UNDERSTANDING BUILDING CRACKS IN THE CONSTRUCTION INDUSTRY AND WHY YOU NEED TO HIRE PROFESSIONALS.

By David Maclaren KIIZA

When you traverse urban setups in Uganda particularly, one would realize that in almost every building; whether very old over 30years or new less than 5 years; cracks are evident.it still beats my understanding as to why buildings constructed before 1962 Uganda independence and early 1970’s barely have cracks unless the structured has or is being tampered with due to remodeling

Nyakasura School Administration block (left) built in the late 1920’s and Dining Hall (right) built in the 1960’s

Talking to Raymond Banyanga a RICS Chartered Quantity Surveyor with JR Consultants and

Eng Sabil Ismael Byaruhanga a Consultant with Ground Control Engineering and Construction Ltd; the two all agreed that  Cracks in a building are of common occurrence because a building component develops cracks whenever stress in the component exceeds its strength hence are classified in to structural and non structural categories. The structural ones include faulty design, faulty construction or overloading which may endanger safety of buildings. The non structural cracks include internally induced stresses depending on width of crack, these are classified in to thin (< 1mm), medium (1mm to 2mm) and wide (> 2mm wide). Internally induced stresses in building components lead to dimensional changes and whenever there is a restraint to movement as is generally the case cracking occurs.

 

However, the most common causes of cracking globally in general are; chemical reactions in construction materials, foundation movements and settling of buildings, changes in temperature and climate, environmental stresses like the nearby movement of trains, earthquakes etc. Faulty and defective design, poor quality materials, incorrect methods of construction, weather and loads of wear and tear can create cracks.

Aging and natural deterioration of the buildings can also result in many types of defects; most of which are localized and non structural in nature. There are few which are severe and can affect the structural safety of the buildings. Identifying the defects earlier on and carrying out timely repair and maintenance to keep the building in sound structural condition is the best way to avoid unnecessary panic.

But are cracks a serious issue? Well, since they occur commonly you might think it’s not, but that’s not right. In reality, if you ask this question to a professional even he might find it difficult to answer instantly. The reason is simple, some cracks are not serious and are easy to repair and some can result into danger of the building collapsing or demand expensive repair costs.

In general, taking cracks seriously is important as they appear in wall’s bricks/block mortar plaster, columns-wall joints, beam-wall joints, slabs, beams, columns, foundations and in many other places of the building structure, And yes, inspecting cracks in a building is not simple ask, hence, take the help of a professional who can diagnose them well.

Most of the buildings develop cracks at some point in time during their lifetime, following these reasons.

Initial shrinkage and/or Moisture movement

Most of the building materials having pores in their structure in the form of intermolecular (ex concrete, mortar, bricks etc) expand on absorbing moisture and shrink on drying. These movements are reversible. Initial shrinkage is partly irreversible and occurs in all building materials like brickwork, concrete shrink in the beginning which is partially irretrievable. Shrinkage cracks in building walls can be reduced by using less rich cement mortar in the masonry and by suspending application of plaster on the brickwork surface, basically when the masonry has experienced most of its early shrinkage and has dried well after proper curing.

Shrinkage cracks in rendering/plastering  can be minimized by using mortar for the plaster which is less rich and perfect for providing resistance in durability and abrasion.

Cracks due to shrinkage affect mainly the appearance and finish and the structural stability is not impaired. These cracks generally get localized near door and window openings or stair case walls. In external walls they run downward from window sill to plinth level or to the lintel of lower story.

 

Thermal movement:

Due to variation in atmospheric temperature, all building materials expand on heat and contract on cooling. Thermal movement in the components of building structure creates cracks owing to tensile  of shear stresses. But, again the scale of movement depends on their molecular structure and other properties. Thermal changes are one of the strongest causes of cracks in buildings and demand attention. Say in India, daytime and seasonal changes are normally of the order of 5 - 20°c and 0 - 25°c correspondingly.

some restraint to movement of building component generates internal stresses resulting in cracks due to tensile or shear stresses. Cracks due to thermal movement could be distinguished from those due to shrinkage or other causes from the criterion that the former open and close alternately with changes in temperature while the latter are not affected by such changes.

Thermal movement depends on color and surface characteristics of exposed budding surfaces. Dark colored and rough textured materials have lower reflectivity and hence rise in temperature is more for these surfaces. concrete roof slabs whose material has low conductivity, thermal gradient is quite appreciable and that causes the slab to arch up and also to move outward due to heat from the sun resulting in cracks in external walls which support the slab and in the internal walls that are built up to the soffit of the slab. In case of framed-structures, roof slab, beams and columns move jointly causing diagonal cracks in walls which are located parallel to the movement, and horizontal cracks below beams in walls which are at right angle to the movement.

