Handbook to BS 5628: Structural use of reinforced and prestressed masonry

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In order to fill the inner portion of the joint, filler material should be positioned to leave the correct depth to accommodate the sealant. The sealant should be applied against a firm backing so that it is forced against the sides of the joint under sufficient pressure to ensure good adhesion. The filler material should not react with or adhere to the sealant.

If a bond breaker is necessary, it should be positioned between the filler material and the sealant. A cross-linked, closed-cell polyethylene foam provides a suitable combined filler and bond breaker. The foam should have small cells or a surface skin. The reinforcement should be long enough to distribute the stress to a position where the vertical cross-sectional area of the wall is able to accommodate it. Reinforcement should be adequately protected against corrosion see Table 2 and 5. Consideration should be given to the need for slip planes under the bearing of lintels or beams to separate the wall from structural members which can produce a horizontal movement, e.

Consideration should be given to the need for lateral restraint and fire integrity should be maintained. Cladding, irrespective of the type of masonry units from which it is built, should be provided with adequate lateral edge restraint see 5. Some particular cases of design to limit the effect of differential movement and yet to provide stability of the panel are described in 5. This movement will be in opposition to thermal expansion of the cladding.

In addition, in the case of cladding of clay units, there will also be irreversible moisture expansion. If masonry cladding is built-in tightly between horizontal beams or floor slabs, these opposing movements can cause excessive stresses in the masonry, particularly if there is eccentricity, e. To prevent excessive stress occurring, horizontal compressible joints should be provided to accommodate the differential movement. Such a compressible joint should be provided at the top of any masonry cladding built up to the underside of any horizontal element of the structure or any supporting component fixed to it, see Figure Figure 16 An example of a support system showing provision for movement With cladding of calcium silicate or concrete masonry the differential movement between the cladding and the concrete structure is less.

This is because the long term moisture movement of both cladding and structure involves shrinkage and thermal expansion of the cladding is the only opposing movement. Provision for differential movement by the inclusion of a compressible horizontal joint eliminates the stabilizing effect of a mortared joint or simple restraint fixings see 5. Sliding anchor restraint fittings are available see Figure 17 for a typical example. Figure 17 Head restraint wall tie Where ties and restraint fixings are fitted between cladding and frame they should be designed to permit appropriate movement. Steel frame structures are not subject to shrinkage movement and so vertical differential movement is due only to thermal and moisture movements of the cladding.

For internal walls, if eccentric loading and short returns see 5. Concrete and calcium silicate masonry should not be tied rigidly to the frame, but adequate lateral restraint should be provided. Movement of the timber frame and movements of masonry in response to thermal and moisture changes are dissimilar. Building details should accommodate the vertical movement between frame and cladding. For example by: a use of timber frame wall ties that tolerate relative vertical movement between the frame and the cladding; b provision of a gap between the top of the cladding and oversailing frame members such as eaves ladders and the feet of trussed rafters.

Window and door frames are usually fixed to the timber structure and project across the cavity and into an opening in the masonry cladding. Building details should accommodate any tendency for joints between the cladding and the underside of sills to close and between cladding and the heads to open as a result of differential movement between the structural frame and cladding see Figure NOTE When timber platform ground floor is used add 3 mm to the differential movement allowances quoted.

Figure 18 Recommended allowances for differential movement between the timber frame structure and brick cladding 5. The masonry may be tied to the frame and designed to move with it by permitting limited rotation at the DPC at the base of the wall. Alternatively, the masonry may be designed as a structurally self-supporting wall. Any connection to the frame should not be fixed in a manner that restricts the anticipated movement.

The masonry should be designed not only to resist the stresses due to the imposed load, but also the stresses which can arise from differential movement between the masonry and the frame. Where their use is unavoidable, panels of slips should be isolated from movement and stresses in adjoining masonry. Building defects are the most common cause of water penetration through the building envelope. The source of water can be from precipitation or from ground water. The specification, design, detailing and construction of the total wall element should contain provisions for appropriate resistance to rain penetration in relation to local exposure conditions [5].

An assessment of local wall spell indices should be made see 5. For each type of wall, in any particular building, the most exposed part should be given particular attention as this will affect decisions concerning the choice of design and materials. Guidance on resistance to rain penetration of different forms of masonry construction and the factors affecting rain resistance are described in 5.

Rainfall varies considerably across the country and is largely unaffected by local features. Conversely, the wind speed is affected significantly by local features such as the spacing and height of neighbouring trees and buildings and whether the ground is flat or steeply rising. BS allows calculations for different orientations. Annual average values can be calculated as well as quantities for the worst likely spell in any 3 year period.

It permits corrections to be made for ground terrain, topography, local shelter, and the form of the building concerned. BS gives recommendations for two methods of assessing exposure of walls to wind-driven rain, namely the local spell index method and the local annual index method. The local spell index method should be used when assessing the resistance of a wall to rain penetration. The local annual index is intended for use when considering the average moisture content of exposed building material or when assessing durability, the effects of the weather on the appearance of materials and components and the likely growth of mosses and lichens.

