Design of Masonry Structures PDF

DESIGN OF MASONRY STRUCTURES Third edition of Load Bearing Brickwork Design A.W.Hendry, B.Sc., Ph.D., D.Sc, F.I.C.E., F.I. Struct.E., F.R.S.E. B.P.Sinha, B.Sc., Ph.D., F.I. Struct.E., F.I.C.E., C. Eng. and S.R.Davies, B.Sc., Ph.D., M.I.C.E., C.Eng. Department of Civil Engineering University of Ed...

DESIGN OF MASONRY STRUCTURES Third edition of Load Bearing Brickwork Design A.W.Hendry, B.Sc., Ph.D., D.Sc, F.I.C.E., F.I. Struct.E., F.R.S.E. B.P.Sinha, B.Sc., Ph.D., F.I. Struct.E., F.I.C.E., C. Eng. and S.R.Davies, B.Sc., Ph.D., M.I.C.E., C.Eng. Department of Civil Engineering University of Edinburgh, UK E & FN SPON An Imprint of Chapman & Hall London · Weinheim · New York · Tokyo · Melbourne · Madras ©2004 Taylor & Francis 1 Loadbearing masonry buildings 1.1 ADVANTAGES AND DEVELOPMENT OF LOADBEARING MASONRY The basic advantage of masonry construction is that it is possible to use the same element to perform a variety of functions, which in a steelframed building, for example, have to be provided for separately, with consequent complication in detailed construction. Thus masonry may, simultaneously, provide structure, subdivision of space, thermal and acoustic insulation as well as fire and weather protection. As a material, it is relatively cheap but durable and produces external wall finishes of very acceptable appearance. Masonry construction is flexible in terms of building layout and can be constructed without very large capital expenditure on the part of the builder. In the first half of the present century brick construction for multi- storey buildings was very largely displaced by steel- and reinforcedconcrete-framed structures, although these were very often clad in brick. One of the main reasons for this was that until around 1950 loadbearing walls were proportioned by purely empirical rules, which led to excessively thick walls that were wasteful of space and material and took a great deal of time to build. The situation changed in a number of countries after 1950 with the introduction of structural codes of practice which made it possible to calculate the necessary wall thickness and masonry strengths on a more rational basis. These codes of practice were based on research programmes and building experience, and, although initially limited in scope, provided a sufficient basis for the design of buildings of up to thirty storeys. A considerable amount of research and practical experience over the past 20 years has led to the improvement and refinement of the various structural codes. As a result, the structural design of masonry buildings is approaching a level similar to that applying to steel and concrete. ©2004 Taylor & Francis 1.2 BASIC DESIGN CONSIDERATIONS Loadbearing construction is most appropriately used for buildings in which the floor area is subdivided into a relatively large number of rooms of small to medium size and in which the floor plan is repeated on each storey throughout the height of the building. These considerations give ample opportunity for disposing loadbearing walls, which are continuous from foundation to roof level and, because of the moderate floor spans, are not called upon to carry unduly heavy concentrations of vertical load. The types of buildings which are compatible with these requirements include flats, hostels, hotels and other residential buildings. The form and wall layout for a particular building will evolve from functional requirements and site conditions and will call for collaboration between engineer and architect. The arrangement chosen will not usually be critical from the structural point of view provided that a reasonable balance is allowed between walls oriented in the principal directions of the building so as to permit the development of adequate resistance to lateral forces in both of these directions. Very unsymmetrical arrangements should be avoided as these will give rise to torsional effects under lateral loading which will be difficult to calculate and which may produce undesirable stress distributions. Stair wells, lift shafts and service ducts play an important part in deciding layout and are often of primary importance in providing lateral rigidity. The great variety of possible wall arrangements in a masonry building makes it rather difficult to define distinct types of structure, but a rough classification might be made as follows: Cellular wall systems Simple or double cross-wall systems Complex arrangements. A cellular arrangement is one in which both internal and external walls are loadbearing and in which these walls form a cellular pattern in plan. Figure 1.1 (a) shows an example of such a wall layout. The second category includes simple cross-wall structures in which the main bearing walls are at right angles to the longitudinal axis of the building. The floor slabs span between the main cross-walls, and longitudinal stability is achieved by means of corridor walls, as shown in Fig. 1.1(b). This type of structure is suitable for a hostel or hotel building having a large number of identical rooms. The outer walls may be clad in non-loadbearing masonry or with other materials. It will be observed that there is a limit to the depth of building which can be constructed on the cross-wall principle if the rooms are to have ©2004 Taylor & Francis  Fig. 1.1 Typical wall arrangements in masonry buildings. ©2004 Taylor & Francis effective day-lighting. If a deeper block with a service core is required, a somewhat more complex system of cross-walls set parallel to both major axes of the building may be used, as in Fig. 1.1(c). All kinds of hybrids between cellular and cross-wall arrangements are possible, and these are included under the heading ‘complex’, a typical example being shown in Fig. 1.1(d). Considerable attention has been devoted in recent years to the necessity for ensuring the ‘robustness’ of buildings. This has arisen from a number of building failures in which, although the individual members have been adequate in terms of resisting their normal service loads, the building as a whole has still suffered severe damage from abnormal loading, resulting for example from a gas explosion or from vehicle impact. It is impossible to quantify loads of this kind, and what is required is to construct buildings in such a way that an incident of this category does not result in catastrophic collapse, out of proportion to the initial forces. Meeting this requirement begins with the selection of wall layout since some arrangements are inherently more resistant to abnormal forces than others. This point is illustrated in Fig. 1.2: a building consisting only of floor slabs and cross-walls (Fig. 1.2(a)) is obviously unstable and liable to collapse under the influence of small lateral forces acting parallel to its longer axis. This particular weakness could be removed by incorporating a lift shaft or stair well to provide resistance in the weak direction, as in Fig. 1.2(b). However, the flank or gable walls are still vulnerable, for example to vehicle impact, and limited damage to this wall on the lowermost storey would result in the collapse of a large section of the building. A building having a wall layout as in Fig. 1.2(c) on the other hand is clearly much more resistant to all kinds of disturbing forces, having a high degree of lateral stability, and is unlikely to suffer extensive damage from failure of any particular wall. Robustness is not, however, purely a matter of wall layout. Thus a floor system consisting of unconnected precast planks will be much less resistant to damage than one which has cast-in-situ concrete floors with two-way