Center for By-Products Utilization MECHANICAL PROPERTIES OF CONCRETE INCORPORATING HIGH-CALCIUM FLY ASH -A REVIEW THE UNIVERSITY OF WISCONSIN -MILWAUKEE 2 MECHANICAL PROPERTIES OF CONCRETE INCORPORATING HIGH-CALCIUM FLY ASH -A REVIEW

This paper provides the state-of-the-art information on high-calcium, ASTM Class C fly ash use in cement-based construction materials, such as high-performance concrete, ready-mixed concrete, and low-strength flowable concrete. The major topics included are: properties of fly ash, effects of fly ash inclusion on fresh and hardened concrete and controlled low-strength materials (CLSM); and, future research needs. The fresh concrete properties discussed are workability, water requirement, bleeding, segregation, air content, time of set, and temperature effect. The hardened concrete properties such as compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, creep and shrinkage, and fatigue strength are described. It is shown that high-strength/high-durability/high-performance concrete containing significant amounts (up to 40% cement replacement levels) of Class C fly ash can be manufactured for strength levels up to 100 MPa. Future research efforts should be directed towards use of high-lime fly ash in blended cements with minimum (less than 20%) portland cement in the blend. Keywords: Fly ash; concrete; strength; splitting tensile strength; flexural strength; modulus of elasticity; fatigue strength; mechanical properties. ACI Fellow Tarun R. Naik is Director of the Center for By-Products Utilization and Associate In accordance with ASTM 618, coal fly ash is classified into two main categories, Class F fly ash (low-calcium) and Class C fly ash (high-calcium). Class F fly ash is produced from combustion of bituminous or anthracite coal, and Class C fly ash is generated from burning of lignite and subbituminous coals. Class F fly ash has been used in concrete for more than half a century. Substantial amount of data regarding Class F fly ash use in concrete are available including the effect of Class F fly ash on strength and durability characteristics of concrete. Since the late 1970s, Class C fly ash has become available in the USA and Canada due to burning of lignite and ubbituminous coals. High-calcium fly ashes are also available in other countries including Spain, Poland, and Greece. 5 Combustion of low-sulfur coals produces improved, low-sulfur, emissions. As a result, due to strict environmental regulations, it is expected that a large number of electric power plants in the USA and elsewhere will utilize low-sulfur coals in the future which will result in increased production of Class C fly ash. Relatively little work had been conducted on the use of Class C fly ash in concrete and other materials until the early 1980s. Research related to the application of large quantities of Class C fly ash in structural grade concrete began at the University of Wisconsin-Milwaukee in 1984 and currently extensive research work is in progress for evaluation of long-term strength and durability performance of high-volume Class C fly ash concrete systems. Pioneering work for production of structural-grade concrete containing high-volume of Class F fly ash was performed by Malhotra and his co-workers PROPERTIES OF FLY ASH Fly ash is a heterogeneous mixture of particles varying in shape, size, and chemical composition. The particles of a Class C fly ash are shown in of CaO, whereas Class C fly ashes normally show total CaO greater than 10%. Up to 43% CaO is found in some fly ashes from Barcelona, Spain. Crystalline mineral phases present in a Class C fly ash may include quartz, periclase, lime, calcium aluminate, calcium sulfate, alkali sulfates, in addition to glass which ranges between 60-90% 7 To a large extent, performance of fly ash in concrete is dependent upon its physical and mineralogical characteristics. Glassy particles are of special importance because they participate in pozzolanic reactions in concrete. In general, reactivity of these particles increases with decreasing particle size MIXTURE PROPORTIONING METHODS Three basic mix-proportioning techniques have been generally used for fly ash concrete systems (2) the addition of fly ash as fine aggregate, the addition method; and, (3) the partial replacement of cement, fine aggregate, and water. A variation of the first method is to replace cement by fly ash by weight and reduce the water content to obtain equal workability. The simple replacement method requires direct replacement of a portion of the portland cement with fly ash either on a weight or volume basis. Generally early age strength is decreased when this method is used, particularly for Class F fly ash. The addition method involves adding fly ash to the mix without reducing the cement content of the no-fly ash concrete mixture. In general, this method increases strength and overall quality at all ages, but does not provide any saving on the cement cost. 8 In the third method, partial amount of cement is replaced by a larger mass of fly ash, with or without adjustments made in fine aggregate, and water content is reduced for a specified workability. This method can be further divided into two techniques: modified replacement, and rational proportioning methods. In the modified technique, the total weight of cement plus fly ash of a fly ash containing concrete mixture exceeds the total weight of portland cement used in a comparable no-fly ash mixture. This method produces early age compressive strengths of fly ash concrete comparable to or greater than plain portland cement concrete without fly ash. The authors have found this method to be very effective in producing concrete with the specified early strength and higher later age strength of fly ash concrete relative to concrete without fly ash. Because of simplicity and effectiveness, this method is probably the best in assuring the specified concrete performance for the fly ash concrete system. Desired high-strength and high-durability is maintained by adding superplasticizer to concrete to produce, high-strength, high-durability concretes at low-water-to-cementitious materials ratios. 9 The third method, partial replacement, rational mixture proportioning, technique assumes that each fly ash possesses a unique cementing efficiency. A mass of fly ash (F) is converted to an equivalent mass of cement as KF, where K is a fly ash cementing efficiency factor. For simplicity K can be assumed to be one, particularly for good quality Class C fly ash. The required strength and workability of fly ash concrete are obtained by applying Abrams' relationship between strength and water-to-cementitious materials ratio (W/(C + KF)) and adjusting the volume ratio of cementitious particles to water and aggregate PROPERTIES OF CONCRETE Addition of fly ash to concrete alters both fresh and hardened concrete properties. Effects of incorporation of fly ash on concrete properties are discussed in the following sections. Fresh Concrete Properties Workability, Cohesiveness, Segregation, and Bleeding--Replacement of cement with fly ash enhances rheological properties of fresh concrete. In general, workability and cohesiveness 10 are improved, and bleeding and segregation are reduced. Incorporation of fly ash in concrete reduces water requirements for a given consistency and increases density of concrete (particularly the transition zone between aggregate and mortar is densified). Consequently, the reduction in water requirement due to fly ash addition causes reduction in bleeding and segregation, and improvement in permeability. However, a well dispersed mixture of cement and fly ash particles is needed to decrease the size of bleed channels and improve the aggregate-paste transition zone microstructure A majority of past investigations had substantiated that inclusion of fly ash in concrete causes decreased water demand, increased workability and pumpability, and decreased bleeding However, Helmuth Time of Setting--It is generally accepted that concrete setting is retarded when Class F fly ash is added to concrete mixtures. Class C fly ashes have shown mixed behavior in regards to setting characteristics of concrete. The initial and final setting times measured in accordance with ASTM 403 may increase, decrease, or remain unaffected due to inclusion of Class C fly ash Air-Entrainment--In order to improve freezing and thawing durability of concrete, air entrainment is included in concrete. Studies have shown that the use of fly ash in concrete increases the air entraining agent (AEA) dosage requirements relative to the control concrete without fly ash The primary reason for the increased AEA dosage rate for fly ash concrete is said to be due to the presence of unburned coal, as measured by loss on ignition (LOI), and/or increased fineness of fly ash compared to cement. Gebler and Kliege