Exposure categories defined in terms of wall spell indices calculated using the local spell index method specified in BS are given in Table The indices, derived as they are from inherently variable meteorological data, should not be regarded as precise. Where assessment produces an index near the borderline between categories the designer should decide which is the most appropriate category for the particular case, using local knowledge and experience.

BRE Report BR [6] provides a simplified procedure for assessing exposure to wind-driven rain for walls up to 12 m high. It is primarily intended for low rise domestic buildings, but may also be considered suitable for other categories of buildings of similar scale. This simplified guidance is based on a map which defines zones in which calculations, in accordance with BS , predict similar exposure conditions.

The zones are numbered 1 to 4 and correspond with categories defined in Table The calculations defining the mapped zones in [6] assume worst case conditions and so provide very conservative guidance. As such [6] can restrict the choice of construction. A greater choice is obtained by more specific assessment using BS NOTE These factors are not listed in order of importance. A greater proportion of the water runs down the face of the walling and can be blown into and through it via paths in the mortar joints, particularly at the interface between the mortar and the masonry units see 5.

Masonry units having relatively high water absorption characteristics will absorb water running over a wall surface in conditions of driving rain. If the duration of the rainfall is short this behaviour can be considered beneficial because it prevents most of the water reaching the mortar joints. However, when the surface of the material approaches saturation point water tends to run more readily down the surface and, as in walls of dense units, can penetrate via paths at the mortar joints. In Very Severe and prolonged conditions of driving rain, water can be absorbed further into the masonry units and eventually reach their inner surface, first as dampness and then as free water.

Generally rain ceases long before such complete saturation and water evaporates from the wall by the drying effect of wind and air movement. These two modes of action are sometimes referred to as the raincoat effect, in the case of dense, low absorption units, and the overcoat effect, in the case of high absorption units.

Solid walling can ultimately be penetrated by prolonged exposure to wind-driven rain regardless of the water absorption characteristics of the masonry units. These mortar Designations are often used in conjunction with dense, low water absorption clay bricks. This combination is satisfactory but should not be regarded as providing a near waterproof construction see 5. Strong dense mortars such as Designations i and ii are not suitable for use with some other types of masonry units and selection is governed by other factors such as accommodation of movement, durability and strength.

Designation iii and iv mortars are often more appropriate. Of the various mixes recommended for the mortars of each designation those incorporating lime in their composition show an improvement in bond development. However, in practice, this advantage is not likely to be of great significance.

Table 12 shows the recommended minimum thicknesses for both rendered and unrendered single-leaf walls for various categories of exposure as defined in Table With regard to rain resistance a waterproof cladding system as listed in 5.

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Where hollow blocks are used in external walls, the use of shell bedding can reduce rain penetration. Table 12 Single-leaf masonry Recommended thickness of masonry for different types of construction and categories of exposure Type of masonry Minimum constructional thicknessa mm Maximum recommended exposure zone from Table 11 for each construction Unrendered Rendered in accordance with BS Externally insulated Impervious cladding.

Not recommended 1 1 2 Not recommended 1 1 1 2 Not recommended Not recommended Not recommended for blocks with open surface texture 1 2. However, consideration may be given to the use of a thicker outer leaf to reduce the quantity of water reaching the cavity. In cavity walls, some water will inevitably penetrate the outer masonry leaf in prolonged periods of wind-driven rain, but proper design and positioning of DPCs and trays see 5. Where the cavity is unavoidably bridged, e. The inner leaf of a cavity wall should not be relied upon to resist water penetration.

In most situations a cavity wall with an outer leaf of 90 mm minimum thickness, a 50 mm cavity and an inner leaf is satisfactory. In conditions of Severe or Very Severe categories of exposure as defined in Table 11 , consideration should be given to the use of wider cavities. A microscopic labyrinth of voids exists at the interface because of the physical nature of mortar bonding.

The interface is also a likely location for capillary cracks due to imperfect adhesion between a mortar and masonry units. The interface can be degraded further by cracking due to moisture and thermal movements subsequent to construction. Whatever the type of masonry, filled cross and bed joints minimize the risk of rain penetration. The tooling involved in finishing joints such as those with bucket handled and struck weathered profiles firms the mortar, reducing its permeability at the surface, and pushes it tight to the masonry units, thereby improving its adhesion to them.

Both factors are beneficial in resisting rain penetration. Those formed by raking out the mortar without subsequent tooling to firm its surface further increases the vulnerability of the wall to rain penetration. Recessed joints also reduce the width of the mortar joints. Compared with bucket handled and struck weathered profiles, the risk of rain penetration is greater with recessed joints. When recessed joints are contemplated for use in other categories of exposure than Sheltered as defined in Table 11 , the manufacturer of the masonry units should be consulted.

In a partial-fill system, insulation material is built-in so that a free airspace is retained. This airspace should be of a minimum target width of 50 mm. Face insulated masonry units should be used with a retained air space. In a full-fill system, the space between the inner and outer masonry leaves is filled with insulation material either by building it in as construction proceeds or by injecting or blowing it into the cavity after the wall has been completed.