reinforcement. Similarly, the detailing of elements and their connections is of great importance. For example, adequate bearing of beams and slabs on walls is essential in a gravity structure to prevent possible failure not only from local over-stressing but also from relative movement between walls and other elements. Such movement could result from foundation settlement, thermal or moisture movements. An extreme case occurs in seismic areas where positive tying together of walls and floors is essential. The above discussion relates to multi-storey, loadbearing masonry buildings, but similar considerations apply to low-rise buildings where there is the same requirement for essentially robust construction. ©2004 Taylor & Francis  Fig. 1.2 Liability of a simple cross-wall structure to accidental damage. ©2004 Taylor & Francis 1.3 STRUCTURAL SAFETY: LIMIT STATE DESIGN The objective of ensuring a fundamentally stable or robust building, as discussed in section 1.2, is an aspect of structural safety. The measures adopted in pursuit of this objective are to a large extent qualitative and conceptual whereas the method of ensuring satisfactory structural performance in resisting service loads is dealt with in a more quantitative manner, essentially by trying to relate estimates of these loads with estimates of material strength and rigidity. The basic aim of structural design is to ensure that a structure should fulfil its intended function throughout its lifetime without excessive deflection, cracking or collapse. The engineer is expected to meet this aim with due regard to economy and durability. It is recognized, however, that it is not possible to design structures which will meet these requirements in all conceivable circumstances, at least within the limits of financial feasibility. For example, it is not expected that normally designed structures will be capable of resisting conceivable but improbable accidents which would result in catastrophic damage, such as impact of a large aircraft. It is, on the other hand, accepted that there is uncertainty in the estimation of service loads on structures, that the strength of construction materials is variable, and that the means of relating loads to strength are at best approximations. It is possible that an unfavourable combination of these circumstances could result in structural failure; design procedures should, therefore, ensure that the probability of such a failure is acceptably small. The question then arises as to what probability of failure is ‘acceptably small’. Investigation of accident statistics suggests that, in the context of buildings, a one-in-a-million chance of failure leading to a fatality will be, if not explicitly acceptable to the public, at least such as to give rise to little concern. In recent years, therefore, structural design has aimed, indirectly, to provide levels of safety consistent with a probability of failure of this order. Consideration of levels of safety in structural design is a recent development and has been applied through the concept of ‘limit state’ design. The definition of a limit state is that a structure becomes unfit for its intended purpose when it reaches that particular condition. A limit state may be one of complete failure (ultimate limit state) or it may define a condition of excessive deflection or cracking (serviceability limit state). The advantage of this approach is that it permits the definition of direct criteria for strength and serviceability taking into account the uncertainties of loading, strength and structural analysis as well as questions such as the consequences of failure. The essential principles of limit state design may be summarized as follows. Considering the ultimate limit state of a particular structure, for ©2004 Taylor & Francis failure to occur: (1.1) where is the design strength of the structure, and the design loading effects. Here !m and !f are partial safety factors; Rk and Qk are characteristic values of resistance and load actions, generally chosen such that 95% of samples representing Rk will exceed this value and 95% of the applied forces will be less than Qk. The probability of failure is then: (1.2) If a value of p, say 10-6, is prescribed it is possible to calculate values of the partial safety factors, !m and !f, in the limit state equation which would be consistent with this probability of failure. In order to do this, however, it is necessary to define the load effects and structural resistance in statistical terms, which in practice is rarely possible. The partial safety factors, therefore, cannot be calculated in a precise way and have to be determined on the basis of construction experience and laboratory testing against a background of statistical theory. The application of the limit state approach as exemplified by the British Code of Practice BS 5628 and Eurocode 6 (EC 6) is discussed in Chapter 4. 1.4 FOUNDATIONS Building structures in loadbearing masonry are characteristically stiff in the vertical direction and have a limited tolerance for differential movement of foundations. Studies of existing buildings have suggested that the maximum relative deflection (i.e. the ratio of deflection to the length of the deflected part) in the walls of multi-storeyed loadbearing brickwork buildings should not exceed 0.0003 in sand or hard clay and 0.0004 in soft clay. These figures apply to walls whose length exceeds three times their height. It has also been suggested that the maximum average settlement of a brickwork building should not exceed 150 mm. These figures are, however, purely indicative, and a great deal depends on the rate of settlement as well as on the characteristics of the masonry. Settlement calculations by normal soil mechanics techniques will indicate whether these limits are likely to be exceeded. Where problems have arisen, the cause has usually been associated with particular types of clay soils which are subject to excessive shrinkage in periods of dry weather. In these soils the foundations should be at a depth of not less than 1 m in order to avoid moisture fluctuations. High-rise masonry buildings are usually built on a reinforced concrete raft of about 600mm thickness. The wall system stiffens the raft and ©2004 Taylor & Francis helps to ensure uniform ground pressures, whilst the limitation on floor spans which applies to such structures has the effect of minimizing the amount of reinforcement required in the foundation slab. Under exceptionally good soil conditions it may be possible to use spread footings, whilst very unfavourable conditions may necessitate piling with ground beams. 