IMIESA June 2021

Page 64

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

C

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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IMIESA June 2021

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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

IMIESA June 2021

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


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Articles inside

Kprime offers higher performance

2min
page 69

Quality you can trust

6min
pages 66-68

How water influences concrete mix designs

6min
pages 64-65

Exploring new avenues to fight corruption

5min
pages 60-61

Precast pole durability

1min
page 63

Founding the right approach

6min
pages 54-55

Geotechnical partnerships that work

6min
pages 56-57

Challenges when building on dolomite

5min
pages 58-59

Piling at Harbour Arch

2min
page 53

The power to perform in the water reticulation market

3min
pages 47-48

Transport solutions that empower

2min
page 49

Shaping the automotive zone

2min
page 52

Moving the City of Tshwane towards zero emissions

5min
pages 50-51

Groundwater: hidden but never forgotten

3min
pages 45-46

Repositioning for growth

3min
page 42

Tshwane’s eco champion tackles Hennops River pollution

3min
pages 43-44

Local community upskilled with new plant installation

2min
page 41

Water Wise Buildings

1min
page 40

Unscheduled shutdowns and process efficiency

1min
page 39

Bridging the chasm between school and life

4min
pages 37-38

AECI Much Asphalt to pioneer low-carbon asphalt

2min
page 36

New material specifications for roadbuilders

2min
pages 31-32

The smart way to monitor bridge loads

2min
page 33

Sabita Awards recognise excellence

4min
pages 34-35

ESMFs support bankability of SADC power projects

4min
pages 28-30

Gabion systems need geotextiles

4min
pages 26-27

Infrastructure news from around the continent

5min
pages 18-19

Unique offering for transport

5min
pages 12-13

National siltation programme to preserve SA’s large dams

6min
pages 16-17

iPAVe saves billions by redefining live deflection measurement

9min
pages 8-11

Ramaphosa’s smart city dream becomes ‘reality in the making’

9min
pages 20-23

Five decades of delivery

6min
pages 14-15

Bosch Holdings celebrates 60 years of innovation in engineering

5min
pages 24-25

President’s comment

2min
page 7
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