Understanding Concrete Cracks and Uniform Settlement

Concrete is a widely used construction material, but it can develop cracks for various reasons. One specific type of cracking occurs in fresh or newly hardened concrete when its settlement is obstructed. Uniform settlement is expected as water evaporates and the mix consolidates. However, if parts of the concrete are prevented from settling uniformly, stress builds up, leading to a particular type of crack.

Formation of Settlement Cracks in Concrete

Settlement cracks happen while concrete is still plastic or very early in the hardening process. As the heavier solid particles in the concrete mixture settle downwards, water rises to the surface (bleeding). This settlement process is normal. However, if something like reinforcing steel bars (rebar), large aggregate particles, or formwork ties obstructs the downward movement of the concrete above it, the concrete gets hung up on these obstructions. The concrete below the obstruction continues to settle, creating a void or reduced density zone just beneath the obstruction and tension above it. When this tension exceeds the concrete's very low strength at this early stage, a crack forms, typically running parallel to the obstruction on the surface.

Analyzing Concrete Crack Types

• Settlement crack: These cracks are directly related to the settlement of concrete around obstructions. They often appear over reinforcing bars or other embedded items shortly after placing the concrete and typically run along the line of the obstruction. This perfectly matches the description in the question.
• Longitudinal crack: Longitudinal cracks usually run parallel to the length of a concrete element. They can be caused by drying shrinkage, thermal contraction, or sometimes structural loads, but are not specifically tied to obstruction during early settlement.
• Shear crack: Shear cracks are caused by shear forces, which tend to make one part of the concrete slide past another. These are typically structural cracks appearing under load, not during the early settlement phase due to obstructions.
• Random crack: Random cracks, often called map cracking or plastic shrinkage cracks, appear as a network of fine cracks on the surface. They are usually caused by rapid drying of the surface before the concrete has gained sufficient strength, and they are not typically associated with underlying obstructions preventing settlement.

Identifying the Correct Crack Type

Based on the analysis, the crack that forms specifically because there is an obstruction to the uniform settlement of hardened or hardening concrete is known as a settlement crack. The obstruction prevents the concrete from settling uniformly, creating differential movement and tension that leads to cracking.

Revision Table: Common Concrete Crack Types

Additional Information on Concrete Cracking Prevention

Preventing concrete cracks is crucial for durability and appearance. Settlement cracks can be minimized by:

• Ensuring proper concrete vibration to help it consolidate around obstructions.
• Using appropriate concrete cover over reinforcing steel.
• Revibrating the concrete after bleeding has occurred and before final finishing, especially over congested rebar areas.
• Using a concrete mix with appropriate workability and reduced bleed potential if necessary.

Understanding the cause of different crack types helps in diagnosing problems and implementing effective preventative measures during the placement and curing of concrete.