BSBG Head of Structures, Steven Bailey, presents a case study highlighting the principle differences between blockwork and brickwork.
We use the word masonry to describe walls built out of masonry units laid on a mortar bed.
Masonry unitsare either:
- Blocks – large hollow or solid units generally made ofconcrete
- Bricks – smaller units either solid or with small cores usually made ofclay
Sizes and dimensional variations
Blocks
Blocks are commonly referred to by theirnominal size(i.e. 400mm long, 200mm wide and 200mm high). Because an allowance is made for 10mm wide mortar joints, the actual size is 390mm x 190mm x 190mm.
The standard nominal sizes for both solid and hollow blocks are:
400 x 300 W x 200 H
400 x 250 W x 200 H
400 x 200 W x 200 H
400 x 150 W x 200 H
400 x 100 W x 200 H
Blocks are formed in steel moulds and since the material is relatively stable, the size of the individual units can be controlled to within small tolerances.
Bricks
Bricks are generally made to a traditional size – 230mm long, 110mm wide and 76mm high. These are the actual dimensions andnominal sizesare not quoted for bricks.
Since bricks are made of clay they can undergo shape changes during manufacture, particularly in the firing process, and individual units can vary considerably in size. Tolerances are measured by placing 20 units together, which measures the average size, but not the variation of individual units.
Material and block densities
Density is typically expressed in kilogram per metre cubed (kg/m3).Material density, as the name suggests, is the actual density of the material – whileblock densityis an average one that accounts for any voids or cores in the block. Material and block density is same for solid blocks/bricks.
Blocks
Typically, concrete blocks have the following material densities:
- 2100 kg/m3– blocks having this density are typically referred to asNormal WeightThe actual block densities for Normal Weight Vertical Hollow blocks vary from 1250kg/m3to1750kg/m3, subject to block thickness
- 1000 kg/m3– blocks having this density are typically referred to asLight WeightThe actual block densities for Light Weight Vertical Hollow blocks vary from 650kg/m3to 800kg/m3, subject to block thickness
- 550 kg/m3– blocks with this density are typically referred to asAutoclaved Aerated Concrete(AAC) blocks. These blocks are manufactured as solid and for this reason their block density is the same as the material one.
Bricks
Bricks are not commonly used in UAE architecture and construction. The fired solid brick material density is approximately 1900kg/m3 while the perforated brick is between 1400 – 1500kg/m3.
READ: BDA Design Note – Brickwork Dimensions Tables
Miscellaneous data
Various other parameters that are typically specified and are outlined in specification F10 are:
- Characteristic compressive strength – this is usually of importance for load bearing walls
- Thermal properties
- Sound transmission – as an example, solid or grout infilled 200mm thick normal weight blocks can provide 58dB insulation; 200mm normal weight hollow blocks – 52dB; and 200mm lightweight hollow blocks – 48dB
- Fire rating – typically 150mm thick normal or light weight blocks either solid or hollow provide a minimum of three hours fire rating.
Mortar and mortar joints
Mortar is a workable paste that binds building blocks together. It is made of cement and sand with different ratios to achieve the required strength. The compressive strength of mortar ranges between 2 to 12 N/mm2. The mortar joints used in concrete block and brick construction are usually 10 mm thick. For AAC block construction, a special type of mortar is used (2-3 mm) and this is a pre-mixed cement-based product with higher quality than normal mortar.
Control joints
Blocks
Control joints are required in a concrete masonry wall to prevent cracking due to shrinkage of the concrete blocks, temperature expansion/contraction and differential settlement of foundations.
Unreinforced concrete masonry
Control joints should be provided at all joints where there is potential for cracking, such as major changes in wall height, changes in wall thicknesses (other than piers), joints in slabs, and T-joints.
Typically, control joints should not be placed at spacing exceeding 6m.
Reinforced concrete masonry
In walls over 3m high, the spacing of control joints should not exceed 16m.
In walls 3m high or less and incorporating capping reinforced concrete beams, control joints must not be used.
Clay masonry
Clay products expand over time as they absorb water. The expansion is not uniform over time; approximately one quarter happens during the first six months, one half in first two years and three quarters over five years. The expansion can reach 1mm per metre over a 15-year period.
Joints are required in clay masonry (brick walls) to allow for the expansion of the clay. The joints should have sufficient width so that they allow for the bricks to expand freely. Joints are also provided in clay masonry to account for temperature expansion/contraction and differential settlement of foundations.
Ties are required on both sides of a joint, but where it is not possible to use them, masonry flexible anchors must be used across the joint.
There are two types of joints:
- Articulated joints – vertical gaps that allow for minor footing movement, to prevent distress and cracking. These joints provide the flexibility needed when a building is on soft soils and typically are not required on stable sites. Joints should not be further than 3m from corners and their width is 10mm for a 3m high wall and 15mm for a 6m high one.
- Control joints – these joints are provided as wall thickness, wall height or footing steps change. They are also typically located near a corner. In internal masonry, control gaps are not usually required, except at re-entrant angles in long walls.
Applications for reinforced blockwork
If a concrete block wall is reinforced and grouted, it can be used as a load bearing wall in both directions – vertical and horizontal. Such a wall is used in a great variety of structures. The applications include retaining walls, housing, industrial buildings, fences, etc. It should be noted that a solid concrete block wall can also be used as load bearing, but predominantly in a vertical direction.
Reinforced blockwork is built by placing reinforcement in the block cores and then grouting. The masonry becomes a composite of block, grout and reinforcement and can have a strength similar to that of the lower concrete grades.
Most 150mm and 200mm thick concrete masonry walls that require a load bearing function in a vertical direction may have only some parts reinforced and grouted – these are always the end and T-junction vertical cores.
Façade concrete masonry walls that need to support heavy pre-fabricated elements will usually require all cores to be reinforced and grouted.
Bathroom and kitchen walls supporting sanitary hardware and cabinets will also typically require all cores to be reinforced and grouted, or solid blocks to be used as alternative. In the case of bathrooms, partial grouting can be applied up to a height of 1.2m above floor level.
The grout used to fill the cores should be properly specified and typically it should not have less than 15MPa characteristic compressive strength, preferably 20MPa. Cement content should not be less than 300kg/m3and coarse aggregate size should not exceed 10mm.
The reinforcement typically consists of 1T12 or 1T16 bars beside all door openings, all corners, junctions and at 1600–1800mm spacings along the wall. Horizontal bar – usually 1T12 is placed under all window sills.
Information required by a structural engineer
The following information is typically required by a structural engineer to enable him to estimate the weight of masonry walls on slabs and also to provide reinforcement details in case load bearing walls are required:
- Masonry wall layout
- Type of masonry – concrete blocks or clay bricks
- Thickness of masonry
- Type of blocks – solid or hollow. In case of hollow, the architect should specify if load bearing function is required
- Material density or specification if the block will be normal weight, light weight or AAC
- Approximate thickness and type of finishes applied to the masonry wall – plaster, tiles, heavy façade elements and any offsets from the wall.
Note:By default a structural engineer will consider 12mm plaster on each face, however anything different to this will need to be advised by the architect.
