Slab level crack or parapet walls cracks
Construction methods in Pakistan that are in practice have unfortunately not witnessed advancement and innovation as compared to rest of the world. Our house construction methods have remained the same through out the years except for minor changes such as introduction of mixing machines and lifts for concrete pouring (at the time when world has moved towards ready mix concrete already long time ago). There was a time not long ago when this process too was carried out in completely manual way using labor force. Blame is to be given to the governments for lack of innovations as there have been no serious steps taken in any tenure for encouraging innovations, vocational training and education, policy making for industry to manufacture or assemble latest construction machinery locally. Fortunately we do have skilled labor but skilled labor without proper vocational education and training can not produce best output.
The house construction methods that we adopt in our daily practice has various flaws that result in defects during or after completing the construction of house. One of the most major defect that arises in 99% of the houses is the horizontal hairline crack along the upper floor slab level visible from outside and vertical hairline cracks in parapet walls as shown in the picture below. Both these cracks occur due to more or less the same reasons i-e thermal expansion of steel and joint between masonry and concrete.
Let us discuss about the slab level crack first. This crack becomes visible even before completely finishing the house. As stated above, major reason for this particular crack is the thermal expansion of steel. This crack occurs in upper floor of the house where slab is directly facing the sun and has no load on it unlike the lower floor slab. There is no method to completely get rid of this crack however there are couple of methods with which the occurrence of this crack can be delayed. One thing that is necessary to mention here is that these cracks are more prominent and visible in structures that are designed on straight line architecture. Houses made in designs such as victorian and spanish styles that consists of cornices, arcs and borders are more likely not to get this crack.
As shown in picture below, this method is most commonly used in straight line architectures as precautionary measure against this crack. A steel mesh is applied before plastering at the slab level using nails and neeru(cement paste). It is allowed to dry and settle down before finally plastering the wall. Though this method will not guarantee that the crack will never appear, but at least it works in many cases.
Similarly for protection against the vertical hairline cracks in parapet walls, only method that will guarantee no cracks is the making of parapet walls in concrete structures. Parapets made in bricks definitely crack with time. Method adopted in our houses for construction of parapets in brick structure normally consists of a steel dowel bar to give support to 4.5 inches thick brick wall. Concrete is poured after every 2-3 ft wherever dowel bar has been placed, that joint where concrete and brick masonry joins, is the place where crack appears after settlement. Hence to avoid this crack, recommended method is to make parapet walls in concrete. However this is an expensive method as compared to normal brick parapet.
These types of hairline cracks that occur in parapet walls or at slab level are not dangerous in any regards for the structural stability of the building. And once these cracks are repaired, there is a very minor chance of them appearing again.