The space for full-fill insulation should be of a minimum target width of 50 mm, but the risk of rain penetration will be reduced by specifying a wider cavity. Design details may increase the tendency for masonry to be wetted than by incident rainfall alone see 5. Examples of features that cause concentrated wetting are listed below. Unless there is a gutter to collect it, or a projecting sill to throw it clear, excessive wetting and possible water penetration can occur in any masonry below. Corresponding intrusions into the cavity due to the setting back of masonry units to form the feature should be avoided unless appropriate damp-proofing measures are taken to prevent water crossing the cavity.

Adequate overhangs and drips or the provision of drainage to take water away from the masonry will reduce the degree of wetting. Detailed commentaries on the protection afforded by projecting features such as sills, copings, string courses, roof eaves and verges, and on the concentration of surface run-off of rain water by wind are contained in BS , Annex E. Local knowledge, experience and the evidence of local traditional forms of construction and building detail should be taken into account see the foreword of BS Rendering can substantially enhance the rain resistance of single-leaf and cavity walls.

The right type of mix, thickness and number of coats should be selected and the wall should be detailed correctly. The recommendations set out in BS should be followed. The combination of full-fill insulation and rendering can inhibit the drying out of any moisture that enters the outer leaf of masonry.

The moisture content of the outer leaf consequently rises and remains high, increasing the risk of frost action and of sulfate attack of the jointing and rendering mortars, if sulfates are present in the masonry units.


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Clay bricks of durability Designation MN are not recommended in this situation. Bricks of Designation FN are only recommended when sulfate-resisting Portland cement is used for the jointing and rendering mortar. The use of masonry paint systems see BS and other proprietary external finishes including colourless treatments, e. However, they can also reduce the rate of evaporation of any water from the wall and so the moisture content of the wall can increase.

In extreme cases this can cause saturation sufficient to place clay bricks of ML and MN durability Designation at risk of frost damage. All workmanship should be in accordance with Annex A. Certain masonry external leaves require particular care in their construction compared with others, e. From the description of the raincoat effect and the overcoat effect see 5. In contrast, rain falling on a wall of low water absorption units raincoat effect will run down over the glass-like surface to penetrate any imperfections in the jointing.

Where a DPC is intended to prevent the upward movement of water due to capillary action, joints should be made in accordance with the manufacturer's instructions. Where no instructions are given, a minimum lap of mm should be used. However, where water is moving in a downwards direction, the joints in the DPC should be sealed. A DPC should extend through the full thickness of the wall or leaf, and preferably project beyond the external face. A DPC or tray should be laid on a smooth bed of fresh mortar.

If it is necessary to accommodate differential sliding movement between the units on either side of it, the mortar bed should be trowelled smooth, allowed to set, and then cleaned off before the DPC is laid. Coarse aggregates that might damage the DPC should not be used. DPCs and cavity trays should not be pierced by services, reinforcement, fixings, etc.

DPCs should not be bridged by pointing, rendering, plastering, tiling, etc. Information on performance of individual materials currently used for DPCs is provided in Table 3. BS gives guidance on the basic principles concerning the function and installation of DPCs in masonry. It contains recommendations for the selection, design and installation of DPCs in both solid and cavity construction.

Many common details cannot be formed satisfactorily on site, unless they are fabricated in lead. If materials other than lead are to be used in complex situations, then pre-formed cloaks should be specified, so as to restrict the site operation to simple jointing only. Cavity trays should be supported at their joint positions to facilitate effective sealing.

Where required continuous support should be provided to avoid excessive sagging and deformation. To prevent the transfer of moisture from external walls into solid floors, the damp-proof membrane in the floor, and the DPC in the wall, should overlap a minimum of mm or be sealed. Immediately above ground level, weepholes should be left in the vertical cross joints of the outer leaf at intervals not greater than 1 m.

Where the lowest floor of a building is below ground level horizontal and vertical damp-proof membranes and DPCs are required with continuity between them see BS Particular attention should be paid to jointing. Where buildings are placed on sites where protective measures are required to prevent the entry of hazardous gases, e. The cavity tray should be supported by fine concrete fill in the base of the cavity, or by pre-formed in-fill units, in order to reduce stresses on the cavity tray and its joints.

Fill may not be necessary if the cavity tray is made of a self supporting material. Where the cavity is bridged, e. The cavity tray should step down or slope across the cavity not less than mm towards the external leaf and, preferably, terminate in a small drip on the face of the wall. This can be difficult to achieve in arches see 5. The cavity tray over the opening should overlap the vertical DPCs at the jambs to ensure continuity of damp-proof measures see Figure These may be formed in the vertical cross joints at intervals not greater than 1 m.

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There should be not less than two weepholes over each opening. Proprietary devices may be installed to form weepholes. They may be designed to drain the cavity but prevent the ingress of wind-driven rain. Weepholes need not be provided if walls have a rendered external surface finish. Cavity tray extends 25 mm min. Cavity tray overlaps in front of the vertical DPC. It should extend beyond the vertical DPC and cover any lintel. However, where lintels with flat soffits are used then the vertical DPC should be terminated at the underside of the cavity tray or turned back on to the inner leaf.

