TYPES AND SIZES
Concrete blocks are defined as solid, cellular or hollow, as illustrated inFig. 2.1.
Concrete blocks are manufactured to various work face dimensions in an extensive range of widths, offer- ing a wide choice of load-bearing capacity and level of insulation. Manufacturers work size dimensions should be indicated as length, width and height, in that order, to BS EN 771-3: 2003 and BS 6073-2: 2008.
However, the National Annex NA (informative) to BS EN 771-3: 2003 and the British Standard particularly note that previous standard UK practice was to specify blocks by length, height and thickness.
The standard work face size, which co-ordinates to three courses of metric brickwork allowing for 10 mm mortar joints, is 440×215 mm (Fig. 2.2), but the other sizes in Table 2.1 are marketed for aes- thetic and constructional reasons. For example, narrow bands of a different colour may be used as visual features within fairfaced blockwork, and foundation wall blocks are normally laid flat. The use of thin- joint masonry offers speedier construction, especially when using large format blocks (Fig. 2.3), which are approximately equivalent in size to two standard units. However, blocks heavier than 20 kg should not be lifted by a single person as this potentially can lead to injury. Within the 20 kg limit are 100 mm wide aircrete blocks with face dimensions 610× 375 mm for speedy construction using the thin-joint system.
Fig. 2.1 Types of concrete blocks
The European Standard (BS EN 771-3: 2003) describes a wide range of aggregate concrete masonry units incorporating either dense or lightweight aggre- gates. Under the European Standard, the minimum
Fig. 2.2 Co-ordinating sizes for blockwork
description for concrete blocks includes the Euro- pean Standard number and date (e.g. BS EN 771-3:
2003), the type of unit (e.g. common or facing), work size dimensions and tolerance category, configuration (e.g. solid or with voids) and compressive strength.
Also, depending on the particular end use, additional description may be required. This may, as appropri- ate, include surface finish, net and gross dry density, co-ordinating size, thermal properties and moisture movement. Tolerance limits for regular-shaped blocks are defined at four levels in Table 2.2. Compressive strengths of concrete masonry units are classified as Category I or Category II. Category I units have a tighter control with only a 5% risk of the units not achieving the declared compressive strength.
The European Standard (BS EN 771-4: 2003) gives the specification for autoclaved aerated con- crete (AAC) masonry units. The maximum size of units within the standard is 1500 mm length×600 mm width×1000 mm height. The tolerance limits on the dimensions are defined inTable 2.3, and are dependent on whether the units are to be erected with standard or thin-layer mortar joints. The standard manufacturer’s description for AAC masonry units must include the European Standard number and date (e.g. BS EN 771-4: 2003), dimensions and tolerances, compressive strength (Category I or Category II, as for concrete
B L O C K S A N D B L O C K W O R K 3 9 Table 2.1 Typical work sizes and strengths for concrete blocks to BS
6073: 2008 Length (mm)
Width (mm)
Height (mm) Aggregate concrete blocks
Coursing blocks
190 90 65
190 90 90
215 100 65
290 90 90
440 90 65
440 100 65
440 90 140
440 100 140
Standard blocks
390 90 190
390 100 190
390 140 190
390 190 190
440 75 215
440 90 215
440 100 215
440 140 215
440 190 215
440 215 215
Aircrete concrete blocks Coursing blocks
215 90–150 65
215 90–150 70
Standard blocks
440 50–350 215
610 50–350 215
620 50–350 215
Notes:
Not all sizes are produced by all manufacturers but other sizes may be available.
Widths quoted by certain manufacturers include: 50, 70, 75, 100, 115, 125, 130, 140, 150, 190, 200, 215, 255, 265, 275, 300 and 355 mm.
Foundation blocks with face sizes of typically 255×290, 255×300, 310×350, 400×215, 440×140 or 440×215 mm are laid flat.
Beam and block floor rectangular units are usually 100 mm thick, with face dimensions of 440×215, 440×350, 440×540, 610×350 or 620×215 mm.
