Implications of Using Manufactured Fine Aggregates in Concrete Pavements

The International Center for Aggregates Research has been performing research on the use of manufactured fine aggregates (MFA) for over 15 years. The first issue was the high amounts of micro fines, i.e. minus 75 μmm, that typically exceed the amounts permitted by ASTM C33 in the production of concrete. Research has generally shown that good quality concrete can be made with higher levels of micro fines when mixtures are properly designed. Water reducers are often required to maintain workability while using acceptable water-cement ratios. The effect of MFA on workability is discussed, including the use of blended natural and MFA. A brief review of properties of concrete made with high micro fines will be presented. Research has also shown that by use of micro fines, proper grading and mixture proportioning it is possible to reduce cement by up to 30% with equal or improved strength and durability with lower cost. The higher cost of water reducer to maintain workability is more than offset by the reduction in cost due to cement reduction. This represents a win-win-win situation since the advantages are (1) improved durability due to lower amounts of paste; (2) lower cost; and (3) reduced CO2 emissions due to lower cement requirement. ICAR research has resulted in recommendations for mixture proportioning using MFA including high levels of micro fines. Micro fines are included in the paste instead of part of the fine aggregates. Guidelines for mix proportions are included. One area that has been of concern is the lower skid resistance associated with the use of carbonate MFA in concrete pavements. Since fine aggregate has the greatest influence on skid resistance, the use of 100% carbonate fines is usually not acceptable since they tend to polish more. Our research using the dynamic friction tester (DFT) on laboratory slabs that are polished, accompanied by testing on field sections using the DFT and a skid trailer, has found that up to 60% or more of carbonate fine aggregate and the balance a good quality silica sand will yield good skid resistance over time.