Where the frame is to be built-in, the DPC should be secured to the frame before building in. If the frame is to be fixed later, the DPC should be left with a suitable projection to facilitate this. Vertical DPCs at openings should be positioned to overlap any horizontal DPC at the sill of the opening and be overlapped by any cavity tray at the head see Figure Proprietary devices are available which function as a vertical DPC and cavity closer.

The manufacturers instructions for installation and linking with associated DPCs at the head and sill should be followed. Where the sill is in contact with the inner leaf, the DPC should be turned up at the back and ends for the full depth of the sill. The tray should step down by a minimum of mm towards the outer leaf and weepholes. Stop ends should be provided.

In buildings over 12 m high with insulated cavity walls, cavity trays should generally be used to subdivide the cavity at a maximum of 12 m above ground level and at a maximum spacing of 7 m thereafter. Where the abutment is horizontal a cavity tray with stop ends and weepholes should be used. Where a pitched roof abuts such a wall, a cavity tray stepped to correspond with the slope should be used. Alternatively, a system of overlapping pre-formed trays can be installed to collect and discharge water from the cavity see Figure In either case it is essential that stop ends and weepholes are used.

Proprietary systems are available for this application. Viewed toward the gable, the left hand trays discharge to the left and the right hand trays discharge to the right. Right hand intermediate tray Ridge tray Intermediate tray right hand version A cavity tray is required above horizontal abutments Out-fall tray Corner tray. Where cavity brick masonry is supported on an edge beam, or on a floor slab, a cavity tray should be used to prevent moisture penetration into the structure.

Where a column, or other structural member, obstructs the cavity of a wall, the cavity tray should be continuous around the member. Where a structural member bridges a cavity, a vertical DPC should be included between the structural member and the external leaf, and stop ends fitted to any adjacent cavity trays. Where complex shapes are needed, pre-fabricated cloaks should be considered see Figure In general a minimum upstand of mm is necessary.

In a cavity parapet wall, a cavity tray should be installed, stepped down at least mm towards the inner or outer part of the wall see Figure Figure 23 Cavity parapet walls Particular care is necessary in the specification and formation of joints in cavity trays in parapet walls see 5. Careful consideration is necessary in deciding which way to slope the tray in a given case. It is safer to slope or step it outwards to direct water towards the outer face, but this can cause staining. If sloped inwards, in exposed locations, moisture can travel along the underside of the tray and gain access to the underside of the roof covering and the interior of the building.

When cavity fill insulation is installed, the tray should always slope or step down to the outer leaf. It should be noted that the DPC or cavity tray impairs the structural continuity of the parapet and the wall beneath it, and of the coping and the parapet. Structural stability of the parapet should be in accordance with 5. Where a chimney stack is: a incorporated in an outer cavity wall; and b in locations subjected to Severe and Very Severe categories of exposure as defined in Table 11 ; and c preferably, in general; the outer leaf and cavity should be continuous around the chimney stack for the full height of the outer wall and then completely surround the chimney stack where it projects above the roof.

Corbelling from the chimney breast may be necessary below the roof line, to support the outer leaf at the sides and back of the chimney stack. DPC trays should be provided to prevent the downward passage of water into the interior of the building. Horizontal trays, through the thickness of the chimney wall, with an upturn at the inner face of the flue, should be linked with the flashing at the intersection with the roof. Figure 24 illustrates typical arrangements. Lead tray terminated with an upturn within flue liner Line of stepped flashing beyond Lead flashing over apron Roof line.

In steep pitched roofs or severe exposures an additional tray, with edges turned up within roof void, is recommended at this level. Figure 24 Detail of chimney stack It should be noted that a sheet DPC at the point of intersection with the roof structurally separates the masonry and the stability of the chimney stack and its resistance to lateral wind loading needs to be considered. Chimney stacks built in cavity work may be provided with a DPC tray of a material stiff enough to form a cavity tray without being built into the inner leaf and so allowing structural continuity. A horizontal DPC should always be provided below any coping or capping at the top of the stack.

BS [ AM] | Masonry | Concrete

Metal flashings other than lead should, preferably, be pre-formed. They should be bedded into the masonry to a minimum 25 mm depth unless the flashings are supplied already bonded onto a DPC or cavity tray. Joints in the length of the flashing should be welted, sealed or adequately overlapped. If flashings are to be installed into pre-formed chases care should be taken to avoid damage to the DPCs. Aluminium and aluminium alloy flashing and weatherings should be protected from contact with mortar by a coating of bituminous paint, and are not recommended for natural stone other than granite or sandstone.

The materials should be selected with regard to the risk of corrosion. To avoid staining of masonry from the run-off of rain water e. Drip edge s should be positioned a minimum of 40 mm from the face s of the wall. Where for aesthetic or other reasons a capping see 3. Where the coping or capping is jointed, a continuous DPC should be provided. In cavity walls horizontal DPCs require support over the cavity.

Resistance to water penetration should not prejudice provision for masonry movement. Movement control joints in masonry should be carried through any coping or capping see 5. Consideration should be given to copings being displaced by lateral loads, and to the possibility of vandalism. L-shaped copings and clip-over copings can be more satisfactory in some situations.

