Effect of Compaction Time on Concrete Compressive Strength

Abstract

Compaction is the process of expelling the trapped air from inside the concrete during pouring, whereby the repositioning of the aggregate particles within the molds increases the density of the concrete. Compaction processes significantly increase the final strength of concrete and enhance the bonding with steel reinforcement. Good compaction also increases the wear resistance and general durability of concrete, reduces permeability and helps reduce shrinkage and creep properties. Proper compaction also ensures that the molds are completely filled - ie no air voids - and the desired finish of vertical surfaces is obtained. When concrete is first placed in the mold, it will contain between 5% and 20% of the trapped air volume, excluding concrete with very low or high subsidence. Although the aggregate particles are covered with mortar, they tend to arch against each other and are prevented from falling or sticking due to internal friction. Thus, the concrete compaction process consists of two stages: First, the aggregate particles are stirred down to fill the mold giving a flat top surface. In the second stage, the trapped air is expelled. This study reviews the two stages of the compaction process, where with vibration, the initial incorporation of concrete can be achieved relatively quickly, and concrete liquefaction at surface levels, which gives the impression that the concrete is compacted. The trapped air may take a little longer to rise to the surface. Therefore, the compaction process must be prolonged until this is achieved, that is, until the air bubbles on the surface disappear. This study aims to determine and estimate the change in the compressive strength of concrete when changing the proportions of the mixture, delaying the pouring time, and increasing the settling time. Since the initial setting time is (45) minutes without any additions, as this stage takes place in a lot of reactions. Therefore, this study recommends that the best time to pour the samples is immediately after mixing, otherwise it is better to leave them until after (20) minutes to obtain homogeneity again.