A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
7. What are the potential problems of using liquid nitrogen to cool
concrete? (FC2)
Based on the results of past research, there is minimal impact on the
properties and performance of concrete by liquid nitrogen (LN). The
addition of liquid nitrogen appears to decrease the slump value of fresh
concrete. However, the slump loss is not caused by liquid nitrogen but by
previous hot concrete mix, i.e. the slump of LN-cooled concrete is the
same of original hot concrete. Liquid nitrogen can also be observed to
extend the setting time of concrete.
The safety of workers is one of the major concerns when using liquid
nitrogen. The extremely low temperature of liquid nitrogen is dangerous to
workers as prolonged contact of liquid nitrogen with skin cause severe
burns and frostbite. Most concrete mixing drums may not be capable to
endure the thermal shock brought about by liquid nitrogen.
8. Should on-site addition of water to fresh concrete be allowed?
(FC2)
The addition of water to fresh concrete in truck mixer upon arrival at the
location of concrete is allowed only if some design mixing water is held
back during initial mixing stage. Addition of water in excess of design
mixing water would definitely cause an increase of water cement ratio
leading to a reduction of concrete strength.
The purpose of adding water to fresh concrete is to increase the
workability to facilitate placement of concrete. The use of water-reducing
agent or superplasticizer could help resolve the problem but extra
attention has to be paid on the segregation issue. In case there is some
buffer in the amount of water in fresh concrete so that addition of water
would not result in exceeding the designed water cement ratio, water
could be added based on the following rule of thumb: 5 liters of water per
m 3 of concrete gives about 25mm increase in slump.
9. What are the reasons in setting maximum and minimum time for
concreting successive lift in water-retaining structures?
Maximum time for concreting successive lifts is required for concrete
structures to reduce the potential differential strains. However, minimum
time is needed to allow for possible shrinkage and thermal contraction to
take place in the event of alternative bay construction before concreting to
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