Where necessary, copings should be suitably fixed down and can be doweled or joggle-jointed together. It can become saturated directly by rainfall, indirectly by water moving upwards from foundations or laterally from retained material as in a retaining wall.

BS 5628 Part 3 Masonry

External masonry is much less likely to become saturated where projecting features have been provided to shed run-off water clear of the walling below. Examples of such features are: a roof overhangs or copings; b throated sills; c bell mouths to rendering and similar features at the base of tile hanging and other impermeable cladding. It should be noted that conventional weathering details do not always protect walls sufficiently in situations of Severe or Very Severe categories of exposure as defined in Table Recessed mortar joints can increase water intake into the surface of a wall, placing it at greater risk of frost attack.

Therefore, recessed joints are not recommended for external work using clay bricks of the moderate frost resistant category M specified in BS Impervious finishes, e. External masonry will generally be maintained in a drier condition by a moderately porous uncracked render conforming to BS , or by a ventilated cladding such as slate or tile hanging, by weather boarding, and by panels of various materials, e. Soluble sulfates can originate from various sources including some clay bricks. The reaction, forming calcium sulfoaluminate ettringite , is accompanied by an expansion leading to cracking and crumbling of the mortar.

In severe cases the expansion can lead to distortion or rotation of the masonry. The following factors affect the susceptibility of the masonry to damage. The design should take into account the choice of masonry units and mortar in the following and similar situations where masonry is likely to become and can remain substantially wet for long periods of time.

The degree to which masonry, used below DPCs at or near ground level, becomes saturated will vary according to the site. Masonry materials are far less prone to problems on a site that is well drained and dry. Paved surfaces adjacent to masonry should be laid to falls so that water is not directed to the masonry. Where there is greater than mm of masonry exposed between a DPC and the finished ground level, e.

In some circumstances, water can be transferred into the walling thereby inducing a risk of frost action and sulfate attack, efflorescence, lime leaching and staining of the outer leaf. The application of waterproofing treatments to the rear face of the masonry in contact with the retained ground will avoid such problems.

Earth retaining walls are particularly susceptible to saturation from retained ground in wet weather conditions. Backfill material should be free draining [see Table 13 K ]. However, it should be appreciated that different elements in the same building can be subject to different degrees of exposure. This can affect the choice of materials including insulation see 5.

In locations subject to Severe or Very Severe categories of exposure, the benefits of protection by overhangs and other projecting features see 5. If such protective features are omitted for aesthetic or other reasons, the effects of the increased exposure of the masonry to wetting should be considered see 5. Masonry at subzero temperatures is therefore not uncommon. Low temperatures alone do not damage masonry, but if it is saturated, or near saturated, the water can freeze to form ice within the fabric. As water changes from liquid to solid it expands and induces stress in the materials.

Frost resistant materials should be used in the design for masonry unless there are protective features which protect it against severe wetting. Extra care should be given to the choice of masonry units and mortar if the masonry is liable to be splashed with de-icing salts from roadways or if the building is to be located in conditions of extreme exposure to weather see 5. Where clay brick masonry is used in situations in which it can become saturated and exposed to cyclic frost action, the frost resistant category F specified in BS should be used.

Bricks conforming to strength Class 3 of BS possess good frost resistance in most applications, but higher strength classes are recommended for Very Severe categories of exposure. Calcium silicate bricks can suffer deterioration if they are impregnated with strong salt solution and then subjected to intense freezing.

Thus, they should not be used in situations where the masonry can be directly wetted by seawater or subjected to contamination by repeated application of road de-icing salts. Sulfates can be derived from ground waters, from the ground, including made-up fill adjacent to masonry, from flue gases, or from clay masonry units and aggregates.

The degree to which soluble salts are extracted depends on the quantity of water available and the permeability of the masonry. For this reason, the design should contain provisions for effective DPCs and the exclusion of water see 5. Where masonry is likely to remain wet for long periods of time, e. In these situations consideration should be given to the use of strong mortar mixes using Portland cement or sulfate-resisting Portland cement. Calcium silicate and concrete masonry do not contain soluble sulfates. However, masonry built of these units can be vulnerable to sulfates from other sources.

Expert advice should be sought regarding the selection of concrete masonry units when it is intended to use them where significant concentrations of sulfates could be present, e. As a result, the masonry will be more likely to become very wet or saturated, so increasing the risk of frost damage or disfiguration. In such cases more durable masonry units and mortar should be selected, and this can in turn govern the choice for the whole building. Examples of architectural features leading to increased local exposure are: a recessed windows with sloping masonry at the bottom; b flush sills; c inadequate or non-existent overhangs at verges; d large expanses of glazing or impermeable cladding with no effective form of construction at the base designed to shed run-off rainwater clear of the masonry beneath; e areas of rendering adjoining the masonry and recessed from it without an efficient seal at the junction, or other detail to prevent the entry of water to the back of the render; f vertical tile hanging, the lower edge of which has little or no projection over the walling below.

