Bricks and Mortar Flashcards
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
What designation mortar is mostly used?
In most buildings / dwellings Designation (iii) or (iv) mortar used
What type of Mortar is available?
Mix on-site, factory prepared wet lime/sand mixes, or ready mixed cement+lime+sand+retarder+colouring agents are common nowadays (usually last 36-48 hours before setting)
What are commonly used additives to mortar?
Plasticisers are commonly used to improve workability (hydrated lime), adhesion, spread – not FAIRY LIQUID as it can cause efflorescence…. a very common practice!
What are Lime Mortars?
Much lower embodied carbon than cement-based mortars. Comprised usually of 1 part lime & 3 parts sand.
What is Lime Mortar effective at?
Very flexible and able to withstand greater movements without cracking. Allows building to ‘breath’, thus fewer issues with dampness - Majority of old/historic buildings made using lime mortar
What is Traditional Mortar made of?
Lime and sand (no cement) – typically ratio of 1:3 or 1:2 by volume – e.g. bucketfuls
What is Traditional Morta effective at?
Plastic mortar, high workability, low strength, flexible and hardens slowly.
Why is cement preferred instead of lime?
Quicker strengths but less accommodating – cracks easily
What are the two different Lime Mortars?
Hydraulic Lime Mortar - Non-Hydraulic Lime Mortar.
What is Hydraulic Lime Mortar?
Hydraulic means to harden underwater and is considered between cement and non-hydraulic mortars.
What is Hydraulic Lime Mortar made of?
The mix contains dicalcium silicate but unlike cement mortars no tricalcium silicate. The dicalcium silicate (with water) provides the setting properties of hydraulic lime. 3 strength grades specified in EN 549: NHL2, NHL3.5 & NHL5 (N/mm2) - the numbers indicate approximate strength
What is Non-Hydraulic Lime Mortar?
Hardens by reaction with atmospheric carbon dioxide - called carbonation. The reaction with carbon dioxide produces calcium carbonate. Usually takes several days to gain initial hardness and therefore can delay construction progress. Commonly known as lime putty. In projects, unspecified amount of PFA or slag added to reduce setting time