1.5 REINFORCED AND PRESTRESSED MASONRY The preceding paragraphs in this chapter have been concerned with the use of unreinforced masonry. As masonry has relatively low strength in tension, this imposes certain restrictions on its field of application. Concrete is, of course, also a brittle material but this limitation is overcome by the introduction of reinforcing steel or by prestressing. The corresponding use of these techniques in masonry construction is not new but, until recently, has not been widely adopted. This was partly due to the absence of a satisfactory code of practice, but such codes are now available so that more extensive use of reinforced and prestressed masonry may be expected in future. By the adoption of reinforced or prestressed construction the scope of masonry can be considerably extended. An example is the use of prestressed masonry walls of cellular or fin construction for sports halls and similar buildings where the requirement is for walls some 10 m in height supporting a long span roof. Other examples include the use of easily constructed, reinforced masonry retaining walls and the reinforcement of laterally loaded walls to resist wind or seismic forces. In appropriate cases, reinforced masonry will have the advantage over concrete construction of eliminating expensive shuttering and of producing exposed walls of attractive appearance without additional expense. Reinforcement can be introduced in masonry elements in several ways. The most obvious is by placing bars in the bed joints or collar joints, but the diameter of bars which can be used in this way is limited. A second possibility is to form pockets in the masonry by suitable bonding patterns or by using specially shaped units. The steel is embedded in these pockets either in mortar or in small aggregate concrete (referred to in the USA as ‘grout’). The third method, suitable for walls or beams, is to place the steel in the cavity formed by two leaves (or wythes) of brickwork which is subsequently filled with small aggregate concrete. This is known as grouted cavity construction. Elements built in this way can be used either to resist in-plane loading, as beams or shear walls, or as walls under lateral loading. In seismic situations it is possible to bond grouted cavity walls to floor slabs to give continuity to the structure. Finally, reinforcement can be accommodated in hollow block ©2004 Taylor & Francis walls or piers, provided that the design of the blocks permits the formation of continuous ducts for the reinforcing bars. Prestressed masonry elements are usually post-tensioned, the steel, in strand or bar form, being accommodated in ducts formed in the masonry. In some examples of cellular or diaphragm wall construction the prestressing steel has been placed in the cavity between the two masonry skins, suitably protected against corrosion. It is also possible to prestress circular tanks with circumferential wires protected by an outer skin of brickwork built after prestressing has been carried out. ©2004 Taylor & Francis 2 Bricks, blocks and mortars 2.1 INTRODUCTION Masonry is a well proven building material possessing excellent properties in terms of appearance, durability and cost in comparison with alternatives. However, the quality of the masonry in a building depends on the materials used, and hence all masonry materials must conform to certain minimum standards. The basic components of masonry are block, brick and mortar, the latter being in itself a composite of cement, lime and sand and sometimes of other constituents. The object of this chapter is to describe the properties of the various materials making up the masonry. 2.2 BRICKS AND BLOCKS 2.2.1 Classification Brick is defined as a masonry unit with dimensions (mm) not exceeding 337.5×225×112.5 (L×w×t). Any unit with a dimension that exceeds any one of those specified above is termed a block. Blocks and bricks are made of fired clay, calcium silicate or concrete. These must conform to relevant national standards, for example in the United Kingdom to BS 3921 (clay units), BS 187 (calcium silicate) and BS 6073: Part 1 (concrete units). In these standards two classes of bricks are identified, namely common and facing; BS 3921 identifies a third category, engineering: Common bricks are suitable for general building work. Facing bricks are used for exterior and interior walls and available in a variety of textures and colours. Engineering bricks are dense and strong with defined limits of absorption and compressive strength as given in Table 2.2. ©2004 Taylor & Francis Bricks must be free from deep and extensive cracks, from damage to edges and corners and also from expansive particles of lime. Bricks are also classified according to their resistance to frost and the maximum soluble salt content. (a) Designation according to frost resistance Frost resistant (F): These bricks are durable in extreme conditions of exposure to water and freezing and thawing. These bricks can be used in all building situations. Moderately frost resistant (M): These bricks are durable in the normal condition of exposure except in a saturated condition and subjected to repeated freezing and thawing. Not frost resistant (O): These bricks are suitable for internal use. They are liable to be damaged by freezing and thawing unless protected by an impermeable cladding during construction and afterwards. (b) Designation according to maximum soluble salt content Low (L): These clay bricks must conform to the limit prescribed by BS 3921 for maximum soluble salt content given in Table 2.1. All engineering and some facing or common bricks may come under this category. Normal (N): There is no special requirement or limit for soluble salt content. 2.2.2 Varieties Bricks may be wire cut, with or without perforations, or pressed with single or double frogs or cellular. Perforated bricks contain holes; the cross-sectional area of any one hole should not exceed 10% and the volume of perforations 25% of the total volume of bricks. Cellular bricks will have cavities or frogs exceeding 20% of the gross volume of the brick. In bricks having frogs the total volume of depression should be Table 2.1 Maximum salt content of low (L) brick (BS 3921) ©2004 Taylor & Francis less than or equal to 20%. In the United Kingdom, calcium silicate or concrete bricks are also used, covered by BS 187 and BS 6073. Bricks of shapes other than rectangular prisms are referred to as ‘standard special’ and covered by BS 4729. Concrete blocks may be solid, cellular or hollow. Different varieties of bricks and blocks are shown in Figs. 2.1 and 2.2. 2.2.3 Compressive strength From the structural point of view, the compressive strength of the unit is the controlling factor. Bricks of various strengths are available to suit a wide range of architectural and engineering requirements. Table 2.2 gives a classification of bricks according to the compressive strength. For low-rise buildings, bricks of 5.2 N/mm 2 should be sufficient. For dampproof courses, low-absorption engineering bricks are usually required. For reinforced and prestressed brickwork, it is highly unlikely that brick strength lower than 20 N/mm2 will be used in the UK. Calcium silicate bricks of various strengths are also available. Table 2.3 gives the class and strength of these bricks available. Concrete bricks of minimum average strength of 21 to 50 N/mm2 are available. Solid, cellular and hollow concrete blocks of various thicknesses and strengths are manufactured to suit the design requirements. Both the thickness and the compressive strength of concrete blocks are given in Table 2.4. 