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Making concrete crack less

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Research Findings on the Mitigation of Plastic Shrinkage Cracking in Concrete by Dr. Ravindra Gettu, Professor, Department of Civil Engineering and Moghul Sirajuddin, Former MS Research Scholar at Indian Institute of Technology Madras.

In recent years there has been a rapid increase in the use of supplementary cementitious materials for high performance and durable concrete. Such materials, like fly ash and slag, being fine and less reactive than cement, increase the water demand and reduce bleeding, which in general is good. Moreover, the early strength gains of concrete may also decrease. On the whole, this makes the concrete more susceptible to plastic shrinkage in hot and dry environmental conditions. Consequently, plastic shrinkage cracking is a serious concern nowadays, especially in slabs, decks and pavements.

A common complaint among builders and contractors is that floors and slabs crack just after the concrete is placed, especially during hot days. Also, this seems to be occurring more often during the past few years rather than a decade or two earlier. Such cracking causes leakage and necessitates costly repair and delays during construction. Cracks that appear within a day or so in concrete can be attributed mainly to plastic shrinkage or contraction that occurs due to high evaporation when the concrete is still wet or plastic. The best way to avoid such cracks is to cure the concrete by covering the surface to  reduce evaporation immediately after placing and then to spray or sprinkle water. However, this may not always be possible or done as efficiently as it should be. Therefore, there is the need to assess the factors that promote such cracking and the ways to mitigate the problem.

The Study

The recent work done at the IITMLafarge Laboratory for Durability and Long-Term Performance of Concrete, subjected several typical concretes to hot-dry wind in a specially designed chamber to assess:

  • The dependence of the cracking on the cements and additives used normally in construction sites in India.
  • Crack mitigation measures such as the addition of special shrinkage reducing admixtures and fibres.
  • The benefit of surface applied curing compounds on plastic shrinkage crack reduction.

The parameters investigated included the rate of evaporation  of water from the surface of the concrete, crack initiation time and crack widths. The study had two major objectives:

  • To investigate the influence of supplementary cementitious materials and blended cements on plastic shrinkage cracking
  • To control plastic shrinkage cracks using shrinkage reducing admixtures, fibres and curing compounds

Materials

Two supplementary cementitious materials – Fly ash (class F) and Granulated blast furnace slag and two blended cements – Portland Pozzolana cement (PPC) and Portland slag cement (PSC) were analyzed. Supplementary cementitious materials replaced ordinary Portland cement at 15% and 30%, by weight, while blended cements completely replaced ordinary Portland cement. The binder content and water binder ratio are fixed as 340 kg/m3  and 0.55, respectively, for the entire study.

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