Common crushing strengths to BS 6073-2: 2008 are 2.9, 3.6, 7.3, 8.7, 10.4, 17.5, 22.5, 30.0 and 40.0 MPa, but some manufacturers supply additional intermediate strengths (e.g. 4.2 MPa).
Fig. 2.3 Thin-joint masonry using large format blocks. Photograph reproduced from GBG 58 by permission of BRE and courtesy of Aircrete Products Association
units) and dry density. Further description for specific purposes may include durability, configuration (e.g.
perforations or tongued and grooved jointing system) and intended use.
Table 2.2 Limit of tolerances on aggregate concrete block sizes
Tolerance category D1 D2 D3 D4
Length (mm) +3 +1 +1 +1
−5 −3 −3 −3
Width (mm) +3 +1 +1 +1
−5 −3 −3 −3
Height (mm) +3 ±2 ±1.5 ±1.0
−5 Notes:
BS 6073: 2008 states that tolerance categories D3 and D4 are intended for use with thin-layer mortar joint systems. Therefore, most units used within the UK conform to tolerance categories D1 and D2.
Closer tolerances may be declared by the manufacturer.
Table 2.3 Limit of tolerances on autoclaved aerated concrete block sizes
Standard joints of general purpose and lightweight mortar
Thin-layer mortar joints
GPLM TLMA TLMB
Length (mm) −5 to+3 ±3 ±1.5
Width (mm) ±3 ±2 ±1.5
Height (mm) −5 to+3 ±2 ±1.0
Notes:
For autoclaved aerated concrete units of category TLMB, the maximum deviation from flatness of bed faces and plane parallelism of bed faces is
≤1.0 mm in each case.
Closer tolerances may be declared by the manufacturer.
MANUFACTURE
Dense concrete blocks, which may be hollow, cellular or solid in form, are manufactured from natural dense aggregates including crushed granite, limestone and gravel. Medium and lightweight concrete blocks are manufactured incorporating a wide range of aggre- gates including expanded clay, expanded blastfurnace slag, sintered ash and pumice. Concrete is cast into moulds, vibrated and cured. Most aerated (aircrete or autoclaved aerated concrete) blocks are formed by the addition of aluminium powder to a fine mix of sand, lime, fly ash (pulverised fuel ash) and Portland cement.
The hydrogen gas generated by the dissolution of the metal powder produces a non-interconnecting cellu- lar structure. The process is accelerated by pressure steam curing in an autoclave (Fig. 2.4). For some prod- ucts, additional insulation is afforded by the filling of voids in the cellular blocks or by bonding on a layer of
extruded polystyrene, polyurethane or foil-faced phe- nolic foam (Fig. 2.1). Standard blocks, typically natural grey or buff in colour, are usually shrinkwrapped for delivery. Different grades of blocks are usually identi- fied by scratch marks or colour codes.
PROPERTIES Density and strength
The British Standard BS 6073-2: 2008 lists common compressive strengths of 2.9, 3.6, 7.3, 8.7, 10.4, 17.5, 22.5, 30.0, and 40.0 MPa for the range of aircrete and aggregate concrete blocks. However, the major- ity of concrete blocks fall in the range from 2.8 to 30 MPa, with associated densities of 420–2200 kg/m3 and thermal conductivities from 0.10 to 1.5 W/m K at 3% moisture content (Table 2.4). Drying shrinkages are typically in the range 0.03–0.05%.
Durability
Dense concrete blocks and certain aerated lightweight blocks are resistant to freeze/thaw conditions below damp-proof course (DPC) level. However, some lightweight concrete blocks, with less than 7 MPa crushing strength should not be used below DPC level, except for the inner skin of cavity construction.
Fixability
Aerated and lightweight concrete blocks offer a good background for fixings. For light loads, nails to a depth of 50 mm are sufficient. For heavier loads, wall plugs and proprietary fixings are necessary. These fixings should avoid the edges of the blocks.
