Engineering properties of high performance concrete containing large volume of Class C fly ash

Abstract

This investigation for the use of large volume of fly ash in concrete in combination with superplasticizer, was for the purpose of optimizing its mechanical properties while reducing its cost. Several concrete mixtures using coarse/fine aggregate ratio of 1.22 and aggregate/binder ratio of 5.1 were investigated. Fly ash was used as a partial replacement of type 10 Portland cement at levels ranging between 20-60% by weight of the total cementitious materials in the mixture. Use of superplasticizer allowed a reduction of the water/binder ratio to 0.28-0.33, while the K-slump of fresh concrete was kept at a practical level of 25%. The effect of fly ash on the development of the compressive strength of the hardened concrete was determined. The selection of a concrete mixture with an optimum fly-ash/cement ratio was based on compressive strength results and cost. Concrete with compressive strength levels of 50 MPa, applicable to mid-rise buildings, mine structural components and bridge construction, was obtainedby taking advantage of the water reducing properties of superplasticizers, and by replacing 50% of the cement with Class C fly ash. The 28-day compressive strength of the resultant concrete was approximately 80% of the strength of the identical control mixture containing no fly ash replacement of the cement; at 56 and 91 days, the strength of the resultant mixture improved and eventually became identical to that of the control mixture. The above results were achieved with a 10% reduction in cost, which is a significant savings for the construction industry. The selected mixture was tested for its engineering properties of strength, elasticity, shrinkage and creep, and the results were compared to the same properties of a control mixture. Creep and shrinkage are important concrete properties in prestressed and reinforced concrete structures. Time-dependent deformation of concrete due to creep and shrinkage, results in partial loss of the prestress force and produces significant changes in deflection. In reinforced concrete structures a slow growth of deflection with time may lead to eventual unsatisfactory performance of the structure. Creep and shrinkage of concrete are affected by time, stress intensity, temperature and humidity. In the present study it is indicated that fly ash concrete produced lower creep and drying shrinkage strains than the control concrete under sustained loads at room temperature while its creep increased with a rise in sustained temperatures. Durability tests were performed using freezing-and-thawing and sulphate resistance experiments. The results were compared to the same properties of a control mixture containing superplasticizer and 100% type 10 Portland cement. The frost resistance of fly ash concrete was found to be comparable tothe control mixture. The presence of a large volume of fly ash improved the sulphate resistance of the hardened concrete. Microstructural studies were concurrently conducted in order to determine and explain the effects of superplasticizer and fly ash in producing high performance concrete.