FAIRFACED CONCRETE BLOCKWORK
Within fairfaced blockwork, an appropriate choice of size is important to both co-ordination and visual scale. Whilst blocks can be cut with a masonry cutter, the addition of small pieces of block, or the widening of perpends over the 10 mm standard, is unaccept- able. The insertion of a thin jumpercourse at floor or lintel height may be a useful feature in adjusting the coursing. Curved blockwork may be constructed from standard blocks, the permissible curvature being dependent on the block size. The oversail between alternate courses should not normally exceed 4 mm in fairfaced work. If the internal radius is exposed, then the perpends can be maintained at 10 mm with uncut blocks, but if the external radius is exposed, the blocks will require cutting on a splay. For tighter curves specialswill be required.
B L O C K S A N D B L O C K W O R K 4 7
Fig. 2.9 Fairfaced blockwork—IDP Offices, Glasgow.Architect: IDP.
Photograph: Courtesy of Ibstock Brick Ltd.
THIN-JOINT MASONRY SYSTEMS
Thin-joint blockwork may be constructed with mor- tar joints of only 2–3 mm, provided that the aircrete or equivalent blocks have been manufactured to fine tol- erances and on-site workmanship is good. The special rapid-setting mortar sets typically within 60 minutes and the full bond strength is achieved after only 2 hours, allowing more courses to be laid each day.
In the case of brick and block cavity construction, the inner leaf is built first, providing a weatherproof enclo- sure as quickly as possible. The outer skin of brickwork can subsequently be built up, using wall ties fixed to the face, either screwed or hammered into the completed blockwork. Bed joints in thin-layer mortar blockwork do not necessarily co-ordinate with those of the brick- work, so conventional cavity wall ties can only be used if they are slope-tolerant.
Usually, inner leaf construction commences with a line of 440×215 mm standard height blocks, with normal bedding mortar to compensate for varia-
tions in the foundation level, followed by the larger 440 or 620 mm×430 mm high blocks, which should weigh less than 20 kg for repeated lifting by one oper- ative. Heavier blocks require mechanical lifting or two-person handling. Thin-joint mortars, consisting of polymer-modified 1 : 2 cement : sand mix with water-retaining and workability admixtures, are fac- tory pre-mixed and require only the addition of water, preferably mixed in with an electrically powered plas- terer’s whisk. The mortar is applied manually with a serrated scoop of the appropriate width or through a pumped system to achieve uniformity. Work should only proceed at temperatures above 5◦C.
The main advantages of thin-joint systems over tra- ditional 10 mm joint blockwork are:
increased productivity allowing storey-height inner leaves to be completed in one day;
up to 10% improved thermal performance due to reduced thermal bridging by the mortar;
improved airtightness of the construction;
the accuracy of the wall, which allows internal thin- coat sprayed plaster finishes to be used;
higher quality of construction and less wastage of mortar.
The acoustic properties of thin-joint mortar walls differ slightly from those of walls constructed with 10 mm mortar joints. Resistance to low-frequency noise is slightly enhanced, whilst resistance to high- frequency sound is slightly reduced.
Completed thin-joint blockwork acts as a mono- lithic slab, which, if unrestrained, may crack at the weaker points, such as near openings. To avoid this, the block units should be laid dry to avoid shrinkage and bed joint reinforcement (1.5 mm thick GRP mesh) should be appropriately positioned. Larger structures require movement joints at 6 m centres.
Certain extruded multi-perforated clay and calcium silicate blocks are designed for use with thin mortar bed joints and dry interlocking vertical joints. One system of clay blocks requires only horizontal propri- etary adhesive joints of 1 mm applied with a special roller tool, as the units are ground to exact dimensions after firing. Blocks may be used for inner and/or outer leaf cavity construction or for solid walls. Whilst this reduces the initial construction time, exposed sides of the units subsequently require plaster or cement render to minimise heat loss by air leakage. Typical block sizes are 300×224 mm and 248×249 mm with widths of 100, 140, 190 and 365 mm.
Fig. 2.10 Selection of bonding patterns for visual blockwork
BOND
A running half-block bond is standard, but this may be reduced to a quarter bond for aesthetic reasons. Block- work may incorporate banding of concrete bricks, but because of differences in thermal and moisture move- ment, it is inadvisable to mix clay bricks with concrete blocks. Horizontal and vertical stack bonds and more sophisticated variations, such as basket-weave bond, may be used for infill panels within framed structures (Fig. 2.10). Such panels will require reinforcement
within alternate horizontal bed joints, to compensate for the lack of normal bonding.
