Bricks and Mortar
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If a brick is classified as ‘OL’, it is?
Not frost resistant and low salt content
If an engineering brick is classified as ‘B’, it has a strength between (N/mm2)?
50 - 70 N/mm2
Typically what is the compressive strength range for bricks (in N/mm2)?
15 - 140 N/mm2
The cement content in lime mortar is typically (%)?
0%
Bricks are usually fired for (hours)?
Between 10 and 40 hours, depending upon kiln type and other variables
What is the current UK (also global standard) brick size?
215mm x 102.5mm x 65mm
Specification for Clay Bricks?
BS EN771 - replaced BS 3921
How do water absorption ranges in bricks?
Ranges from very low (dense) to 0 to 25% water absorption for very porous bricks
Name different types of bricks
Solid - no holes, cavities or depressions Frogged - depressions usually not greater than 20% of the total volume Perforated - holes not exceeding 25% of gross volume for use in structural applications Facing Bricks - appearance, frost resistance & strength – the majority of bricks nowadays Engineering Bricks - dense & low water absorption, higher strength, frost resistance Structural & Domestic Brickwork - has low stresses generally
What does Frost resistant (F) mean?
Resistant in all situations - repeated freezing and thawing
What does Moderately frost resistant (M) mean?
Durable except when saturated & repeated freezing /thawing
What does Not frost resistant (O) mean?
Liable to damage by the frost – hence suitable for internal use only
What does fired brick consist of?
Sodium, potassium, magnesium sulphates exist in the fired brick – with time these may slowly attack the hardened mortar
What are the two categories for salt content in bricks?
‘Low’ (L) & ‘Normal’ (N) FL ~ Frost resistant (F) & low salt (L) FN ~ Frost resistant (F) & normal salt (N) ML ~ Moderate (M) & low salt (L) OL ~ Not frost resistant (O) & low salt (L)
What is efflorescence?
It is white staining on brickwork due to salts crystallizing on the surface in springtime
How does moisture movement affect bricks?
Clay bricks slowly expand with time whereas mortar shrinks – the net effect is a slow expansion of walls – require movement joints at regular intervals to accommodate expansion and prevent stress build-up in wall
How does a chemical attack affect bricks?
Bricks generally have very good resistance to chemicals - mortar is the weak link
Are bricks fire resistant?
Bricks have very good fire resistance as they are fired in the kiln at 900°C+
What are clays composed of?
Silica and alumina with varying amounts of metallic oxides. Metallic oxides promote the fusion of the particles at lower temperatures.
How does Metallix Oxices affect clays?
Metallic oxides promote the fusion of the particles at lower temperatures. Metallic oxides (particularly those of iron, magnesium and calcium) also influence the colour of the fired brick.
What happens to bricks in high temperatures?
The firing of clay bricks Clay, unlike metal, softens slowly and melts or vitrifies gradually when subjected to rising temperatures.
What does Vitrification do to clay?
Vitrification allows clay to become a hard, solid mass with relatively low absorption.
What does the firing process involve?
The firing process involves decomposition of clay minerals to component oxides & re-crystallization into new compounds - durable, glassy matrix formed
What are the requirements of modern mortar?
Adhere to bricks/stone/blocks, high plasticity & workability, adequate stiffening rate, good durability, frost, sulphate attack and rain penetration resistant, appearance and colour - Architects specification