4 Factors Affecting Autogenous Shrinkage of Concrete

The autogenous shrinkage of concrete is the chemical processes of Portland cement hydration lead to a type of shrinkage, which is observed in sealed specimens, i.e., at no moisture loss.

Factors affecting the autogenous shrinkage of concrete are mainly cement, mineral admixtures, water-cement ratio, and aggregates.

The nature of cement affecting the autogenous shrinkage of concrete

The root cause of autogenous shrinkage of concrete is the hydration of cement. The hydration rate of cement, hydration combined with water is the key to affecting the size of autogenous shrinkage. Among the cement mineral composition, C3A has the fastest hydration rate and the highest binding water content, followed by C4AF and C3S, while C2S hydration has the least effect.

Therefore, under the same conditions, the higher the content of C3A and C4AF in cement, the greater the autogenous shrinkage, and the higher the content of C2S, the smaller the autogenous shrinkage. The finer the cement, the larger the specific surface area, the faster the hydration rate, the more combined water, and the corresponding increase in autogenous shrinkage.

Water-cement ratio affecting the autogenous shrinkage of concrete

The water-cement ratio is an important factor influencing the size of autogenous shrinkage, the size of the concrete water consumption is directly affected by the water-cement ratio. The larger the water-cement ratio, the smaller the autogenous shrinkage, and the smaller the proportion of the total shrinkage.

Mineral admixture affecting the autogenous shrinkage of concrete

The composition, activity, and fineness of mineral admixtures are closely related to the size of concrete shrinkage. Different admixtures have different effects on autogenous shrinkage. Silica fume is a superfine active admixture, which accelerates the hydration reaction of cement, making the concrete more active, and making the concrete structure denser, so the greater the amount of silica fume, the greater the autogenous shrinkage.

The effect of slag on autogenous shrinkage is still more disagreement. It is generally believed that when the surface area of slag is less than 400m2/kg, it is beneficial to reduce concrete shrinkage. With the increase in the amount of slag, since the shrinkage decreases when the specific surface area is greater than 400m2/kg, slag activity increased significantly, with the increase in the amount of slag since the shrinkage increases, but when the amount of slag is greater than 75%, since the shrinkage of concrete since the shrinkage is reduced due to the reduction in the activity of cementitious materials.

Although fly ash is an active mixed material, it hydrates very slowly in the cement slurry system. Therefore, under the same water-cement ratio conditions, replacing part of the cement with fly ash is equivalent to increasing the early effective water-binder ratio. Thus, fly ash can reduce the early self-drying rate inside the concrete, significantly reducing the early autogenous shrinkage late fly ash continues to hydrate so that the degree of self-drying inside the cement stone increased.

However, at this time the concrete has a high modulus of elasticity and a very low autogenous creep coefficient. Therefore, the same degree of self-drying produced by the autogenous shrinkage is much smaller compared to the early stage. This effect of fly ash can be referred to as the energy hysteresis release effect.

Coarse and fine aggregates

In concrete, aggregate plays the role of the skeleton and inhibits the shrinkage of cement. On the one hand, the increase in aggregate relatively reduces the amount of slurry. On the other hand, the elastic deformation of aggregate caused by autogenous releases the tensile stress formed by the shrinkage deformation, thereby inhibiting the autogenous shrinkage of cement.

Also to read:

7 Steps to Solve Cement Additive Adaptability Problems

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