04 Walls Flashcards
What are the dimensions of a brick?
215mm x 102.5mm x 65mm
Name some different types of brick bond.
- English bond
- Flemmish bond
- Header bond
- Stretcher bond
- English garden wall bond
- Flemmish garden wall bond
How does English bond differ from Flemish bond?
- English Bond - alternate rows of headers and stretchers (one of the strongest bonds but requires more facing bricks)
- Flemish Bond - alternate headers and stretchers (not as strong as English bond but uses less facing bricks and is considered to be aesthetically superior)
What is the difference between a common, facing and engineering brick?
- Common brick - basic clay brick, no attempt at colour/texture control or special surface treatment
- Facing brick - more attractive, sides more accurately finished and smoothed, colours more uniform
- Engineering brick - used where strength, low water porosity or acid (flue gas) resistance is needed
What is the function of mortar in brick/block walls?
The material that binds bricks/blocks together, helping to distribute the load through a wall and seals the bricks/blocks against water ingress
How is mortar made?
Made by mixing a fine aggregate (usually sand) and a binding agent (traditionally lime, nowadays cement) with water
How is mortar expressed in terms of its component parts?
Expressed as either a cement/sand ratio (i.e. 1:3) or a cement/lime/sand ration (i.e. 1:1:6)
Explain the purpose of using a plasticiser in a mortar mix?
Lime or a liquid plasticiser is often added to the mortar to allow the mix to flow better, thus increasing workability (a 1:1:6 mix is the same strength as a 1:3 mix without lime)
What are the implications of using a weaker mortar mix in constructing a wall?
A weaker mix (proportionally less cement):
- More flexible - able to resist the stresses imposed by movement
- If cracking occurs, it will tend to happen at the weaker mortar joints, allowing for an easier/cheaper repair than repairing/replacing the bricks
- Too weak and the mortar will become porous and may crush under high compression forces
What are the implications of using a stronger mortar mix in constructing a wall?
- Increased strength and durability
- Absorb less water and are more resistant to frost attack
- Reduced ability to accommodate movement - tendency to shrink and crack, allowing water into the wall
- Reduced workability
Why may using a strong mortar mix be detrimental when repointing an older building?
A strong mortar can lead to damage in the brick/stone itself, as it fails to perform as a sacrificial item or fails to allow water to evaporate adequately
What factors would you consider when specifying the type of mortar to be used?
- Structural requirements
- Ability to accommodate movement
- Resistance to frost during construction
- Resistance to rain penetration
- Better adhesion
- Characteristics of the brick
What guidance is available when specifying mortar?
BS 998-2 (Table NA.1, showing 4 categories with varying attributes)
What is pointing?
Act or process of repairing or finishing joints in brickwork, masonry etc. with mortar
What is a DPC?
Damp Proof Course - impermeable layer of material to prevent rising damp, most often polyethylene or bitumen-polymer
When did damp proof courses become compulsory in buildings?
Mandatory in all British buildings since the Public Health Act in 1875
When did damp proof membranes become compulsory in buildings?
Made compulsory by the National Building Regulations in 1965
Name some different materials that have been used for DPCs and DPMs over the years.
Early DPCs included (many become brittle with age and fail due to building movement):
- Tar and sand
- Hessian soaked in tar
- Lead
- Copper
- Slate
- Two/three courses of engineering bricks
What are the main requirements for DPCs and DPMs in relation to the building regulations?
- Part C requires the DPC to sit at least 150mm above ground level
- Part C requires DPMs to be continuous with the DPC on the inner leaf
When were cavity walls introduced?
- Introduced by the Victorians as early as 1850
- More commonly introduced in the 1920s
What is the purpose of cavity walls?
- Prevents water reaching the internal skin
- Improves thermal efficiency - air is a good insulator and addition of cavity wall insulation improves this further
Describe the typical elements that make up a cavity wall.
- Outer leaf (i.e. facing bricks)
- Cavity
- Insulation board
- Inner leaf (i.e. aerated blocks)
- Wall ties
- DPC (on each leaf, 150mm minimum from ground level)
- Backfill (weak concrete mix below ground)
Where would the partial fill insulation be placed in the cavity wall?
Held against the inside leaf by retaining washers clipped on the walls ties
Why would you backfill a cavity?
- Prevents the inner and outer leaf being squeezed together when trenches are backfilled
- Must stay well below DPC level and be struck away from the internal leaf so water falls to the outer leaf
