Concrete is one of the most widely used materials in construction, known for its strength, durability, and versatility. However, even the most expertly poured concrete can develop cracks during the drying process, often leaving builders and property owners concerned. Understanding why concrete cracks when drying can help manage expectations and implement strategies to minimise cracking.
The science behind drying concrete
When concrete is poured, it undergoes a chemical process known as hydration, where water reacts with cement to form a hard, durable matrix. This process is not the same as “drying.” In fact, concrete doesn’t simply dry out—it cures. During curing, moisture is retained within the mix to ensure proper hydration and to allow the material to reach its full strength.
Cracking often occurs due to internal and external stresses placed on the concrete as it cures. Below are the primary reasons for cracking:
1. Shrinkage during drying
Concrete contains water that begins to evaporate as it cures. This evaporation causes the concrete to shrink slightly. If the shrinkage is uneven or if parts of the concrete are restrained by the ground or other structural elements, tension builds up. Once the tension exceeds the tensile strength of the concrete, cracks can form.
This phenomenon is especially common in larger concrete slabs, as the volume of water loss and the potential for uneven drying increases with size.
2. Rapid moisture loss
If concrete loses moisture too quickly, it can lead to surface cracking known as plastic shrinkage cracks.
Rapid moisture loss can occur due to:
- Hot, dry, or windy weather conditions
- Insufficient curing measures (e.g., lack of protective coverings or curing compounds)
- Poor water-to-cement ratio in the mix
Proper curing practices, such as keeping the surface damp or using curing membranes, can help slow down evaporation and reduce the risk of cracking.
3. Thermal changes
Temperature fluctuations during curing can cause thermal expansion and contraction. If the concrete expands or contracts too quickly, it may crack. This is particularly an issue for large pours, as the center of the slab may retain heat while the surface cools and contracts.
4. Poor mix design
The composition of the concrete mix plays a significant role in its behaviour. Too much water, improper aggregate selection, or low-quality cement can weaken the mix, making it more prone to cracking. Additionally, adding too much water to make the mix easier to work with can increase shrinkage during curing.
5. Improper placement and finishing techniques
Mistakes during the pouring and finishing process can contribute to cracking. Overworking the surface, not compacting the mix properly, or pouring on improperly prepared ground can lead to stresses and vulnerabilities in the final structure.
Can cracks be prevented?
While it’s nearly impossible to prevent all cracks, there are several steps that can minimise their occurrence:
- Proper curing: Ensure the concrete cures slowly by covering it with damp blankets, plastic sheeting, or applying curing compounds.
- Control joints: Install control joints to direct cracking along predetermined lines, reducing random cracks.
- Ideal weather conditions: Pour concrete in moderate weather and avoid extreme heat or cold.
- Good mix design: Use a high-quality mix with the appropriate water-to-cement ratio.
- Professional installation: Hire experienced professionals who understand the importance of proper placement and finishing techniques.
What to do about cracks in concrete
Not all cracks are a sign of structural failure. Hairline or surface cracks are often cosmetic and can be repaired with patching materials or sealants. However, larger cracks or those that extend
Concrete cracking during drying is a natural result of the material’s behaviour under stress, but it doesn’t have to be a cause for alarm. By understanding the reasons behind cracking and implementing preventive measures, you can ensure a stronger, more durable concrete structure. Whether you’re working on a small patio or a large commercial project, proper planning and care are essential to minimising cracks and achieving a long-lasting finish.
If you’re looking for expert advice or professional concrete services, contact us at. Our experienced team can guide you through every step of your project, ensuring the best possible results.