1. Introduction

1 1. IntroductionIn a survey conducted by BCA in year 2003 involvingabout 10,000 private residential units, water seepagethrough external walls was found to be a commondefect faced by homeowners. The survey findings alsoshowed that the use of single layer brickwall is themost common cause of water seepage through externalwalls. Almost 90% of the water seepage occurredthrough cracks in the plastered brickwalls. In general,water seepage through external walls occurred withinthe first five years of building seepage through the external walls isunacceptable to the occupants.

2 The problem is furthercompounded by Singapore s high humidity andabundant rainfall throughout the year. High windspeeds experienced by high-rise buildings alsoincrease the likelihood of water seepage. Buildingenvelopes must, therefore, be adequately designedand constructed to prevent ingress of are various external wall systems used in thelocal industry, including precast concrete walls, castin-situ reinforced concrete walls, brickwalls, curtainwalls, cladding walls, concrete block walls, to volume constraint, this publication focuses onprecast concrete walls, cast in-situ reinforced concretewalls and plastered brickwalls.

3 It provides industrygood practices to help achieve durable and effectivewaterproofing of the building Design of External GENERALThe ingress of rainwater impinging on external wallsusually occurs though joints and cracks in the a lesser extent, seepage through absorption andpermeation may also occur depending on the materialand thickness of the of external walls is usually achieved byusing suitable materials, providing adequate wallthickness, designing proper construction details, aswell as providing surface rendering and finishes whichserve as barrier against water ingress.

4 Where externalwalls are exposed to severe weather conditions, moreextensive surface waterproofing should be chapter focuses on the design aspects of precastconcrete walls and plastered CHOICE OF EXTERNALWALL SYSTEMThe common types of external walls include cast in-situ reinforced concrete (RC) walls, precast concretewalls and masonry design of external wallsFigure : Common types of external wall systemReinforced Concrete (RC) WallPrecast Concrete WallMasonry BrickwallWater seepage usually occurs through the fabric ofbuilding elements or through the gaps between theseelements.

5 For effective watertightness, the wall systemshould be constructed with impervious material andwith minimal joints. Large precast concrete panelsand cast in-situ RC construction with low water cementratio, in general, have better watertightness performancethan brickwalls, which have extensive joints betweenlayers of bricks. In addition, precast walls have manyother benefits over cast in-situ concrete walls andbrickwalls, such as better quality, higher buildabilityand better , the use of precast concrete walls is stronglyrecommended for external wall EXTERNAL PRECAST WALLSP recast joints are the weakest links in ensuringwatertightness of the external precast concrete include joints between precast concrete elements,between precast and cast in-situ elements, as well asbetween precast elements and window/door framesor other fittings.

6 Joint detailing and the use of suitablesealant should, hence, be carefully considered duringthe design stage. Joints with complicated profiles aredifficult to seal and this may affect the watertightnessof the building are two types of joint detailing employed forprecast concrete walls, namely the one-stage joint andtwo-stage joint. One-stage joint is a simple butt jointwith sealant applied against a backer rod at the externalface of the wall. One-stage joint offers only a singleline of defence against water seepage. Pressure dropmay occur across the one-stage joint and water mayseep through micro cracks or hairline cracks.

7 Two-stage joint (Figure ), on the other hand, providestwo defence lines against water ingress. Experience hasshown that two stage joints give better watertightnessperformance than one-stage of external walls2 Figure : Two-stage joint detailingTwo-stage horizontal joint (Sectional view)Two-stage vertical joint (Plan view) HORIZONTAL JOINTSFor horizontal joints of precast walls, joggled joints(as shown in Figure ) provide better watertightnessperformance than the butt joints. The matchingupstand and downstand profile of the joggled jointprevents inflow of water through the building height of the upstand and downstand is dependenton the exposure conditions, wall thickness and typeof precast wall (load-bearing or non load-bearing).

8 Itgenerally ranges from 35mm for mild exposure to100mm for severe exposure shows the typical details for a horizontaljoggled joint. The joint is sealed with high strengthnon-shrink grout and an approved self-adhesivecompressible waterproofing strip. In addition, anapproved flexible cementitious waterproofingmembrane may be used at the inner corner of theintersection between the wall panel and beam/slabfor enhanced watertightness stripNon-shrink groutINTERNALEXTERNALEXTERNALINTERNALS ealantBacker rodInfill concreteThe gap at the external wall face is usually not sealedto allow incident water in the joint to drain off.

9 If thisgap is also to be sealed (eg. for aesthetic reasons), thejoint can be fully sealed using non-shrink grout, witha backer rod and an appropriate sealant installed atthe exterior end of the joint (Figure ). Alternatively,the joint can be sealed with non-shrink grout at theinterior end with a sealant installed at the exterior endof the joint. For such sealing system, the sealant mustbe discontinued at regular intervals (at intersectionswith the vertical joints) to drain off incident waterthat has managed to seep into the gap (Figure ).For load bearing walls, the entire horizontal jointmust be sealed, for example, with non-shrink grout(Figure ).

10 3design of external wallsFigure : Typical sectional view of horizontal joggled joint (for non load-bearing walls)Approved self-adhesivecompressible waterproofing stripINTERNALEXTERNALHigh strength non-shrink groutApproved flexible cementitiouswaterproofing membrance15mm100mm100mmTypicalstorey levelCast in-situ beamFillet[A][C][B]15mmSlope 1:10 Precast panel[A] 35mm[B] 35mm[B][C] 1design of external walls4 Figure : Typical sectional view of horizontal joggled jointa) Fully sealed horizontal jointb) Sealing of external gap (For non load bearing walls)Joint fully sealed withnon-shrink groutBacker rodSealantApproved self-adhesivecompressible waterproofing stripHigh strengthnon-shrink groutBacker rodSealant must bediscontinued atvertical VERTICAL JOINTSThe two-stage joint detailing is recommended forvertical joints as it provides various water-resistantbarriers.