International Concrete Abstracts Portal

SP79 It's a virtual encyclopedia of what mineral by-products do to concrete--and why. 1196 pages of essential information in 62 clearly defined, easy reference chapters. Key subjects include: effects on durability, fiber reinforced concrete, fly ash, pozzolans, sulfate resistance and concrete workability. With the rapidly increasing usage of by-product additives in concrete, every designer and builder needs the answers which these volumes provide.

Early-age strength development of concrete in which part of the portland cement has been replaced by low-calcium fly ash tends to be slow, because fly ash acts as a relatively inert component during this period of hydration, though at later ages it contributes significantly to strength development. It was considered that the problem of low early-age strength of portland cement-fly ash concrete could be overcome by the incorporation of small amounts of condensed silica fume, a very fine and more rapidly reactive pozzolan. This report presents the results of an investigation on the early-age strength development of concrete incorporating 30% low-calcium fly ash, and to which small amounts of condensed silica fume have been added. The amounts of the fume ranged from 0 to 20% by combined weight of the portland cement plus fly ash. A total of thirty 0.06-m3 concrete mixtures with water-(cement + fly ash) ratios ranging from 0.40 to 0.80 were made; 240 cylinders were tested in compression and 180 prisms were tested in flexure. A supplementary series of six concrete mixtures was made to deter-mine the effect of silica fume and fly ash on the long-term strength development of concrete. Test data showed that the incorporation of condensed silica fume increased the compressive strength of concrete at all ages as compared with the compressive strength of the control concrete (70% portland cement + 30% fly ash). At 7 days, the loss of compressive strength due to the partial replacement of cement by fly ash was completely overcome by the addition of 10% condensed silica fume for concretes with water-(cement + fly ash) ratios ranging from 0.40 to 0.60; 15 to 20% was required for concretes with higher water-(cement + fly ash) ratios, At 28 days, regardless of the water-(cement + fly ash) ratio, the effect was generally achieved with less than 5% silica fume addition. The laterage strength development of portland cement-fly ash concrete did not appear to be impaired by the use of condensed silica fume indicating availability of sufficient lime for the fly ash pozzolanic activity.

A low heat of hydration blended cement for structural applications has been developed. This cement is composed of 50 percent of a Canadian cement, CSA Type 20, (ASTM Type II), 35 percent of a slowly reactive slag, and 15 percent of condensed silica fume. The heat of hydration measured at 20°C of this blended cement is 30 percent lower than that of the pure Type 20 cement whereas the 28 d compressive strength of standard mortars is about the same. The peak temperature of an insulated mass of concrete having a 28 d compressive strength of 30 MPa is reduced from 44.4°C to 34.4°C when using this composition instead of the Type 20 cement. A microstruc tural study has shown th at when using this blended cement the CSH formed is ve ry dense and amorpho us-like.

In order to reduce the density for load bearing structural members and to improve the strength properties a new binder matrix has been investigated (1, 2). The binder matrix is composed of cement, variable amounts of condensed silica fume, high range water reducer and air entraining agent. The use of heavy duty grade expanded clay as lightweight aggregate in combination with artificially introduced air voids in the concrete was found to be ad-vantageous. With these combinations it was possible to attain an air content up to 40 % of the concrete volume and density / com-pressive strength values ranging from 1.1 t/m3 / 10 MPa to 1.8t/m3 60 MPa. The main properties of such lightweight concrete were investigated including strength and density, bond of embedded steel, modulus of elasticity and length change. In this paper the main results are presented and discussed. It can be concluded that cement-condensed silica fume blends can be used for the production of low density high strength lightweight concrete for structural use.

The good ability of concrete to protect embedded steel against corrosion is mainly due to the high alkalinity of the pore water of the concrete, which provides a passive, noncorroding state of the embedded steel If this passivity of the steel is broken - either by carbonation or by the presence of chloride ions the corrosion of the steel will mainly be controlled by the electrical resistivity of the concrete and the rate of oxygen transport through the concrete to the steel. The present paper describes a study on how additions of up to 20% condensed silica fume by weight of cement affect the rate of carbonation, electrical resistivity and rate of oxygen transport through water-saturated concrete. It was found that the effect of silica on the parameters studied varied within wide limits, The rate of oxygen transport was only slightly affected while the rate of carbonation was somwhat reduced. The most pronounced effect of silica was found on the electrical resistivity which is increased by up to 190-1600% for cement contents varying from 100 to 400 kg/m3, respectively.