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In devising a suitable method to seal up cracks detected on concrete surface, it is of paramount importance to determine if further movement would be expected for the cracks. If the crack is not expected to move further, it is sufficient to brush cement grout into it. For wider cracks, other materials like latex-cement mixture may be considered for sealing the crack.
The rate of hydration of cement paste is related to the placing temperature of concrete. The rate of heat production is given by the empirical Rastrup function:
H= Hox 2r(T-T1)
Ho = Rate of heat production at a reference temperature
T = Temperature where rate of heat production H
T1 = Temperature where rate of heat production Ho
r = 0.084
There are two principle mechanisms in transferring shear forces across a reinforced concrete crack, namely, aggregate interlock and dowel action.
The aggregate interlock refers to the interaction between rough surfaces of the crack. The shear stiffness of aggregate interlock is influenced by the axial tensile stiffness of the reinforcement.
In many standards and code of practice of many countries, the allowable size of crack width is normally limited to less than 0.5mm for reinforced concrete structure to enhance the durability of concrete. The limitation of crack width can serve the aesthetic reason on one hand and to achieve durability requirement by avoiding possible corrosion of steel reinforcement on the other hand.
In the calculation of thermal movement, the following formula is used in
most codes:
wmax=sa(T1+T2)/2
where wmax= maximum crack width
s = maximum crack spacing
a = coefficient of thermal expansion of mature concrete
T1= fall in temperature between peak of hydration and ambient
temperature
T2= fall in temperature due to seasonal variation
For the purpose of defining the serviceability crack width limit state, the maximum design surface crack widths for the exposure conditions defined in BS8007 should be taken to be the following:
The maximum design surface crack widths for direct tension and flexure or restrained temperature and moisture effects are:
1) severe or very severe exposure: 0.2 mm;
2) critical aesthetic appearance: 0.1 mm.
To answer this question, one must fully understand the effect of formwork on the temperature of concreting structure. Without doubt, with better insulation of structure by timber formwork, the overall rise of temperature and hence the peak hydration temperature is also increased. However, for a well-insulated structure, the temperature gradient across concrete element is reduced. Therefore, the use of well-insulated formwork (like timber) increases the maximum temperature and reduces the temperature gradient across the structure at the same time. Hence, whether steel or timber formwork should be used to control thermal cracking is dependent on the restraints and the size of section.
Let’s take a circular column as an example to illustrate this.
When the temperature is rising, the inner concrete’s temperature is higher than outer concrete’s temperature and the inner concrete is expanding. This induces pressure to the outside and the induced compressive stress will result in formation of radial cracks near the surface of concrete.
The purpose of critical steel ratio is to control the cracking pattern by having concrete failing in tension first. If steel reinforcement yields first before the limit of concrete tensile strength is reached, then wide and few cracks would be formed. In the calculation of critical steel ratio, the thickness of the whole concrete section is adopted for analysis.
Construction joints are introduced during construction and they are not intended for movement to take place. If the concrete panel on each side of the construction joint has been designed to have maximum crack width of 0.2mm, theoretically this joint would also behave in the same way as its adjoining material.