Fig. 2.4 Manufacture of aerated blocks
B L O C K S A N D B L O C K W O R K 4 1
Table 2.4 Typical relationship between density and thermal conductivity for concrete blocks
Nominal density (kg/m3) 2200 2000 1800 1600 1400 1200 1000 900 800 750 700 600 500 460 420
Typical thermal conductivity (W/m K) 1.5 1.2 0.83 0.63 0.47 0.36 0.27 0.24 0.20 0.19 0.17 0.15 0.12 0.11 0.10 Notes: Blocks of differing compositions may vary significantly from these average figures and manufacturers’ data should be used.
Thermal insulation
The Building Regulations Approved Document Part L (2006 Edition) requires new dwellings (Part L1A) and other new building types (Part L2A) to be com- pliant with an overall energy and carbon performance, the Target Emission Rate (TER) based on the whole building (Chapter 7, page 246). IndividualU-values for elements are therefore not set, except for exten- sions on existing dwellings (Part L1B) and other existing buildings (Part L2B) where an indicativeU- value of 0.30 W/m2K is the standard for new exposed walls. The limiting area-weighted U-value standard for wall elements in new buildings is 0.35 W/m2K, but to achieve the Target Emission Rate overall, most buildings will require wallU-values within the range 0.27–0.30 W/m2K.
The following material combinations achieve a U-value of 0.27 W/m2K (Fig. 2.5).
Partially filled cavity
102.5 mm fairfaced brickwork outer leaf 50 mm clear cavity
50 mm foil-faced polyurethane foam (= 0.023 W/m K)
100 mm lightweight blocks (= 0.15 W/m K) 12.5 mm plasterboard on dabs (= 0.16 W/m K).
Fully filled cavity
102.5 mm fairfaced brickwork outer leaf
100 mm full-fill cavity of blown mineral wool (= 0.038 W/m K)
100 mm lightweight blocks (= 0.15 W/m K) 13 mm dense plaster.
Similarly, aU-value of 0.27 W/m2K can be achieved with 100 mm external fairfaced blockwork as an alternative to fairfaced brickwork, provided that the
Fig. 2.5 Typical blockwork construction achievingU-values of at least 0.27 W/m2K
necessary additional thermal resistance is furnished by slightly increased cavity insulation. The thin-joint mortar system for inner leaf blockwork gives slightly enhancedU-values compared to the equivalent stan- dard 10 mm joint blockwork construction.
Rendered solid wall construction can also achieve a U-value of 0.27 W/m2K (Fig. 2.5).
Solid wall
16 mm external render
215 mm high-performance lightweight blocks (= 0.11 W/m K)
50 mm lining of 9.5 mm plasterboard (= 0.16 W/m K) backed with 40 mm phenolic foam insulation (= 0.023 W/m K).
The following material combinations achieve a U-value of 0.20 W/m2K. These tighter specifications are required to achieve higher ratings in respect of the Code for Sustainable Homes.
Partially filled cavity
102.5 mm fairfaced brickwork outer leaf 50 mm clear cavity
75 mm foil-faced polyurethane foam (= 0.022 W/m K)
100 mm lightweight blocks (= 0.15 W/m K) 12.5 mm plasterboard on dabs (= 0.16 W/m K).
Fully filled cavity
102.5 mm fairfaced brickwork outer leaf
150 mm full-fill cavity of blown mineral wool (= 0.038 W/m K)
100 mm lightweight blocks (= 0.15 W/m K) 13 mm dense plaster.
Solid wall
16 mm external render
70 mm phenolic foam insulation (= 0.023 W/m K) 215 mm high-performance lightweight blocks (= 0.11 W/m K)
12.5 mm plasterboard on dabs (= 0.16 W/m K).
For domestic construction the appropriate Robust Details should be used to ensure compliance with thermal and sound requirements of the Building Reg- ulations.
Phase change material blocks
Phase change materials (PCMs) incorporated into aer- ated concrete blocks offer some additional thermal stability to the internal environment by absorbing excessive summer heat, which is then released during the cooler periods. This phase change at 26◦C effec- tively increases the thermal capacity of the lightweight blocks. One manufacturer colour codes the phase change material blocks green for easy identification.
