Mass Concrete and Materials used for mass Concrete
Mass Concrete
Mass Concrete can be defined as 'Any volume of concrete with dimensions large enough to require measures to be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimum cracking'.The mass concreting technique is used for a massive structure such as dams and large bridge piers and foundation. Mass cocnrete is basically concrete with a higher proportion of coarse aggregate and a lesser proportion of cement, Large size aggregates are preferred for mass cocnrete. Strength, economy, uniformity, and all other factors considered for normal concrete should be taken into account for mass concrete also. Mass cocnrete should be properly designed, placed and cured to obtain a durable structured with more economy, In mass concrete work, the major problem is temperature shrinkage.
Concrete intended for large structures must have excellent control of early setting in order to prevent shrinkage, cracking, and the problems associated with placement and workability of concrete.
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| MASS CONCRETE |
Materials used for mass Concrete
- Portland cement with low heat of hydration
- Aggregate
- Pozzolanas
- Mixing Water
- Admixture
- Coolants
Portland Cement With Low Heat Of Hydration
The minimum amount of portland cement should be used to achieve the desired strength for both heats of hydration and economy.Pozzolanas
The addition of pozzolanas such as fly ash assists in reducing the heat of hydration, workability, and delayed strength gain.Aggregate
Fine and coarse aggregate should meet the IS requirement of grading. In the case of structural cocnrete for large beam and column, 20-30 mm aggregate is suitable. For dams, 40 mm aggregate can be used. The fine aggregate to total aggregate ratio should be low.Mixing Water
Mixing water should be very low to give low slump(0-50 mm)Admixture
Water reducing and retarding admixture is very useful. Mass cocnrete to be pumped should be provided with high range water reducing and retarding admixture to provide of 12-18 mm after addition of superplasticizers.Coolants
The Temperature of the mass concrete dam should be controlled between 5C and 20C in hot weather conditions. Addition of finely chopped ice instead of water will help. Aggregate should be kept damp and under shade. Steel forms should be sprayed with cold water.Temperature control during the process of mass concrete
Temperature control of mass concrete
The Temperature control in mass cocnrete can be achieved in two ways- Control of the maximum temperature of the concrete
- Artificial cooling of cocnrete to its final stable temperature. Both these techniques are intended to avoid build-up of the thermal build-up of thermal stresses.
In general, serious volume change stresses can be avoided by controlling the temperature drop from maximum to final.
The time duration for initial and fine cooling and the rate of temperature drop are largely dependent upon the diffusivity of concrete. Diffusivity provides a method for comparing the temperature rise within a black by taking into consideration the losses of heat to the dies and the exposed top of a lift. A measure of the diffusivity of cocnrete is necessary for use in the past-cooling measure of the diffusivity of concrete. by scheduling construction.
The temperature rise in cocnrete mass can be controlled by pre-cooling the concrete materials, limiting the amount of heat liberated, and immediate cooling after placement. Pre-cooling the concrete material can lower the maximum temperature but will not substantially change the amount of temperature rise associated with cement hydration.
Cooling the coarse and fine aggregate is of greatest benefit since these materials comprise the bulk of concrete. For maximum benefits, the stock of aggregate will considerably lower the temperature. Immersion of sand in water is not helpful because of the subsequent moisture control problem while batching,.
In Some cases, crushed ice has been substituted for part of the mix water to lower the batch temperature.
Properties Of Mass Concrete
- Workability
- Durability
- Watertightness
- Strength
Workability
Workability is that property of concrete which determines the amount of useful internal work necessary to produce full compaction. workability should be optimum for mass concrete. Uniformity of workability is essential in mass concrete works because the economy generally requires low cement content.Durability
The ability of concrete to resist weathering action, chemical attack, and abrasion are known as durability. It depends on mix design, workmanship, placing and curing. Concrete with a low water-cement ratio correctly consolidated and properly cured provides durable concrete. The durability of concrete is dependent on the properties of materials. Chemical resistance of concrete is improved by careful selection of materials. Weathering durability can be improved further by entrainment of a minute are bubbled into concrete.Watertightness
Watertightness is an important property of concrete in hydraulic structures such as dams, etc. Concrete should be impermeable and this can be achieved by using good quality aggregate, low water-cement ratio, good consolidation during placing, and proper curing Lean mixes used in mass cocnrete for dams, pozzolanas can be added to the concrete mix to improve watertightness.
`Strength
The strength of concrete is defined as the resistance that concrete provides against load so as to avoid failure. It depends on the water-cement ratio, quality of aggregates, compaction, curing, etc.Sometimes it is economical to add pozzolana or use portland pozzolana cement instead of ordinary cement concrete. They provide ultimate strength and water tightness of concrete. Pozzolanas also increase the plasticity and workability of concrete.