A Closer Look at Prevailing Civil Engineering Practice - European ...

A Closer Look at Prevailing Civil Engineering Practice, What, Why and How
Vincent T. H. CHU
Wet-mix method involves the pumping of ready mixed concrete to the
nozzle. Compressed air is introduced at the nozzle to impel the mixture
onto the receiving surface. The wet-mix method is more favorable for large
volume placements. Rebound is less than in the dry-mix method. The
nozzleman does not need to be concerned with the control on water
addition.
20. What is the difference between no-fines concrete, lightweight
concrete and lean concrete
Pervious concrete is sometimes called "no fines" concrete. It is designed
with high porosity and allows water to pass though. It is commonly used in
concrete pavement so as to reduce surface runoff and allow the recharging
of ground water. The high porosity is achieved by a network of
interconnected void. "No fines" concrete has little or no fines and contains
just enough cement paste to cover the surface of coarse aggregates while
maintaining the interconnectivity of voids.
Lightweight concrete is characterized by low density (1,400kg/m 3 to 1,800
kg/m 3 ) and is made of lightweight coarse aggregates. In some cases, even
the fine aggregates are also lightweight too. The primary use of lightweight
concrete is to reduce the dead load of concrete structures.
Lean concrete, which is also known as cement bound material, has low
cementitious material content. It has low concrete strength and is
commonly used as roadbase material.
21. What is the purpose of applying spatterdash before rendering and
plastering
Spatterdash is a mixture of one part of cement to one and a half parts of
coarse sand with enough water. The mixture is thrown forcibly onto the wall
so that the impact removes the water film at the interface between
spatterdash and the substrate leading to the improvement in adhesion. The
spatterdash should cover the substrate surface completely and form a
rough texture. Spatterdash serves as an effective mechanical key to
prevent rendering and plastering material from sliding or sagging. The
roughness of spatterdash improves adhesion by providing a positive “key”
for plaster to grip. The improper application of spatterdash affects the
subsequent bonding of rendering with substrate.
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