Mass Concrete

ByDr. Adel El Kordi

ProfessorStructural Engineering

Department Faculty of EngineeringAlexandria University

CIVE 570Spring 2012

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Definition

•Concrete in a massive structure, a beam, columns, pier, lock, or dam where its volume is of such magnitude as to require special means of coping with the generation of heat and subsequent volume change.•When dimensions are > 1m or 3ft, temperature riseshould be considered. Mass concrete columns and footings

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The temperature rise depends on:1.Concrete placing temperature.

2.Cement composition, fineness, and content.3.Aggregate content and CTE (Coeff. Pf Thermal Expansion).

4. Section thickness.5. Formwork type and time of removal.

6. Ambient conditions.7.Supplementary cementing materials.

Temperature Rise

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Temperature RiseTemperature Rise

Many factors are influential in the temperature rise of concrete:

1- The heat of hydration being the most important factor. The hydration of cement generates

heat (500J/g of CEM I). As a general rule a 5°C to 9°C

temperature rise per 45 kg of Portland cement can be expected from the heat of hydration (ACI

Committee 211 1997).

2- The amount of cement in the mix.

Many factors are influential in the temperature rise of concrete:

1- The heat of hydration being the most important factor. The hydration of cement generates

heat (500J/g of CEM I). As a general rule a 5°C to 9°C

temperature rise per 45 kg of Portland cement can be expected from the heat of hydration (ACI

Committee 211 1997).

2- The amount of cement in the mix. 5

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A part of mixing water was introduced into concrete as crushed ice so that the temperature of in-place fresh concrete was limited to 6°C. Generally, the lower the temperature of concrete when it passes from a plastic state to an elastic state, the less will be the tendency toward cracking.

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Cooling of water or using ice

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A severe case of thermal cracking in a concrete footing.

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To minimize thermal stresses:1. Aggregate with low coefficient of thermal expansion2. Cement with low C3S and low C3A3. Insulating forms4. Cast concrete at night / early morning5. Use ice instead of water6. Pre-cooling aggregate and cement7. Post cooling – embedded pipes8. Provide joints (for expansion and movement)9. Less amount of cement10. Use liquid nitrogen11. Use thin layers12. Fly ash and slag can reduce the heat of hydration

Thermal Stresses in Concrete

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Surface Insulation: The purpose of surface insulation is notto restrict the temperature rise, but to regulate the rate oftemperature drop so that the stress differences due to steeptemperature gradients between the concrete surface and theinterior are reduced.

Thermal Stresses in Concrete

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Temperature gradient

In the case of casing mass concrete, the internal temperature rises and drops slowly, while the surface

cools rapidly to ambient temperature.

In the case of casing mass concrete, the internal temperature rises and drops slowly, while the surface

cools rapidly to ambient temperature.

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Surface contraction due to cooling is restrained by the hotter interior concrete that doesn’t contract as rapidly as the surface. This restraint creates tensile stresses that can crack the surface of the concrete. The width and depth of

cracks depends upon the temperature differential, physical properties of the concrete, and the reinforcing steel.

Temperature gradient

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•The key to reducing thermal cracking is to reduce the peak temperature and to control the

temperature differential between the hot core and the cool surface.

•Reducing the peak temperature effects the time it takes for an element to reach a stable temperature

and effects the temperature differentials. Excessively high internal concrete temperatures

(>70°C) may also lead to DEF (Delayed Ettringite Formation) and durability issues.

•The key to reducing thermal cracking is to reduce the peak temperature and to control the

temperature differential between the hot core and the cool surface.

•Reducing the peak temperature effects the time it takes for an element to reach a stable temperature

and effects the temperature differentials. Excessively high internal concrete temperatures

(>70°C) may also lead to DEF (Delayed Ettringite Formation) and durability issues.

Temperature gradient

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• A temperature differential limit attempts to minimize excessive cracking due to differential volume change. A limit of 20°C is the industry room.• So, our target is that the core temperature should be ≤ 70°C and the temperature of top surface of the concrete should be ≥ 50°C, Thus the differential temperature will be < 20°C

• A temperature differential limit attempts to minimize excessive cracking due to differential volume change. A limit of 20°C is the industry room.• So, our target is that the core temperature should be ≤ 70°C and the temperature of top surface of the concrete should be ≥ 50°C, Thus the differential temperature will be < 20°C

Our Target

Concrete insulating a column.

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