Concrete is a dynamic material that changes its consistency with time. The workability of the fresh concrete decreases during the process of hydration, transportation and placement at working site. This process is expressed as slump loss.
slump loss in concrete is a problem that costs time and money for the concrete placing contractor. The slump loss causes difficulties in placing the concrete and achieving the required finish. To overcome this, the contractors often add water at the jobsite to make the concrete workable. This is not a good practice because it has negative impact on the strength and durability of the concrete.
In this article we will discuss the major reasons for slump loss in concrete and ways to avoid it. The first thing to do is to test the concrete mix in conditions similar to the ones at the construction site, this will ensure that the workability of the concrete in site meets design requirements.
Time is the most important factor in determining slump loss. During the normal slow congealing of concrete called setting, it progressively loses its slump before hardening begins. The abnormal slump loss, however, occurs at a rate much higher than normal and this seems to be amplified at elevated temperature.
One way to reduce slump loss is by adding set retarding admixtures, especially lignosulfonates. These admixtures decrease the water demand by slowing down hydration and therefore increase the workability of the concrete. It is also advisable to use PCE polymers, which have better slump retention than PNS, PMS or LS superplasticizers.
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