Archival technical content: this article preserves historical values, classifications, equations and methods for testing bricks and roof tiles. It is not a current product specification, structural or fire-resistance calculation, energy assessment, roof detail or construction instruction. A project must use declared products, applicable standards, manufacturer data and calculations by authorised professionals.
Today, Savo Kusić focuses on wooden windows, wood-aluminium windows, custom-made windows, doors and requests for quotation. This article remains as a technical archive and does not represent an offer of bricks, roof tiles, masonry or roofing work.
Masonry bricks, pipes, containers and other objects have been made from clay and clay mixtures since ancient times. Through gradual heating to high temperatures and gradual cooling, shapes made from raw materials become more or less solid bodies. The variety of clay deposits allows fired bricks to be made in different qualities.
Masonry bricks (fired bricks)
1. Structure, external appearance and dimensions
To obtain bricks with as few cracks as possible and accurate dimensions, sand, crushed brick and dry clay powder are sometimes added to the raw mixture. Clay from some deposits contains lime in larger lumps. Even when the amount is not large, the lime should be sufficiently dispersed and crushed because it may slake after firing when water penetrates the brick. The resulting increase in volume can cause spalling if a lime lump remains too large.
The external appearance of wall bricks largely depends on the clay composition and firing. Small deviations from the nominal dimensions are important because uneven thicknesses and widths of joints can significantly affect the strength of the wall. The classic dimension of a solid wall brick in the original text is 250 × 120 × 65 [mm], three-quarters 185 × 120 × 65, and halves 120 × 120 × 65 mm. Allowed deviations from the intended dimensions are stated as a maximum of ± 4%.
2. Strength of wall bricks
The text describes strength testing primarily under compressive load, because masonry bricks are mainly used in building elements subject to compression. The brick is placed between two steel surfaces and loaded until it breaks. Bending testing is also mentioned to encourage the production of crack-free bricks.
The historical table shows 15 types of masonry brick divided into three groups according to unit weight. It also shows the period designations, upper limit of unit weight, minimum strength and frost-resistance requirements.
Laboratory testing: a brick breaking under a press can eject sharp fragments. Equipment, guarding, sample placement, measurement, personal protective equipment and result criteria must follow the applicable procedure under the supervision of trained personnel.
3. Compressive strength of brick walls
When different types of bricks were used under the same conditions, the compressive strength of the wall built from stronger bricks was higher. As the thickness of the joints increases, the resistance of the wall decreases. Uneven joints can significantly reduce the compressive strength, which is why the correct shape and dimensions of the bricks are important. The text also states that the compressive strength decreases with an increase in the wall section.
The relationship between brick, mortar and masonry strength for even joints and good bonding is shown by a historical equation and table. The relationship between wall strength and permissible load in the text is 2,7 to 4,8. The coefficient is intended to cover additional stresses from eccentric loads and workmanship defects that are not included in the calculation. The value e = 10 kg/cm2 is given for a well-built wall; for masonry with workmanship defects, the text provides for a lower and, if necessary, negative value.
Not a design value: historical ratios, units and equations must not be used to size a wall or assess an existing structure. Bearing capacity depends on the declared properties of the units and mortar, geometry, connections, slenderness, loads, foundations, seismic action, moisture and quality of execution, supported by a valid calculation from an authorised engineer.
4. Brick and wall elasticity under pressure
The elasticity of fired masonry bricks varies widely. The original text states a lower modulus-of-elasticity limit of E = 100 000 kg/cm2, described as 1 GPa, for bricks with a compressive strength of about 300 kg/cm2, described as 3 MPa; for other bricks it was significantly lower. For clinker, E up to 4,5 GPa is stated.
The deformability of the mortar is important to the wall’s elasticity; lime mortar is much more flexible than cement mortar. More archival content is available in the article on cement setting and storage.
The modulus of elasticity of the wall according to earlier tests in Stuttgart went down to E = 40 MPa with lime mortar and went up to almost 1600 MPa with cement mortar. Those values were determined on concrete and brick walls, with joint thicknesses of 10-12 mm.
5. Shrinkage and swelling of wall bricks
Shrinkage and swelling of wall bricks can be greater than natural stone. As an example, for wall bricks of low strength, swelling up to 0,19 mm/m is stated.
