Case 1
- Introduction
- Causes of Defects
- Good Practices
- Standards
- Maintenance and Diagnostics
- Remedial
- Similar Cases
- References
Good Practices
Dampness
Design
The material selection for exposed surfaces, and their regularity and surface texture should be detailed to
prevent moisture retention. Areas prone to high moisture exposure shall be made impervious with adequate waterproofing systems. Also such surfaces should be without impediments (e.g. built-in cabinet, fixed equipment/services) to ensure access for inspection and to ensure ease of cleaning. Access for adequate cleaning should be provided in accordance with BS 8221- 1, SS 509-1 or equivalent. Ventilate to prevent moisture retention on floor/wall/ceiling. For natural ventilation, opening > 5% of floor area. For mechanical ventilation, air exchange rate > 20 air changes per hour, in accordance
with “CP on Environmental Health”.
Construction
Exterior surfaces of porous building material (e.g., cellulose, brick, stone, cement rendering) can develop biological growth. Avoid such growth as much as possible with treatments of anti-algae/anti-fungus solutions and allow to dry before painting/repainting (SS 652: B.5.2.1). Improve ventilation and remove sources of dampness to dry out the substrates as thoroughly as possible during painting works in
accordance with BS 6150, SS 542 or equivalent. Ensure access ducts are connected to the mechanical
ventilation system. The exhaust system shall dispel the air directly outdoors.
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 [2]
Sloping walls and other such external features should be avoided. If the wall is to be left exposed and uncoated, the quality of concrete used should be higher.
Material
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 [5]. Table 2 shows the various concrete grade to be achieved.
| 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 [5]
Construction
- Ready mix concrete is preferred over site mixed concrete to achieve consistency.
- Check for quality of concrete before placing [6]. 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 minimise 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.
Concrete should be cured sufficiently.
Reinforcement
Design
Sufficient concrete cover should be provided to prevent corrosion of reinforcement due to the ingress of sulfates (Table 2).
| Conditions 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. | |||||
| 2) Cover should be not less than the nominal value corresponding to the relevant environmental category plus any allowance for loss of cover due to abrasion. |
Table 3: Limiting values of the nominal cover of normal weight aggregate concrete [3]
Material
All high yield reinforcement bars should comply with SS 2 and welded steel fabric should comply with SS 32 [5]. 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 [7]
- 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
Water repelling coatings
It is encouraged to protect the exposed concrete wall with algae resisting paints or water repellents so that water and algae cannot adhere to the surface.
Impregnation systems/hydrophobia
- Prevent penetration of water and solutions into concrete without hindering the escape of internal moisture from concrete
- Materials used [7]:
- silicon organic solutions
- resins
- oils
- Sealers
- Heavy barrier against penetrating solutions but may hinder the escape of internal moisture
- Materials used [7]:
- epoxy resins (EP)
- polyurethane resins (PU)
- unsaturated polyester resins (UP)
- Coatings
- compared to sealers, they provide additional protection against mechanical influence.
- increased resistance to diffusion of internal moisture
- Materials used [7]:
- plastic modified cement systems and resins
- Paints
- algae resistance to Singapore standard SS 345[5].