Causes of Early-age Cracking in Concrete

One of the challenges faced by building construction practitioners is the issue of presence of cracks in concrete for as early as seven days after construction.

This piece, taken from The Constructor's blog explains early-age cracks in concrete and how to tackle this anomaly:

Early-age cracking is observed during the first seven days after concrete pouring. However, such cracks may also take more than a week to be visible in reinforced concrete slabs. Therefore, cracks appearing within 60 days after the concrete placement are considered early-age cracks.

The early-age cracks do not cause structural failure in concrete if the structural tolerances are not exceeded. Nevertheless, they can lead to premature corrosion of reinforcing steel bars and spalling of concrete cover, resulting in increased maintenance cost and potentially reduced life span.

Freshly-placed concrete can suffer from early-age cracking based on the composition of the concrete mix, exposure environment, hydration rate, and curing condition. Therefore, a proper understanding of the causes is essential for considering suitable measures to avoid early-age cracking in concrete.

Causes of Early-age Cracking in Concrete

The following are the causal factors of early-age cracking in concrete:

1. Internal Concrete Temperature

Cement hydration?generates the heat of hydration, which can lead to early-age crack development. If the internal temperature reaches around 50?oC, the ettringite can become unstable and dissolve. After the internal temperature decreases, the ettringite will expand and can lead to internal and external cracking.??

Early-age Transverse Cracks in Reinforced Concrete Column developed within Three Days After Concrete Placement

2. Temperature Gradient

A temperature gradient occurs when concrete is exposed to a high temperature that cannot be dissipated. In this case, a high-temperature gradient between the internal and external surfaces of concrete structural elements creates thermal stresses.

If the concrete member is restrained, the thermal stress at the early age of concrete will easily exceed concrete’s tensile strength and develop cracks.

Restraints in concrete can be due to a change in section depth like a junction between a web and flange in a T-beam or coffer slab, top reinforcements in beams or slabs, stirrups in columns, formwork ties, and bridge of coarse aggregate between form and narrow sections.

3. Autogenous Shrinkage

Autogenous shrinkage can generate large tensile stress in concrete at an early age, especially in high-early-strength concrete.

Autogenous shrinkage occurs after its driving force is produced by chemical shrinkage. Chemical shrinkage occurs when the volume of hydration product becomes smaller than the original volume of reacted constituents, which are cement and water. The ultimate chemical shrinkage of a typical hydrating?Portland cement?paste can be in the order of 10% by volume.

When additional water for curing is not readily available after the cement set, the chemical shrinkage will be accompanied by the formation of empty capillary porosity within the hydrating cement paste, known as self-desiccation. The menisci in remaining partially filled pores will, in turn, create autogenous stresses that produce autogenous shrinkage, leading to early-age cracking.

The cracks are commonly developed around internal restraints such as?steel reinforcements?and aggregates or through the depth of the concrete member when sufficient external restraint is available.

Autogenous Shrinkage

4. Plastic Settlement

Plastic settlement is a common factor that leads to early age cracking. It occurs when solid particles in a concrete mixture settle under gravity and bleeding water and move upward.

The downward movement of solid particles creates stress if the concrete is locally restrained from settling. So, when this stress exceeds the tensile strength of recently poured concrete, early age cracking occurs. The cracks are developed at the source of restraint in concrete.

Early-age cracking due to plastic settlement commonly occurs in forms involving a sudden change in the depth of concrete. This is because settlement is greater in the deeper section and creates cracks around the point of change in the formwork section.

Plastic Settlement of Newly Placed Concrete

5. Drying Shrinkage

Drying shrinkage can also cause early-age cracking if the shrinkage is restrained internally, externally, or a combination thereof. Drying shrinkage distributes non-uniformly throughout the concrete member’s thickness, resulting in differential shrinkage.

The differential shrinkage produces axial movement and warping. These two movements create axial and bending stresses if the concrete is restrained, leading to early-age cracking. Drying shrinkage also creates residual stress in small elements.

Drying Shrinkage Cracks in Concrete

6. External Loading

External loading such as vibration, traffic, and wind can create extra stress on the concrete and initiate early age cracks if the tensile strength of concrete is surpassed.?

The tensile strength of concrete at an early age is considerably low and can be surpassed easily. That is why the effects of external loading should not be ignored after?concrete placement.

7. Concrete Creep

Concrete creep is another cause of early-age cracking in concrete. When concrete undergoes time-dependent movement due to the application of internal or external stresses, aggregate in concrete acts to restrain these movements. In this case, the concrete may suffer from cracking.??

Coupled with steps taking to address the causal factors explained above, one of the methods used in minimizing the occurrence of early-age cracking in concrete is internal curing.

Building professionals are enjoined to make adequate preparation and provision for preventing this building and construction challenge as not doing so early enough will result in increased maintenance cost in the nearest future.

Credit: The Constructor