6. Thermal expansion and conductivity
Compared to some types of natural stone, for example granite, well-fired bricks show relatively little and gradual expansion under the influence of heat, according to the text.
Thermal conductivity depends on the type of brick. The dense, heavy sintered clay clinkers conduct heat more quickly than the porous, lightweight bricks, so in a historical overview, bricks are classified by bulk density. The table shows wall thicknesses for three areas of thermal insulation at the time.
Thermal properties: the shown thicknesses are not modern proof of meeting the energy requirements. Declared coefficients of all wall layers, resolved thermal bridges, moisture and condensation, as well as calculation according to current regulations are required.
7. Protection of walls from fire
If the wall is exposed to high temperatures, the text states that strength drops significantly at temperatures above 300 °C. Damage occurs especially during extinguishing and sudden cooling of hot bricks. A wall with facing plaster is described as more resistant, because the plaster layer slows down heating and cooling. For this purpose, the historical text mentions lime mortar with 10% gypsum and cleaning the joints by scraping before plastering.
Fire safety: historic temperature and mortar composition are not proof of fire resistance classification. The resistance of the assembly depends on the declared system, thickness, cavities, load, joints, penetration and method of execution. A wall exposed to fire or sudden cooling must be inspected by an authorized professional before reuse.
8. Behaviour under chemical exposure
Well-fired brick, especially facing brick and clinker, is described in the original text as resistant to wastewater and groundwater. The text also mentions the historical use of fired-brick walls to protect concrete structures from aggressive water. The durability of such protection depends on crack-free bricks with sufficient impermeability, suitable mortar and sealed joints.
Aggressive environments: protection cannot be selected from the material name alone. Water or soil analysis, known concentration and temperature, compatibility of brick, mortar and joints, penetration and maintenance details, and a design for the particular exposure are required.
Roof tiles
The text distinguishes several methods of making roof tiles:
- extruded roof tiles made from a flat strip of clay, including plain tiles with a nib — beaver-tail tiles or pan tiles — and extruded grooved tiles;
- pressed roof tiles made from a plastic clay body, including interlocking, grooved and channelled interlocking tiles.
1. Structure, external appearance and dimensions
Carefully prepared brick clay is required for the production of roof tiles. The fracture structure should be evenly and finely porous, and the surface should be as smooth as possible for less moss retention and easier water runoff.
For appearance, the upper surface is sometimes coated with coloured clay — an engobe — which should remain permanently bonded to the tile.
Roof tiles must have the correct shape and dimensions so that the roof is sufficiently closed at the joints. The original text states that an airtight fit is unnecessary, but that gaps should allow ventilation of the loft and evaporation of condensed water. This is why it mentions longitudinal grooves or ribs on beaver-tail tiles.
2. Strength of roof tiles
Because of stresses during transport, in the roof and during installation, the text specifies a mid-span load test with supports 25 cm apart. For beaver-tail tiles, a minimum load of 50 kg is specified for five tested pieces.
3. Water permeability
Under the historical criterion, roof tiles beneath a 5 cm water column are required to let droplets through, on average, no earlier than after 90 minutes. The text notes that the permeability of many tiles decreases significantly over time.
4. Resistance to weather influences
Roof tiles should withstand a freezing test. In addition to repeated freezing of wet tiles, damage can result from crystallisation of soluble salts introduced by rainwater. To prevent damage, the text suggests limiting soluble salts and shaping and laying tiles to avoid deposits. It also states that tiles on a lower-pitched roof may deteriorate faster than those on a steeper roof.
Roof work: historical criteria do not replace the manufacturer’s declaration, design pitch, substructure, fixing, ventilation, penetration details and load checks. A roof presents fall and fall-through hazards; do not walk on tiles or carry out work without an access plan, collective fall protection and a qualified contractor.
Paving clinker
For paving public roads, walkways and floors in chemical factories, the historical text describes fired bricks with high compressive and bending strength, resistance to wear, regular dimensions and no coarse cracks formed during firing. Deviations from the nominal dimensions are limited so that the joints are even and narrow, and the pavement is flat.
Paving: the choice of clinker and substrate must include the expected load, frost, water and drainage, wear and slip resistance, flatness and accessibility. Chemically loaded floors require a special compatibility check of the complete system.