Rising Dampness

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Good Practices

Design

Rising dampness is caused by moisture ascending through capillaries in masonry and concrete walls. The smaller the diameter of the capillaries the higher the dampness will rise. Implement proper waterproofing
design detailing for reinforced concrete structures. To avoid rising dampness, use suitable DPM/DPC for the site ground conditions in accordance with SS 637 (formerly CP 82) or equivalent.

Construction

Provide adequate damp-proof course/membrane at a height of at least 150 mm above the surrounding finished floor level, to prevent upward movement of moisture through capillary action or rainwater bouncing off the ground. Provide adequate surface drainage and adequate coating, and/or hydrophobic materials, and/or chemical injection as moisture barrier in accordance with BS 8215, BS 6576 or equivalent.

Concrete

Design

An understanding of the environment/atmospheric conditions should be taken into consideration during the design stage.

Environment	Exposure conditions
Mild	Concrete surfaces protected against weather or aggressive conditions
Moderate	Exposed concrete surfaces but sheltered from severe rain or severe traffic Concrete surfaces continuously under non-aggressive water Concrete in contact with non-aggressive soil Concrete subject to condensation
Severe	Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensation Concrete surfaces occasionally exposed to light traffic
Very severe	Concrete surfaces occasionally exposed to seawater spray (directly or indirectly) Concrete surfaces exposed to corrosive fumes and heavy traffic
Most severe	Concrete surfaces frequently exposed to seawater spray (directly or indirectly) and heavy traffic
Abrasive	Concrete surfaces exposed to abrasive action

Table 1: Classification of exposure conditions

Material

Ensure a good mix of dense concrete to reduce porosity and permeability. For exposed surfaces, it is encouraged to provide appropriate flashing, roofing or other water-throwing designs to reduce dampness. If necessary, a layer of damp-proof course or water-resistant coating might be useful to act as a barrier against water penetration.
Durability and impermeability are the main factors affecting the performance of the concrete with time in a structure. Durability is, by and large, closely related to impermeability. Often, concrete is subject to mishandling, misuses and lack of care to prevent it to achieve its full potential
Specifying a dense concrete with minimum water cement ratio
Suitable grading of all suitable materials combined to provide the least amount of voids.

Concrete Grade	30	35	40	45	50
Minimum cement content (kg per m³)	275	300	325	350	400
Maximum cement content (kg per m³)	550	550	550	550	550
Maximum % of Fine Aggregate to Total Aggregate	50	50	50	50	50
Maximum water to cement ratio	0.55	0.50	0.45	0.40	0.40

Table 2: Designed mix of concrete

Cement should comply with SS EN 197 series while coarse and fine aggregates used should comply with SS EN 12620. All aggregates shall be stored in clean places. Table 2 shows the various concrete grade to be achieved.

Construction

Ready mix concrete is preferred over site mixed concrete to achieve consistency.
Check for quality of concrete before placing. e.g. water cement ratio, slump test, etc.
Place the concrete carefully. If concrete is placed directly from a truck or concrete pump, place concrete vertically into position. Do not allow the concrete to fall more than 1 to 1.5 metres
Ensure thorough compaction of the concrete during placement.

Quality control

Avoid following during concreting to minimize cracks:

Avoid excessive manipulation of the surface, which can depress the coarse aggregate, increase the cement paste at the surface, or increase the water-cement ratio at the surface.
DO NOT finish the concrete before it has completed bleeding.
Do not dust any cement onto the surface to absorb bleed water.
Do not sprinkle water on the surface while finishing.

Reinforcement

Design

Sufficient concrete cover should be provided to prevent corrosion of reinforcement. (Table 3).

Condition of exposure	Nominal cover
Mild	25	20	20	20	20
Moderate	–	35	30	25	20
Severe	–	–	40	30	25
Very severe	–	–	50	40	30
Most severe	–	–	–	–	50
Abrasive	–	–	–	see note 3	see note 3
Maximum free water/cement ratio	0.65	0.60	0.55	0.5	0.45
Minimum cement content (kg/m³)	275	300	325	350	400
Lowest grade of concrete	C30	C35	C40	C45	C50
1) This table relates to normal-weight aggregate of 20mm nominal size. Adjustments to minimum cement contents for aggregates other than 20 mm nominal maximum size are detailed in BS EN 206+A2.

Table 3: Limiting values of the nominal cover of normal weight aggregate concrete

Material

All high yield reinforcement bars should comply with SS 2 and welded steel fabric should comply with SS 32 [3]. Reinforcement can be protected further by using following methods:

removal of rust and mill scale before embedment
use of non-metallic coatings such as epoxy resins and solvent containing acrylic resins
use of metallic coatings such as zinc and nickel
Cathodic protection
use of corrosion inhibitors
use of corrosion resistance reinforcement (e.g. stainless steel)
use of low permeability concrete, with improved resistance to chloride ion ingress

External Rendering

Plastering may serve to provide long term protection to the external wall against water penetration [1].

Other protective coatings may include:

solvent borne finishes provide greater tolerance to adverse weather/substrate conditions during application
Surface waterproofing treatments
- include: silicones and acrylic resins solutions
Pigmented coatings form better barrier to water penetration
- include: bituminous emulsion-waterproofing coat and pigmented rubber polymer