Case 1

Good Practices

Design

Specify the use of a local green-certified breathable paint system to reduce trapped moisture and to
avoid wetness and dampness of basement walls. Fungi-resistant paint is recommended. Avoid the use of alkyd based paint on concrete surfaces that may lead to saponification (i.e., the formation of oily patches) in accordance with BS EN 1504-2 or equivalent. The most probable reason for paint blistering in a basement is due to moisture entrapment within the concrete substrate beneath the paint film. With time
when the paint loses its elasticity, it may be too brittle to withstand the forces imposed by the moisture and air entrapments, resulting in hairline cracks and eventually peeling and flaking. The performance of a paint system in a basement requires adequate adhesion to the substrate. Surface preparation of the substrate is hence critical. Specify the paint system and the criticality of surface preparation and paint application in accordance with SS 542.

Construction

Ensure adequate curing of the substrate before paint application to avoid shrinkage cracks. Permissible
moisture content should refer to the paint manufacturer’s recommendation. Clean the surface and use clean tools. Avoid prolonged storing, inadequate stirring, use of incompatible thinner/solvent, or mixing with leftover paints from previous batches in accordance with BS 6150 and SS 542 or equivalent. If it is brush-applied, apply the second coat at right angles to the first coat to help eliminate pinholes and avoid chemical attacks in accordance with BS 8000-0 and SS 150 or equivalent. Ensure that coatings, preferably
locally green certified, are always applied in a minimum of three coats. The material should possess
some degree of flexibility (i.e., be elastomeric) to reduce the risk of cracking due to thermal/moisture movements in accordance with BS 8000-0, SS 150, SS 542 and SS 554 or equivalent.

Construction Joint

Design:

Specify the use of expansion joints where the effects of temperature and moisture are too large to absorb as a strain in order to separate framed structures from joints’ sections in accordance with BS 6093 or equivalent (SS 652: A.2.1.1). Refer to the selection and specification of sealants in accordance with BS 6213 or equivalent (SS 652: A.2.1.2).

Refer to BS EN ISO 11600 for a systematic performance classification scheme for sealants applied in building and construction (SS 652: A.2.1.3). Provide internal/external waterstops to accommodate differential movements (Flanges of waterstops tied to Reinforcement in adequate intervals) (SS 652: A.2.1.4). For working joints (joints where movements are expected), to accommodate lateral, transverse and shear movements, use internal waterstops with a centrebulb. Larger centrebulbs accommodate larger
movements.

Specify proper sealant design at joints to prevent seepage caused by differential movements in accordance with BS EN 15651-3, SS 637 (formerly CP 82) or equivalent. Water penetration is prevented
by incorporating sealants, sealing strips, gaskets and baffles in accordance with BS 8102 or equivalent. Conventional sealants typically have +/− 25% joint movement, and sealants are not meant for joint width
greater than 25 mm.

Provide comprehensive design detailing of joints to avoid mould and plant growth; discolouration due to UV radiation and biological, physical or chemical action; showing of internal structure; and dust collection in accordance with BS 6093 or equivalent. Consider fit-for-immersion sealant for horizontal joints. Sealants designed for immersion should typically be able to mitigate mould growth.

Construction:

Seal construction joints to withstand multi-directional stresses. Ensure proper installation of waterstops.
The adoption of pure bentonite waterstop is recommended since it can seal capillary cracks in the cold
joint (SS 652: A.5.1.1). Provide proper surface preparation for the installation of sealants and gaskets
(ensure that gaskets should not be stretched during installation) in accordance with BS 6093 or

equivalent. Slow down the drying of concrete to avoid plastic shrinkage and provide joints/planes of weakness to confine cracking to determined positions in accordance with BS 6093 or equivalent. Change joint gap width from the time of erection to sealing. Hence, the best time for sealant application in movement joints is when the joint gap is at the mean trending to the maximum in accordance with BS 6093 or equivalent (SS 652: A.5.1.4).

The use of wet joints is essential in minimising water seepage through joint areas. Ensure its proper installation with a suitable backer rod and sealant application. For vertical joints, a baffle provides the overlapping needed to stop water seepage. The baffle needs to extend beyond the groove and not be shortened due to creep in accordance with BS 6093 or equivalent.

Construction joints in basement structure should be protected by waterstops. These waterstops used should be compatible with the waterproofing membrane. They can be placed internally or externally. In addition, the choice of waterstop design should be appropriate with the characteristics of the joint to which they are applied to (Table 1). Generally, external waterstops should be at least 240mm in width in order to be effective.

Table 1: Characteristics of joints

Joint Waterproofing Solution – Courtesy of Sika

Waterproofing

Design

Investigation of the ground condition should be made prior to the selection of the waterproofing system to ensure that the system chosen is able to provide sufficient protection to the basement. The design of the detailing is also very important to the performance of the basement.

The waterproofing system chosen has to be able to withstand the water table and its contaminations. Therefore, it is advisable to carry out site inspections so as to determine the soil conditions before choosing the waterproofing system.

Design considerations, such as drainage, are rather important to the long term performance of the waterproofing since it would minimize the hydrostatic pressure on the waterproofing. Effective drainage design should be capable of collecting, draining and discharging water away from the basement envelope. They should consist of the three fundamental items:

1. A perforated drainage pipe installed with perforation set downward in a bed of gravel that allows water drainage. These pipes are usually made of PVC or vitreous clay.
2. Coarse gravel installed around and over the drainage pipings for percolation and collection of water.
3. Sump pits or other drain fields installed to collect and discharge the drained water from pipes

In addition, back filling should slope away about 3m from the structure to avoid ponding.

Construction

The application of the waterproofing membrane or use of admixture in basement waterproofing should follow strictly with the specifications. It is of paramount importance if patented systems are used. The ratio of various chemicals must be precise. The application tools used should be those that are compatible with the chemical.

Care should be taken to prevent the accidental punctuation of waterproofing from steel fixing or nails and debris during construction. In vertical walls, 6mm thick polyethylene sheets are provided as a slip-sheet to protect waterproofing treatment from being damaged due to slippage during backfilling

Material

Waterproofing system selection of basement structures should be considered through defining the expectations on the level of water-tightness in relations to the proposed usage of the basement and the methods of construction. Table below shows the relationship between the proposed usage of the basement, the expected performance level and the form of construction.

Selection Method

Construction sequence

Painting

Material

Use of a paint system, which allows moisture to pass through the coating to reduce the moisture pressure, is another alternative to minimize paint defects caused by moisture (Table 1)[1-3, 12-13].

Table 1: Paint system for moist area

Construction

Prior to the application or re-application of paint system, the following practices should be followed to make sure the surface is clean and dry.

1. Grind surface imperfections and rinse with clean water and wipe dry (Figure 4).
2. Check the moisture of the substrate. The moisture content should not be more than 6% (Figure 5).

Quality Control

Quality assurance can be carried out as below:

Source: BCA Good Industry Practice-Painting