NCM *Setting and Hardening of Concrete

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NCM *IntroductionConcrete is made by the combination of cement, water, and aggregate of various sizes to make a workable slurry that has the consistency of a thick milk shake.The binding quality of Portland cement paste is due to the chemical reaction between the cement and water, called hydration. Portland cement is not a simple chemical compound, it is a mixture of many compounds. Four of these make up 90% or more of the weight of portland cement: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. In addition to these major compounds, several other play important roles in the hydration process.

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NCM *Chemical composition of cement

NamePercent by Weight Chemical FormulaTricalcium silicate50%3Ca0 SiO2Dicalcium silicate 25%2Ca0 SiO2Tricalcium aluminate10%3Ca0 Al2 O3Tetracalcium aluminoferrite10%4Ca0 Al2 Fe2 O3Gypsum5%CaSO4 H2O

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NCM *Different types of cement contain the same four major compounds, but in different proportions.When water is added to cement, the chemical reaction called hydration takes place and contributes to the final concrete product. The calcium silicates contribute most to the strength of concrete. Tricalcium silicates are responsible for most of the early strength (first seven days).The original dicalcium silicate hydrates, which form more slowly, contribute to the strength of concrete at later stages

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NCM *Even though the chemical reactions may be complete at the surface of the concrete, the chemical reactions at the interior of the concrete take much longer to complete. The strength of the concrete keeps growing as long as the chemical reactions continue.

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NCM *SettingSetting time is defined as the transition from a fluid state to a plastic state.

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NCM *The stiffening of the concrete after it has been placed. A concrete can be 'set' in that it is no longer fluid, but it may still be very weak; you may not be able to walk on it. Setting is due to early-stage calcium silicate hydrate formation. The terms 'initial set' and 'final set' are arbitrary definitions of early and later set; there are laboratory procedures for determining these using weighted needles penetrating into cement/concrete paste.

Setting (contd)

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NCM *Setting (contd)Initial setting time of cement paste is defined as the time when the paste has gained enough rigidity to no longer be in a fluid state.It has been found that this transition can occur in less than one hour or could take up to 24 hours.

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NCM *Setting (contd)The final setting time is when rigidity has increased to a point that the paste becomes a solid of very low strength.In general, cement exhibit initial set in 2 to 4 hours and final set in 5 to 8 hours. Final set is then followed by substantial increases in strength, referred to as the hardening stage.

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NCM *HardeningThe process of strength growth and may continue for weeks or months after the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement hydrates.

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NCM *Hardening time is when concrete has a sufficient bearing capacity to support construction loads. This hardening of concrete may occur in a few hours, or could take up to 2-3 weeks.

Hardening (contd)

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NCM *Factors that affect the setting and hardening time include, but are not limited to Type and amount of cementitious material (cement and fly ash); Water content; Presence of admixtures (accelerators, air entrainers, and water reducers);

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NCM *Volume of fill; Properties of surrounding soil (permeability and degree of saturation); Ambient temperature; and Curing conditions.

Factors that affect the setting and hardening time include, but are not limited to (contd)

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NCM *HOT WEATHER CONCRETING

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NCM *High temperature during construction means that extra precautions need to be taken to ensure successful concrete pours. Hot weather, especially when combined with low humidity and high winds, can make pouring difficult

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NCM *The ideal temperature for laying concrete is around 60F (15.6C), and anything above 80(26.7C)and 90F (32.2C) is considered hot weather concreting. When the aggregate and the water reach a higher temperature the concrete does not mix as well, and shrinkage of the concrete is greatly increased.

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NCM *Listed below are the potential problems for freshly mixed or plastic concrete:Increased demand for waterFaster rate of setting, increased difficulty with handling, transporting, compacting, finishing and a greater risk of cold jointsRate of slump loss increasedIncreased jobsite water additionsIncreased plastic shrinkage cracking

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NCM *Listed below are the potential problems for hardened concrete:Decreased compressive strength at 28 days and later (sometimes up to 30%) resulting from higher water demand and/or higher temperature levels at time of placementIncreased incidence of drying shrinkage and differential thermal crackingDecreased durabilityVariability of surface aestheticsIncreased permeability

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NCM *Methods which will help control mix temperature and heat are:Using crushed ice in the water added to the mixLiquid nitrogen Sprinkling aggregates stockpilesStoring aggregates in a cooled placeIncorporation of fly ashMoisten sub-grade before placementIncorporation of retarding admixturesApplying evaporation retardant to the concrete surface to reduce evaporation rates at the surfaceCure the concrete immediately and thoroughly.

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NCM *Retarding admixture can help:Reduce water requirementDecreased rate of setImproves workabilityIncreases strengths both compressive and flexuralReduce peak heat of hydration

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NCM *Curing and protection

After placement and finishing operations are completed, procedures must continue to protect the concrete from high temperatures, direct sun, low humidity, and winds.

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NCM *COLD WEATHER CONCRETING

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NCM *Concreting in cold weather requires special knowledge and skills to ensure accelerated hardening and high quality in plain- and reinforced-concrete structures. Cold Weather Concreting familiarizes concrete specialists with the characteristic features of concrete in cold weather, including the effects of frost, methods for hardening in subzero temperatures, and other challenges in cold-weather concreting

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NCM *ContdCold weather precautions should be taken when the temperature drops below 4oC. Concrete can be placed safely throughout the winter months if certain precautions are taken. The concrete mixture and its temperature should be adapted to the construction procedure and the ambient weather conditions.

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NCM *Listed below are a few common sense tips for concreting in cold weather.Discuss with your ready mix producer the possibilities of using hot water, increasing the cement content, or adding an accelerating admixture. Order air entrainment if the concrete is exposed to freeze-thaw conditions during or after construction. Plan ahead. Have insulation and heaters ready and available when temperatures are below freezing. Do not place concrete on frozen sub-grade; thaw the sub-grade with steam or protect with insulation.

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NCM *Adding approximately a gallon of water per cubic yard of concrete can increase set time up to one-half hour; adding approximately two gallons can increase set time up to an hour. Provide triple insulation thickness at corners and edges of walls and slabs. Concrete gains very little strength at low temperatures. Fresh concrete must be protected against the disruptive effects of freezing until the concrete attains a compressive strength of about 500 psi. Listed below are a few common sense tips for concreting in cold weather. (contd)

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NCM *Do not expose concrete surfaces to a sudden temperature drop; gradually reduce insulation or enclosure temperature to control concrete cooling (not more than a 50 degree Fahrenheit drop in 24 hours). Avoid using unvented heaters, carbon dioxide from the heaters can cause soft, dusting floors. Keep heaters attended at all timesleave forms in place as long as possible. Even within heated enclosures, forms serve to distribute heat more evenly and help prevent drying and local overheating. All formwork must be free from snow and iceListed below are a few common sense tips for concreting in cold weather. (contd)

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NCM *Assignment 1Describe how the cold weather will affect concrete on these particular conditions:Delay in setting and hardeningFreezing of concrete at early ageFreezing and thawinglow temperature but above 0CLow temperature at the time of concreting but below 0C after concretingTemperature below 0C at the time of concreting and during hardening periodHardened concrete subjected to alternate freezing and thawing

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