CEMENT & CONCRETE
How water influences concrete mix designs Water is an integral par t of any concrete mix design, and not without reason, says John Roxburgh. Here, he explains why the quantity and state of the mixing water are so impor tant.
plastic shrinkage cracking if the surface is allowed to dry out. So, although there are some very good reasons for reducing the amount of mixing water in concrete, one should always consider the important roles this water plays in the whole concreting process.
Hydration
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oncrete mix design usually places significant emphasis on the reduction of the mixing water. By reducing it, less cement can be used, thereby lowering the cost of the concrete. Less water in relation to the cement also increases the strength of the concrete. A reduction in water will also reduce the overall shrinkage in the plastic and hardened concrete, and will ensure that the hardened concrete is less permeable and consequently more durable. Furthermore, reducing the amount of mixing water will help reduce settlement in the compacted concrete and thereby reduce the chance of plastic settlement cracks. But there are some negative concerns about including too little mix water. For example, the plastic concrete could be more susceptible to
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First, mixing water is needed in the concrete for the hydration of the Portland cement clinker. Water reacts with the clinker to form a cement gel, which is the ‘glue’ that holds the sand and stone together and gives the concrete its strength. Typically, around 28 kg of water is theoretically needed to hydrate 100 kg of Portland cement clinker. However, during the hydration process, some of the mixing water is ‘trapped’ in the cement gel structure and can never be made available for the hydration process. The result is that richer concrete mixes, with water to cement ratios of below about 0.56, may self-desiccate. This means that all the available mixing water is used up and the concrete would then require additional curing water to continue the hydration process. Interestingly, most concretes – particularly stronger ones – end up with a proportion of the cement clinker unhydrated. This bodes well for when the concrete is eventually recycled. Crushed, this unhydrated cement
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Interestingly, most concretes – particularly stronger ones – end up with a proportion of the cement clinker unhydrated.” then becomes available if the recycled concrete is used as aggregate in new concrete. When fly ash and slag are used as cement extenders, the mixing water will help facilitate the pozzolanic reaction of the fly ash and optimise the slag’s role as a latent hydraulic binder.
Workability Second, mixing water is also the major lubricant within the plastic concrete mix. Adding more mixing water to a mix will generally increase the workability of the concrete. In the slump test, which is a useful tool in assessing concrete workability, a rule of thumb that often applies is that adding 10 litres of water to a cubic metre of concrete will double its slump, while removing 10 litres of water from a cubic metre of concrete will halve its slump. This shows the critical lubricating effect of mixing water.
Air voids In modern concrete mix design, admixtures also play an important role in the workability