Concrete, mortar, bricks, and wood are common building materials that are porous in nature. As a result, they tend to expand when they absorb moisture from the atmosphere and shrink when they dry out. These changes occur cyclically during moisture changes. However, building materials can undergo irreversible changes during their initial conditions as a result of changes in moisture content.
Irreversible movement in building materials includes the initial shrinkage or plastic shrinkage of cement and lime-based materials upon initial drying. Additionally, there is the initial expansion of burnt clay bricks and other clay products upon removal from kilns. These types of cracks are commonly observed in concrete due to moisture changes and can be classified into four types: initial shrinkage cracks, plastic shrinkage cracks, plastic settlement cracks, and initial expansion of concrete.
It is essential to understand these types of cracks to prevent any potential damage to the structure. These cracks may occur as a result of the irreversible movement caused by the initial changes in moisture content. Hence, it is crucial to ensure proper curing of concrete to minimize the impact of moisture changes. Proper curing can prevent or minimize the formation of cracks in concrete and other building materials.
Types of Cracks in Concrete due to Moisture Change
Initial Shrinkage in Concrete and Masonry
Building materials that are cement or lime-based, such as concrete, mortar, masonry units, masonry, and plaster, undergo initial shrinkage that is partly irreversible. This phenomenon occurs during the setting process when the moisture in the materials dries out, and it is a significant cause of cracking in building structures. Initial shrinkage occurs only once in the lifetime of concrete and mortar and is therefore an important factor to consider during construction.
The effect of initial shrinkage on concrete and mortar depends on various factors. Firstly, the cement content is an essential factor that affects shrinkage. The higher the cement content, the greater the shrinkage of concrete and mortar. Secondly, the water content also plays a crucial role in the shrinkage of these materials. When the water quantity used in the mix increases, the shrinkage also increases.
The maximum size, grading, and quality of aggregate also affect the shrinkage of concrete and mortar. An increase in the maximum size of aggregate with good grading reduces the water-cement ratio required for the same workability requirement of concrete, thus reducing the porosity of the material. This, in turn, reduces the initial shrinkage of concrete.
Proper curing from the start of initial setting to at least 7 to 10 days is another essential factor in reducing initial shrinkages. The moisture provided through curing helps concrete and masonry to expand, and when they dry up, the final shrinkage is less.
The surface area of aggregates also affects shrinkage. As the surface area of concrete increases with the increase in fine aggregates, the required water quantity for the required workability also increases. With an increase in water quantity, the shrinkage of concrete and masonry increases when they dry up.
The chemical composition of cement is also an important factor in shrinkage. Cement with a higher proportion of tricalcium silicate and a lower proportion of alkalis has less shrinkage than rapid hardening cement with a lower proportion of tricalcium silicate and a higher proportion of alkalis.
The temperature of fresh concrete and the relative humidity of surroundings also affect the shrinkage of concrete and mortar. Concreting done in mild winter has much less cracking tendency than concreting done in hot summer months. In cement concrete, one-third of the shrinkage occurs in the first 10 days, half within one month, and the remaining half within a year. Therefore, shrinkage cracks in concrete continue to occur and widen up to a year period.
Plastic shrinkage of concrete
Cracks that appear on the surface of concrete before it sets are caused by plastic shrinkage of the material. This happens due to the settling of heavy aggregates at the bottom of the concrete and the rise of water to the surface. This settling and rising process is known as the bleeding of concrete. As long as the rate of evaporation is lower than the rate of bleeding, there will be a continuous layer of water at the surface, known as the “water sheen,” which prevents shrinkage from occurring.
However, if the concrete surface loses water faster than the bleeding action brings it to the top, then the top layer of the concrete will shrink. Because concrete in a plastic state cannot resist any tension, cracks will develop on the surface. This type of cracking is common in slabs.
The degree to which plastic shrinkage occurs in concrete is influenced by several factors. One of these factors is the temperature of the concrete. Higher temperatures can lead to increased rates of evaporation, which can cause the concrete to dry out more quickly and result in greater shrinkage.
Another important factor is exposure to heat from the sun’s radiation. This can cause the surface of the concrete to heat up and increase the rate of evaporation, leading to greater shrinkage. The amount of heat that the concrete is exposed to will depend on factors such as the time of day and the angle of the sun.
The relative humidity of the ambient air is also a significant factor in plastic shrinkage. When the air is dry, it can absorb more moisture from the concrete and increase the rate of evaporation. This can cause the concrete to shrink more quickly and to a greater extent than if the air were more humid. The velocity of wind can further exacerbate this effect by increasing the rate of evaporation from the concrete surface.
Overall, the extent of plastic shrinkage in concrete is dependent on a combination of these factors. Engineers and construction professionals need to carefully consider the environmental conditions when designing and pouring concrete to minimize the risk of shrinkage and ensure the durability and longevity of the structure.
Plastic settlement cracks
When concrete is poured, the settling of large aggregates can lead to the development of plastic settlement cracks on the surface. However, in reinforced concrete, the presence of reinforcement can sometimes impede the settling of aggregates. As a result, the concrete above the reinforcement can be subjected to excessive pressure, causing it to crack and form voids beneath the reinforcement. These voids can weaken the overall structure and compromise its integrity. Therefore, it is important to carefully consider the placement and distribution of reinforcement to avoid the formation of such cracks and voids.
Plastic settlement cracks are normally observed:
Over form work tie bolts, or over reinforcement near the top of section. In narrow column and walls due to obstruction to sedimentation by resulting arching action of concrete due to narrow passage. At change of depth of section. rewrite only given context in paragrahsChatGPT
When constructing a formwork for columns, there are two common methods for placing tie bolts. One is to place them over the formwork, while the other is to place them near the top of the reinforcement section. The choice of method may depend on various factors such as the design requirements and the construction materials used.
However, in narrow columns and walls, there may be obstructions that hinder the natural settlement of the concrete. In such cases, the concrete may undergo arching action due to the narrow passage, leading to difficulties in sedimentation. This can pose a challenge during the construction process and may require additional measures to ensure the structural integrity of the column or wall.
Another situation where placement of reinforcement may need to be considered is at a change of depth of the section. This can occur in structures such as beams or columns where the cross-sectional shape or dimensions change abruptly. The reinforcement needs to be placed in such a way that it effectively supports the load distribution and ensures the stability of the structure.
Initial expansion of concrete:
During the manufacturing of clay bricks, they are fired at a high temperature which causes the removal of not only the intermolecular water but also the water that is a part of the clay’s molecular structure. As the bricks cool down, they become moisture-hungry and begin to absorb moisture from the surrounding environment. This absorption of moisture results in gradual expansion of the bricks, and most of this expansion is irreversible.
It is commonly accepted that the initial expansion of the bricks stops after the first three months. However, if these bricks are used in brickwork before the initial expansion has ceased, irreversible expansion can occur which may lead to cracking in the masonry. Therefore, it is important to wait until the initial expansion has ceased before using such bricks in brickwork to avoid any potential damage.