**6. Internal frost damage mechanisms**

The durability of concrete refers to its ability to withstand deterioration due to harsh environmental conditions. These conditions can act alone or together and

#### *Concrete Performance in Cold Regions: Understanding Concrete's Resistance… DOI: http://dx.doi.org/10.5772/intechopen.99968*

include heating and cooling, freezing and thawing, wetting and drying, chemical attacks, and abrasion. Deterioration due to freezing/thawing causes D-cracking to occur [23]. Under harsh service conditions, the durability of reinforced concrete structures is related to concrete frost resistance. Frost resistance tests are accompanied by the accumulation of residual dilation deformations affected by temperature-humidity stresses, ice formation and other factors. It is affected directly by the porosity, which is an integral part of the concrete structure which is formed as a result of cement hydration [24]. If the porous material is so wet that the theory of hydraulic over-pressure governs the freezing phenomenon, pore water is squeezed into the larger air-filled pores and the external environment surrounding the sample and causes there an abrupt increase of relative humidity. However, if the pore system is not filled with pore water to the extent that hydraulic pressures are induced into the material then after the first freezing of the pore water an underpressure is formed in the pore system and the sample contracts [25], **Figure 7**.

D-cracking is a type of freeze/thaw damage in concrete pavements, it occurs due to the poor-quality coarse aggregates. By increasing the wet level of coarse aggregate reaching saturation, it becomes more susceptible to damage during freezing/ thawing cycles. Pressure builds up inside of the coarse aggregate as a result of water freezing inside its pores. If the pressure due to the expansion of the water within the pores of the coarse aggregate is higher than its internal strength, the coarse aggregate will crack. Then the deterioration process accelerated due to the increased potential water availability, where the interfacial transition zone between the coarse aggregate and cement matrix is supposed to be slightly thicker. Its porosity can be 100% and that increases and accelerates the freeze/thaw deterioration [23, 26, 27].

Sicat et al. [28], investigated experimentally the real-time deformational behavior of the interfacial transition zone in concrete during freeze/thaw cycles. They observed that due to its high porosity and weak strength of the interfacial transition zone, its deformation is higher than that of cement matrix and aggregate. The deformation has been experimentally proven by taking a closer look at the unbroken sections of the interfacial transition zone of the specimens by electron microscope after the freeze/thaw cycles in **Figure 8**. This deformation is clearer in the case of wet specimens as well as much significant in higher water to cement ratios.

It is observed that the deterioration due to freeze/thaw cycles increases by increasing the number of cycles, and sometimes a certain number of freeze/thaw cycles is required before the deterioration occurs, and that could be the effect

**Figure 8.**

*Interfacial transition zone after freeze/thaw cycles (from left to right 30%, 50%, and 70% w/c); (a) dry conditions, (b) saturated conditions [28].*

of fatigue. As each freeze/thaw cycle is added to cumulative internal deterioration this is similar to the normal mechanical fatigue with constant load cycles. However, it considers as low-cycle fatigue, where normally the freeze/thaw cycles are less than 300.

The reason behind the fatigue hypothesis is that one often notices a development of damage of the type shown in **Figure 9** when the material is tested in so-called "open" freeze/thaw, i.e. in a test where the specimen has access to water, during freezing and/or during thawing. In some cases -curves D, P, C in **Figure 9** damage increases progressively with the number of freeze/thaw cycles already from the first cycle, in other cases, all other curves a certain number of freeze/thaw cycles are needed to initiate.

The residual compressive strength of concrete after the effect of the freeze/thaw cycle is tested by Lu et al. [30] as shown in **Figure 10, Left**. It shows the changing of compressive strength after 0, 25, 50, and 75, freeze/thaw cycles respectively when the strain rate remained constant. They observed that the compressive strength decreased linearly with an increasing number of freeze/thaw cycles, where after

#### **Figure 9.**

*Reduction in E-modulus of cement mortar specimens repeatedly frozen in the air to* −*15°C and thawed at water +5°C, 2 cycles per day [29].*

*Concrete Performance in Cold Regions: Understanding Concrete's Resistance… DOI: http://dx.doi.org/10.5772/intechopen.99968*

**Figure 10.**

*Left: Compressive strength versus freeze/thaw cycles. Right: Compressive strength versus strain rates [30].*

#### **Figure 11.**

*Typical stress–strain curves for concrete at strain rate 1.0 × 10–2/s under different freeze/ thaw cycles [31].*

repeated freeze/thaw cycles, the cracks in the concrete will pass through each other, and their strength will gradually decrease, and finally even completely lost. However, the compressive strength increases with strain rate in a nearly linear progression as shown in **Figure 10, Right**. Under freeze/thaw cycles, crack propagation becomes faster with a higher stress rate, however, enhancement of concrete strength could happen due to the coarse aggregate that stops the crack extension.

They find a decrease in the residual compressive strength of concrete if the concrete was subjected to a fatigue compression loading prior to freeze/thaw cycling and this reduction was increased by increasing the fatigue cycles. Fatigue cycles cause microcracks and that increase in the irreversible tensile strain due to freeze/ thaw cycles afterwards. Han and Tian [31], presented the stress–strain relationships after different freeze/thaw cycles as plotted in **Figure 11**. They concluded that the concrete deformation experienced four deformation stages: compaction, elastic deformation, plastic deformation, and finally fracture. The peak load decreases as the freeze/thaw cycling increase where microcracks occurs as a result of freeze/ thaw cycling, its number affects the microcrack number and width.
