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Hi Mass concrete is one of challenges with rate of pour as well as temperature control of concrete. below parameters govern the Temperature control from a Concrete supplier point of view :
1) ENsure that Design has blended cementatious content. For example a mix with Fly Ash can reduce the heat of hydration to a very good extent and also helps in permeability of concreting through pumps.
2) Ensure that the water used during manufacture of concreting is chilled water (if ambient temp more than 32). Ideally the chilled water temperature shall not exceed 4 degrees at discharge point.
3) Ensure at site that no trucks are kept detained for long time due to unnecessary requirement.
4) Moisture control of aggregates in shaded aggregate bins at Concrete yard helps in reducing the TC as well.
5) Insulation of pipelines for chiller/water tanks help loosing the loss of temp control in chillers/water tanks.
Once the above features in concrete yard are taken care, there are some site level controls like using thermocol for thermal energy on concrete, curing with cold water, planning the pour maximum at night time if possible etc.
There are two source of heat- one from hot surroundings and another from hydration reaction of concrete.
Measures for surrounding heat are:
- cast at night or early in the morning
- use insulating materials (semi hard rubber) under the formwork
- shade the casting with offset to avoid direct sunlight and hot gust of wind
Measures for hydration heat are:
- design a concrete mix that is helpful with hot weather
- A retarder or admixture or specialized material (some volcanic material has such properties or artificially designed) to control hydration reaction or heat generation.
- use cold water, ice or may be dry-ice (I don't have much idea about dry-ice)
- cast in multi-layer, allow to cool layer by layer otherwise curing by flooding will not work.
- moist sand, moist thick blanket can also be used for curing, special material(one side reflective material and other side water soaking soft porous material sandwiched together with glue) can also be used.
I think curing concrete after casting is very important for preventing cracking
Low heat material
Low-heat generating concrete mixtures are always a wise choice for mass concrete to minimize potential thermal problems. Low-heat generating concrete mixes use the maximum allowable level of low-heat pozzolans— such as Class F fly ash or slag—as cement replacements, and the minimum amount of total cementitious materials that achieves the project requirements. Class F fly ash generates about half as much heat as the cement that it replaces and is often used at a replacement rate of 15 to 25%. Ground granulated blast-furnace slag is often used at a replacement rate of 65 to 80% to reduce heat. The reduction in heat generation achieved depends on the concrete temperature, and should be evaluated on a case-by-case basis
Pre-cooling of Concrete
The concrete temperature at the time of placement has a great impact on the maximum concrete temperature. Typically, for every 1 F (0.6 C) reduction or increase in the initial concrete temperature, the maximum concrete temperature is changed by approximately 1 F (0.6 C). As an example, to reduce the maximum concrete temperature by approximately 10 F (6 C), the concrete temperature at the time of placement should generally be reduced by 10 F (6 C). Methods to precool concrete include shading and sprinkling of aggregate piles (as appropriate), use of chilled mix water, and replacement of mix water by ice. Efforts to cool aggregates have the most pronounced effects on the concrete temperature because they represent 70 to 85% of the weight of the concrete. Liquid nitrogen can also be used to precool concrete or concrete constituents. This option can significantly increase the cost of concrete; however, it has been used to successfully precool concrete to 34 F (1 C) for highly specialized mass concrete placements.
Post Cooling of concrete
Cooling pipes in mass concrete are sometimes used to reduce maximum concrete temperatures and to quickly reduce interior temperatures. This method can have high initial and operating costs, but benefits can often outweigh these costs if cooling pipe size, spacing, and temperatures are optimised properly
Surface Tension
Insulation or insulated formwork is often used to warm the concrete surface and reduce the temperature difference, which in turn minimizes the potential for thermal cracking. For most mass pours, surface insulation does not appreciably increase the maximum concrete temperature, but it can significantly decrease the rate of cooling. Insulation is inexpensive, but resulting delays from the reduced cooling rate can be costly. Insulation often has to remain in place for several weeks or longer. Removing it too soon can cause the surface to cool quickly and crack. Many types of insulation materials are available, and insulation levels can be optimised to meet required temperature differences and maximise the rate of cooling.
Controlling temperature in mass concrete sometime makes drastic changes in placed concrete so we have to consider at least following parameters before ordering concrete for site pouring.
I hope the above shall do the needful of your interest.
