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Several factors like poor concrete quality, improper structural design, inappropriate steel bar spacing, large slab span, improper aggregates, etc. are responsible for the development of cracks in RCC slabs. While the issues pertaining to improper structural design can be eliminated in the design stage, the other factors can be avoided in the construction stage of the project.

What are the Causes of Cracks in Reinforced Concrete Slab?

1. Poor Concrete Quality

Compromising with the concrete quality is one of the reasons for crack developments in reinforced concrete slabs. Poor concrete quality results in lower concrete strength, specifically, the tensile strength. As a result, the tensile strength of concrete reaches its maximum limit at a very low magnitude of stress. 

Incorrect water to cement ratio, inadequate concrete mixing, improper placement of concrete, and insufficient consolidation are factors that can jeopardize the concrete quality. Therefore, prepare and pour concrete according to the designated mix and follow proper concrete placement procedure.

2. Improper Structural Design

Another cause of crack development in an RCC slab is the low reinforcement ratio due to errors in the design stage. A lower reinforcement ratio yields a lower slab capacity to support loads. As a result, the RCC slab cracks at smaller loads.

Patterns of Cracks in Reinforced Concrete Slabs Due to Poor Concrete Quality
Figure-1: Patterns of Cracks in Reinforced Concrete Slabs Due to Improper Structural Design

3. Wrong Steel Bars Spacing

When the spacing between primary and distribution reinforcement is greater than the designated spacing, then cracks may develop in the RCC slab.

4. Insufficient Concrete Cover

Insufficient concrete cover reduces the required protection for steel bars. As a result, chloride attacks would cause steel corrosion, leading to cracking of concrete along steel bars.

Cracks Due to Insufficient Concrete Cover
Figure-2: Cracks Due to Insufficient Concrete Cover

5. Improper Curing

Improper curing of concrete may cause concrete shrinkage and subsequently initiate cracks. Moreover, it could reduce the strength of the concrete. The cracks due to improper curing can be shallow fine cracks that are parallel to each other.

Shrinkage Cracks Appear on Slabs are Parallel to each Other
Figure-3: Shrinkage Cracks Appear on Slabs are Parallel to each Other

6. Large Slab Span

When the span of the slab is longer than that of applicable standards, then the development of cracks, especially flexural cracks, is highly likely.

7. Errors in Formwork Placement

Improper installation of formworks could also be one of the reasons for crack development in the RCC slab.

8. Wrong Material Selection

The selection of right concrete constituent, such as aggregate, is essential in reducing the risk of crack initiation in the reinforced concrete slab. The use of alkali-aggregate in concrete can cause the development of cracks. The map of cracks due to alkali-aggregate is shown below.

Cracks in Reinforced Concrete slab Due to Alkali-aggregate reactions
Figure-4: Cracks in Reinforced Concrete slab Due to Alkali-aggregate reactions

9. Severe Environmental Conditions

Sulfate attacks occur when the reinforced concrete slab is constructed in harsh environmental conditions, like, coastal areas. The pattern of cracks due to sulfate attack can be observed in the following figure.

Cracks Caused by Sulfate Attack
Figure-5: Cracks Caused by Sulfate Attack

FAQs

What are the causes of cracks in reinforced concrete slabs?

Several factors like poor concrete quality, improper structural design, inappropriate steel bar spacing, insufficient concrete cover, large slab span, improper aggregates, and inadequate curing regime are responsible for the development of cracks in RCC slabs.

How does poor concrete quality lead to crack development in reinforced concrete slabs?

Compromising with the concrete quality is one of the reasons for crack developments in reinforced concrete slabs. Poor concrete quality results in lower concrete strength, specifically tensile strength. As a result, the tensile strength of concrete is surpassed at a lower stress than expected. 

How do environmental conditions initiate cracks in Reinforced concrete slabs?

Sulfate attacks occur when the reinforced concrete slab is constructed in harsh environmental conditions, like, coastal areas. The sulfate attack causes crack development in concrete.

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