2.2.4 Absorption Bricks contain pores; some may be ‘through’ pores, others are ‘cul-de-sac’ or even sealed and inaccessible. The ‘through’ pores allow air to escape in the 24 h absorption test (BS 3921) and permit free passage of water. However, others in a simple immersion test or vacuum test do not allow the passage of water, hence the requirement for a 5 h boiling or vacuum test. The absorption is the amount of water which is taken up to fill these pores in a brick by displacing the air. The saturation coefficient is the ratio of 24 h cold absorption to maximum absorption in vacuum or boiling. The absorption of clay bricks varies from 4.5 to 21% by weight and those of calcium silicate from 7 to 21% and concrete units 7 to 10% by weight. The saturation coefficient of bricks may range approximately from 0.2 to 0.88. Neither the absorption nor the saturation coefficient necessarily indicates the liability of bricks to decay by frost or chemical action. Likewise, absorption is not a mandatory requirement for concrete bricks or blocks as there is no relationship between absorption and durability. ©2004 Taylor & Francis  Fig. 2.1 Types of standard bricks. ©2004 Taylor & Francis  Fig. 2.2 Concrete blocks. Table 2.2 Classification of clay bricks according to compressive strength and absorption Table 2.3 Compressive strength classes and requirements of calcium silicate bricks ©2004 Taylor & Francis  Table 2.4 Compressive strength and thickness of concrete blocks 2.2.5 Frost resistance The resistance of bricks to frost is very variable and depends on the degree of exposure to driving rain and temperature. Engineering bricks with high compressive strength and low absorption are expected to be frost resistant. However, some bricks of low strength and high absorption may be resistant to frost compared to low-absorption and highstrength brick. Bricks can only be damaged provided 90% of the available pore space is filled with water about freezing temperature, since water expands onetenth on freezing. Hence, low or high absorption of water by a brick does not signify that all the available pores will become filled with water. Calcium silicate bricks of 14 N/mm2 or above are weather resistant. In the United Kingdom, frost damage is not very common as brickwork is seldom sufficiently saturated by rain, except in unprotected cornices, parapets, free-standing and retaining walls. However, bricks and mortar must be carefully selected to avoid damage due to frost. Table 2.7 shows the minimum qualities of clay and calcium silicate bricks to be used for various positions in walls. Precast concrete masonry units are frost resistant. 2.2.6 Dimensional changes (a) Thermal movement All building materials expand or contract with the rise and fall of temperature. The effect of this movement is dealt with in Chapter 13. ©2004 Taylor & Francis Fig. 2.3 Expansion of kiln-fresh bricks due to absorption of moisture from atmosphere. (b) Moisture movement One of the common causes of cracking and decay of building materials is moisture movement, which may be wholly or partly reversible or, in some circumstances, irreversible. The designer should be aware of the magnitude of this movement. Clay bricks being taken from the kiln expand owing to absorption of water from the atmosphere. The magnitude of this expansion depends on the type of brick and its firing temperature and is wholly irreversible. A large part of this irreversible movement takes place within a few days, as shown in Fig. 2.3, and the rest takes place over a period of about six months. Because of this moisture movement, bricks coming fresh from the kiln should never be delivered straight to the site. Generally, the accepted time lag is a fortnight. Subsequent moisture movement is unlikely to exceed 0.02%. In addition to this, bricks also undergo partly or wholly reversible expansion or contraction due to wetting or drying. This is not very significant except in the case of the calcium silicate bricks. Hence, the designer should incorporate ‘expansion’ joints in all walls of any considerable length as a precaution against cracking. Normally, movement joints in calcium silicate brickwork may be provided at intervals of 7.5 to 9.0 m depending upon the moisture content of bricks at the time of laying. In clay brickwork expansion joints at intervals of 12.2 to 18.3 m may be provided to accommodate thermal or other movements. The drying shrinkage of concrete brick/blockwork should not exceed 0.06%. In concrete masonry, the movement joint should be provided at 6 ©2004 Taylor & Francis  Table 2.5 Moisture movement in different building materials m intervals as a general rule. However, the length of the panel without movement joint should not exceed twice the height. Some indication of reversible or irreversible movement of various building materials is shown in Table 2.5. The EC6 gives guidance for the design values of dimensional changes for unreinforced masonry, which are given in Chapter 4 (section 4.4). 2.2.7 Soluble salts (a) Efflorescence All clay bricks contain soluble salts to some extent. The salt can also find its way from mortar or soil or by contamination of brick by foreign agents. In a new building when the brickwork dries out owing to evaporation of water, the dissolved salts normally appear as a white deposit termed ‘efflorescence’ on the surface of bricks. Sometimes the colour may be yellow or pale green because of the presence of vanadium or chromium. The texture may vary from light and fluffy to hard and glassy. Efflorescence is caused by sulphates of sodium, potassium, magnesium and calcium; not all of these may be present in a particular case. Efflorescence can take place on drying out brickwork after construction or subsequently if it is allowed to become very wet. By and large, efflorescence does not normally result in decay, but in the United Kingdom, magnesium sulphate or sodium sulphate may cause disruption due to crystallization. Abnormal amounts of sodium sulphate, constituting more than 3% by weight of a brick, will cause disruption of its surface. Brick specimens showing efflorescence in the ‘heavy’ category are not considered to comply with BS 3921. (b) Sulphate attack Sulphates slowly react in the presence of water with tricalcium aluminate, which is one of the constituents of Portland cement and ©2004 Taylor & Francis hydraulic lime. If water containing dissolved sulphate from clay bricks or aggregates reaches the mortar, this reaction takes place, causing mortar to crack and spall and thus resulting in the disintegration of the masonry. Sulphate attack is only possible if the masonry is exposed to very long and persistent wet conditions. Chimneys, parapets and earth- retaining walls which have not been properly protected from excessive dampness may be vulnerable to sulphate attack. In general, it is advisable to keep walls as dry as possible. In conditions of severe exposure to rain, bricks (L) or sulphate-resistant cement should be used. The resistance of mortar against sulphate attack can be increased by specifying a fairly rich mix, i.e. stronger than grade (iii) mortar (1:1:6) or replacing lime with a plasticizer. Calcium silicate and concrete units do not contain significant amounts of sulphate compared to clay bricks. However, concrete bricks of minimum 30 N/mm2 strength should be used in mortar for earth-retaining walls, cills and copings. 2.2.8 Fire resistance Clay bricks are subjected to very much higher temperatures during firing than they are likely to be exposed to in a building fire. As a result, they possess excellent fire resistance properties. Calcium silicate bricks have similar fire resistance properties to clay bricks. Concrete bricks and blocks have 30 min to 6 h notional fire resistance depending on the thickness of the wall. 2.3 MORTAR The second component in brickwork is mortar, which for loadbearing brickwork should be a cement:lime:sand mix in one of the designations shown in Table 2.6. For low-strength bricks a weaker mortar, 1:2:9 mix by volume, may be appropriate. For reinforced and prestressed brickwork, mortar weaker than grade (ii) is not recommended. 2.3.1 Function and requirement of mortar In deciding the type of mortar the properties needing to be considered are: Development of early strength. Workability, i.e. ability to spread easily. Water retentivity, i.e. the ability of mortar to retain water against the suction of brick. (If water is not retained and is extracted quickly by a high-absorptive brick, there will be insufficient water left in the mortar joint for hydration of the cement, resulting in poor bond between brick and mortar.) ©2004 Taylor & Francis  Table 2.6 Requirements for mortar (BS 5628) ©2004 Taylor & Francis  Proper development of bond with the brick. Resistance to cracking and rain penetration. Resistance to frost and chemical attack, e.g. by soluble sulphate. Immediate and long-term appearance. 