Building materials such as concrete, mortar, bricks, wood etc are porous in nature and expand by absorbing moisture from atmosphere and shrink on drying. These changes in building materials are cyclic in nature and are caused during moisture changes.
But building materials undergo irreversible changes due to change in moisture content during its initial conditions. These initial changes cause shrinkage or expansion of materials. For example, irreversible movement in materials are shrinkage of cement and lime based materials on initial drying i.e. initial shrinkage/plastic shrinkage and expansion of burnt clay bricks and other clay products on removal from kilns i.e. initial expansion.
Initial Shrinkage in Concrete and Masonry:
Initial shrinkage of building materials which is partly irreversible, occurs in all building materials that are cement / lime based such as concrete, mortar, masonry units, masonry and plaster etc. This initial shrinkage is one of the main causes of cracking in building structure.
Initial shrinkage as the name suggests occurs only once in the lifetime of concrete and mortar, i.e. during the time of construction when the moisture dries out during setting process. Initial shrinkage is most significant cause for the cracks in structures.
The effect of initial shrinkage in concrete and mortar depends on following factors:
- a) Cement content – Shrinkage of concrete and mortar increases with richness of mix.
- b) Water content – Increase in water quantity used in the mix increases the shrinkage.
- c) Maximum size, grading and quality of aggregate –With increase in the maximum size of aggregate with good grading, the water-cement ratio decreases for the same workability requirement of concrete. The use of less water reduces the initial shrinkage of concrete due to reduction in porosity.
For example, the shrinkage of cement sand mortar is 2 – 3 times that of cement concrete using 20mm maximum size of aggregate and 3 – 4 times that of cement concrete using 40mm maximum size of aggregate.
- d) Curing of concrete and masonry – Proper curing from the start of initial setting to at least 7 to 10 days helps in reducing initial shrinkages. The moisture provided through curing helps concrete and masonry to expand and thus, when they dry up, the final shrinkage is less.
- e) Surface area of aggregates -Surface area of concrete increases with increase in fine aggregates and this requires large water quantity for the required workability. With increase in water quantity, the shrinkage of concrete and masonry increases when they dry up.
- f) Chemical composition of cement – Shrinkage is less for the cement having greater proportion of tri-calcium silicate and lower proportion of alkalis i.e. rapid hardening cement has greater shrinkage than ordinary port-land cement.
- g) Temperature of fresh concrete and relative humidity of surroundings – With reduction in the ambient temperature the requirement of water for the same slump/workability is reduced with subsequent reduction in
Concreting done in mild winter have much less cracking tendency than the concreting done in hot summer months.
In cement concrete 1/3rd of the shrinkage take place in the first 10 days, ½ within one month and remaining ½ within a year time. Therefore, shrinkage cracks in concrete continues to occur and widens up to a year period.
Plastic shrinkage of concrete:
Cracks appearing on the surface of concrete before setting of concrete is due to plastic shrinkage of concrete. The cause of shrinkage cracks in concrete is due to settlement of heavy aggregates at the bottom of concrete and rise of water to the surface. This process continues till the concrete has set and it is called as bleeding of concrete.
As long as the rate of evaporation is lower than the rate of bleeding, there is a continuous layer of water at the surface known as “water sheen”, and shrinkage does not occur. When the concrete surface looses water faster than the bleeding action bring it to the top, shrinkage of top layer takes place, and since the concrete in plastic state can’t resist any tension, cracks develops on the surface. These cracks are common in slabs.
The extent of plastic shrinkage in concrete depends on:
- Temperature of concrete,
- Exposure to the heat from sun radiation,
- Relative humidity of ambient air and velocity of wind.
Plastic settlement cracks
Plastic settlement cracks occurs on concrete surface due to settlement of large aggregates. In the case of reinforced concrete, sometimes the settlement of aggregates is obstructed by reinforcement. These obstructions break the back of concrete above them forming the voids under their belly.
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.
Initial expansion of concrete:
When the clay bricks are fired during manufacturing, due to high temperature not only the intermolecular water but also water that forms a part of molecular structure of clay is driven out. After burning, as the temperature of the bricks falls down, the moisture hungry bricks starts absorbing moisture from the environment and undergoes gradual expansion, bulk of this expansion is irreversible.
For the practical purpose it is considered that this initial expansion ceases after first three months.
Use of such bricks before cessation of initial expansion in brickwork, will cause irreversible expansion and may lead to cracking in masonry.