Effect of Fine Natural Aggregates on the Mechanical Properties of Roller Compacted Concrete for Pavement Applications

Roller compacted concrete is distinct from ordinary concrete by virtue of its zero slump. Over the past several years, the increasing usage of RCC for pavement construction has been noted. The use of RCC for pavements is relatively a new technology and is still under development. However, early trials have proven the success of the use of RCCP, which encouraged engineers to apply it in various fields.Although roller compacted concrete pavement (RCCP) has enjoyed numerous applications of roads and large floors in Europe, Canada and United States since 1970s, it has not been used in Iran appropriately.According to ACI 325.10R, RCCP is defined as a relatively stiff mixture of aggregates, cementitious materials, and water, which is compacted by vibratory rollers and the use of aggregate fractions finer than the 75 micrometers (NO. 200) sieve, if non plastic, may be beneficial. Very fine aggregates (smaller than 75 µm) play an influential role in conventional concrete, but its effect on in both the fresh and hardened states of RCC was not investigated. Generally the presence of very fine particles limited in PCC for four main reasons:1. Reducing workability due to large surface area2. Preventing proper bond between cement paste and aggregates3. Absorbing water and reducing strength 4. Increasing cracking resistanceAccordingly, most standards limit the very fines content to only a few percent. ASTM C33 limits the fines content in fine aggregate to 5%. The European standard for aggregates (EN 12620) permits four levels of fines content in fine aggregate (3, 10, 16 and 22%). In addition, specific limits can be established at different location according to different conditions.In the case of sand manufacturing, it is difficult to produce sand with very low fines content and to do so it requires modern washing system and a huge amount of water that is invaluable in hot and dry climates. Also as a result the amount of waste fines that accumulated annually for instance in the U.S alone is in excess of 100 million tons that have considerable environmental impact. In order to extend the usage of RCCP, a comprehensive laboratory investigation was carried out to evaluate the workability and mechanical properties of RCCP containing high-volume and low-volume non-plastic aggregates fractions finer than the 75 µ. After determining the optimum percent of non-plastic natural filler, concrete mixtures were made with 2.5%, 7.5%, 10% and 15% non-plastic aggregates finer than sieve #200, in order to check the possibility of using unwashed aggregates and the effects of these very fine materials on mechanical properties of RCCP. Also the variables such as cement content and superplasticizer have been evaluated and discussed. Various plots and tables showing the interrelations between the studied variables were included in the paper.Results indicated that it is feasible to use unwashed aggregates in RCCP mixes but a few limitations should be carefully considered. Also natural filler caused to improve the mechanical properties of RCC. In addition one of the major findings was that the main factor that controlled most of hardened properties is achieving sufficient compaction and unlike conventional concrete increasing cement content may not result in strength increase. Based on the results, the optimum aggregates proportion and W/C ratio have been found in order to make RCC for pavement applications that meet all specified limits in the ACI 325.10 state-of-the-art report and ACI 211.3R-97. Finally the optimum percentage of super plasticizer was determined.