2.3.2 Cement The various types of cement used for mortar are as follows. (a) Portland cement Ordinary Portland cement and rapid-hardening cement should conform to a standard such as BS 12. Rapid-hardening cement may be used instead of ordinary Portland cement where higher early strength is required; otherwise its properties are similar. Sulphate-resistant cement should be used in situations where the brickwork is expected to remain wet for prolonged periods or where it is susceptible to sulphate attack, e.g. in brickwork in contact with sulphate-bearing soil. (b) Masonry cement This is a mixture of approximately 75% ordinary Portland cement, an inert mineral filler and an air-entraining agent. The mineral filler is used to reduce the cement content, and the air-entraining agent is added to improve the workability. Mortar made from masonry cement will have lower strength compared to a normal cement mortar of similar mix. The other properties of the mortar made from the masonry cement are intermediate between cement:lime:sand mortar and plasticized cement:sand mortar. 2.4 LIME: NON-HYDRAULIC OR SEMI-HYDRAULIC LIME Lime is added to cement mortar to improve the workability, water retention and bonding properties. The water retentivity property of lime is particularly important in situations where dry bricks might remove a considerable amount of water from the mortar, thus leaving less than required for the hydration of the cement. Two types of lime are used, non-hydraulic or semi-hydraulic, as one of the constituents of mortar for brickwork. These limes are differentiated by the process whereby they harden and develop their strengths. Non-hydraulic lime initially stiffens because of loss of water by evaporation or suction by bricks, and eventually hardens because of slow carbonation, i.e. absorption of carbon dioxide from the air to change calcium hydroxide to calcium carbonate. Semi-hydraulic lime will harden in wet conditions as a result of the presence of small quantities of compounds of silica and alumina. It ©2004 Taylor & Francis  hardens owing to chemical reaction with water rather than atmospheric action. In the United Kingdom, the lime used for mortar must conform to BS 890. 2.5 SAND The sand for mortar must be clean, sharp, and free from salt and organic contamination. Most natural sand contains a small quantity of silt or clay. A small quantity of silt improves the workability. Loam or clay is moisture-sensitive and in large quantities causes shrinkage of mortar. Marine and estuarine sand should not be used unless washed completely to remove magnesium and sodium chloride salts which are deliquescent Fig. 2.4 Grading limits of mortar sand (BS 1200). ©2004 Taylor & Francis and attract moisture. Specifications of sand used for mortar, such as BS 1200, prescribe grading limits for the particle size distribution. The limits given in BS 1200 are as shown in Fig. 2.4, which identifies two types of sand:sand type S and sand type G. Both types of sand will produce satisfactory mortars. However, the grading of sand type G, which falls between the lower limits of sand S and sand G, may require slightly more cement for a particular grade of mortar to satisfy the strength requirement envisaged in BS 5628 (refer to Table 2.6). 2.6 WATER Mixing water for mortar should be clean and free from contaminants either dissolved or in suspension. Ordinary drinking water will be suitable. 2.7 PLASTICIZED PORTLAND CEMENT MORTAR To reduce the cement content and to improve the workability, plasticizer, which entrains air, may be used. Plasticized mortars have poor water retention properties and develop poor bond with highly absorptive bricks. Excessive use of plasticizer will have a detrimental effect on strength, and hence manufacturers’ instructions must be strictly followed. Plasticizer must comply with the requirements of BS 4887. 2.8 USE OF PIGMENTS On occasion, coloured mortar is required for architectural reasons. Such pigments should be used strictly in accordance with the instructions of the manufacturer since excessive amounts of pigment will reduce the compressive strength of mortar and interface bond strength. The quantity of pigment should not be more than 10% of the weight of the cement. In the case of carbon black it should not be more than 3%. 2.9 FROST INHIBITORS Calcium chloride or preparations based on calcium chloride should not be used, since they attract water and cause dampness in a wall, resulting in corrosion of wall ties and efflorescence. 2.10 PROPORTIONING AND STRENGTH The constituents of mortar are mixed by volume. The proportions of material and strength are given in Table 2.6. For loadbearing brickwork the mortar must be gauged properly by the use of gauging boxes and preferably should be weigh-batched. ©2004 Taylor & Francis  Recent research (Fig. 2.5) has shown that the water/cement ratio is the most important factor which affects the compressive strength of grades I, II and III mortars. In principle, therefore, it would be advisable for the structural engineer to specify the water/cement ratio for mortar to be used for structural brickwork; but, in practice, the water/cement ratio for a given mix will be determined by workability. There are various laboratory tests for measuring the consistency of mortar, and these have been related to workability. Thus in the United Kingdom, a dropping ball test is used in which an acrylic ball of 10 mm diameter is dropped on to the surface of a sample of mortar from a height of 250 mm. A ball penetration of 10 mm is associated with satisfactory workability. The test is, however, not used on site, and it is generally left to the bricklayer to adjust the water content to achieve optimum workability. This in fact achieves a reasonably consistent water/cement ratio which varies from one mix to another. The water/cement ratio for 10mm ball penetration, representing satisfactory workability, has been indicated in Fig. 2.5 for the three usual mortar mixes. It is important that the practice of adding water to partly set mortar to restore workability (known as ‘knocking up’ the mix) should be prevented. 2.11 CHOICE OF UNIT AND MORTAR Table 2.7 shows the recommended minimum quality of clay or calcium silicate or concrete bricks/blocks and mortar grades which should be used in various situations from the point of view of durability. 2.12 WALL TIES In the United Kingdom, external cavity walls are used for environmental reasons. The two skins of the wall are tied together to provide some degree of interaction. Wall ties for cavity walls should be galvanized mild steel or stainless steel and must comply to BS 1243. Three types of ties (Fig. 2.6) are used for cavity walls. Vertical twist type made from 20 mm wide, 3.2 to 4.83 mm thick metal strip ‘Butterfly’—made from 3.15 mm wire Double-triangle type—made from 4.5 mm wire. For loadbearing masonry vertical twist type ties should be used for maximum co-action. For a low-rise building, or a situation where large differential movement is expected or for reason of sound insulation, more flexible ties should be selected. In certain cases where large differential movements have to be accommodated, special ties or fixings have to be used (see Chapter 13). In specially unfavourable situations ©2004 Taylor & Francis Fig. 2.5 Effect of water/cement ratio on the compressive strength of mortar of grades I, II and III. non-ferrous or stainless-steel ties may be required. BS 5628 (Table 6) gives guidance for the selection and use of ties for normal situations. 