Concrete shrinkage, along with structural restraints, leads to crack development in restraint structural members. The structural restrictions are commonly due to the other parts of the structure. Concrete observes cracks when the stresses imposed upon the structure surpasses the concrete tensile strength.
Cracks may propagate at much lower stresses than are required to cause crack initiation. Thus, reducing the amount of shrinkage would improve both strength and durability of concrete. Shrinkage is nothing but the reduction of concrete volume due to the loss of moisture.
If the volume reduction happens before the concrete hardens, then it is termed as plastic shrinkage. However, when volume reduction occurs because of moisture loss after the concrete hardens, it is called drying shrinkage. Finally, concrete can experience carbonation and autogenous shrinkage as well.
Reasons for Concrete Shrinkage
The best strategy to tackle any problem is to eliminate the factors that lead to the given issue, such as concrete shrinkage in this case. The following factors are the culprit for concrete shrinkage.
1. Excess Water in Concrete
The workability and handling of concrete needs an adequate amount of water, but the free water in the concrete is precisely the essential cause of concrete shrinkage.
2. Environmental Factors
Environmental factors like temperature, humidity, and wind velocity are also responsible for concrete shrinkage.
3. Concrete Quality
Concrete quality with respect to w/c ratio, cement content, heat of hydration, aggregate, and compaction level.
4. Geometry of Structural Member
Geometrical factors such as the surface-area-to-volume ratio of concrete and depth of concrete can influence concrete shrinkage greatly. The concrete members smaller in size would suffer higher shrinkage compared to larger structural members.
Besides, the larger the concrete surface, the wider the concrete area exposed to the environment, and, as a result, the possibility of shrinkage is higher.
How to Reduce Shrinkage of Concrete at Construction Site?
1. Reduce Water Quantity
The reduction of water quantity can play a crucial role in the reduction of drying shrinkage, but practice reasonable cautions to maintain workability needed to place and compact concrete. Tests have shown that decreasing water to cement ratio from 0.45 to 0.35 leads to the reduction of concrete shrinkage by 50%.
Surface crazing on walls and slabs is an excellent example of shrinkage from drying on a small scale. Crazing usually occurs when the surface layer of the concrete has higher water content than that of the interior concrete. The result is a series of shallow, closely spaced, fine cracks.
Reduce both drying shrinkage and settlement cracking by decreasing the concrete's water content as the wall is placed from the bottom to the top. Using this procedure, bleed water from the lower portions of the wall will tend to equalize the water content within the wall. This procedure needs careful control and proper consolidation.
2. Add Additives
The former reduces water content and concrete shrinkage, and the latter declines the internal effect of shrinkage. Additionally, use shrinkage reducing admixture, which compensates for concrete shrinkage by creating expansive crystals.
Use the optimum practical quantity of aggregate in the concrete mix to reduce drying shrinkage to minimum possible value. The higher the aggregate stiffness, the more effective it is in reducing the shrinkage of the concrete.
That is why concrete containing sandstone aggregate has a higher shrinkage rate—about twice that of concrete containing granite, basalt, or limestone.
4. Protect Concrete Surface from Rapid Drying
Covering concrete surface would reduce or eliminate the influence of environmental factors on concrete until the curing process begins. The use of ice or chilled water can help to decline batched concrete temperature. Employ temporary windbreaks during windy days to decrease wind velocity.
5. Time of Concreting
The selection of proper time for concreting may reduce concrete shrinkage, especially plastic shrinkage. For instance, pour concrete at night during summer to avoid high temperatures that concrete may be exposed to during the daytime.
6. Proper Concrete Compaction
Compact concrete properly to avoid shrinkage of concrete. Prevent concrete re-tempering at the construction site since it increases drying shrinkage because of the increase of water content.
7. Proper Curing
The adoption of proper curing mechanisms can reduce concrete shrinkage. Adequate curing allows concrete to hydrate appropriately and gain designated strength.
Reinforced concrete slabs would particularly benefit from adequate curing because it enables the concrete to achieve higher tensile capacity before drying out take place. Steam curing can reduce concrete shrinkage.
8. Adequate Contraction Joints and Steel Detailing
Proper placement of contraction joints in combination with appropriate detailing of steel bars can reduce the detrimental effect of concrete shrinkage; control crack development.
FAQs on how to reduce concrete shrinkage in construction site
Shrinkage is the reduction of concrete volume due to the loss of moisture.
If the volume decrease happens before the concrete hardens, then it is termed plastic shrinkage. However, when volume reductions occur because of moisture loss after the concrete hardens, then it is called drying shrinkage. Finally, concrete can experience carbonation and autogenous shrinkage as well. Read more
1. Presence of excess water in concrete when it dries.
2. Environmental factors like temperature, humidity, and wind velocity.
3. Concrete quality with respect w/c ratio, cement content, heat of hydration, aggregate, and compaction level.
4. Geometrical factors such as surface area to volume of concrete and depth of concrete can influence concrete shrinkage. Smaller size concrete member would suffer higher shrinkage compare with large structural members.
Shrinkage reducing admixture, which compensates for concrete shrinkage by creating expansive crystals. Read more
Plastic shrinkage and drying shrinkage are the most common types that have an immense effect on concrete.