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
Sawing cannot be implemented too late. When the volume of concrete is
reduced significantly owing to drying shrinkage or thermal contraction, it
induces tensile stress owing to the restraint of such reduction. When the
tensile stresses exceed the tensile strength of concrete, cracking would
result and form in other locations instead of the planned location of
contraction joints.
20. For box-out of concrete carriageway, should square box-out or
round box-out be adopted? (C3)
Isolation joints are introduced where the pavement contains manholes and
other structures. The joints allow independent movement of the pavement
and roadside structures without any connection which otherwise could
result in damage.
Square box-outs are commonly used in pavement and it suffers from the
demerit that cracks are observed to appear at the corners of the box-out.
Hence, to prevent crack formation at corners, the use of rounded box-outs
could eliminate such occurrence. In fact, some form of reinforcement could
be introduced to reduce the crack formation in square box-outs. For
instance, fillets or reinforcing bars and fabric could be placed at interior
corners of square box-outs to hold the cracks together should cracks grow.
21. What is the significance of ten percent fines value in testing
sub-base material?
Ten percent fines value is a measure of the resistance of aggregate
crushing subjected to loading and it is applicable to both weak and strong
aggregate. Fine aggregates are defined as those passing 2.36mm sieve.
The test aims at looking for the forces required to produce 10% of fine
values (i.e. weight of fines aggregates/weight of all aggregates = 10%).
This test is very similar to Aggregate Crushing Test in which a standard
force 400kN is applied and fines material expressed as a percentage of the
original mass is the aggregate crushing value.
Granular sub-base is subjected to repeated loadings from truck types. The
stress level at the contact points of aggregate particles is quite high. The
sub-base in pavement is a structural layer used for distribution of traffic
loads into larger area. As such, it is of paramount importance that the
sub-base material should itself not be disintegrated under severe traffic
loads. Ten percent fines value can be used to reveal the aggregate
properties when subjected to mechanical degradation.
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