Watertightness

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Good Practices

Design

Specify and ensure joint sealant performance are of suitable form to withstand air penetration during assembly, transportation, installation and operation of the external wall system in accordance with BS EN
12152, BS EN 13830, SS 654 or equivalent. Adopt the recommendations for watertightness of external walls in accordance with SS 637 (formerly CP 82) or equivalent. Refer to the provisions for watertightness of precast concrete slab and wall panels in accordance with BS EN 13369, SS 677 or equivalent. Adopt the preventive measures for water leakage and weatherproof joints in accordance with BS EN 12154, BS EN 13830, SS 654 or equivalent.

Adopt the guidelines for calculations of thermal performance of windows, doors and shading devices in accordance with ISO 15099, ISO 12631 or equivalent. For the recommended
thermal performance of a curtain wall (overall U value in accordance with standards), refer to BS EN ISO 12631, BS EN 13830, SS 654 or equivalent. To ensure airtightness and watertightness, specify the sealing of the gap between the wall and window frame. For gap size 7–10 mm, specify a durable sealant, e.g., polyurethane or silicone. For gap size 11–25 mm, specify grouting of a durable material, e.g., a non-shrink grout. Consider specifying a layer of waterproofing membrane to be applied over the external surface of the joint areas between the wall and window frame before plastering or skim-coating the external walls.

Construction

Measure air permeability of joints between precast concrete external wall components with reference to ISO 6589 or BS EN 12153 and SS 654, SS 212, SS 268 or equivalent. Test weather resistance of the external façade against acceptable performance criteria. Detect water leakage through the façade by simulating rainwater penetrations under pressure using a watertightness test. Maintain watertightness, which will otherwise cause damage to the façade or other building elements, in accordance with ISO 7361 or equivalent, and watertightness of joints between two prefabricated ordinary concrete external wall
components in accordance with ISO 7729 or equivalent.

Keep an attic stock on-site for future uncertainties, especially important for reflective or low-emissivity coated glass as replacement stock may result in colour or reflectivity matching problems in accordance with ASTM C1401-14 or equivalent. The gaps between the window frame and wall must be properly sealed to prevent water seepage at these locations. Depending on the size of the gap, waterproof grouting or sealant can be used. Test method for weathertightness (air leakage and watertightness) of aluminium alloy windows in accordance with BS 4873, SS 212 or equivalent. For cleaning of fenestration products, adopt the manufacturer’s installation instructions. Maintenance requirements should be passed on during commissioning stages.

Concrete

Design

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

EnvironmentExposure conditions
MildConcrete surfaces protected against weather or aggressive conditions
ModerateExposed 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
SevereConcrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensation
Concrete surfaces occasionally exposed to light traffic
Very severeConcrete surfaces occasionally exposed to seawater spray (directly or indirectly)
Concrete surfaces exposed to corrosive fumes and heavy traffic
Most severeConcrete surfaces frequently exposed to seawater spray (directly or indirectly) and heavy traffic
AbrasiveConcrete 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 Grade3035404550
Minimum cement content (kg per m3)275300325350400
Maximum cement content (kg per m3)550550550550550
Maximum % of Fine Aggregate to Total Aggregate5050505050
Maximum water to cement ratio0.550.500.450.400.40

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 exposureNominal cover    
Mild2520202020
Moderate–35302520
Severe––403025
Very severe––504030
Most severe––––50
Abrasive–––see note 3see note 3
Maximum free water/cement ratio0.650.600.550.50.45
Minimum cement content (kg/m3)275300325350400
Lowest grade of concreteC30C35C40C45C50
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 (eg. 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.

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