NCM 2007 1

Setting and Hardening concrete

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Setting

Setting time is defined as the transition from a fluid state to a plastic state.

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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, for example. 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 paste.

Setting (cont’d)

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Setting (cont’d)

It has been found that this transition can occur in less than one hour or could take up to 24 hours.

Initial set of cement paste is defined as the time when the paste has gained enough rigidity to no longer be in a fluid state.

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Setting (cont’d)

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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Dormant period

Setting

Addition of water

Plastic and workable paste

Initial set

Stiff and unworkable paste

Final set

Rigid solid gaining strength with time

Initial setting time min 45

minute

Final setting time max 10h.

Schematic description of setting and hardening of a cement paste.

Hardening

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Hardening

The 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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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 (cont’d)

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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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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 (cont’d)

NCM 2007 11

HOT WEATHER CONCRETING

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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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The ideal temperature for laying concrete is around 60°F (15.6ºC), and anything above 80°(26.7ºC)and 90°F (32.2ºC) 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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Listed below are the potential problems Listed below are the potential problems for freshly mixed or plastic concrete:for freshly mixed or plastic concrete:

Increased demand for water Faster rate of setting, increased difficulty with

handling, transporting, compacting, finishing and a greater risk of cold joints

Rate of slump loss increased Increased jobsite water additions Increased plastic shrinkage cracking Entrained air content control increased

difficulty

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Listed below are the potential problems Listed below are the potential problems for hardened concrete: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 placement

Increased incidence of drying shrinkage and differential thermal cracking

Decreased durability Variability of surface aesthetics Increased permeability

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Methods which will help control mix Methods which will help control mix temperature and heat are:temperature and heat are:

Using crushed ice in the water added to the mix Liquid nitrogen Sprinkling aggregates stockpiles Storing aggregates in a cooled place Incorporation of fly ash Moisten sub-grade before placement Incorporation of retarding admixtures Applying evaporation retardant to the concrete

surface to reduce evaporation rates at the surface

Cure the concrete immediately and thoroughly.

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Retarding admixture can help:

Reduce water requirement Decreased rate of set Improves workability Increases strengths both compressive and

flexural Reduce peak heat of hydration

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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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COLD WEATHER CONCRETING

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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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Cont’d

Cold 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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Listed below are a few common sense Listed below are a few common sense tips for concreting in cold weather.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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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 Listed below are a few common sense tips for concreting in cold weather. (cont’d)concreting in cold weather. (cont’d)

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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 times leave 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 ice

Listed below are a few common sense tips for Listed below are a few common sense tips for concreting in cold weather. (cont’d)concreting in cold weather. (cont’d)