Case 2
- Introduction
- Causes of Defects
- Good Practices
- Standards
- Maintenance and Diagnostics
- Remedial
- Similar Cases
- References
Good Practices
Depending on the exposure e.g. a structure subjected to Cl– attack (coastal or offshore) or high CO2 (carpark or heavy industrial), of which the concrete cover shall be higher with better concrete quality, the general requirements are as follows:
(a) The minimum concrete cover to the top and bottom steel reinforcement of the slab shall be 20mm and 30mm respectively.
(b) The minimum concrete cover to main reinforcement bars of the beam shall be 35mm.
(c) The minimum concrete cover to main reinforcement bars of the column shall be 60mm.
(d) The minimum concrete cover to the steel reinforcement bar at the inner face of water tank which is in contact with water shall be 40mm.
(e) The minimum concrete cover to main reinforcement bar of the pilecap shall be 75mm measured from the side and top face of pilecap and 50mm measured from the pile head.
(f) The minimum concrete cover to the main reinforcement bars of the footing shall be 75mm.
Designer must ensure high constructability and the Constructor must ensure that the tradesmen for formwork/steel placing/concreting are trained and capable, to achieve the concrete cover as required.
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. |
Table 1: Classification of exposure conditions
Material
Recommendations for concrete to resist sulphate attacks are given in BRE Digest 250 and BS EN 1992-1-1+A1. Generally the concrete should be:
- Good quality, well designed, well placed and compacted
- Low water cement ratio
- Use of air-entrainment to the extent of about 6%
- High pressure steam cured
- Provided with a protective coating
- Sufficient cover
| 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 due to the ingress of sulfates and chlorides (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. | |||||
| 2) Use of sulphate resisting cement conforming to BS 4027 (Withdrawn). If exposed more severe exposure conditions, the covers should be increased by 10mm. | |||||
| 3) 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
Material
All high yield reinforcement bars should comply with SS 2 and welded steel fabric should comply with SS 32. 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
Surface protection systems
- Surface protection systems may be applied to improve resistance against water seepage, chemical attacks and enhance concrete durability. This would enhance protection against corrosion of reinforcement.
- Impregnation systems: function by lining the pores of concrete, reducing the capillary absorption capacity of substrate and lowering the amount of water that can be absorbed.
- Prevent penetration of water and solutions into concrete without hindering the escape of internal moisture from concrete
- Materials used:
- silicon organic solutions
- resins
- oils
- Sealers
- Heavy barrier against penetrating solutions but may hinder the escape of internal moisture
- Materials used:
- 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:
- plastic modified cement systems and resins