5 Critical Reasons For Concrete Cracking
Concrete cracking affects not only the buildings’ appearance but also the safety of their structure and lifespan. This phenomenon is caused by 5 critical reasons, including drying shrinkage, self-shrinkage, plastic shrinkage, thermal shrinkage, and autogenous(chemical) shrinkage.
1. Drying Shrinkage Caused By Water Loss
Drying shrinkage mostly happens when the water in the capillary or gel pores of concrete is lost in an unsaturated air environment. High-performance concrete is less likely to dry compared with ordinary concrete due to its low porosity. However, the cumulative effect of drying shrinkage gets some serious power in mass concrete. The water loss in concrete is like that in the human body, which will cause changes in the internal structure. When the stress generated by these changes gets above the tensile strength of the concrete, cracks can result.
2. Plastic Shrinkage During Initial Hardening
Plastic shrinkage can be found in the plastic stage before hardening. High-performance concrete features a low water-to-gel ratio, less free water, and fine mineral admixtures that are more sensitive to water, which means they do not bleed and lose water quickly. These make high-performance concrete more likely to suffer from plastic shrinkage. The concrete loses water on its surface before it completely sets, remaining in a stable plastic state on the inside. Such a difference creates a tensile stress on the surface. Once the stress grows to more than the tensile stress, cracks occur. Although quite thin, they are numerous and densely distributed on the surface of the concrete.
3. Self Shrinkage - Humidity Changes To Blame
The self-shrinkage refers to when the humidity decreases in the closed internal structure of concrete along with the cement hydration. Such a phenomenon results in unsaturated water in the pores. As a result, it creates negative pressure and triggers the self-shrinkage of concrete. Because of the low water-to-gel ratio, high-performance concrete could see higher strength in its early stage and faster loss of water. The relative humidity of the pore system goes under 80 percent. Meanwhile, the dense structure of high-performance concrete stops moisture from the outside and thus aggravates self-shrinkage.
4. Thermal Shrinkage - Damage from Thermal expansion
Large-volume concrete projects that are demanding on strength require much more cement. This brings more heat of hydration and heats the system more quickly to around 35 to 40℃. On top of the initial temperature, the highest temperature may even surpass 70 to 80℃. Concrete has properties of thermal expansion and cold contraction and a CTE(Coefficient of thermal expansion) of 10×10-6/℃. When the temperature drops by 20 to 25℃, we can calculate the cold shrinkage of about (2 – 2.5)×10-4, while the ultimate tensile value of concrete is only 1 – 1.5×10-4. So, the stress caused by cold shrinkage can easily exceed the tensile strength of concrete. Consequently, cracks appear and extend from the surface to the interior of the concrete, seriously affecting its structure.
5. Autogenous shrinkage- Side Effect of Hydration
Autogenous shrinkage is also called chemical shrinkage. During the cement hydration, the absolute volume of the cement-water system decreases and forms many pores. However, hydration can be limited in high-performance concrete owing to its lower water-to-gel ratio and additional fine mineral admixtures. So, the chemical shrinkage would be less than in ordinary concrete. Noticeably, the cracks formed from autogenous shrinkage still have impacts on the microscopic structure of the concrete. Combined with other factors, it can also be a trigger for cracks.
In addition to the factors above, another main reason for concrete cracking is temperature contraction stress. The stress is caused by temperature fluctuation and shrinkage that happens when the large volume of cement used in mass concrete releases hydration heat.
Prevention and Control - How to Fight Against Concrete Cracking
1. Optimize the Concrete Mix Ratio
- Cement
- Prioritize low- and medium-heat cement to reduce hydration heat.
- Limit the amount of cement while retaining the strength and performance of concrete, reducing temperature rise.
- Aggregates
- Choose high-quality aggregate with moderate particle size.
- Use more aggregate and less cement mortar to reduce concrete shrinkage. For example, well-graded aggregates and medium sand could effectively increase the density of concrete.
- NOVASTAR Polycarboxylate superplasticizer(PCE) is a high-performance water-reducing agent and is highly soluble in water. It can improve the concrete flow and reduce the amount of cement without increasing water consumption. A low dosage of this water reducer could bring good fluidity to the concrete. Also, the content of chloride ions and alkali of Polycarboxylate superplasticizer(PCE) is quite low, making concrete more durable.
2. Improve The Construction Process
- Pour the cement by layers or sections to control layer thickness and pouring speed. This is to help the heat inside the concrete be evenly distributed and avoid thermal stress or temperature gradients.
- Compact the concrete to guarantee an ideal density and thus prevent concrete cracking.
- Cover moisture-insulating material, such as a plastic film, after pouring the concrete to reduce evaporation and cracking.
- Control the temperature inside and outside the concrete by spraying water on the surface. This can control the temperature of concrete and decrease the thermal stress.
