Case 3
- Introduction
- Causes of Defects
- Good Practices
- Standards
- Maintenance and Diagnostics
- Remedial
- Similar Cases
- References
Good Practices
Design
Adopt SS EN 1992-1-1, SS EN 1992-1-2, CP 65-1 or equivalent for the design of concrete. (SS 652: A.1.1.1)
Specify admixtures (e.g., water-reducing agent, pozzolanic products, pore refiner, etc.) to reduce permeability. Alternatively, for corrosion control in special areas with a high risk of water penetration, an electrochemical treatment can be specified; a process where electrochemical drying of concrete occurs by passing a current through the reinforcement, similar in principle to cathodic protection in accordance with BS EN 1504-9 or equivalent.
Construction
For the construction of basements, refer to the guidelines and provisions stipulated in BS 8004, EN 1997 or equivalent. Maintain water-cement ratio and the recommended aggregate grading during basement construction. Verify aggregate quality in accordance with ASTM C33/C33M-16e1 or equivalent. Use corrosion-resistant bars and corrosion inhibitors. Apply proper vibration (compaction) and curing. Consider concrete sealing to avoid exposing the aggregate in concrete work (mitigate pitting, scaling, spalling, powdering or chalking of concrete).
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 |
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. 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 |
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. |
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