DUUSEC Engineering Services

WHY BUILDINGS CRACK(PART 1)

Reasons for the cracks on your building

We all desire to own houses that are structurally safe and aesthetically captivating but this is not so easily achieved. Due to some unavoidable reasons and faulty construction processes cracks starts to develop on various structural and non-structural components of the building with time. Cracks on buildings poses eyesore to property owners, occupants and neighbour of/to such a building. It creates a sense of danger even after such cracks have been treated. Cracks may only affect the appearance of the building only, however a crack maybe the first sign of a serious defect which may affect the serviceability or stability (structural integrity) of the building. The appearance of cracks will also negatively affect the value of the building, its insurance value, its marketability and can be the subject of litigation.

Cracking is an unavoidable reaction to the inability of a structure to bear its design loads or movements to which it is subjected. It is a common phenomenon to see various crack patterns in buildings during its construction, at completion, when subjected to super imposed load or during its service life. Building components develops cracks when its strength is exceeded by stresses.

TYPES OF CRACKS

 Cracks in buildings can be generally classified into structural and nonstructural cracks.

1.    Structural Cracks: These arises from wrong design, erroneous construction steps or overloading (such as addition of floors not in the original design) and may endanger the safety of a building. e.g. cracking of beams, columns, slabs, staircase, footings etc.

2.    Non-structural Cracks: These results from internally induced stresses in buildings materials and might initially not jeopardize the safety of a building but may look unpleasant, or may create an impression of faulty work or may create a sense of danger. In some cases, due to passage of moisture through the cracks the internal finishes may be damaged thus adding to the cost of maintenance, or causes corrosion of reinforcements, thereby adversely affecting the structural rigidity of such building with time. e.g. Cracks on walls.

CAUSES OF CRACKS

The following are the major causes of cracks that occurs in buildings:

a.   Foundation Settlement: it is normal buildings settle under its own weight with time. This settlement has to be uniform and tolerable, differential settlement is a principal cause of cracks in buildings especially when the right type of foundation is not used for the building. During construction if over-dig and backfill material is not properly compacted it will settle over time. Other reasons for settlement includes decay of organic matters present in topsoil, change in moisture content of soil below and around the foundation and overload from the superstructure. This settlement will cause any concrete poured atop the soil to settle with it thereby leading to severe crack problem.

b.   Poor structural design and specification: it is important that at the stage of building design, the designer (structural) first takes the existing environmental conditions around the site into consideration. A geotechnical (soil) investigation will help in determining the type of foundation, the type of concrete material and grade of concrete to be used depending on the subsoil chemical and moisture condition. It is critical for the architect and structural engineer ascertain the competence level of the contractor in executing their designs. Neglect of this important aspect could have an adverse effect on buildings.

c.   Poor construction practice (workmanship): building owners most often neglect the need to engage competent technical persons during construction of their buildings. For a building to be safe and healthy, it is absolutely necessary to maintain good construction practice at all the stages to completion without cutting corners. This is evident in the following;

                     i.    selection of poor quality (cheap) materials.

                   ii.    Improper selection of materials.

                 iii.    Wrong and inadequate proportioning of concrete and mortar constituents.

                 iv.    Inadequate control of batching, mixing, transportation, placing, finishing and curing during concrete production.

                   v.    Inadequate quality control and supervision during the various stages.

d.   Thermal movement: Building material are prone to expansion on heating and shrinking on cooling. This depends on factors such as temperature variation, member dimensions, physical properties of the material and coefficient of thermal expansion of the material. Majorly it is the external walls especially those with thin dimension and the roof (concrete roof, parapet wall) exposed to direct solar radiation and substantive thermal variation that are prone to cracking.

e.   Moisture movement: most of the building materials that have pores in its structure (i.e intermolecular spaces) will expand on absorption of moisture and shrink on drying. It is the combination of this shrinkage and restraint forces from subgrade or other building materials that develops tensile stresses on the building element. Cracks will develop if this combined stresses exceeds the tensile strength of the material (concrete or mortar).

f.     Indiscriminate alterations and impositions: indiscriminate alteration of building designs and addition of loads by interior decorators e.t.c have led to the collapse of so many buildings. Imposing of more suspended floors not contained in the initial building design at the instance of the client.

g.   Poor maintenance: every structure needs proper maintenance after a certain period of time from its construction completion. If such is not in place minor defects not identified early could lead to further deterioration.

h.   Movement due to Chemical reactions: building materials (concrete, mortar) may crack as a result of expansive reactions between aggregate containing active silica and alkali derived from cement hydration. The alkali silica reaction results in the formation of swelling gel, which tends to draw water from other portions of concrete. This causes local expansion which results in cracks in the structure.

i.     Growth of vegetation:

Tree roots generally grow and spread horizontally on all sides to the extent of height of the tree above the ground at maturity. If trees are located close to a wall, it can cause cracks in the building due to expansive action of roots growing under the foundation or death and decay of such roots. Plants could grow in fissures of walls, because of seeds contained in bird droppings, or carried by wind or other means. If such plants  growth is not effectively eliminated, these may lead to development of severe cracking of wall.