What is Retempering of Concrete?

In concrete industry retempering is a common and confusing term. Retempering is remixing of concrete mass which is observed undue stiffening and loss of workability due to delay in placing concrete after mixing. Now question is why to delay as we know very tiny setting time of cement. Placing of concrete can be delayed due to concrete delivery from mixing plant to

-construction site of lengthy tunnels

-road construction work where long haul is required.

-manual transpor-tation of concrete in hilly region

This partially set concrete or undue stiffened concrete is sometimes rejected by engineer in charge. But from economical consideration concrete is costly material and from environmental consideration wastage of natural aggregate is occurred. So it is important to justify actually this stiffened concrete is workable and not harm desire strength and durability properties.

As defined above, retempering need some clarification like:

-addition of water

-addition of cement

i.e. sometimes additional water and cement are required to achieve desire property.

But many specifications, provided in the codes all over the world, do not allow retempering. Say I.S. Code did not allow remixing of partially hardened mortar/concrete. It requires renewed mixing with or without use of additional cement, water or aggregate.   

An observation on properties of retempered concrete shows that a very wet mix having delay of one hour gains a strength increment of 2-15% when retempering is applied. But further delay renders a decrease in strength.

Some research shows that retempering with additional water that is required to have desire slump i.e. maintaining water/cement ration as per design may be allowed.

Thus it can be concluded that if initial workability and the workability derived from retempering of concrete with addition of water is same, the strength loss is small.