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
14. What is the significance of isolation joints
Isolation joints isolate slabs or concrete structure from other parts of
structure. The presence of isolation joints allows independent vertical or
horizontal movement between adjoining parts of the structure. Otherwise,
the structure may experience cracking owing to the restrained movement
caused by directional connection between adjoining concrete structures.
15. What are the reasons of occurrence of dusting and scaling of
concrete surface
Dusting refers to the formation of loose powder arising from the
disintegration of concrete surface. For dusty concrete surface, they can
be easily scratched by nails. The cause of dusting can be related to
finishing works carried out before completion of concrete bleeding. The
working back of bleeding water to concrete surface produces a low
strength layer with high water cement ratio. It may also arise owing to
inadequate curing and inadequate protection of freshly placed concrete
against rain, wind and snow.
Scaling is the occurrence of peeling of hardened concrete surface as a
result of freezing and thawing effect. Scaling occurs in non-air-entrained
concrete as air entrainment is normally adopted to protect concrete
against freezing and thawing. Spaying of sodium chloride deicing salts is
also another common cause of scaling.
16. What is the purpose of adding polyethylene film in interior slabs
sitting on grade for building structures
Membrane materials, such as polyethylene film, are commonly used to
reduce vapour transmission from soils to concrete slab. They are often
termed “vapour retarder” and are placed on the underside of concrete
slabs sitting directly on soils for building structures. Protection from
moisture is essential because floors inside buildings are normally covered
with carpet and tiles and penetration of water vapour through concrete
slabs could result in the failure of adhesives in tiling, discoloration of
flooring products and fungal growth.
17. Should on-site addition of water to fresh concrete be allowed
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
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