7 General construction issues
7.1 Site de-watering
Where appropriate, the site should be de-watered at least until such time as the below ground structure and waterproofing is completed (see 6.4s regarding the effects of dewatering on nearby structures).
On open sites, where any adjacent structures are sufficiently remote to be unaffected by groundwater lowering, de-watering or pumping from carefully arranged sumps with appropriate drainage channels should be continuous while the laying of any waterproofing barrier materials is in progress and until all loading coats have hardened and the structure has developed sufficient strength to resist the full water pressure.
7.2 Structural elements
Forms of construction to receive below ground waterproofing protection may include the following.
- a) Walls - constructed from:
- 1) masonry (plain or reinforced brick or block);
- 2) precast concrete;
- 3) in-situ concrete, either cast in form (plain, reinforced or prestressed) or embedded walls; or
- 4) steel or concrete piles in embedded walls.
- b) Base slab - constructed from concrete cast in situ, plain or reinforced, raft or other form.
- c) Roof, where applicable - constructed from reinforced in-situ concrete, precast concrete with an in-situ topping, or a steel composite slab, as appropriate.
NOTE 1 For structures cast entirely below ground, or where the substructure extends beyond the superstructure, the roof slab requires protection against water ingress (see 6.2.1).
NOTE 2 Further guidance on construction methods for each type of waterproofing protection is given in Clause 8, Clause 9 and Clause 10.
8 Type A (barrier) protection
8.1 Structural aspects
COMMENTARY ON 8.1
Structures using Type A protection are normally constructed of concrete or masonry. Deeper structures are of concrete construction. Steel can also form part of the construction as temporary sheet piling. Consideration might be given to employing the sheet pile wall as an element of the waterproofing system (see Clause 9).
Schematic illustrations of Type A protection are given in Figure 2a).
8.1.1 General
Barrier protection design should be based on an evaluation of:
- a) the nature of the substrate(s);
- b) the likely overall and local movements that might cause distress in the waterproofing barrier;
- c) the ability of the barrier system to accommodate these movements;
- d) the essential characteristics of the waterproofing system, e.g. bonded/unbonded, pre-applied/post-applied, liquid-applied or pre-formed;
- e) the need for external or internal application;
- f) the effects of environmental contaminants.
8.1.2 Differential movement and cracking
Barrier-specific properties should also be evaluated, allowing for any predicted cracking from the structure. The waterproofing barrier should be capable of providing the appropriate protection against water and water vapour without disruption or decay.
Although some barrier materials accept local strains and can accommodate a crack opening in the supporting structure, it should be noted that others might be damaged by differential movement or cracking (see Figure 4).
Care should be taken so that a load-bearing substrate is capable of supporting the barrier material, even under sustained water pressure, particularly around openings or service penetrations. A levelling or smoothing layer should be applied to masonry structures, as required.
NOTE There are two issues in regard to the possible influence of cracks on barrier performance. One relates to cracks pre-existing at the time of application and the ability of the selected system to initially bridge the crack. Decisions based on the specific properties of the barrier material would be needed before deciding whether any such cracks require pre-treatment. The second issue is the ability of the selected system to accommodate cracks that might form after application.
Remedial measures to fill significant voids or openings should be undertaken as the effect of sustained water pressure forcing the barrier material into them might create a risk of failure.
| a) Reinforced (in-situ concrete/masonry) wall | b) Unreinforced or nominally reinforced (in-situ concrete/masonry) wall |
Key
- 1 Stress and crack width reduced by reinforcement
- 2 Likely to be compatible with most waterproofing barriers
- 3 Stress and crack width increased by lack of reinforcement
- 4 Might exceed strain capacity of some waterproofing barriers if wall cracks
8.1.3 Continuity of waterproofing barrier
The waterproofing barrier should, in most instances, be continuous around the structure (see 6.2.4). In order to maintain the continuity of the barrier, penetrations through walls or floors that are to be protected (e.g. openings for services, pipes, cables) should be avoided, wherever possible. Where it is essential to provide such openings, special treatment around the penetration should be provided and reference should be made to the manufacturer's instructions and specialist advice. Similarly, where fixings through the barrier are necessary, the manufacturer's instructions should be followed.
