The Constructor

Preplaced Aggregate Concrete – Uses for Concrete Repair

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Preplaced aggregate concrete is made by forcing into the voids of a mass of clean, graded coarse aggregate densely prepacked in formwork. This concrete is used where placing conventional concrete is extremely difficult, such as where massive reinforcement steel and embedded items are present, in underwater repairs, concrete and masonry repairs, or where shrinkage of concrete must be kept to a minimum.

Figure 1(a): Preplaced Aggregate Concrete Repair to Concrete Wall

Figure 1(b) Preplaced Aggregate Concrete

For the purpose of repair method using preplaced aggregate concrete, grout typically consists of sand, cement, pozzolana, plasticizer / superplasticizer and air entraining agents (for anticipated freeze and thaw problem, if required). The pozzolana and the plasticizer / superplasticizer are used to impart flowability to the grout.

The coarse aggregate is washed to remove all fines and screened just prior to placement. Grout is then injected through forms to provide the cementing matrix. Grouting is begun at the bottom of the pre-placed aggregates.

Characteristics of the grout are affected by the water content, sand grading, cement, pozzolana and the types and amount of admixtures. For each design of grout mixture, there are optimum amounts of fillers and admixtures to produce the best pumpability or consistency.

Proper proportioning of the structural grout mix components is necessary to get the required strength and durability of the finished preplaced aggregate concrete. Trial mix design is necessary for each job.

In underwater repair, injection of grout at the bottom of the preplaced aggregate concrete displaces water, leaving a homogeneous mass of concrete with minimum of paste wash out.

In such applications, addition of anti-wash admixtures minimizes the paste wash out. For underwater PAC, the quality of underwater should also be tested to determine its influence on PAC over a period of time for taking appropriate corrective action.

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