CommunicationSimple test method to assess the relative effectiveness of plasticising chemical admixtures
Introduction
Due to the demanding performance levels essential for concrete structures built in harsh environments, high-performance concrete containing supplementary cementitious materials, such as silica fume (SF), is frequently specified. Because of their individual physical and chemical properties, supplementary cementitious materials can offer considerable mechanical and durability performance improvements [1], [2], [3]. It is well documented in [4], [5], however, that in order to optimise the potential benefits of SF in concrete, it should always be used in conjunction with a plasticising chemical admixture. This is because it is essential to thoroughly disperse the material throughout a given mix, as flocculates of the material may otherwise be formed, and also to compensate for the potential high water demand of the mixture caused by its high surface area. Ensuring dispersion is particularly the case for SF, which is purposely preconditioned to form particle agglomerates as its inherent high particle fineness may otherwise cause handling problems. If plasticising admixtures are not used, it is likely that desired workability, mechanical, and durability performance levels will not be achieved, owing to the higher water demand of the mixture, poor material dispersion, or both [6].
It is important to realise, however, that all plasticising chemical admixtures do not behave similarly in concrete, with performance levels often depending on Portland cement (PC) and admixture compatibility [4]. Therefore, until such time as compatible suites of admixtures and binder materials are available commercially, a major design requirement when proportioning concrete containing supplementary cementitious materials is to choose an admixture that provides optimum levels of particle dispersion. Against this background, this paper proposes a quick and simple test method that is capable of assessing the relative effectiveness of plasticising chemical admixtures for use in concrete containing supplementary cementitious materials.
Section snippets
Theory of hydrometer test [7]
Mainly used for soil analysis, the hydrometer test determines the particle size distribution (PSD) of fine-grained materials. In the test, material is mixed in water, and a deflocculating agent is added to ensure full particle dispersion. When this specimen is agitated in a graduate filled with water, the various particles settle at rates according to their size. As a result, an initially uniform density of the suspension will begin to vary, becoming denser at the bottom of the graduate where
Phase 1: hydrometer test series
Using the procedure outlined above, a series of hydrometer tests was carried out in Phase 1 to assess the dispersive effectiveness of three separate plasticising chemical admixtures (labelled A, B, and C). Meeting the recommended dosages for each, the quantity of admixture used in each case was 0.75% by mass of the total binder content. Admixtures A and B were high-range water-reducing admixtures, whereas admixture C was an ordinary water reducer. All materials were of sulfonated naphthalene
Hydrometer test
Decay of final hydrometer readings with time and PSD curves are given in Fig. 2(a) and (b), respectively. Original results obtained and the data steps undertaken to enable the construction of these curves are additionally given in Table 1 for the PC/10% SF control mixture.
As expected, the general trends in Fig. 2(a) indicate decreasing hydrometer readings with time, reflecting the gradual settlement of the PC and SF particles. Also, apparent from Fig. 2(a) is the influence of using plasticising
Conclusions
(1) A simple test method to assess the relative effectiveness of plasticising chemical admixtures for use in concrete has been proposed. Using a hydrometer, the test measures the capability of admixtures to disperse the individual particles of PC/SF combinations.
(2) From a study carried out on three different admixtures, the proposed test method clearly differentiated between PC/SF combinations containing no admixture, normal-range water-reducing admixtures, and high-range water-reducing
Acknowledgements
The South African Foundation for Research Development and Alpha are gratefully acknowledged for supplying materials and funding the reported research project. Thanks are also given for the technical guidance and contributions made by Alpha representatives Mr. V. Blackbeard and Mr. P. Pretorius.
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