Self-desiccation, or chemical drying, is a common occurrence in all concretes. It’s a fact that the hydration reaction between cement and internal water, continue throughout time. The water in capillaries will use for hydration, and internal drying continues. This phenomenon is identified as self-desiccation in concrete.
The fundamental cause of chemical shrinkage is the hydration reaction. The main ingredients of the cement, C3S, C2S, C3A, C4AF reacts with water and develop C-S-H hydrate. This C-S-H occupies less volume than its original components. This volume reduction is the cause of chemical shrinkage.
The effect of self-desiccation is significant in the first few months. It occurs irrespectively of the water-cement ratio. However, concretes having water-cement ratios below 0.38 show a noticeable impact of it. You can observe micro-cracking due to shrinkage stress by self-desiccation.
There are advantages as well as disadvantages of self-desiccation.
- Reduce saturation level: In some situations, we need to dry the concrete quickly. In cases like wooden flooring, the drying time is vital for the progress. Wood flooring on wet pavement can grow unpleasant fungus. Self-desiccation inside the concrete will reduce free water content in the concrete and reduce the drying time.
- Increase the resistance to frost damage: Self-desiccation in concrete always try to empty the capillaries, making them perfect spaces for air voids. This air void structure is the key resistance for the freeze and thaw attack.
- Decrease the risk of reinforcement corrosion: Reduction of free water content due to self-desiccation helps to minimize the corrosion risk. Steel corrosion is always lesser in drier places.
- Shrinkage effect: Some water-cement ratios can show significant shrinkage due to self-desiccation. This is common in high-performance concrete as well as cement grouts. High cement content in both of these mixes shows substantial shrinkage during hydration.
- Induce stress and microcracking: There can be stresses and cracks induce during hydration because of chemical shrink. High strength concrete can undergo significant shrinkage stresses like 40 MPa. When the concrete tensile capacity is not enough to bare this stress, a shrinkage crack propagates.
- Long term durability: Induced stress and the occurrence of cracks can directly affecting the life span of the concrete. Self-desiccation imposes such undesirable conditions in the concrete.
One of the significant features of chemical shrinkage is its continuous action. It starts at the instant of mixing and continues until hydration stops. Throughout the liquid, stiffening, and hardening times, chemical shrinkage continues. However, there is another type of shrinkage dominate during stiffening period called autogenous shrinkage
Autogenous shrinkage occurs in the concrete because of the relative humidity (RH) decrease during the hydration. When the pore water consumes for hydration, pore pressures decrease. This is an indirect occurrence of the self-desiccation process in the concrete.
At the plastic stage of concrete hardening, autogenous shrinkage dominates its effect. Drying out pore water becomes thinner and thinner due to self-desiccation. Pore holes try to shrink because of the surface tension of the available water.
7 main factors affecting the self-desiccation and shrinkage.
Curing method affects the evaporation rate. Wet blankets and ponding are the best methods to retain internal water. When evaporation increases, shrinkage increases. Reducing internal water evaporation using proper curing methods will help in developing stiffer and tighter microstructure in the concrete which will resist shrinkage more
Humidity level inside the pores is considered as the internal humidity of concrete. When humidity decrease autogenous shrinkage increases
Surface area to volume ratio of materials
Higher surface area of cement shows that the cement particles are fine. This will allow the cement to react quickly and hydrates at a higher rate. It increases the chemical shrinkage of the concrete. However, more surface area in the aggregates means that they are fine and irregular in shape. This will decrease the permeability in the concrete causing minimum drying shrinkage.
Slump and w/c
Shrinkage is caused due to the loss of water in the paste. At higher slumps, the paste volume is high. As a result, concrete with higher slumps shows higher shrinkage.
Similarly, with water-cement ratios, mixes with low w/c below than 0.38 show a considerable amount of chemical and autogenous shrinkages. Increasing the water-cement ratio results in reducing shrinkage risk. However, drying shrinkage is prominent in concrete with high water-cement ratios. Large and interconnected pore holes in such concrete dry out quickly and subject to shrinkage
Fine aggregate proportion
Total aggregate content, as well as the fine aggregate proportion, has a positive impact on shrinkage. It is known that cement paste causes the shrinkage during hardening. However, aggregate doesn’t support any shrinkage. Improving aggregate content in the concrete reduces the paste content as well as the shrinkage.
High cement content results in high paste content. This will increase chemical and autogenous shrinkage. Expansion and shrinkage because of hydration heat is also dominating in cement rich concrete.
Having high air content is beneficial for shrinkage movement. But, these air voids improve the permeability as well as the drying shrinkage.
How to face the desiccation shrinkage
In most cases, shrinkage is not beneficial for the concrete. It can develop surface cracks as well as deep cracks. Shrinkage cracks are one of the reasons we use isolation joints in concrete pavements. Few possible approaches are listed below to reduce the effect of shrinkage in the concrete
Try to avoid rapid hardening & finer ground cement if possible.
Use moderate water-cement ratio mixes. The range of 4 to 4.5 is best to use for outdoor and indoor applications
Accept some bleeding in the concrete. This will encourage free water to move out during the initial setting. At the fresh stage, shrinkage will not be an issue for the concrete.