Case 2
- Introduction
- Causes of Defects
- Good Practices
- Standards
- Maintenance and Diagnostics
- Remedial
- Similar Cases
- References
Good Practices
Design
Tall slender concrete structures should be designed with due consideration of the effects of lateral deflection and be within acceptable vibration limits in accordance with BS EN 1992-1-1, SS EN 1992-
1-2, CP 65-1 or equivalent. Any deflection/deformation of the concrete structure due to vertical loading
should be compatible with the degree of movement acceptable by other elements (i.e., the finishes,
services, partition, glazing and cladding) in accordance with BS EN 1992-1-1 or equivalent. Limit State Method should be considered for design and verification of structure for durability of façade structures. Refer to modelling of deterioration process in accordance with ISO 13823 or equivalent.
The causes, effects and methods of prevention and repair for cracks (e.g., longitudinal cracks, transverse
cracks, cracks above prestressing strands, web crack at or near the prestressing strands, corner crack, and miscellaneous cracks) in precast concrete wall panels should be considered and applied in accordance with BS EN 13369, SS 677 or equivalent. Limit the design crack width with reference to CP 65-2 or equivalent. Laboratory mechanical tests should be carried out to measure deformations on horizontal joints between load-bearing walls and concrete floors in accordance with ISO 7845
Construction
Use two-stage joints for precast façade construction to ensure higher watertightness performance, since doing so will avoid seepage through hairline cracks, as is the case with one-stage joints in accordance with SS EN 1992-1-2 or equivalent. Seal off horizontal joints for load bearing walls with non-shrink grout.
Minimise cracks in rendered brick walls by using appropriate mix ratio, thickness and number of coats.
Provide bonding bars at interfaces between different materials in order to minimise cracks (e.g., where a brick wall abuts concrete). Alternatively, the bonding bars can be cast together with the concrete member.
At the completion of the construction stage, minor repair work or fixing adjustments may be acceptable. Enhance the durability of vulnerable parts of construction; ensure that surfaces exposed to water are freely drained; provide adequate cover to steel; use protective coatings for either steel or the concrete, or
both in accordance with BS EN 1992-1-1, SS EN 1992-1-1, SS EN 1992-1-2, CP 65-1 or equivalent. Make accessible components of which predicted service life is less than the design life of the structure for inspection, and replace them in accordance with ISO 13823 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 [6]
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 [7]. 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 [7]
Construction
Ready mix concrete is preferred over site mixed concrete to achieve consistency.
Check for quality of concrete before placing [8]. 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.
| 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 [6]
Material
All high yield reinforcement bars should comply with SS 2 and welded steel fabric should comply with SS 32 [7]
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 [9]
- 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