CONSTRUCTION
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Building Cracks
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ou may have wondered why a building or buildings you might have observed formed unequivocal cracks. It is evident that many property owners have felt a sense of unease with the onset of cracks apparent on their asset(s). Frankly speaking, many builders demonstrate tenuous clues as to the reason their construction go sour, and many have slight knowledge in mitigative measures to avoid the apparent unpleasing appearance of a completed construction with random or alternating cracks on their building. There exist a vast number of reasons why buildings crack. It is imperative to say that only the prominent reasons will accrue to the content herein presented. Notably, cracks in buildings tend to exist prevalently in components such as walls and floors. Therefore, you as the engineer, contractor or the observer will need to fathom the intertwining of the workings of a building structure to cause walls or floors to crack. Wall Cracks Cracks in walls tend to appear due to faulty brickwork. This is not only the consequence of poor workmanship by the bricklayer but also questionable bricks’ or blocks’ quality, and the mistake of making up for inappropriate bricks or blocks with the mortar or paste used within the brickwork. In many constructions, low strength bricks or blocks are deployed, and builders resort to mixing mortars that are of higher strength in an attempt to compensate for low strength exhibited by the bricks or blocks. Consequently, this resolution culminates into the cracking of the bricks or
blocks themselves. The mechanics behind a strong brick or block with a slightly lower strength mortar is that bricks utilise the space left between them for expansion and using a lower strength mortar permits the expansion to occur. When the strength regime is reversed, that is, the brick or block displays lower strength than the mortar, the weakest component will succumb and allow for the crack to form. Another factor that is normally misconceived by many is to assume that the wall will become weak because of weak mortar. This is unfortunately not the case owing to Poisson’s ratio, in that the less the height of a structural component, the more load it will be able to resist. This is evident in the cylinder concrete test samples versus the cube concrete test samples. The latter yielding a larger strength than the former. Furthermore, walls crack due to the phenomenon of differential settlement. This results from what engineers call “ground consolidation”. When a structure is built it tends to squeeze the ground on which it is erected. Depending on the compact state of the soil, some parts of the ground will subside more than others. This difference of settlement causes stress on the walls. The stress is then relieved on the wall by finding the weakest spots, which in buildings happen to be in doors and windows. That is why windows and doors should have joints in them to allow this phenomenon to occur without any implications. Another good strategy to circumvent this problem is to properly select the correct kind of foundation, either from
Construction & Mining Magazine, Vol 5 | Edition No 2
the many options of shallow foundations or the three available deep foundations options. A Structural Engineer will be very handy for this application. Additionally, a shear failure of the soil can occur, leading to considerable settlement and impending failure of the structure. The right choice of foundation is also paramount in this case. Moreover, when walls are of a reasonable length, it is advisable to insert expansion joints at moderate intervals. If these joints are not catered for, the wall will definitely expand, as all materials do due to temperature fluctuations. Major pressure (or stress) buildup occur under such circumstances and this results into a crack on any weak points along the length of the wall. So please, help yourself with that expansion joint and have a lasting smile as you view your crackless building walls. Floor Cracks Floors made of concrete have a tendency of cracking. But let’s be pragmatic and real; concrete will always crack. The main issue is controlling the sizes of your cracks. These can range from miniscule to large eye fretting cracks. The manner in which you treat the concrete is the determining factor in the size of the cracks. Concrete by property shrinks after hardening, resulting in what is termed “shrinkage cracks”. The main solution is to add a minimum reinforcement to concrete spaced in a correct manner. It would be better, to a certain