2.13 CONCRETE INFILL AND GROUT The mix proportion by volume for reinforced and prestressed masonry should be 1:0 to cement:lime:sand:10 mm maximum size aggregate. ©2004 Taylor & Francis  Table 2.7 Durability of masonry in finished constructiona (BS 5628) (A) Work below or near external ground level ©2004 Taylor & Francis  Table 2.7 (Contd) ©2004 Taylor & Francis ©2004 Taylor & Francis  Table 2.7 (Contd) ©2004 Taylor & Francis ©2004 Taylor & Francis  Table 2.7 (Contd) ©2004 Taylor & Francis ©2004 Taylor & Francis  Table 2.7 (Contd) ©2004 Taylor & Francis ©2004 Taylor & Francis  Table 2.7 (Contd) ©2004 Taylor & Francis ©2004 Taylor & Francis  Fig. 2.6 Metal wall ties suitable for cavity walls. ©2004 Taylor & Francis Table 2.8 Chloride content of mixes Table 2.9 Characteristic tensile strength of reinforcing steel The maximum size of the aggregate can be increased depending on the size and configuration of the void to be filled with concrete. In some cases it would be possible to use concrete design mix as specified in BS 5328 for reinforced and prestressed masonry. In reinforced and prestressed masonry, the bricks or blocks coming in contact with concrete will absorb water from the mix depending on its water retentivity property, and hence maximum free water/cement ratio used in BS 8110 may not be applicable. In order to compensate for this and for free flowing of the mix to fill the space and the void, a slump of 75 mm and 175mm for concrete mix has been recommended in BS 5628: Part 2. In prestressed sections where tendons are placed in narrow ducts, a neat cement or sand:cement grout having minimum compressive strength of 17 N/mm2 at 7 days may be used. ©2004 Taylor & Francis Fig. 2.7 Typical short-term design stress-strain curve for normal and low- relaxation tendons. The mix must conform to the limit prescribed by BS 5628: Part 2 for maximum total chloride content as in Table 2.8. 2.14 REINFORCING AND PRESTRESSING STEEL 2.14.1 Reinforcing steel Hot-rolled or cold-worked steel bars and fabric conforming to the relevant British Standard can be used as reinforcement. The characteristic strengths of reinforcement are given in Table 2.9. In situations where there is risk of contamination by chloride, solid stainless steel or low-carbon steel coated with at least 1 mm of austenitic stainless steel may be used. 2.14.2 Prestressing steel Wire, strands and bars complying to BS 4486 or BS 5896 can be used for prestressing. Seventy per cent of the characteristic breaking load is allowed as jacking force for prestressed masonry which is less than the 75% normally allowed in prestressed concrete. If proper precautions are taken, there is no reason why the initial jacking force cannot be taken to 75–80% of the breaking load. This has been successfully demonstrated in a series of prestressed brick test beams at Edinburgh University. The short-term design stress-strain curve for prestressing steel is shown in Fig. 2.7. ©2004 Taylor & Francis 3 Masonry properties 3.1 GENERAL Structural design in masonry requires a clear understanding of the behaviour of the composite unit-mortar material under various stress conditions. Primarily, masonry walls are vertical loadbearing elements in which resistance to compressive stress is the predominating factor in design. However, walls are frequently required to resist horizontal shear forces or lateral pressure from wind and therefore the strength of masonry in shear and in tension must also be considered. Current values for the design strength of masonry have been derived on an empirical basis from tests on piers, walls and small specimens. Whilst this has resulted in safe designs, it gives very little insight into the behaviour of the material under stress so that more detailed discussion on masonry strength is required. 3.2 COMPRESSIVE STRENGTH 3.2.1 Factors affecting compressive strength The factors set out in Table 3.1 are of importance in determining the compressive strength of masonry. Table 3.1 Factors affecting masonry strength ©2004 Taylor & Francis 3.2.2 Unit/mortar/masonry strength relationship A number of important points have been derived from compression tests on masonry and associated standard tests on materials. These include, first, that masonry loaded in uniform compression will fail either by the development of tension cracks parallel to the axis of loading or by a kind of shear failure along certain lines of weakness, the mode of failure depending on whether the mortar is weak or strong relative to the units. Secondly, it is observed that the strength of masonry in compression is smaller than the nominal compressive strength of the units as given by a standard compressive test. On the other hand, the masonry strength may greatly exceed the cube crushing strength of the mortar used in it. Finally, it has been shown that the compressive strength of masonry varies roughly as the square root of the nominal unit crushing strength and as the third or fourth root of the mortar cube strength. From these observations it may be inferred that: 1. The secondary tensile stresses which cause the splitting type of failure result from the restrained deformation of the mortar in the bed joints of the masonry. 2. The apparent crushing strength of the unit in a standard test is not a direct measure of the strength of the unit in the masonry, since the mode of failure is different in the two situations. 3. Mortar withstands higher compressive stresses in a brickwork bed joint because of the lateral restraint on its deformation from the unit. Various theories for the compressive strength of masonry have been proposed based on equation of the lateral strains in the unit and mortar at their interface and an assumed limiting tensile strain in the unit. Other theories have been based on measurement of biaxial and triaxial strength tests on materials. But in both approaches the difficulties of determining the necessary materials properties have precluded their practical use, and for design purposes reliance continues to be placed on empirical relationships between unit, mortar and masonry strengths. Such relationships are illustrated in Fig. 3.1 and are incorporated in codes of practice, as set out in Chapter 4 for BS 5628 and Eurocode 6. 3.2.3 Some effects of unit characteristics The apparent strength of a unit of given material increases with decrease in height because of the restraining effect of the testing machine platens on the lateral deformation of the unit. Also, in masonry the units have to resist the tensile forces resulting from restraint of the lateral strains in the mortar. Thus for given materials and joint thickness, the greater the height of the unit the greater the resistance to these forces and the greater ©2004 Taylor & Francis Fig. 3.1 Relationship between brick crushing strength and brickwork strength for various mortar strengths. Based on test results. the compressive strength of the masonry. A corollary of this proposition is that, for a given unit height, increasing the thickness of the mortar joint will decrease the strength of the masonry. This effect is significant for brickwork, as shown in Fig. 3.2, but unimportant in blockwork where the ratio of joint thickness to unit height is small. It follows from this discussion that the shape of a unit influences the strength of masonry built from it, and if units are laid on edge or on end ©2004 Taylor & Francis  Fig. 3.2 Effect of joint thickness on brickwork strength. the resulting masonry strength will be different from that of masonry in