REINFORCEMENT
Blockwork will require bed-joint reinforcement above and below openings where it is inappropriate to divide the blockwork up into panels, with movement joints at the ends of the lintels. Bed-joint reinforcement would be inserted into two bed joints above and below such openings (Fig. 2.11). Cover to bed reinforcement should be at least 25 mm on the external faces and 13 mm on the internal faces. Combined vertical and horizontal reinforcement may be incorporated into hollow blockwork in accordance with BS 5628-2: 2005, where demanded by the calculated stresses. Typical sit- uations would be within retaining basement walls, and large infill panels to a framed structure.
MOVEMENT CONTROL
Concrete blockwork is subject to greater movements than equivalent brickwork masonry. Therefore, the location and form of the movement joints requires greater design-detail consideration, to ensure that inevitable movements are directed to the required loca- tions and do not cause unsightly stepped cracking or fracture of individual blocks. Ideally, such movement joints should be located at intersecting walls, or other points of structural discontinuity, such as columns.
Additionally, movement joints are required at changes in thickness, height or loading of walls, above and below wall openings, and adjacent to movement joints in the adjoining structure (Fig. 2.12). External unrein- forced non-load-bearing concrete masonry walls with a length to height ratio of 3 : 1 or less must be separated into a series of panels with vertical movement-control joints at approximately 9 m centres or more frequently for masonry walls with a length to height ratio of more than 3 : 1 (NA to BS EN 1996-2: 2006).
Wall ties should allow for differential movement between the leaves in cavity construction and should be spaced at 900 mm horizontally and 450 mm vertically, for 50–75 mm cavities.
MORTARS
The mortar must always be weaker than the blocks to allow for movement.
B L O C K S A N D B L O C K W O R K 4 9 The usual mixes for standard 10 mm joints are by
volume:
cement/lime/sand 1 : 1 : 5 to 1 : 1 : 6 cement/sand + plasticiser 1 : 5 to 1 : 6 masonry cement/sand 1 : 4 to 1 : 5
Fig. 2.11 Reinforced blockwork
Fig. 2.12 Blockwork movement joints
Below DPC level a stronger mix is required and sulphate-resisting cement may be necessary depending on soil conditions.
cement/sand 1 : 4
cement/lime/sand 1 : 1.5 : 4.5
Where high-strength blockwork is required, stron- ger mortars may be necessary. Mortar joints should
be slightly concave, rather than flush. Bucket handle and weathered or struck joints are suitable for external use, but recessed joints should only be used internally.
Coloured mortars should be ready mixed or carefully gauged to prevent colour variations. Contraction joints should be finished with a bond breaker of polythene tape and flexible sealant. For expansion joints, a flexible filler is required, e.g. bitumen-impregnated fibreboard with a polythene foam strip and flexible sealant. Where blockwork is to be rendered, the mortar should be raked back to a depth of 10 mm for additional key.
Masonry should not be built when the temperature is at or below 3◦C and falling or unless it is at least 1◦C and rising (BS 5628-3: 2005).
FINISHES Internal finishes
Plaster should be applied normally in two coats to 13 mm. Blocks intended for plastering have a tex- tured surface to give a good key. Dry lining may be fixed with battens or directly with adhesive dabs to the blockwork. Blockwork to be tiled should be first ren- dered with a cement/sand mix. Fairfaced blockwork may be left plain or painted. Where standard blocks are to be painted, the appropriate grade should be used.
Unfired-clay blocks should be finished with breath- able materials, such as clay or lime plaster, clay boards, limewash or highly vapour-permeable paint.
External finishes
External boarding or hanging tiles should be fixed to battens, separated from the blockwork with a breather membrane. For external rendering a spatterdash coat should be applied initially on dense blockwork, fol- lowed by two coats of cement/lime/sand render. The first 10 mm coat should be the stronger mix (e.g.
1 : 1 : 6); the 5 mm second coat must be weaker (e.g.
1 : 2 : 9). Cement/sand mixes are not recommended as they are more susceptible to cracking and crazing than mixes incorporating lime. The render should termi- nate at damp-proof course level with a drip or similar weathering detail.
FOUNDATIONS
Foundation blocks laid flat offer an alternative to trench fill or cavity masonry. Portland cement blocks
should not be used for foundations where sulphate- resisting cement mortar is specified, unless they are classified as suitable for the particular sulphate condi- tions. Sulphate and other chemically adverse ground conditions are classified in the BRE Special Digest 1 (2005) from DS1 (Design Sulphate Class 1) to the most aggressive, DS5. Foundation blocks can be of either dense or appropriate lightweight concrete, the latter providing enhanced floor edge insulation. Inter- locking foundation blocks, with a tongue and groove vertical joint, slot together with only bed-joint mor- tar being required. A handhold makes manipulating these blocks on site much easier than lifting stan- dard rectangular blocks. Thicknesses within the range 255–355 mm are standard.