Concrete gains roughly 70 per cent of its 28-day strength by 7 days. If the 28-day strength is 40 megapascals, what is the approximate 7-day figure?
Answer: A
Seventy per cent of 40 is 28 megapascals. Proportions of this kind are used on site to judge whether a pour is developing strength as expected before the full result is available. Site decisions often cannot wait the full twenty-eight days.
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Strength gain in concrete continues after 28 days because
Answer: B
Belite in particular reacts slowly, and blended cements continue longer still through the pozzolanic reaction. The 28-day figure is a convention for specification rather than the final strength of the material. Long-term gain is one argument in favour of blended cements.
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State whether the following is true or false.
The 28-day strength of concrete is its maximum possible strength.
Answer: C
Strength continues to rise for months and sometimes years, particularly with blended cements. Twenty-eight days is a convenient point for specification rather than any limit of the material. Later strengths are sometimes specified where the loading allows.
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Temperature affects the rate of hydration such that concrete cured warm
Answer: D
Rapid early hydration produces a coarser and less uniform gel, so the ultimate strength can be below that of concrete cured cool. It is one of the few cases where going faster leaves you worse off in the end. Steam-cured precast units are designed with this trade in mind.
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Assertion (A): Concrete cured at a high temperature may finish weaker than the same mix cured cool.
Reason (R): Rapid early hydration produces a coarser and less uniformly distributed gel.
Answer: A
Both are true and the reason gives the mechanism: the product forms too quickly to distribute itself evenly through the space available, leaving a microstructure that carries load less efficiently. Steam curing trades ultimate strength for early turnaround.
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Maturity, as used in concrete practice, combines
Answer: B
Correct answer: temperature and time to estimate strength gain
Maturity treats strength development as a function of both how long the concrete has been curing and how warm it was, since hydration accelerates with temperature. Combining the two into a single index - degree-hours above a datum - allows the in-place strength to be estimated from thermocouples rather than waiting on cubes, which is how formwork striking and post-tensioning times are decided on site.
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State whether the following is true or false.
Maturity methods allow in-place strength to be estimated without testing cubes.
Answer: C
Recording temperature against time in the actual member gives an estimate of how far hydration has gone. Calibration against cubes of the same mix is still required before the method can be relied upon. The relationship differs for every mix and must be established first.
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A concrete is held at 20 degrees for 3 days and then 30 degrees for 2 days. Using a datum of minus 10 degrees, what is its maturity in degree-days?
Answer: D
Three days at 30 above datum gives 90, and two days at 40 above datum gives 80, making 170 degree-days. Subtracting the datum before multiplying is the step that is most often forgotten. The datum represents the temperature below which hydration effectively stops.
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Autogenous shrinkage differs from drying shrinkage in that it occurs
Answer: A
The hydration products occupy less volume than the cement and water they replace, so the paste contracts even when sealed. It is most significant in low water to cement ratio mixes, where there is little free water to buffer it.
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State whether the following is true or false.
Autogenous shrinkage is most pronounced in high water to cement ratio concrete.
Answer: B
It is most pronounced at low ratios, where there is little free water and self-desiccation sets in as hydration proceeds. High-strength concrete is therefore more prone to it than ordinary mixes. Internal curing with saturated lightweight aggregate can offset it.
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