Case 4

Cause of Defects

• All new constructions would experience settlement due to exertion of loads on freshly compacted soil for the initial 5 years of its life. Even settlement would not threaten the structure’s integrity. However, uneven settlement may cause cracking if the movements are not accommodated. With the lack of provision of a waterproofing membrane, these cracks would provide a passage for water seepage when the slab is subjected to hydraulic pressure from water-bearing facilities such as planter boxes.
• Poor quality concrete having honeycomb voids, without the provision of a waterproofing membrane, may provide a better passage for water seepage when the slab is subjected to hydraulic pressure. Honeycombs may be formed due to poor workmanship during casting as well as insufficient vibration provided after being placed, allowing excessive segregation in the plastic concrete. As the heavier components settle, the water level would rise. When the water dries up, voids will be formed.
• During the post-construction stage, the slab can be subjected to different temperature gradients at different times of the day. This temperature change experienced by the slab may cause periodical expansion and contraction, which results in differential thermal movement. The structure may hence, seeks relief through cracking at their joints when they are not provided for the movement.
• Shrinkage cracks typically result from the failure to cure the concrete properly. However, the cause may also be the result from poor site preparation, improper mixing, placing and finishing of concrete.
• When water pressure builds up against the concrete structure, these crack lines will present as relief points for water seepage. Therefore, the absence of a waterproofing system allows ground water to seep onto the basement floor through capillary action.
• Water seepage into a basement could take place through (a) porous concrete, (b) cracks, (c) joints, and (d) penetrations.
• Concrete is not waterproof. It consists of micropores formed during the migration of excess water during the hydration process. As the excess water dries off, they form channels or capillary pores. When subjected to water pressure, the pores allow the passage of water, resulting in leakage.
• The porosity of concrete, coupled with crack lines, allows the ingress of water, and, when re-surfaced, may result in the formation of efflorescence (deposit of salt) and/or calthemite (deposit of calcium carbonate).
• This is exacerbated by the formation of cracks due to the weak tensile strength of concrete against stresses resulting from shrinkage, differential settlements, etc.