Early-age cracking in concrete, which becomes apparent within the initial seven days post-pouring, poses concerns for structural integrity. Even though these cracks may not compromise the structure if within tolerances, they can pave the way for issues like premature corrosion and spalling, leading to increased maintenance expenses. This article delves into the causes of early-age cracking and their implications.
Internal Concrete Temperature :
Cement hydration generates heat, potentially causing cracks if the internal temperature hits around 50°C. This heat can destabilize ettringite, leading to internal and external cracking once the temperature decreases.
Temperature Gradient :
Exposure to high temperatures without adequate dissipation can create a temperature gradient. This gradient, when significant, results in thermal stresses that surpass the concrete’s tensile strength, causing cracks. Restraints in concrete, like changes in section depth, contribute to this phenomenon.
Autogenous Shrinkage :
Autogenous shrinkage, driven by chemical shrinkage, generates tensile stress, particularly in high-early-strength concrete. Chemical shrinkage occurs when hydration product volume decreases. Lack of water for curing intensifies this, leading to autogenous shrinkage and subsequent cracking, especially around internal restraints.
Plastic Settlement :
Plastic settlement occurs when solid particles settle under gravity and bleeding water moves upward. Stress is created when settling is restrained, resulting in early age cracking. Forms with sudden depth changes are susceptible to this, as settlement is greater in deeper sections.
Drying Shrinkage :
Drying shrinkage, if restrained internally or externally, causes non-uniform distribution throughout the concrete. This leads to differential shrinkage, axial movement, and warping, creating stresses that can cause early-age cracking.
External Loading :
Vibration, traffic, and wind, as external loads, can exert additional stress on concrete. If this surpasses the low tensile strength of early-age concrete, cracking may occur. Ignoring the effects of external loading post-placement is not advisable.
Concrete Creep :
Concrete creep, a time-dependent movement due to internal or external stresses, can lead to early-age cracking when aggregates act to restrain these movements.
Understanding the diverse causes of early-age cracking is crucial for implementing preventive measures. By addressing these factors, we can enhance the longevity and durability of concrete structures, minimizing maintenance costs and potential risks.