 

Elastic Deformations

Structural components of a building such as walls, columns, beams and slabs, generally consisting of materials like masonry, concrete, steel etc, undergo elastic deformation due to load in accordance with Hook's law, the amount of deformation depending upon elastic modulus of the materials, magnitude of loading and dimensions of the components. If RCC slabs, RCC lintels over openings and masonry in plinth and foundation have good shear resistance, cracking in question would not be very significant.

 

Creep Movement

Gradual and slow time-dependent distortion of the concrete structure under constant loads is known as creep. It can generate extreme stress and lead to the development of cracks. In concrete, extent of creep depends on a number of factors, such as water and cement content, water cement ratio, temperature, humidity, use of admixtures and pozzolanas, age of concrete at the time of loading and size and shape of the component. Creep increases with increase in water and cement content, water cement ratio, and temperature; it decreases with increase in humidity of the surrounding atmosphere and age of material at the time of loading. In case of brickwork, amount of creep depends on stress/strength ratio and, therefore, creep in brickwork with weak mortar, which generally has higher stress/strength ratio, is more. Another reason for greater creep in case of brickwork with weak mortar is that weak mortar has greater viscous flow than a strong mortar. In brick work, creep may cease after 4 months while in concrete it may continue up to about a year or so. However, in concrete, extent of creep is related to the process of hardening and thus most of the creep takes place in the first month and after that its pace slows down. That means creep strain can be reduced by deferring removal of centering and application of external load. The mechanism of creep is still vague and not visibly understood. At low-stress levels, it is believed to be due to seepage and viscous low and at high-stress levels, it can be due to micro cracking and inter-crystalline slip.

 

Chemical Reaction

Chemical reactions in building materials increase their overall volume and internal stress which results in cracks with materials involved weakening due to chemical reactions. Carbonation in cement-based materials, sulphate attack on cement products, alkali-aggregate reaction, and corrosion of reinforcement in concrete are few of the most common types of chemical actions on building materials

 

Foundation Movement and Settlement of Soil

Shear cracks happen due to huge differential settlement in the foundation because Structures constructed on expansive soils are vulnerable to swelling on absorbing moisture and shrink on drying due to change in moisture content of the soil are highly prone to cracking. Shear cracks occur due to any of the following causes.

·       Unequal bearing pressure under different parts of the structure

·       A low factor of safety in the design of foundations

·       Bearing pressure being in excess of the safe bearing strength of the soil

·       Local distinction in the nature of supporting soil

These are serious cracks as they occur due to foundation movement of the corner and end of the building structure, hence demand special measures to prevent such cracks.

 

Cracking Due To Vegetation

The existence of any form of vegetation can be a reason of cracks in the walls of a building mainly due to the expansive action of roots growing in the brick masonry or under the foundation. Simply put, the roots of trees set in the vicinity of a wall can create cracks in walls due to the growth of roots under foundation. The cracks occur in clay soil due to moisture contained by roots.

The deterioration process in concrete starts with the penetration of several aggressive agents and is a major cause of cracks on walls. Essentially, it dictates the capacity of concrete to survive weathering action, chemical attack, or any other process of deterioration. Hence, low permeability is the main factor to concrete resilience. There are several factors which control concrete permeability, water-cement ratio, curing, air voids due to deficient compaction, use of admixtures, micro-cracks due to loading, cyclic exposure to thermal differences, and oldness of the concrete.

The permeability of cement mixture is a function of water-cement ratio given great quality materials, suitable proportioning and good construction practice. the permeability of the concrete is a straight function of the interconnection and porosity of pores of the cement paste.

 

Structural Design

Poor or bad structural design and specifications are another striking cause of the cracks in concrete works. The designer needs to consider all the environmental aspects which include soil investigations; this will enable the designer to come up with a proper robust design of the foundation.

 

Poor Workmanship

Inferior mixing of construction materials, like sand, cement, and aggregate cause cracks on the walls, slabs, beams, etc. Poor workmanship is normally a result of a lack of proper supervision, ignorance, negligence, carelessness, negligence, and many others or a combination of all these.

 

Poor and/or no maintenance of buildings

It is crucial to always take good care of your house, and this can be done by doing regular maintenance works. This ensures the building structure is intact.

 

Natural Forces

Earthquakes, tremors, winds, rains, wooding like the Kerala Havoc and many others can cause cracks in the buildings.

 

For diagnosis, the following information is to be collected and studied; location, shape, size, depth, behavior and other characteristics, specification of job, time of construction, past history, when the cracks first came to notice and  whether the cracks are active or static. This will help all stakeholders to understand the type of crack he/she will be dealing with in the construction industry.

The author is a Civil Eng, Statistician & Petroleum Engineer

@kdavidmaclaren