What is the very first crack that occurs in any RCC member, especially if constructed during summer?

Question

Prepp Team · Accepted Answer

Understanding Early Cracks in RCC Members

When concrete is placed, it contains a significant amount of water. As the concrete hardens and dries, this water evaporates. This loss of moisture causes the concrete to shrink. If this shrinkage is restrained (for example, by the foundation, reinforcing steel, or adjacent structural elements), tensile stresses develop within the concrete. Since concrete is strong in compression but weak in tension, these tensile stresses can exceed the concrete's tensile strength, leading to cracks.

Why Shrinkage Cracks Appear First

Shrinkage cracks are often the very first type of cracks to appear in reinforced cement concrete (RCC) members, especially shortly after placing and during the curing period. There are primarily two types of shrinkage cracks that occur early:

Plastic Shrinkage: This occurs while the concrete is still plastic (fresh and wet). It happens when water evaporates rapidly from the surface of the concrete, faster than it can be replaced by bleed water rising to the surface. This is particularly common in hot, dry, or windy conditions. The rapid drying causes the surface layer to shrink, leading to shallow cracks that often appear in a random pattern.
Drying Shrinkage: This occurs after the concrete has hardened, as it continues to lose moisture over a longer period. These cracks typically develop later than plastic shrinkage cracks but are still a common cause of early cracking.

Given the scenario mentioning construction during summer, the conditions (high temperature, potentially lower humidity, wind) are highly conducive to rapid evaporation of surface water, making plastic shrinkage a very likely candidate for the very first crack.

Analyzing Other Crack Types

Let's consider why the other options are less likely to be the very first crack:

Settlement crack: These cracks are caused by the differential settlement of the supporting structure or foundation. While they can occur relatively early if foundation issues are present, they are related to the ground or support rather than the concrete's inherent material behavior during curing, which is what causes shrinkage cracks first in the member itself.
Corrosion spalling crack: These cracks occur much later in the life of an RCC member. They are caused by the corrosion (rusting) of the reinforcing steel, which expands and pushes the concrete cover outwards, leading to spalling and cracking. This process takes time, typically years, and is not the very first crack during construction or initial curing.
Flexural crack: These cracks occur when the member is subjected to bending loads that induce tensile stresses exceeding the concrete's tensile strength in the tension zone. Flexural cracks typically appear after the concrete has gained sufficient strength and the structure is subjected to service loads. They are load-induced, not primarily caused by material behavior during initial hardening and drying in the absence of significant load.

Therefore, considering the initial stage of concrete construction and the emphasis on summer conditions promoting rapid drying, shrinkage cracks, particularly plastic shrinkage cracks, are the most probable very first type of crack in an RCC member.

Summary of Early Crack Types

Crack Type	Typical Cause	Likelihood of Being the Very First Crack	When it Occurs
Shrinkage Crack (Plastic)	Rapid surface water evaporation from fresh concrete	High (especially in hot/dry conditions like summer)	Within minutes to a few hours after placing/finishing
Shrinkage Crack (Drying)	Moisture loss from hardened concrete	Moderate (can occur relatively early but often later than plastic shrinkage)	Days, weeks, or months after hardening
Settlement Crack	Differential movement of support/foundation	Possible, but less directly related to concrete material behavior during initial curing compared to shrinkage	Can occur early or later depending on foundation conditions
Corrosion Spalling Crack	Rusting of reinforcing steel	Very Low	Years after construction
Flexural Crack	Bending under load	Low (occurs when subjected to significant load after hardening)	When sufficient bending load is applied

Revision Table: Key Concepts for RCC Cracks

Concept	Description
RCC	Reinforced Cement Concrete; concrete with embedded steel reinforcement.
Shrinkage	Volume decrease in concrete due to moisture loss or chemical reactions.
Plastic Shrinkage	Shrinkage occurring while concrete is still in its plastic state due to rapid surface drying.
Drying Shrinkage	Shrinkage occurring after concrete hardens due to continued moisture loss.
Tensile Strength of Concrete	Concrete is weak in tension, making it susceptible to cracking under tensile stress.
Curing	Maintaining adequate moisture and temperature in concrete to ensure proper hydration and strength development, which helps minimize shrinkage.

Additional Information on Preventing Early RCC Cracks

Preventing or minimizing early shrinkage cracks, especially plastic shrinkage cracks, is crucial for the durability and appearance of RCC structures. Several measures can be taken:

Proper curing practices are essential, particularly in hot weather. This includes covering the concrete surface with wet coverings (like burlap), using curing compounds, or ponding water on the surface to prevent rapid moisture loss.
Adding synthetic or steel fibers to the concrete mix can help control and bridge early cracks.
Using a proper concrete mix design with minimum water content necessary for workability.
Preparing the subgrade or formwork properly to avoid excessive absorption of water from the fresh concrete.
Erecting temporary windbreaks or sunshades in hot, windy conditions to reduce evaporation rates.
Avoiding over-finishing the surface, which can bring too much fine material and water to the top.

By understanding the causes of shrinkage and implementing preventive measures, the occurrence of early cracks in RCC members can be significantly reduced.