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CONCRETE: TO CRACK OR "NOT" TO CRACK (PART 1)

Portland cement is a widely used building material. Concrete mixes are designed for a particular job using ingredients that are largely obtained locally, that is, portland cement, admixtures, aggregates, and water. A wide range of strength characteristics of the hardened concrete is possible through a proper combination of these ingredients. Commonly used concrete has a compressive strength ranging between 25 Mpa and 100 Mpa (3600 psi and 14500 psi). Although concrete is being used for its compressive strength - its main advantage - concrete exhibits low tensile strength, which turns out to be its main disadvantage. Because of its low tensile strength, approximately 10% of its compressive strength, concrete often shows signs of cracking during its service life.

Cracking due to Shrinkage of Concrete

The process of shrinkage depends strongly upon the composition of the concrete, but typically, concrete shrinks by approximately 0.05% over a period of a few months.

Although this shrinkage ratio seems very small, the induced tensile forces due to shrinkage can easily exceed the tensile strength of the concrete and crack it. Can we eliminate such an undesirable situation? The answer is no. But there are ways to control the amount and location of these cracks.

Cracking of Sidewalks

Before the concrete hardens, the builder provides grooves at regular intervals. These grooves are called expansion joints and are meant to create weaknesses along the sidewalk. During the shrinkage process, the sidewalk will crack at its weakest locations, which is along the expansion joints. Because the cracks are located along the joints, they become "invisible" to us. Nonetheless, uncontrolled cracking is often observed, despite the presence of expansion joints.

To provide a proper weakness point along the sidewalk, the depth of the expansion joint must be at least one-fifth of the thickness of the sidewalk. If the joint is too shallow, random cracking will appear in the vicinity of the expansion joint. Also, it he joints are spaced too far apart, they will not be able to relieve the stress build-up due to shrinkage and cracking will occur somewhere between the expansion joints. For sidewalks, a spacing of 1m to 2m is generally sufficient to control the appearance of cracking.

Cracking of Basement Floors

Expansion joints are seldom used in residential construction. Builders typically use a low-strength concrete mix for the construction of concrete floors. Using a lower concrete strength means that the concrete will be more susceptible to cracking. An 8m by 12m basement floor would show signs of cracking at a spacing of approximately 3m, with cracks emanating from areas of weakness, such as floor drains and services. Since the performance of the basement floor is not affected by the presence of shrinkage cracks, remedial work is seldom warranted.

 
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