Question

1- Sketch a typical atmospheric pressure steam curing cycle identifying the main steps and

requirements for each step.

2- What is meant by the characteristic strength of concrete?

3- Compressive strength specimens could be cured by standard curing method or on site

beside the concrete elements. What is meant by standard curing? Explain when each curing

4- Discuss the effect of curing, time and temperature, on compressive strength of concrete.

State the minimum moist curing period according to ACI 308.

method is used.

Answer 1

1)Please refer the sketch attached at the end for detailed answer

2)Charecteris strength Definition as per Code:

Charectristic strength is defined as the strength below which not more than 5% of the test results are expected to fail.This means for example,if the charecteristic strength of the concrete is 35N/mm2 and stress of 35N/mm2 is applied on the cube then the chance of failure of the cube is only 5%.

3)Cubes should always be cured at the standard curing method.This is because,the main intention if testing the cube is to identify whether the concrete has any issue in gaining the strength under the standard curing conditions.If the cubes are cured at the site,then we will not be able to identify whether the cubes failed because of the defect in concrete ir improper curing.Standard curing method is nothing but curing the cubes in the laboratory under the Temperature and water quality/parameters as mentioned in the code.

4)Concrete undergoes a chemical reaction to gain strength.During this process heat is produced and the main purpose of curing is to reduce this heat,so that the concrete gains the required strength.The temperature of the water should be less than the temperature of the heat produced due to heat of hydration.Curing should be done once the initial setting time of the concrete is completed and till the active chemical reaction undergoes within the concrete.If the curing is not done,then more heat will be produced within the concrete which in turn evaporates the water the main requirement  for chemical reaction .Hence the required compressive strength will not be gained.

Required curing period as per ACI 308 is 7 to 10 days

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