The best evidence of ability to withstand frost damage is provided by brick masonry which has been in service for some years. Such cappings give relatively little protection to the masonry beneath, which can become saturated for up to 1 m below the capping level, depending on the water absorption of the masonry units used. It is strongly recommended that parapets and chimneys should be protected by copings and DPCs see 5. Cappings of brick masonry and tile creasing, even though flaunched with mortar, cannot be relied upon to keep out moisture and require an effective DPC beneath them.

Where possible, a one-piece coping, with weathered top and ample overhang, properly throated, is preferred see 5. Reference to experience of durability in service of masonry units and mortar produced from local constituent materials in the geographical area concerned can provide valuable guidance. The recommendations above are for finished work; during construction, masonry units, mortar and recently finished work may need protection see BS Nevertheless, it should be durable enough for use in the intended location.

Few natural stones will not give adequate service between eaves and DPC in buildings of domestic scale. Durability will need to be assessed for the more exposed elements of a building, e. Particular care should be exercised when selecting a stone for which there is no previous local experience of its satisfactory use. There are few established test methods for natural stone.

Water run-off from limestone and magnesian limestone can attack sandstone, some granites and some bricks. Masonry material combinations should be chosen that are not vulnerable to such attack. Alternatively, building detail should prevent the flow of water from limestone to other masonry materials.

For wall ties see Table Bolts, nuts, screws, etc. Reinforcement for non-structural use should be appropriate to the category given in Table 2. For guidance, see BS Joists should not project into a cavity. Table 13 Durability of masonry in finished construction Masonry condition or situation Quality of masonry units and appropriate mortar designations Clay units Calcium silicate units Concrete bricks Concrete blocks Remarks.

Design of Prestressed Girder for Bridge - Prestressed Girder Reinforcement Details

High risk of saturation without freezing High risk of saturation with freezing In building In external works. Some types of autoclaved aerated concrete block may not be suitable. The manufacturer should be consulted. If b made with dense sulfate ground conditions exist, the aggregate complying recommendations in 5. The masonry most vulnerable autoclaved aerated in A2 and A3 is located between mm block see remarks above, and mm below, finished ground in iii level.

In this area masonry will become wet As for A1 in ii or iii and can remain wet for long periods of time, particularly in winter. In conditions of highly mobile As for A1 in ii groundwater, consult the manufacturer on the selection of materials 5. If sulfate ground conditions exist, the recommendations of 5. DPCs of clay units are unlikely to be suitable for walls of other masonry units, as differential movement can occur see 5.

Handbook to BS 5628: Part 2: Structural use of reinforced and prestressed masonry

Table 13 Durability of masonry in finished construction continued Masonry condition or situation Quality of masonry units and appropriate mortar designations Clay units Calcium silicate units Concrete bricks Concrete blocks Remarks. Walls should be protected by roof overhang and other projecting features to minimize the risk of saturation. However, weathering detail may not protect walls in conditions of Very Severe driving rain see 5. Certain architectural features, e. Where Designation iv mortar is used it is essential to ensure that all masonry units, mortar and masonry under construction are protected fully from saturation and freezing see A.

Where FN clay units are used in Designation ii mortar for C2, sulfate-resisting cement should be used see 5. Rendered walls are usually suitable for most wind-driven rain conditions see 5. Where FN or MN clay units are used, sulfate-resisting cement should be used in the mortar and in the base coat of the render see 5. Where Designation iv mortar is used it is essential to ensure that all masonry units, mortar and masonry under construction are fully protected from saturation and freezing see A.

FL or FN in i High risk of or ii saturation, e. Most parapets are likely to be severely exposed irrespective of the climatic exposure of the building as a whole. Some types of BS or BS ; or autoclaved aerated concrete block may not c having a compressive be suitable. Where FN clay units are used in F2, sulfate-resisting cement should be d most types of used see 5.

Single-leaf walls should be rendered only on one face, the other being left free to breathe. All parapets should be provided with a coping. Table 13 Durability of masonry in finished construction continued Masonry condition or situation H Chimneys Quality of masonry units and appropriate mortar designations Clay units Calcium silicate units Concrete bricks Concrete blocks Remarks.

Chimney stacks are normally the most exposed masonry on any building. Brick masonry b made with dense and tile cappings cannot be relied upon to aggregate conforming to keep out moisture. The use of a coping is BS or BS ; or preferable. Some autoclaved aerated concrete blocks may be unsuitable for use in I.

Where b made with dense cappings or copings are used for chimney aggregate conforming to terminals, the use of sulfate-resisting BS or BS ; or cement is strongly recommended c having a compressive see 5. Masonry in free-standing walls is likely to be severely exposed, irrespective of climatic conditions. Some types of autoclaved d most types of aerated concrete block may also be autoclaved aerated unsuitable. The manufacturer should be block see remarks consulted. Because of possible contamination from the ground and saturation by ground waters, in addition to subjection to Severe b made with dense climatic exposure, masonry in retaining aggregate conforming to walls is particularly prone to frost and BS or BS ; or sulfate attack.

It is strongly recommended that such walls d most types of be backfilled with free draining materials. Where FN or MN clay masonry see remarks units are used, the use of sulfate-resisting cement may be necessary see 5. Some types of autoclaved aerated concrete block are not suitable for use in K1.

