Marc Rached

Mixture Design for Minimizing Cement Content in Pavement Concrete

Pavement concrete containing aggregates that do not meet ASTM C33 often experiences workability problems. One of the reasons is the lack of proper guidelines for proportioning concrete that contains aggregates with poor shape, texture, and grading. Commonly used methods, such as ACI 211, were developed for aggregates that meet ASTM C33; these methods are not adequate for optimizing mixtures containing aggregates such as manufactured sands. This paper presents a simple mixture proportioning method for designing pavement concrete with low slump. The proposed method has the potential to minimize the cement content of paving mixtures with any available source or combination of aggregate without affecting performance. Several combinations of aggregates were tested and the results for slump, compressive strength, and modulus of elasticity are presented in this paper.

Polish Resistance of Fine Aggregates in Portland Cement Concrete Pavements

Many state agencies have limited the use of some sources of fine aggregates in portland cement concrete pavements because of possible skid issues related to the incorporation of such sands. The Texas Department of Transportation, for example, has adopted the acid-insoluble residue (AIR) test to identify polish-resistant aggregates. Because calcium carbonate is soluble in acid, no carbonate sand passes the AIR test, which has a minimum limit of 60% AIR in Texas. This paper describes research that evaluated the polish resistance of aggregates with a laboratory concrete performance test. Concrete slabs made with different fine aggregates were evaluated for skid with the use of a circular track meter, a dynamic friction tester, and a three-wheel polishing device. For each concrete slab tested, the change in texture and friction was monitored for 160,000 polishing cycles. To ensure that the values obtained in the laboratory related to field performance, test sections constructed with 100% limestone sand and blended sands were also evaluated. Results show that some aggregates that failed the AIR test performed as well as some siliceous fine aggregates that passed the AIR test. Other aggregate tests such as the fine aggregates micro-Deval have been shown to relate more closely to the concrete performance tests performed under laboratory conditions. Results obtained from laboratory and field testing showed that blending small quantities of siliceous sand with limestone sands considerably increased the skid resistance of concrete.

A Study on the Minimum Paste Volume in the Design of Concrete Mixture

(Received May 29, 2008, Revised September 24, 2008, Accepted October 31, 2008)Abstract: Optimization of concrete mixing system is very important for the production of quality mixture of concrete and requiresvery complicated, specialized knowledge as there are a variety of variables that influence the result. One of the methods of opti-mizing the concrete mixing system is to minimize the volume of cement paste which, in turn, means maximizing the volume ofaggregate. The purpose of this study is to determine the minimum volume of cement paste used in the design of concrete mixtureand to design the optimum concrete mixing system based on the fluidity of mortar and concrete. In determining the minimum vol-ume of cement paste, experiments of mortar and concrete were performed based on their workability, material segregation andbleeding. Type of aggregate, granularity distribution and sand percentage were used as test parameters and measurements weretaken of the distribution of granularity, usage of HRWRA, minimum volume of paste and drying shrinkage and compressivestrength of concrete.Keywords: minimum paste volume, granularity of aggregate, fluidity, drying shrinkage, compressive strength

Optimizing Aggregates to Reduce Cement in Concrete Without Reducing Quality

Of the constituents of concrete, cement is the most expensive and has the largest carbon footprint. High cement content in concrete affects the durability of concrete. Shrinkage in concrete generally increases with increase in cement content; however, increasing the aggregate content provides more volume stability. Cement content can be reduced with the proper selection of aggregates and use of admixtures. Two approaches are considered: (a) optimizing aggregate gradation by varying aggregate and paste content and (b) replacing cement with mineral fillers (microfines). For each mixture, the high-range water-reducing admixture dosage was adjusted to maintain a constant slump (workability). The effects on slump, compressive strength, drying shrinkage, permeability, and abrasion were measured. Results show that both methods can be used to reduce cement content, maintain or improve performance, and reduce cost, with a more sustainable concrete as the end result.