【Haichuan Chemical Construction Technology】What are the types of concrete shrinkage? What impact does it have on the structure?

There are four types of shrinkage in concrete, with each type being more concealed than the previous one. ◆ Drying shrinkage is the most common and noticeable form of shrinkage. As concrete hardens, water evaporates from the capillaries; this loss of water leads to a reduction in volume and the generation of shrinkage stresses. Water evaporates earlier than the strength of the concrete increases, which results in cracks appearing on the surface. This is the most common type of shrinkage on construction sites, and measures such as maintaining moisture levels and using covers to retain water are taken to address it. ◆Self-shrinking: the hidden threat of high-strength concrete. The hydration reaction of cement consumes water on its own; once the free water inside is used up, the relative humidity drops and the volume of the concrete decreases. Since there is no water to evaporate outward, the concrete starts to shrink on its own. This is a self-drying process that does not require sunlight or wind – the concrete shrinks on its own. High-strength concrete has a low water-cement ratio and little free water, so self-shrinking accounts for more than half of the total shrinkage. Even if the surface of the concrete is well-moistured, it still shrinks internally, and cracks will develop from within outward. ◆Thermal contraction: a time bomb in mass concrete. The heat generated during hydration is released rapidly, causing the internal temperature to rise instantly and the volume to expand. When heat is dissipated and the temperature drops, the volume contracts again. A sudden drop in the external ambient temperature can also cause the surface to contract. If the contraction stress resulting from these temperature differences exceeds the material’s tensile strength, cracks often form across the cross-section. Controlling temperature, monitoring temperature differences, and using cooling water pipes are ways to deal with thermal contraction. ◆Carbonation shrinkage: the most easily overlooked chronic disease. In the air, carbon dioxide penetrates into concrete, dissolves in the remaining water to form carbonic acid, and then reacts with the substances produced by the hydration of cement, resulting in an irreversible reduction in volume. Carbonation is more active at appropriate relative humidity levels; the amount of shrinkage resulting from it isn’t large, but it can combine with drying shrinkage to push things to the point where cracks appear. Over time, the many crack patterns that appear on the surface are related to this phenomenon. The four types of shrinkage mentioned above generally do not act alone; rather, they occur simultaneously within the same structure, building up over time until the tensile stress exceeds the material’s tensile limit. Once cracks appear, water, oxygen, and carbon dioxide can penetrate through these cracks, exacerbating the shrinkage effect. The cracks extend from shallower areas on the surface to deeper layers, thereby affecting the structure’s integrity, impermeability, and durability.

Focusing on just one type of shrinkage is equivalent to creating an easy path for the other three types. Dry shrinkage can be controlled by maintaining proper moisture levels through proper care; spontaneous shrinkage requires internal stabilization and expansion compensation; thermal shrinkage is managed by controlling the heat generated through the mix ratio as well as managing temperature differences; carbonation shrinkage can be slowed down by ensuring high density and using surface coatings to prevent the advance of carbonation. If a crack prevention strategy does not take these different types of shrinkage into account, then cracks will surely find a way to break through those defenses. In the approach of using large-volume high-strength concrete, four measures are implemented simultaneously: controlling the heat of hydration in the mix design, installing temperature monitoring points, covering the surface for water retention during curing, and applying a surface coating to prevent carbonation. It’s not that they are afraid of cracks; rather, they understand that cracks arise from the combined effect of various types of shrinkage forces. By blocking just one crack, it’s as if other factors that can cause cracks are allowed to take effect. What is least considered is not the strength of the concrete, but those various contraction forces whose names we don’t even know; they operate silently, yet together they are sufficient to crack a decent piece of concrete from one end to the other.

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