Some concrete blocks are not suitable for use in K2. Table 13 Durability of masonry in finished construction concluded Masonry condition or situation Quality of masonry units and appropriate mortar designations Clay units Calcium silicate units Concrete bricks Concrete blocks Remarks. Where FN or MN clay units are used, sulfate-resisting cement should be used. If sulfate ground conditions exist the b made with dense recommendation in 5. The c having a compressive manufacturer should be consulted. The d most types of manufacturer should be consulted. Improved workability of mortars containing less cement can be achieved by incorporation of lime, plasticizers or entrained air or any combination of these.

In Table 14, mortar mixes are grouped in four Designations. Within each Designation, mixes produce mortars of approximately equal strength and durability. As cement content is reduced, strength is reduced, but such mortars are more able to accommodate minor movements of the masonry. Adhesion to dry absorbent units can be considerably improved by the incorporation of a water-retaining admixture.

The lime used should be non-hydraulic to BS In effect, the air bubbles serve to increase the volume of the binder paste, filling the voids in the sand, and this improves the working qualities. The incorporation of lime and air-entraining agents in a mortar mix combines the workability benefits of the lime with the freeze-thaw durability benefit of the air-entrainment.

Alternatively, the benefits of air-entrainment can be obtained by the use of air-entrained cement:sand mixes, or by the use of mixes based on masonry cement. All factory-made and masonry cement-based mortars are air-entrained. Table 14 Mortar mixes Types of mortar Binder constituents Cement:lime:sanda Masonry cement:sanda Cement:sanda plasticized. Masonry cement containing a Portland cement and lime in the approximate ratio , and an air entraining additive Masonry cement containing a Portland cement and inorganic materials other than limeb and an air entraining additive NOTE 1 The range of sand volumes is to allow for the effects of differences in grading on the properties of the mortar.

Mortars incorporating both lime and air-entrainment can be used with any sands within the BS and gradings. NOTE 2 Air entrainment to improve the durability and the working properties of the mortar is recommended. It may be achieved by the use of air-entrained cements, either masonry cements or improved cements, by the addition of plasticizer to the site mixer, or by the use of a factory made mortar. Improved cements are Portland cements modified for use in masonry containing a relatively small amount of air entrainment.

They are produced for use in masonry and similar applications.


  1. The Economist (21 September 2013)?
  2. Mothers and Sons: Stories.
  3. Prehistory of the Indo-Malaysian Archipelago.
  4. Free Handbook To Bs 5628: Structural Use Of Reinforced And Prestressed Masonry.
  5. The Compton and Duane Effects.
  6. Principles and dynamics of the Critical Zone;
  7. NOTE 3 BS contains recommendations for other mortar mixes that are more suitable for the repair of the masonry of historic buildings. Cements for mortar and the standards to which they should conform are listed in 5. Although where frost conditions are anticipated there would be some advantage in accelerating the setting of mortar, in practice no suitable admixtures are known that are free from other undesirable effects. In particular, calcium chloride, or admixtures based on this salt, can lead to subsequent dampness or corrosion of embedded metals, including wall ties. Therefore, calcium chloride and admixtures containing calcium chloride should not be added to mortars.

    There is little experience of the successful use of any admixture intended to provide frost protection by depressing the freezing point of the mixing water. Some substances that might be contemplated for this purpose, e. Styrene butadiene rubber SBR admixture may be used to make special mortars with high bond properties. Water retaining admixtures may be used for mortars where high suction masonry units are involved. They are especially useful for plasters and rendering mortars.

    Where mortars are to be specified by strength or where special category construction is to be used, the proportions should be determined from tests see BS , Annex A. In practice, the designer has the following three options: a to specify the Designation and type of mortar and leave the contractor free to batch mix to obtain adequate workability; b to specify actual mix proportions to be used for a particular sand or provide sufficient guidance on the grading of sand to enable the contractor to determine where, within the range, the sand should be proportioned.

    In such cases, consideration should be given to using sands having a particle distribution towards the coarser end of the BS and grading envelope. Such sands can be found among those conforming to grade M of BS ; c to specify the lowest mix proportions for each type and Designation, e. The mortar Designation and type should be selected by reference to structural requirements and taking into account the type of construction, position in the building, degree of exposure see 5.

    The mortars in Table 14 have been selected to provide the most suitable mortar that will be readily workable to allow the bricklayer or blocklayer to produce satisfactory work at an economic rate, be sufficiently durable and be able to assist in accommodating the strains arising from minor movements within the wall. Where a mortar designation richer than the minimum Designation recommended for durability in Table 13 is required for structural reasons, careful consideration should be given to the accommodation of movement see 5. Proportioning by mass will produce more consistent mortars than proportioning by volume.

    Fire resistance is the time from the start of the tests laid down in BS to inclusive, until failure occurs under any one of the listed criteria, i. This time ranges from 30 min to 6 h and is a property of the complete element of structure. Table 15 gives notional fire resistances of walls for various types of construction. Other forms of construction may be used, provided evidence of satisfactory performance in use, based on the results of standard fire resistance tests, is produced.

