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  • Writer's pictureSi Shen

Crack control of concrete (1)

Crack control of concrete using reinforcement to achieve water-tightness


Concrete is like a cranky toddler. It needs a lot of love and care, and yet if there is one tiny bit it is unhappy about, it releases its resentment by cracking. There is a plethora of reasons of why concrete may crack. Too wet, too dry, too hot, too cold, too much rebar, too little rebar. When concrete cracks, it may stain its aesthetic purity, damage its durability, or reduce its ability to serve its intended functional purpose. Particularly for underground concrete structures, when it cracks, it leaks.

From the design perspective, how do we prevent concrete from cracking?

Firstly, let me list out the various reasons concrete may crack.

1. Structural cracks. These are cracks caused by loadings on the structure, either bending or tension.

2. Early age thermal crack. Concrete releases heat as it solidifies. The heat can be enormous for large size concrete pours. It feels hot if you put your hand on the surface. Some time after the concrete is place, it cools down. As we know, things expand when they get hot, and contract when they cool down. It is the cooling down period that causes concrete to shrink, and if it is prevented from shrinking, it cracks. For example, if new concrete is poured on top of old concrete, the old concrete will prevent the new concrete from shrinking, by friction. Then the new concrete cracks. Early age thermal behavior is a complex concept and I will make a separate blog post about this topic.

3. The workmanship cracks, which include cracks that are a result of on-site execution, such as cracks caused by lack of compaction or inadequate curing process. This type of cracks is beyond the control of the designer.



Both structural cracks and early age thermal cracks can be prevented or reduced by putting reinforcement bars in the concrete. When designing structures such as deep basement, tube stations or shafts, water tightness is usually a key design requirement, which can be achieved by either justifying the concrete is uncracked, or controlling cracks in concrete down to a specific level.

From water-tightness perspective, there are two types of crack: through cracks, and non-through cracks. Non-through cracks are caused by bending, where one side of the element is in tension and the other side is in compression. Concrete is considered water-tight for non-through cracks as part of the section is sealed in compression. Through cracks are caused by direct tension, i.e. forces that pulls it apart. In this case leaks happen.



As a designer, if you are designing the concrete to be water-tight, in the first instance think of a way to avoid direct tension, i.e. through cracks. If it can not be avoided, then you need to follow Eurocode 2-3, to design rebars to control through cracks down to a specific level, so that ‘self-healing’ can be achieved. Self-healing means after the crack leaks for a bit, it gets blocked up either by fine particles or chemical reactions.

To calculate the amount of rebar required, guidance in Eurocode 2-3 and CIRIA guide C660 can be followed. Simply put, crack width = Crack Spacing X Strain. In simpler terms, for a given amount of shrinkage, the more number of cracks exist in the concrete, the smaller width each of the crack is going to be, because there are more cracks to share the total shrinkage. The details of the design is too detailed and will be another full blog post. According to Eurocode 2-3, the crack width needs to be controlled down to a level between 0.05mm to 0.2mm, depending on the hydraulic pressure on the structure.



A simple trick in designing crack-control rebar is to use smaller size bars. For a given amount (i.e. tonnage) of reinforcement, smaller bars have larger surface area which provides more grip (friction) over its surrounding concrete, better preventing it from cracking.

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