which the units are laid on their normal bed faces. The masonry strength will also depend on the type of unit: a highly perforated unit is likely to be relatively weak when compressed in a direction parallel to its length and thus result in a correspondingly lower masonry strength. This is illustrated in Table 3.2 which gives some results for brickwork built with various types of bricks. From this table it can be seen that, although there is a substantial reduction in brickwork strength when built and stressed in directions other than normal, this is not proportional to the brick strength when the latter is compressed in the corresponding direction. No general rule can be given relating brickwork to brick strength when compressed with the units laid on edge or on end. Special considerations apply to masonry built with hollow blocks in which the cores may be unfilled or filled with concrete. In the former case the mortar joint may cover the whole of the bed face of the block (full-bedded) or only the outer shells (shell-bedded). The strength of full-bedded blocks is taken to be that of the maximum test load divided by the gross area of the unit and the masonry strength is ©2004 Taylor & Francis  Table 3.2 Compressive strength of bricks and prisms compressed in different directionsa calculated as if the unit was solid. The strength of shell-bedded masonry should be calculated on the basis of the mortared area of the units. Conventionally, the compressive strength of hollow block masonry built with the cores filled with concrete is taken to be the sum of the strengths of the hollow block and the concreted core tested separately. However, even when the materials are of approximately the same nominal strength, this rule is not always reliable as there can be a difference in the lateral strains of the block and fill materials at the ultimate load, resulting in a tendency for the fill to split the block. Various formulae have been devised to calculate the strength of filled block masonry, as for example the following which has been suggested by Khalaf (1991) to give the prism strength of this type of masonry: (3.1) where fb is the unit material compressive strength, fmr is the mortar compressive strength and fc is the core infill compressive strength. 3.3 STRENGTH OF MASONRY IN COMBINED COMPRESSION AND SHEAR The strength of masonry in combined shear and compression is of importance in relation to the resistance of buildings to lateral forces. Many tests on masonry panels subjected to this type of loading have ©2004 Taylor & Francis been carried out with a view to establishing limiting stresses for use in design. Typical results are shown in Fig. 3.3. It is found that there is a Coulomb type of relationship between shear strength and precompression, i.e. there is an initial shear resistance dependent on adhesion between the units and mortar augmented by a frictional component proportional to the precompression. This may be expressed by the formula: (3.2) where τ0 is the shear strength at zero precompression, µ is an apparent coefficient of friction and σc is the vertical compressive stress. This relationship holds up to a certain limiting value of the vertical compression, beyond which the joint failure represented by the Coulomb equation is replaced by cracking through the units. For clay bricks this limit is about 2.0 N/mm2. The shear strength depends on the mortar strength and for units with a compressive strength between 20 and 50 N/ mm2 set in strong mortar the value of t0 will be approximately 0.3 N/mm2 and 0.2 N/mm2 for medium strength (1:1:6) mortar. The average value of µ is 0.4–0.6. The shear stresses quoted above are average values for walls having a height-to-length ratio of 1.0 or more and the strength of a wall is calculated on the plan area of the wall in the plane of the shear force. Fig. 3.3 Typical relationship between shear strength of brickwork and vertical precompression from test results. ©2004 Taylor & Francis That is to say, if a wall has returns at right angles to the direction of the shear force, the area of the returns is neglected in calculating the shear resistance of the wall. 3.4 THE TENSILE STRENGTH OF MASONRY 3.4.1 Direct tensile strength Direct tensile stresses can arise in masonry as a result of in-plane loading effects. These may be caused by wind, by eccentric gravity loads, by thermal or moisture movements or by foundation movement. The tensile resistance of masonry, particularly across bed joints, is low and variable and therefore is not generally relied upon in structural design. Nevertheless, it is essential that there should be some adhesion between units and mortar, and it is necessary to be aware of those conditions which are conducive to the development of mortar bond on which tensile resistance depends. The mechanism of unit-mortar adhesion is not fully understood but is known to be a physical-chemical process in which the pore structure of both materials is critical. It is known that the grading of the mortar sand is important and that very fine sands are unfavourable to adhesion. In the case of clay brickwork the moisture content of the brick at the time of laying is also important: both very dry and fully saturated bricks lead to low bond strength. This is illustrated in Fig. 3.4, which shows the results of bond tensile tests at brick moisture contents from oven-dry to fully saturated. This diagram also indicates the great variability of tensile bond strength and suggests that this is likely to be greatest at a moisture content of about three-quarters of full saturation, at least for the bricks used in these tests. Direct tensile strength of brickwork is typically about 0.4N/mm2, but the variability of this figure has to be kept in mind, and it should only be used in design with great caution. 3.4.2 Flexural tensile strength Masonry panels used essentially as cladding for buildings have to withstand lateral wind pressure and suction. Some stability is derived from the self-weight of a wall, but generally this is insufficient to provide the necessary resistance to wind forces, and therefore reliance has to be placed on the flexural tensile strength of the masonry. The same factors as influence direct tensile bond, discussed in the preceding section, apply to the development of flexural tensile strength. ©2004 Taylor & Francis Fig. 3.4 Variation of brick-mortar adhesion with moisture content of bricks at time of laying. If a wall is supported only at its base and top, its lateral resistance will depend on the flexural tensile strength developed across the bed joints. If it is supported also on its vertical edges, lateral resistance will depend also on the flexural strength of the brickwork in the direction at right angles to the bed joints. The strength in this direction is typically about three times as great as across the bed joints. If the brick-mortar adhesion is good, the bending strength parallel to the bed joint direction will be limited by the flexural tensile strength of the units. If the adhesion is poor, this strength will be limited mainly by the shear strength of the unit-mortar interface in the bed joints. The flexural tensile strength of clay brickwork ranges from about 2.0 to 0.8N/mm2 in the stronger direction, the strength in bending across the bed joints being about one-third of this. As in the case of direct tension, the strength developed is dependent on the absorption characteristics of the bricks and also on the type of mortar used. Calcium silicate brickwork and concrete blockwork have rather lower flexural tensile strength than clay brickwork, that of concrete blockwork depending on the