    If the required fire resistance of a loadbearing cavity wall with a thickness taken from Table 15 is more than 2 h, the imposed load should be shared by both leaves; otherwise, if the imposed load is carried by the exposed leaf only, the minimum thickness of the exposed leaf should be that given for loadbearing single-leaf walls.

    For panel walls which are to provide fire resistance where edge isolation is necessary, special consideration should be given to the edge details. Where movement joints or edge clearances are required for walls designed to resist fire, they should be filled with a non-combustible material, such as mineral fibre, which still allows the movement joint to function.

    Consideration should be given to non-combustible cover strips fixed to both faces of the wall on one side of the joint. Table 15 Notional fire resistance of walls Material Masonry unit type Finisha Minimum thickness of masonryb for notional period of fire resistance mm 6h A Loadbearing single-leaf walls 4h 3h 2h 90 min 60 min 30 min. NOTE Non-loadbearing walls are assumed to carry no load other than their own weight and edge restraint. Loadbearing walls may carry any load up to that which produces the maximum permissible design stresses. The finish should be not less than 13 mm plaster or rendering on each face of a single-leaf wall and on the exposed faces of a cavity wall.

    Plasterboard of an equivalent thickness may be substituted for fire resistance periods up to 2 h. VG is vermiculite:gypsum plaster 1. Perlite may be substituted for vermiculite for clay bricks and other materials with similar surfaces. The thickness represents either the work size of the unit, or, where applicable for solid walls, the sum of the work sizes of two units together with the work size of the joint between them.

    The minimum thickness given is suitable for 75 mm brick-on-edge construction with a completely solid unit with plane faces. The number of cells is that in any cross-section through the wall thickness. Class 1 aggregates for concrete blocks include limestone, air-cooled blastfurnace slag, foamed or expanded slag, crushed brick, well-burnt clinker, expanded clay or shale, sintered pulverized-fuel ash and pumice.

    Class 2 aggregates for concrete blocks include all gravels and crushed natural stone, except limestone. These thicknesses may be reduced to mm for walls built with cellular bricks. Thicknesses given here are minimum thicknesses for each leaf in millimetres mm. These thicknesses may be reduced to 90 mm if the load is distributed over both leaves.

    Table 15 Notional fire resistance of walls continued Material Masonry unit type Finisha Minimum thickness of masonryb for notional period of fire resistance mm 6h B Non-loadbearing single-leaf walls 4h 3h 2h 90 min 60 min 30 min. Solid brick None see 3. Table 15 Notional fire resistance of walls continued Material Masonry unit type Finisha Minimum thickness of masonryb for notional period of fire resistance mm 6h B Non-loadbearing single-leaf walls continued 4h 3h 2h 90 min 60 min 30 min.

    Table 15 Notional fire resistance of walls concluded Material Masonry unit type Finisha Minimum thickness of masonryb for notional period of fire resistance mm 6h C Load-bearing cavity wallsg continued 4h 3h 2h 90 min 60 min 30 min. Solid brick see 3. NOTE Where the manufacturers values are not available, the user is referred to [7] for guidance. Insulation may be positioned externally, internally or within any cavity. The designer should ensure that the construction selected does not conflict with other recommendations of this code. Thermal insulation materials should be specified and installed in accordance with the relevant British Standards and the manufacturers instructions and should be suitable for the particular category of exposure.

    NOTE For guidance on the selection of insulation and the design of insulated masonry, the user is referred to [7]. This is characterized by a drop in the internal surface temperature of the wall in the region of the cold bridge. Further information can be found in [6] and [8]. Detailed information is given in BS The risk of condensation may be assessed using various calculation procedures, e. If interstitial condensation is predicted then the designer should assess the suitability of a construction by considering the position in the construction where condensation occurs, the quantity of condensate expected and the likely effect of the condensate on adjoining materials.

    For example, condensation on the cavity face of the external leaf of a cavity masonry wall is generally inconsequential. Structure borne sound can originate from impact on a surface or from airborne sound impinging on the surface of the structure. However, when designing walls, the origin is generally assumed to be airborne sound rather than impact sound. Sound generated in the air in one room radiates to the surfaces of the enclosing structure and is transmitted through the structural elements, sound separating walls, and flanking walls and floors to an adjacent room, where the sound is normally transmitted through the air to the ear.

    The sound insulation of a single-leaf masonry wall is largely related to its mass per unit area, provided that there are no direct air paths through it. The sound insulation of a cavity wall is related to its mass per unit area, the width of the cavity, and the rigidity and spacing of any wall ties. A cavity wall, with a nominal cavity width of 50 mm and leaves connected by wire butterfly ties as recommended in 5. If more rigid ties are used, or a greater number of ties per square metre are used, the sound insulation of the wall is reduced.

    Direct air paths around the separating wall or floor should be avoided. Window reveals should be sealed to prevent direct transmission along the cavity. Care should be taken to avoid air paths in floors which penetrate separating walls. Unplastered walls in attics or roof spaces should be well built with all bed joints and cross joints filled with mortar. For detailed recommendations, designers are directed to BS During this fire, Andrew was the policies she was noteworthy in the study: the none comes positive university in the series and run of automated reputations in Canada.

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