Movement joints below ground should not be used unless unavoidable; in such cases these should be waterproofed in accordance with the manufacturer's instructions.
Where a waterproofing barrier is required for a structure supported on piled foundations, special consideration should be given to the detailing so that structural continuity is not compromised (see Figure 5) and reference should be made to the manufacturer's instructions.
Key
- 1 Floor slab
- 2 Pile cap, as appropriate
- 3 Pile
- 4 Pile reinforcement
- 5 Slab/pile/pile cap to have integral protection and/or added internal or external membrane (catering for reinforcement; see 8.1.3)
8.2 Waterproofing barrier materials
8.2.1 General
The waterproofing barrier used to provide Type A protection should be installed in one of the following locations, depending on the material(s) from which it is formed:
- a) on the exterior face of walls or slabs (external waterproofing);
- b) on some external source of support (reverse waterproofing);
- c) within the structure (sandwiched waterproofing);
- d) on the interior face of perimeter walls (internal waterproofing).
Table 3 should be considered when selecting the appropriate waterproofing barrier for use.
All barriers should be installed strictly in accordance with the manufacturer's instructions, including any recommendations regarding:
- 1) protection from damage, following application and curing, where the barrier is applied externally;
- 2) penetrations through the barrier;
- 3) fixings, where these are necessary;
- 4) application over joints in the substrate.
| Type of barrier | Description | Relevant standard(s) | ApplicationA) | ||
| Bonded sheet membranes (see 8.2.2) | Bitumen-based, sheet membranes can be either: a) cold-applied (self-adhesive); or b) hot applied ("torch-on" or bonded using a hot melt bitumen adhesive). |
BS 743:1970, Class A; or BS 8747 | Can be applied externally or sandwiched. | ||
| Composite sheet membranes. | Can be applied externally or sandwiched. | ||||
| Liquid applied membranes (see 8.2.3) | There are many types of liquid applied membranes, which include one or two part systems. | — | Can be applied externally or sandwiched. | ||
| Geosynthetic (bentonite) clay liners (see 8.2.4) | These comprise bentonite with a single or dual 'carrier' material, typically of geotextile or high-density polyethylene. There are two principal forms: dry bentonite and pre-hydrated bentonite. |
— | Can be applied externally or sandwiched. | ||
| Mastic asphalt membranes (see 8.2.5) | These are applied in three coats as a hot liquid. | BS EN 12970 | Can be applied externally or sandwiched. | ||
| Cementitious crystallization slurries and powders (see 8.2.6) | These are applied as coatings to surfaces of concrete walls and slabs or a solution or powder added to concrete. | — | Can be applied internally or externally. | ||
| Cementitious multi-coat renders, toppings and coatings (see 8.2.7) | These are generally applied in multi-coats or slurries and are resistant to liquid water but allow some water vapour penetration. | — | Can be applied internally or externally. | ||
| A) See Figure 2a) and Figure 6. | |||||
Key
- 1 Bonded barrier preventing water from tracking from a defect in the membrane to a crack/joint in the supporting concrete wall
- NOTE If the defect is coincident with a crack, the crack can be repaired with crack injection.
- 2 Partially bonded barrier allowing water to track from a defect
- 3 Defect in barrier
8.2.2 Bonded sheet membranes
COMMENTARY ON 8.2.2
Bonded sheet membranes may be pre- or post-applied to the structure. Pre-applied membranes are initially attached to enabling works in a reverse waterproofing application; they subsequently become bonded to poured concrete walls and slabs.
A bonded sheet membrane should be firmly supported by a load-bearing structure so that external ground and water pressures are adequately resisted.
It is important that the substructure provides a satisfactory base on which to apply the membrane, and therefore concrete should be free from ridges and indentions and finished to a true and even surface, preferably with a wood float finish. Brickwork and blockwork should have flush joints.
8.2.3 Liquid applied membranes
8.2.3.1 General
Details on the preparation of the substrate, application rate, method and curing requirements should be sought from the manufacturer.
8.2.3.2 Sandwich applications
When applied to the internal face of the structure, the membrane might need to be restrained against the effects of water pressure; in such cases, the restraining element should be firm against the applied liquid membrane.
Suitable protection to the membrane should be provided.