The phase change material is described inChapter 12 (page 337).
Fire resistance
Concrete block construction offers good fire resistance.
Solid unplastered 90 mm blocks can give up to 60 min- utes’ fire protection when used as load-bearing walls;
certain 150 mm and most 215 mm solid blocks can achieve 360 minutes’ protection. Dense, lightweight and autoclaved aerated concrete blocks with less than 1% organic material are automatically categorised as Euroclass A1 with respect to reaction to fire.
Sound insulation
The Building Regulations 2000 Approved Document E (2003) recognises the need to provide adequate sound insulation both between and within dwellings also between rooms in hostels, hotels and residential accommodation. The regulations require minimum airborne sound insulation of 45RwdB for separating walls and 40RwdB for internal bedroom or WC walls.
The passage of airborne sound depends on the density and porosity of the material. The use ofRobust Details or Pre-Completion Testing is required to demonstrate compliance. The following alternative systems should perform to the required airborne insulation standard for separating walls of new build dwellings.
12.5 mm plasterboard on dabs 8 mm render
100 mm dense (1600–2200 kg/m3) or lightweight (1350–1600 kg/m3) blockwork
75 mm clear cavity only linked by appropriate wall ties
100 mm dense (1600–2200 kg/m3) or lightweight (1350–1600 kg/m3) blockwork
8 mm render
12.5 mm plasterboard on dabs.
These alternatives perform to the required standard only if there are no air leaks within the construction,
B L O C K S A N D B L O C K W O R K 4 3 all joints are filled, the cavities are kept clear except for
the approved wall ties and any chasing out on opposite sides of the construction is staggered. Vertical chases should, in any case, not be deeper than one third of the block thickness. Horizontal chases should be restricted to not more than one sixth of the block thickness, due to the potential loss of structural strength.
Sound absorption
The majority of standard concrete blocks with hard surfaces are highly reflective to sound, thus creating long reverberation times within building enclosures.
Acoustic absorbing concrete blocks are manufactured with a slot on the exposed face which admits sound into the central cavity (Fig. 2.1). Since the void space is lined with sound-absorbing fibrous filler, incident sound is dissipated rather than reflected, significantly reducing reverberation effects. Acoustic control blocks in fairfaced concrete are suitable for use in swimming pools, sports halls, industrial buildings and auditoria.
SPECIALS
Most manufacturers of blocks produce a range ofspe- cials to match their standard ranges. Quoins, cavity closers, splayed cills, flush or projecting copings, lin- tel units, bullnose ends and radius blocks are generally available, and other specials can be made to order (Fig.
2.6). The use of specials in fairfaced blockwork can greatly enhance visual qualities. Matching full-length lintels may incorporate dummy joints and should bear on to full, not cut, blocks.
FAIRFACED BLOCKS
Fairfaced concrete blocks are available in a wide range of colours from white, through buff, sandstone, yellow, to pink, blue, green and black. Frequently the colour is all through, although some blocks have an applied sur- face colour. Most blocks are uniform in colour, but there is some variability with, for example, flecked finishes. Textures range from polished, smooth and weathered (sand- or shot-blasted) to striated and split face (Fig. 2.7), the latter intended to give a random variability associated more with natural stone.
Glazed masonry units are manufactured by the application of a thermosetting material to one or more faces of lightweight concrete blocks, which are then heat-treated to cure the finish. The glazed blocks are available in an extensive range of durable bright colours and are suitable for interior or exterior use.
Fig. 2.6 Block specials
Where required, profiled blocks to individual designs can be glazed by this system. Most manufacturers pro- duce a range of specials to co-ordinate with their standard fairfaced blocks, although, as with special bricks, they may be manufactured from a different batch of mix, and this may give rise to slight varia- tions. In specific cases, such as individual lintel blocks, specials are made by cutting standard blocks to ensure exact colour matching.