Curing Definition The process of prevention of loss of moisture from fresh concrete while maintaining satisfactory temperature regime (BS 8110) Fig: The process of curing by using sheets, watering and spraying chemicals

Curing Objectives Elements To stop the loss of water Maintain the temperature Elements Moisture Temperature Time

Curing Advantages of curing To avoid premature drying of concrete due to Solar radiation Wind Fig: Solar radiation affecting the fresh concrete

Curing Advantages of curing Avoid leaching out by rain or flowing water Fig: Leaching of concrete

Curing Advantages of curing Protection against rapid cooling in first few days Avoid high internal thermal/temperature gradient Fig: Thermal shock to concrete

Curing Advantages of curing Protection against low temperature or frost in cold regions Fig: Low temperature or snow

Curing Advantages of curing Surface zone development Strong Impermeable Crack free Durable The thickness of surface zone affected by curing is 20mm to 50mm(CIRIA 1997)

Curing Advantages of curing Keeping the concrete saturated So long time that Water filled holes/voids Filled by products of hydration Instead by air

Curing Rate of evaporation

Curing Curing Mechanics When RH > 95% Hydration is adequate Reduction in porosity adequate When 95% > RH > 80% Hydration is slow No reduction in porosity When RH < 80% No Hydration No reduction in porosity (Neville1995)

Curing Methods of Curing Form work retention Spraying with water Ponding Application of curing membrane Plastic sheets Tenting against dry winds

Curing Covering with wet sand Periodically wetted cotton or Hessian mat Water absorbant cover

Curing Formwork Retention Cost will increase Very good in cold weather Prevent the thermal cracking 4.Construction time may delay

Curing Impermeable covering Polyethylene sheet in contact with surface It should be suspended a few centimeter above Sides must be sealed Effective high relative humidity chamber must be maintained White sheets are preferable Surface spraying with water before to maintain any small loss of water

Curing Absorbent cover Wet sand and Hessian mat keep the concrete wet until wet Takes the moisture of concrete when dry Not recommended for large sections

Ponding and spraying Possible on flat surfaces Spraying requires Continues water supply Clean nozzles Wind direction and velocity Drainage way

Curing Ponding and spraying Risk of thermal shock ACI_308_92 The temperature of water should not be more than 11oC cooler than concrete temperature Avoid in below freezing temperatures Water should be same as drinking water

Curing membranes Natural / Synethic Resins Waxes Solvents Hand/ sprayed applied sealing compounds Applied in liquid form after free water disappear Natural / Synethic Resins Waxes Solvents

Curing Membrane mechanics Greatly reduce the evaporation When rate of evaporation> rate of bleed Wetting of surface is necessary They can be removed if finishing operation is necessary

Curing Duration It is a function of Plastic shrinkage Temperature control Durability Strength Size of structural element Type of concrete Environment condition Specification requirement

Curing Duration

Curing Duration

Curing Duration

Curing Duration

Maturity of Concrete Definition: It is the sum of product of temperature (above datum level -11oC at which hydration stops) and time over which this temperature occurs M= Sum (T.▲t) It is used to predict the rate of strength development

Strength Development of Concrete Effect of temperature Low temperature decreases early strength development High temperature at early age, Increases the early strength Decreases ultimate strength

Strength Development of Concrete Effect of Curing Strength development requires hydration Hydration requires water Loss of water due to evaporation affects hydration

Monitoring the strength development Standard Procedure Casting cubes or cylinders along with structural members Strength of sample is different from actual structural member Due to difference in maturity Difference in compaction Difference in curing

Monitoring the strength development Standard Procedure Significant temperature gradient across the section Upper surface zone is weak due to water migration Generally 10% higher strength in slab soffits as compared to top when tested by pull out test In flexural members upper zone carries compressive stresses

Monitoring the strength development Maturity measurement Cubes cured along with the structure Temperature matched curing Measurement by Coma probes Measurement by Windsor probe Break out test TNS test Pull out test or LOK test Rebound hammer Coring Tables of formwork striking times

Monitoring the strength development Maturity measurement Cubes cured along with the structure Cubes generally gives lower strength Cubes follow environment temperatures Concrete inside structural member is hot Strength inside is higher In large sections difference is bigger

Monitoring the strength development Maturity measurement Tables of formwork striking times They are conservative They are based on lowest rate of strength gain They can be un economical

Monitoring the strength development Temperature matched curing Procedure A sensor is put in freshly made concrete section A water filled tank having cubes, stirrer and heater And control system Control system detects temperature difference It heats the water to same temperature as in section Cubes have same maturity as section

Monitoring the strength development Maturity measurement Measurement by Coma probes