Following my last articles on Lorca’s earthquake, perhaps it could be a good idea to examine some typical building failures, with the purpose of differentiating idiosyncrasies between Spanish and British construction techniques.
Let us take for instance exterior walls. Normally exterior walls in the UK are load bearing walls while here in Spain with the exception of old buildings, recent construction particularly around the Costas, have a reinforced concrete structure and their external walls as a rule.
Work as infill panels made of cavity walls construction, normally composed of 12,5 cm hollow brick wall, rendered on it external face, 4 to 5 cm of cavity, filled with thermal insulation and a 4 to 7 cm partition walling made up of hollow brick plastered inside.
Often when we are called by British clients to inspect their property around Costa Blanca and Costa Cálida due to the appearance of cracks it is difficult to convince them that the origins of those cracks are not necessarily due to foundations problems, as there are other causes that may affect external walls and produce cracking.
Cracks and fractures, and the lack of stability of external brick walls facades are symptoms of physical or mechanical failure caused mainly by eccentric compression, bending produced by a deformation of the structure, which external walls can barely absorb because of its rigidity and consequently fractures occur, usually in areas affected by shear forces which take place orthogonally to the direction of movements.
The following characteristics of the cracks are very useful to be able to diagnose its causes.
- The size: width, length, depth of the cracks can tell us a lot about its cause.
- Location: Whether they occur under sills, lintels, corners, panels, etc.
- Edges: What type of edges do they have? Are they sharp, blunt, uneven, etc.
- Age and significance: Is the cracking active or has it stabilized?
Causes of fractures and cracking
The cracks and fissures occur most commonly on external walls and partitions and they can be classified due to the characteristics of materials:
- Excessive shrinkage due to moisture absorption by using new brick straight from the factories.
- Oxidation which have occurred in metal components such as steel or iron railings.
- Movement that have occurred due to the link (or lack of) between the structure and the wall itself.
- Deflection of the structure that supports an external wall.
- The structure that supports a cantilevered façade has an important deflection, and the wall falls a few centimetres but the slab below does not have the same deflection, so the external wall is detached from the top floor and, in general, presents an outwards bulge.
- A decrease on the thickness of the external wall as it surrounds externally the concrete columns.
- Partial support for external walls situated on the edge of the slab
Due to the lack of expansion joints
- Cracks occur when expansion or contraction by temperature and humidity fluctuations are not allowed in the construction techniques.
- Incorrect handling of bricks.
- Manufacture and / or improper lying of the mortar.
Due to other causes
- The expansion of the slope forming layer in flat roofs.
- When lintels are under sized and their deflection becomes important.
To deal with all the above problems it should ensured that the bricks resistance and mortar adhesion are adequate for the conditions used.
The mortar.
The quality for this component depends on:
Its resistance. This means that the amount of cement should not be allowed to be very high and water-cement ratio not excessively low.
Plasticity of mortar is achieved with the addition of a dose of lime, or retardant additives.
Adherence of the mortar to the bricks is improved by a suitable consistency, a good water retention capacity and a limited time of use.
Cracking prevention.
In relation to materials to avoid building failure it is advisable to:
- Use bricks on external facades that have been stored for at least during six months and never straight from the kiln.
- To prevent excessive water absorption from the mortar used, bricks can be treated with water repellent.
- Avoid excessive water in the mortar or excessive sand dust.
- Ensure a minimum porosity and voids and / or recesses in the faces of the bricks do not used bricks that have smooth surfaces which will be in contact with the mortar.
- Wet the pieces through irrigation or soaking in water for several minutes before using them.
In relation to the structure
- Limit the deflection on the concrete beams that supports brick walls.
- Limit the deflection between the concrete slabs that sustain the floors.
- Limit the deflection on a cantilever structures such as covered balconies.
- Limit the deflection of lintels.
Regarding the lack of stability of exterior cavity walls this should not have altered its entire thickness when covering the external face of a column. If for whatever reason it is not possible to do this, it is advisable to place a steel mesh horizontally between courses to reinforce the brick wall layers and better still have the entire brick wall past the front of the slab front.
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