Weathering Cracks in Construction: Causes and Prevention

Weathering cracks in construction are the cracks that occur in concrete or other building materials due to long-term exposure to weather conditions such as temperature changes, freeze-thaw cycles, and moisture. These cracks may appear on the surface or internally and can weaken the structure over time. They are a common issue in many buildings and structures, especially in areas with extreme weather conditions.

Cracking due to weathering refers to the damage that occurs to concrete structures as a result of environmental factors such as freezing and thawing, wetting and drying, and heating and cooling.

Freezing and Thawing (FNT):

When the free water present in the concrete freezes, it expands and exerts pressure on the surrounding concrete, leading to cracking. This is known as freezing and thawing (FNT) damage.

Heating and Cooling (HNC):

High temperatures can also damage concrete, causing it to lose strength and leading to general spalling and flaking of the surface. If the aggregates used in the concrete have a high coefficient of thermal expansion, the damage will be greater.

Wetting and Drying (WND):

When concrete undergoes volume changes due to repeated wetting and drying, excessive stresses may develop leading to the formation of cracks and disintegration of the concrete. This is known as wetting and drying (WND) damage.

The Effects of Freezing and Thawing on Weathering Cracks

Freezing and thawing can cause significant damage to concrete structures. The process involves the following:

During the freezing phase, capillary water present in the concrete comes out to the surface and forms ice lenses parallel to the surface.
The formation of ice lenses exerts pressure on the concrete, which can result in cracking.
If the moisture within the capillaries of the concrete freezes, it can develop water pressure and cause cracking.
In addition, if the aggregates used are saturated beyond the critical degree of saturation, the expansion of absorbed water during freezing may also cause cracking of the concrete.

Impact of Wetting and Drying and Heating and Cooling on Weathering Cracks

Excessive volume changes due to these effects can cause cracks and lead to the deterioration of concrete. Additionally, heating and cooling can cause damage to concrete in the following ways:

Fire and frost-actions: These can damage concrete, resulting in spalling and flaking from the surface.
Loss of strength: With temperature increases above about 300°C, concrete gradually loses strength.
Aggregates: If the aggregates used in the concrete have a high coefficient of thermal expansion, they can cause greater damage.

Controlling Weathering Cracks Caused by Freezing and Thawing

Use the lowest practical water-cement ratio and total water content.
Ensure adequate air entrainment to control freezing damage.
Use durable aggregate to minimize the effect of freezing.
Adequately cure concrete prior to exposure to freezing conditions.
Properly design concrete structures with consideration of important thermal properties such as thermal conductivity, thermal diffusivity, specific heat, and coefficient of thermal expansion.

Understanding Thermal Conductivity in Construction

Thermal conductivity is the measure of a material’s ability to conduct heat through it.
It is measured in joules per second per square meter of area of the body, when the temperature difference is 1°C per meter thickness of the body.
The conductivity of concrete depends on various factors including the type of aggregate, moisture content, density, and temperature of concrete.
When the concrete is saturated, the value of thermal conductivity ranges from about 1.4 to 3.4 J/m2·S·°C/m.
The wet density of concrete made with different aggregates varies from 2240 to 1590, and the higher the density of concrete, the higher the value of concrete thermal conductivity.

What is Thermal Diffusivity?

Thermal diffusivity is a measure of the ability of a material to conduct heat and distribute it through its mass. It is a property that is important in construction, as it affects the ability of materials to regulate temperature and resist damage from temperature fluctuations. Thermal diffusivity is measured in square meters per second (m²/s) and is dependent on the material’s thermal conductivity, density, and specific heat capacity. Materials with high thermal diffusivity can distribute heat more quickly and uniformly, while materials with low thermal diffusivity are slower to distribute heat and may experience greater temperature fluctuations.

Coefficient of Thermal Expansion in Concrete

The coefficient of thermal expansion is the change in unit length per degree of temperature and depends on the mix proportion in concrete. For hydrated cement paste, the coefficient of thermal expansion varies between 11 x 10^-6 to 20 x 10^-6 per °C, while for aggregates, it ranges between 5 x 10^-6 and 12 x 10^-6 per °C. The type and content of aggregate used in concrete influence the value of the coefficient of thermal expansion, with limestone and Gabbros having lower values and gravel and Quartzite having higher values. The average value for concrete can be assumed as 10 x 10^-6 per °C.

Example Calculation

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Assuming a coefficient of thermal expansion for concrete of 10 x 10^-6 per °C and a diurnal temperature variation of 25°C, the thermal shrinkage strain would be 250 x10^-6. According to Lowe, cracks develop in concrete at a differential strain of 200 x10^-6, which means that the differential strain, in this case, is 250 x 10^-6, causing a high degree of micro-cracking in the concrete.

Using the standard formula for modulus of elasticity, which is stress/strain, with a modulus of elasticity of 3.5 x 104 N/mm², and a differential strain of 250 x 10^-6, the resulting tensile stress would be 8.75 N/mm².

According to IS 456-2000, the modulus of elasticity of concrete is given by the relation:

Modulus of Elasticity = 5000 x √ fck N/mm², which in this case is 3.5 x 104 N/mm².

The flexural strength can be calculated using the formula 0.7 x √ fck, which results in 4.94 N/mm².

Since the tensile strength of concrete is 3.5 N/mm², a tensile stress of 8.75 N/mm² is more than double the tensile strength of concrete, leading to a high degree of micro-cracks in the concrete.