How Big A Concrete Floor Slab Can Be Casted Without Cracks?

This article is to discuss what is the optimum size a concrete floor slab can be casted in a single panel without any cracks and defects especially in the industrial floor sector which demand for high quality floor. Even though the technology behind the construction of concrete slab has improved over the years, besides expecting a floor to be built flat and level, one of the major challenges facing the construction industry is how to built a ‘crack-free slab. Concrete being a vagarious material tends to crack due to various reasons but mainly caused by drying shrinkage, thermal movement and restraints. Therefore, concrete floor cracking may be caused by a single factor but also combination of two or more factors. As such designers and builders can only endeavor to mitigate, minimize, or control cracks but almost impossible to guarantee constructing a large concrete slab with no cracks.

1.0 Why big slabs are preferred?

If slabs are casted in smaller panels and without restraints, cracks can easily be avoided but to construct a large size panel, for example 1,000m2 in a single pour successfully without any form of cracks and defects will be a challenge. Large slabs are preferred as it speed up the job, contain less construction joints and contraction joints which are potential weakness on the slab if not properly constructed as shown in Figure 1. The other advantage of minimum joints is to reduce curling of slab at edges as shown in Figure 2. Such curling defects will disrupt smooth operation of lift-trucks and spalling at joints resulting in continuously costly repair.

2.0 Construction of concrete floor slab (slab-on-grade)

Concrete has high compressive strength but low tensile strength and tensile strength of the concrete is only about 10% of the compressive strength, therefore concrete will cracks when the tensile stress within exceeds the tensile strength of the concrete. Hence, one way to mitigate cracks is to lower the tensile stress in concrete and to increase the tensile strength of the concrete. When cracks occurred, blame goes to poor workmanship, poor quality of concrete supplied or improper design. In actual facts, any cracks resulted could be man-made or caused by factors to be discussed as follows.

Common factors which may contribute to cracking in floor slab.

2.1 Ground settlement

Subgrade is the original ground and it is best to carry out a soil investigation to study the soil bearing capacity to support the intended design load. Any filling materials selected for filling low shall be suitable for easy filling and well compaction. A well-compacted sub-base will facilitate loaded vehicles to roam the construction site freely without rutting the surface and also a simple proof rolling method to ensure the subgrade and sub-base are well compacted to prevent settlement cracks.

2.2 Contraction joints

As thicker slab is stiffer and will tend to produce less cracks compared to a thinner one unless the later is specially designed. Likewise, a thinner slab will also tend to curl more than a thicker slab of similar mix design and working condition. Therefore, to avoid random cracking, contraction joints are created on weaken planes on the slab surface at locations where high stresses accumulate and prone to cracks. In fact, contraction joints are simply pre-determine man-made straight line cracks created to release stresses on the floor surface to induce cracks when the slab commence to shrink, being straight line man-made cracks, nobody will complain. If contraction joints are not provided to disipate the stresses, random cracks will be unavoidable.The contraction joints are carry out using mechanical diamond saw machines the next day or the same day if a green blade is available. The depth of cut is normally one-fourth of the slab thickness and the saw-cutting is carried out immediately when the concrete gained sufficient hardnesss, normally between 8 to 12 hr after the floor is finished. To mitigate random cracks, contraction joint spacings of maximum 36 times the slab thickness, up to a maximum of 5.5meter have produced acceptable results(ACI 302.1R-04). If a slab is 150mm thick, the contraction joints need to be spaced at every 5.4meter.

Contradictly, wider contraction joints spacing of up to 16m x 16m have been successfully carried out reinforced with 20kg/m3 of steel fibres without any form of cracks.

2.3 Type of reinforcement and positioning

To select welded mesh or steel bars as reinforcement for crack control is one task but placing and positioning the reinforcement location right is the challenge. The purpose of reinforcement is to provide additional strength where needed and if cracks happened, it supposed to keep the cracks tight and preventing it from opening and becoming wider. If the location of reinforcement is wrongly placed, for example, placed lower than mid section of slab, it will defeat it’s purpose and may not performed. It is best to position about 30mm from the surface for crack control. To overcome such difficult to position the reinforcement right, steel fibres or synthetic fibres are used as a replacement for better crack control. Generally, fibres will provide better crack control as they are 3-dimensionally distributed within the concrete slab to absorb any tensile stresses which occur at early stage when the young concrete has yet to achieve the tensile strength to resist cracks. When early micro-cracks developed, the fibre strand will intersect and blocked the micro-cracks from developing into macro-cracks as the concrete shrinks.

2.4 Concrete mix design