classifications and Causes of Movements in Masonry Buildings
Movements in masonry buildings are deformations caused to it due to various reasons. Types and causes of such movements in masonry buildings is discussed.
There are different factors for example, temperature variations and changes in moisture content, that lead to generate or create deformations in masonry structures. These movements must be considered to prevent their detrimental influence on the masonry buildings.
Substantial stresses might be generated due to restriction of masonry members by interconnection with those masonry elements which have different movement.
Movement restriction of masonry, which is a brittle material, induces fracture and develops cracks through which water can penetrate and might damage the building fabrics.
Remedial techniques are bothersome and costly most of the time, so taking movements of masonry structures into considerations at design stages is extremely significant.
Types and Causes of Movements in Masonry Buildings
Following are the types and causes of movement in masonry buildings:
- Moisture variations
- Temperature changes
- deformation due to applied loads
- Foundation movements
- Chemical reactions in materials
Fig.1: Masonry Structure
Movements in Masonry Buildings due to Moisture variations
Changing in moisture content lead to create dimensional changes in masonry materials and this is the case for all types of building materials apart from metal. These dimensional variations could be permanent or in another word irreversible for example, clay brick undergo permanent long term moisture expansion which it is at ultimate value after the unit have cooled.
Not only does the moisture expansion rate of clay brick reduce with time but also it considerably depends on the type of the clay and firing degree.
British Standard provided guidance on movements due to moisture variations and stated that, typical anticipated movement range in fired clay unit is commonly less than 0.02%.
Long term expansion happened because of absorption of moisture from atmosphere. Both external and internal walls absorb moisture but it is considerably quicker in the former.
All sorts of masonry materials demonstrate reversible shrinkage or expansion with varying moisture content at entire stages of their life span. Table-1 provides typical values of moisture movements of masonry material, concrete and steel.
Table-1: Moisture movement indices for masonry material, concrete, and steel
|Materials||Reversible moisture movement, %||Irreversible moisture movement, %|
|Calcium silicate brickwork||0.01-0.05||-0.01-0.04|
|Concrete block or brickwork||0.02-0.04||-0.02-0.06|
|Aerated, autoclaved brickwork||0.02-0.03||-0.05-0.09|
|Dense aggregate concrete||0.02-0.10||-0.03-0.08|
Movements in Masonry Buildings due to Temperature Changes
Movements due to thermal variations are based on the coefficient of expansion of the material and the range of temperature that masonry elements will be exposed.
The evaluation of temperature ranges is complicated because it is based on other material properties for example thermal capacity and reflectivity value, but values for coefficient of thermal expansion are provided in Table-2.
Table-2: Coefficient of thermal expansion values of masonry materials, concrete and steel
|Materials||Coefficient of thermal expansion, 10-6/oC|
|Calcium silicate brickwork||8-14|
|Concrete block or brickwork||7-14|
|Aerated, autoclaved brickwork||8|
|Dense aggregate concrete||10-14|
Moreover, when a masonry wall is restrained at its edges, temperature changes will produce compression stresses because there is no room for free linear expansion to take place.
The intensity of these stresses is depending on modulus of elasticity of the material, temperature variations, and coefficient of thermal expansion.
The distribution of stress along the restrained edges of the masonry wall in not uniform in real life structure, so crack development is likelihood. Nonetheless, full restrain in masonry edges is not possible that is why thermal variations might lead to sort of movements or deformations instead of pure cracking.
It is should be mentioned that, contraction of masonry wall due to cooling may lead to initiate cracks because the wall as a whole is prevented to gain its previous position.
Deformations due to Applied Loads
Deformations due to applied load include creep, shrinkage and elastic movements.
When a masonry element such as pier is subjected to axial compressive loads its height is slightly decreased, and after the load is removed it might return to its original position. In this case the pier is behaved elastically.
However, when small permanent deformation takes place after the vertical compressive load is removed, the pier behavior would be plastic and this phenomenon is called creep.
Clay brickwork does not creep under normal loadings and therefore, it should not exhibit considerable sign of creep. In the design of masonry structures, taken creep into account is more crucial with reinforced members for which estimation of initial elastic deformation and deformations due to permanent loading is required.
Deformations due to Foundation Movements
Generally, movements resulted from foundation are the most usual reason of cracking in masonry walls. Masonry buildings that built on clay soil are most likely to subject to foundation movements, due to frequent decrease and increase in moisture content of the soil under the masonry structure.
Soil settlement on infilled sites and mining operation influences are the source of building masonry defects in some areas. It is extremely crucial to take precautionary measures with regard to foundation design if these problems are expected. For example, make sure that foundation level is one meter below the surface which is the most elementary solution.
Additionally, to prevent foundation problems in mining subsidence and weak soils, more adequate and elaborate measures are required to solve these issues and avoid structural damage in the future for which remedial process might be uneconomical and difficult.
Fig.2: Masonry Building Constructed on Soil
Movements in Masonry Buildings due to Chemical Reactions in Materials
In most situations, masonry materials are not subjected to chemical attacks but problems might arise due to sulfate attack on mortar, concrete blocks, corrosion of wall ties, and other steel components installed in the masonry buildings.
Mortar or concrete expansion due to sulfate attack could lead to disintegration of masonry. Clay bricks or ground water might be the source of soluble salts and the attack will occur if the masonry is continuously saturated.
Movement due to chemical reactions can be dealt with successfully by choosing constituents of cement properly for example, employing sulfate resistance cement below damp proof course level when the problem originated from underground water.