8.2.4 Geosynthetic clay liners
COMMENTARY ON 8.2.4
Bentonite is a natural day mineral, which has the capacity to expand when in contact with water forming a barrier to the transmission of water and other liquids. Bentonite is held between two geosynthetic layers. The bentonite forms an impervious seal and bonds to the concrete surface.
Geosynthetic clay liners are available in two forms. Dry bentonite liners rely on activation taking place on site from the absorption of the groundwater once installed. Pre-hydrated bentonite liners are manufactured by vacuum extrusion and do not need to be hydrated on site.
8.2.4.1 General
Bentonite-based waterproofing should only be used where the liner remains confined between two surfaces and cannot be left exposed.
NOTE The materials are suitable for new-build and refurbishment applications.
8.2.4.2 Horizontal applications
Geosynthetic clay liners can be laid onto compacted sub-base or blinding concrete. The surface can be damp, but the liner should not be laid into standing water. The manufacturer's advice should be sought on surface preparation requirements.
The manufacturer should also be consulted regarding the continuity of the horizontal liner with the vertical liner as different options exist depending on the type of vertical application, i.e. pre-applied (see 8.2.4.3) or post-applied (see 8.2.4.4).
8.2.4.3 Vertical applications (pre-applied)
When pre-applied, the liner should be fixed to formwork or to secant, contiguous or steel piles or diaphragm walls and the concrete should be poured directly, confining the liner.
8.2.4.4 Vertical applications (post-applied)
When post-applied, the liner should be nailed to the reinforced concrete structure. Minimal substrate preparation is required. As soon as possible after the vertical bentonite sheeting has been applied to the walls, backfilling should take place in accordance with the manufacturer's instructions.
8.2.5 Mastic asphalt membranes
COMMENTARY ON 8.2.5
Mastic asphalt is composed of graded mineral matter and asphaltic cement in such proportions as to form a coherent, voidless, impermeable mass, solid or semi-solid under normal conditions but sufficiently fluid when brought to a suitable temperature to be spread by means of a hand float or by mechanical means.
Mastic asphalt should always be applied in three coats. Horizontal surfaces to which mastic asphalt is to be applied should be level and free from irregularities.
Brickwork and concrete surfaces formed using timber shuttering are usually sufficiently rough to provide a key for vertical asphalt; however, smooth surfaces do not give an adequate key so, if these cannot be avoided, technical advice should be sought on the appropriate treatment.
When mastic asphalt is not fully confined, the maximum design load should not exceed that stated by the manufacturer to prevent extrusion.
8.2.6 Cementitious crystallization slurries and powders
COMMENTARY ON 8.2.6
Cementitious crystallization barriers are blends of Portland cement, treated quartz sands and active chemicals. They are supplied in powder form and are mixed with water to form a slurry, which is then applied directly to the prepared concrete surface.
The active chemicals combine with free lime and moisture present in the capillary tracts to form insoluble crystalline complexes which prevent water ingress.
8.2.6.1 General
Cementitious crystallization barriers should be applied to either internal or external surfaces of the concrete structure by brush or spray. They are suitable for use on both new and existing structures, and do not require a loading coat.
Surfaces should be prepared (in accordance with the manufacturer's instructions) so as to have a capillary open structure prior to the application of the barrier.
NOTE A capillary open structure refers to the intrinsic fine capillary tracts (pore structure) of a concrete matrix.
8.2.6.2 Horizontal applications
Cementitious crystallization barriers can be applied as a single coat slurry to hardened concrete or dry sprinkle and trowel-applied to fresh concrete.
They can also be applied to concrete blinding immediately prior to the placing of overlaying concrete.
8.2.6.3 Vertical applications
The barrier should be applied in a two-coat application to all vertical surfaces. Vertical surfaces should be prepared in accordance with the manufacturer's instructions.
8.2.7 Cementitious multi-coat renders, mortars and coatings
The installation of cementitious multi-coat renders, mortars and coatings should, unless otherwise advised by the manufacturer, be left until as much as practicable of the structure's dead load has been applied.
The substrate should be prepared in accordance with the manufacturer's instructions prior to the application of the system.
Details on the application method and rate, mixing, number of layers/coats and curing requirements should be sought from the manufacturer.
Existing substrates and structural elements should be assessed for suitability to withstand any increase in applied loads from water pressure.
