Concrete-environment interaction: The key to durable service life performance

N.R. Shattaf and R.N. Swamy

The exposure conditions in the coastal areas of many parts of the world are recognised as one of the most aggressive climatic conditions which pose severe challenges to the concrete technologist and design engineer to ensure long-term durable service life performance of the concrete material and concrete structure. In this type of exposure, the quality and extent of initial water curing, and the subsequent interaction between concrete and the environment can often control and dictate concrete material performance resulting in premature deterioration and low durable service life of concrete structures. The overall aim of this paper is to develop a better understanding of the influence of this initial water curing, and the subsequent hot, dry/humid salt-contaminated environment on the engineering properties of concretes designed for high durability. In the tests reported here, high durability concretes containing silica fume and various amounts of ground, granulated blast furnace slag were developed. These concretes were then exposed to a laboratory indoor environment (23 + 2oC, 70 + 10% RH) and the outdoor environment (24-45oC, 30-95% RH). The tests carried out over a period of 540 days related to engineering properties such as compressive strength, dynamic modulus, pulse velocity and moisture/thermal movements. It is shown that under variable water curing and subsequent severe exposure conditions, the vulnerability of concrete to material and structural damage begins to occur long before compressive strength loss is identified. It is shown that loss in dynamic modulus which will have a direct effect on the long term performance of concrete structures occurs long before losses in compressive strength. The results presented in this study show that compressive strength is not an indicator of durable service life and it cannot be used to predict structural performance in real life exposure conditions.

Load-moment interaction envelopes for design of tall stacks - A limit state approach

K.S. Babu Narayan and Subhash C. Yaragal

Use of particle packing for mix proportioning of reactive powder concrete

J.K. Dattatreya, K.V. Harish and M. Neelamegam

Particle packing deals with into the problem of selecting appropriate sizes and proportions of particulate materials to obtain a compact mixture. The optimisation of the packing density of granular components of concrete has become an important factor for achieving improvements in mechanical properties of conventional concrete. Granular components of concrete viz., aggregates and fillers occupy as high as 70 to 85% of the concrete volume. Therefore, controlling the granular mixture proportion will help to minimise the volume of void space, thereby ensuring higher strength and better workability due to improved macro-mechanical properties Further, with the same w/c ratio, smaller amount of water and hence lesser cement paste is needed when the granular fraction is more densely packed, which in turn reduces the occurrence of weak interface between the aggregate and the cement paste thus enhancing the strength and durability of the concrete Since high-strength concretes are made with a low w/c, they require a large amount of cement in the mixing process that may cause severe creep and drying shrinkage and other associated problems. As the requirement for maximum mobilisation of strength increases as in the case of ultra high strength and reactive powder concretes, the requirement for optimisation of packing density data plays a vital role. Theoretical studies suggest that with better packing of the ingredients of concrete, one can achieve a higher compressive strength (up to 20 %) for the same mix composition and w/c ratio. In the present study, an attempt has been made to experimentally determine the packing densities of unitary, binary, and ternary combinations of the three grades of standard sand as well as various combinations of sand fractions with filler material, such as, quartz powder and binding powders, such as cement and silica fume in order to optimise the proportions of reactive powder concrete mixes. The experimentally determined packing density results for the various granular mixtures are compared with the particle packing densities predicted by two of the popular theoretical models proposed by Dewar and de Larrard. These studies showed that 20% addition of quartz to the sand fractions results in the highest packing density. Also, the packing density computations using Dewar and Larrard's methods bear good correlation with the experimentally determined packing densities. It is also concluded that when compared to single size sand fractions G1, G2 and G3 with packing densities of 0.63 to 0.68, ternary mixtures with and without quartz yielded increased packing density of 0.76 to 0.81 (11 to 28%) and the compressive strength increased by about 4-10%. Even though, the increase in compressive strength is less, it should be noted that in case of very high strength concretes, even this order of improvement is a significant step. This aggregate combination will be beneficial in combination with chemically reactive binders and steel fibres in producing at ultra high strengths.