be used. Air entraining admixtures (AEA) are typically not used with RCC due to the significant dosages that would be required to achieve a typical entrained air content (i.e. 5-8%). RCC appears to yield acceptable freeze-thaw performance in the field without the use of AEA.