compressive strength of the unit and the thickness of the wall. ©2004 Taylor & Francis 3.5 STRESS-STRAIN PROPERTIES OF MASONRY Masonry is generally treated as a linearly elastic material, although tests indicate that the stress-strain relationship is approximately parabolic, as shown in Fig. 3.5. Under service conditions masonry is stressed only up to a fraction of its ultimate load, and therefore the assumption of a linear stress-strain curve is acceptable for the calculation of normal structural deformations. Various formulae have been suggested for the determination of Young’s modulus. This parameter is, however, rather variable even for nominally identical specimens, and as an approximation, it may be assumed that (3.3) where is the crushing strength of the masonry. This value will apply up to about 75% of the ultimate strength. For estimating long-term deformations a reduced value of E should be used, in the region of one-half to one-third of that given by equation (3.3). Fig. 3.5 Typical stress-strain curve for brick masonry. ©2004 Taylor & Francis 3.6 EFFECTS OF WORKMANSHIP ON MASONRY STRENGTH Masonry has a very long tradition of building by craftsmen, without engineering supervision of the kind applied to reinforced concrete construction. Consequently, it is frequently regarded with some suspicion as a structural material and carries very much higher safety factors than concrete. There is, of course, some justification for this, in that, if supervision is non-existent, any structural element, whether of masonry or concrete, will be of uncertain strength. If, on the other hand, the same level of supervision is applied to masonry as is customarily required for concrete, masonry will be quite as reliable as concrete. It is therefore important for engineers designing and constructing in masonry to have an appreciation of the workmanship factors which are significant in developing a specified strength. This information has been obtained by carrying out tests on walls which have had known defects built into them and comparing the results with corresponding tests on walls without defects. In practice, these defects will be present to some extent and, in unsatisfactory work, a combination of them could result in a wall being only half as strong in compression as it should be. Such a wall, however, would be obviously badly built and would be so far outside any reasonable specification as to be quite unacceptable. It is, of course, very much better for masonry to be properly built in the first instance, and time spent by the engineer explaining the importance of the points outlined below to the brick- or blocklayer and his immediate supervisor will be time well spent. 3.6.1 Workmanship defects in brickwork (a) Failure to fill bed joints It is essential that the bed joints in brickwork should be completely filled. Gaps in the mortar bed can result simply from carelessness or haste or from a practice known as ‘furrowing’, which means that the bricklayer makes a gap with his trowel in the middle of the mortar bed parallel to the face of the wall. Tests show that incompletely filled bed joints can reduce the strength of brickwork by as much as 33%. Failure to fill the vertical joints has been found to have very little effect on the compressive strength of brickwork but does reduce the flexural resistance. Also, unfilled perpendicular joints are undesirable from the point of view of weather exclusion and sound insulation as well as being indicative of careless workmanship generally. ©2004 Taylor & Francis (b) Bed joints of excessive thickness It was pointed out in discussing the compressive strength of brickwork that increase in joint thickness has the effect of reducing masonry strength because it generates higher lateral tensile stresses in the bricks than would be the case with thin joints. Thus, bed joints of 16–19 mm thickness will result in a reduction of compressive strength of up to 30% as compared with 10mm thick joints. (c) Deviation from verticality or alignment A wall which is built out of plumb, which is bowed or which is out of alignment with the wall in the storey above or below will give rise to eccentric loading and consequent reduction in strength. Thus a wall containing a defect of this type of 12–20 mm will be some 13–15% weaker than one which does not. (d) Exposure to adverse weather after laying Newly laid brickwork should be protected from excessive heat or freezing conditions until the mortar has been cured. Excessive loss of moisture by evaporation or exposure to hot weather may prevent complete hydration of the cement and consequent failure to develop the normal strength of the mortar. The strength of a wall may be reduced by 10% as a result. Freezing can cause displacement of a wall from the vertical with corresponding reduction in strength. Proper curing can be achieved by covering the work with polythene sheets, and in cold weather it may also be necessary to heat the materials if bricklaying has to be carried out in freezing conditions. (e) Failure to adjust suction of bricks A rather more subtle defect can arise if slender walls have to be built using highly absorptive bricks. The reason for this is illustrated in Fig. 3.6, which suggests how a bed joint may become ‘pillow’ shaped if the bricks above it are slightly rocked as they are laid. If water has been removed from the mortar by the suction of the bricks, it may have become too dry for it to recover its originally flat shape. The resulting wall will obviously lack stability as a result of the convex shape of the mortar bed and may be as much as 50% weaker than should be expected from consideration of the brick strength and mortar mix. The remedy is to wet the bricks before laying so as to reduce their suction rate below 2kg/m2/min, and a proportion of lime in the mortar mix will help to retain water in it against the suction of the bricks. ©2004 Taylor & Francis Fig. 3.6 Effect of moisture absorption from mortar bed. Movement of bricks after laying results in ‘pillow’ shaped mortar bed. (f) Incorrect proportioning and mixing of mortar The effect of mortar strength on the strength of masonry may be judged from Fig. 3.1 from which it may be seen with bricks having a crushing strength of SON/mm2 that reducing the mortar strength from 11N/mm2 to 4.5N/mm2 may be expected to reduce the brickwork strength from 14 N/mm2 to 11N/mm2. This corresponds to a change in mortar mix from 1:3 cement:sand to 1:4.5 or about 30% too little cement in the mix. A reduction in mortar strength could also result from a relatively high water/cement ratio whilst still producing a workable mix. It is therefore important to see that the specification for mortar strength is adhered to although there is an inherent degree of tolerance sufficient to accommodate small errors in proportioning and mixing the mortar. The use of unsuitable or an excessive amount of plasticizer in place of lime will produce a porous and possibly weak mortar and has to be guarded against. 3.6.2 Workmanship defects in concrete blockwork Most of the studies on the effect on the compressive strength of masonry, on which the above discussion is based, have been carried out on clay brickwork walls. Some of the factors described, however, apply also to concrete blockwork including the need to fill bed joints and for walls to be built accurately in terms of verticality, planeness and alignment. Excessively thick joints are less likely to be significant in blockwork but the need to meet the specified mortar mix or strength is equally important. Protection against adverse weather conditions is again necessary. ©2004 Taylor & Francis