During Cold weather, cold weather concreting the rate of hardening and setting of concrete is very much retarded when the temperature falls below 21 deg. C. At about 10 deg. C. the action of setting slows down to about one-half of what it is at 21 deg. C. In addition to the slowing down or stopping of hydration and hardening, there is also the danger of disintegration of unset concrete due to the disruptive effect set up by the expansion of the mixing water as it freezes cold weather concreting.
During cold weather concreting shall be abandoned when the temperature falls below 4.5 deg. C, The most convenient method is to heat the mixing water and, for very low temperatures to heat the aggregate as well. Heat the mixing water to 66 deg. C. (150 deg. F.). On no account shall the hot water be added to cement alone. Aggregates may be heated to 21 deg. C. Mixer drum may also be warmed. The cement must not be heated when used for cold weather concreting. Fresh concrete must not be allowed to freeze. If concrete is frozen, setting and hardening cease. Avoid the use of frozen aggregate. The concrete placed shall be protected against frost by suitable covering. Cold weather concreting damaged by frost shall be removed and work redone. n increase of cement content of the mix by about 20 to 25 percent, use of rapid hardening cement with an admixture of calcium chloride or, high alumina cement are usually recommended. With high alumina Cement concreting can proceed without any further precautions provided that the temperature is not at freezing point or below and the materials are not frozen.
“Accelerators” are used in cold weather concreting to increase the rate of hardening and thereby reduce the likelihood of failure. They accelerate the hydration of the cement and increase the rate of evolution of heat; thus the temperature of the concrete is raised and the freezing point of the mixing water is lowered, enabling concreting to be carried out when the air temperature is near or slightly below freezing point.
As far as practicable, the use of accelerators or admixtures should be avoided. Calcium chloride is the most commonly used material for accelerating hardening of the concrete and is perhaps the most reliable, which may be used up to 2 percent max : (prefer 1.5 percent) of the weight of cement. Quantity in excess of this proportion is harmful.
In no circumstances should this chemical be added to high alumina cement?
Calcium chloride is a white deliquescent and hygroscopic salt commercially available at low cost in flakes or granular form and delivered in moisture-proof bags or airtight drums, and should be stored in a dry place. It is dissolved in the mixing water to which cement is added afterward. Calcium chloride should not be placed in contact with water or mixed dry with aggregate. Calcium chloride shall not be used where reinforcement is provided in the concrete.
The use of calcium chloride approximately halves the setting time; the concrete must be placed in position and finished with a minimum of delay because of the rapid setting.
Most concrete has a w/c of 0.45 to 0.60, which means there is substantial excess water that will not react with cement. Eventually, the excess water evaporates, leaving little pores in their place. Environmental water can later fill these voids. During freeze-thaw cycles, the water occupying those pores expands and creates stresses which lead to tiny cracks. These cracks allow more water into the concrete and the cracks enlarge. Eventually, the concrete spalls – chunks break off. The failure of reinforced concrete is most often due to this cycle, which is accelerated by moisture reaching the reinforcing steel. Steel expands when it rusts, and these forces create even more cracks, letting in more water
When portland cement is mixed with water, heat is liberated. This heat is called the heat of hydration, the result of the exothermic chemical reaction between cement and water. The heat generated by the cement’s hydration raises the temperature of concrete. During normal concrete construction, the heat is dissipated into the soil or the air and resulting temperature changes within the structure are not significant. However, in some situations, particularly in massive structures, such as dams, mat foundations, or any element more than about a meter or yard thick, the heat can not be readily released. The mass concrete may then attain high internal temperatures, especially during hot weather construction, or if high cement contents are used demonstrates the effect of element size on concrete temperature with time due to the heat of hydration. Temperature rises of 55°C (100°F) have been observed with high cement content mixes.
(2) These temperature rises cause expansion while the concrete is hardening. If the temperature rise is significantly high and the concrete undergoes nonuniform or rapid cooling, stresses due to a thermal contraction in conjunction with structural restraint can result in cracking before or after the concrete eventually cools to the surrounding temperature. Contractors often insulate massive elements to control temperature changes. As a rule of thumb, the maximum temperature differential between the interior and exterior concrete should not exceed 20°C (36°F) to avoid crack development.
(3) The potential for thermal cracking is dependent on the concrete’s tensile strength, coefficient of thermal expansion, temperature difference within the concrete, and restraint on the member When comparing concretes of equal cement content but different water-cement ratios, mixes with higher water-cement ratios have more water and microstructural space available for hydration of the cement (more of the cement hydrates and it hydrates at a faster rate), resulting in an increased rate of heat development. The increase in heat of hydration at 7 days resulting from an increase in water-cement ratio from 0.4 to 0.6 is about 11% for Type I cement. The effect is minimal for moderate- and low-heat cement.
(4) The water-cement ratio effect is minor compared to the effect of cement content. However, a lower water-cement ratio in concrete achieved by increasing the cement content results in greater heat generation. Higher temperatures greatly accelerate the rate of hydration and the rate of heat liberation at early ages (less than 7 days). Chemical admixtures that accelerate hydration also accelerate heat liberation and admixtures that retard hydration delay heat development. Mineral admixtures, such as fly ash, can significantly reduce the rate and amount of heat development.
cold weather concreting