Readymix drivers play a crucial role in sound practice

Transporting concrete is an often overlooked but critical part of good concrete practice and has a major influence on downstream quality.

John Roxburgh, senior lecturer: School of Concrete Technology, Cement & Concrete SA

Batching, transporting and the placing of concrete form a close triangle of dependence. The rate at which a concreting job can be done depends on the slowest part of this triangle,” explains John Roxburgh, senior lecturer: School of Concrete Technology, Cement & Concrete SA.

“How you batch and prepare fresh concrete will have a direct effect on its transport, and how the concrete is transported will affect the placing and concrete end product,” Roxburgh continues.

An especially important aspect of transporting concrete is to ensure that, at the time of placement, the concrete has the desired workability for ease of placement and compaction.

Reasons for loss of workability

Roxburgh says there are four main reasons why concrete may lose workability during transport: • water evaporation from or bleeding out of the fresh concrete • hydration of the cement causing structural build-sup in the concrete • the ‘wearing off’ of the admixtures • the segregation of the components of the fresh concrete. “Water is the major lubricant in fresh concrete and a rule of thumb is that the concrete slump is halved for every 10 litres of mixing water lost per cubic metre of concrete. Most water is lost through evaporation into the air and through the hydration of the cement. Hot, dry and windy ambient conditions will speed up the loss of water,” he states.

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Cement gel growth

The hydration of the cement with water not only results in loss of the lubricating mixing water, but also results in cement gel growth. The structural build-up of the cement gel in the mix leads to stiffening and loss of workability. Any increase in the temperature of the concrete or local ambient temperature will speed up this process.

“Although admixtures – particularly plasticising and retarding admixtures – are wonderful modern tools used for