Slab thickness is a major technical concern. This problem is generally a result of variations in the granular base elevations which can cause excessive localized thinning of the slab and, therefore, a reduction in the performance of a floor.
Elevation control of granular bases is a difficult task. Laser graders are available for a wide variety of equipment, however sometimes manual leveling is the only practical choice, as in limited access areas such as small rooms and sloping granular bases. In small or restricted spaces, elevation control may be possible through the use of a compatible fine screening layer as a finish course for the granular base or through increased slab thickness. Specifiers should consider the limitations of each site condition (access and aggregate sizing) and should consider the use of thicker floors to offset any anticipated larger variations in granular base elevations (adding the variation in the base to the slab thickness required).
This problem in most cases be avoided through a pre-construction inspection of the granular base elevations. It is the responsibility of the earthworks subcontractor and general contractor to ensure this is performed correctly prior to ordering a concrete pour. All too often, concrete pours are scheduled without this critical item being checked thoroughly. As would be expected, little can be done to correct this problem in the middle of a large concrete placement.
The Canadian Standard CSA A23.1 Concrete Materials and Methods of Concrete Construction stipulates that the maximum variation in the elevation of the granular base shall not exceed ± 10 mm (± 3/8″) and the acceptable thickness tolerances for slabs on grade, worded as follows:
The thickness of a slab on grade shall be acceptable if the average thickness is not more than 10 mm less than the specified thickness, and no thickness measurement is more than 20 mm less than the specified thickness. The slab thickness shall be determined from randomly located cores obtained from each floor placement at a rate of one core for every 100 m2 of floor area, except that no core shall be taken closer than 1 m to any vertical obstruction. The thickness of the slab at each location shall be the average length of the core. When calculating the average thickness of the slab, cores more than 20 mm longer than the specified thickness shall be considered to have a length 20 mm more than the specified thickness.
1. Additional testing should be undertaken in areas of unacceptable thickness results to determine the extent of corrective action.
2. Core measurements should be taken within 7 days of each floor placement or as soon as practical.
More care is required in the field to avoid future performance problems. Please read the CSA A23.1 Standard for full details on these and many other floor requirements.
Specifiers are urged to incorporate “single source” clauses in their specifications, which call for the concrete flooring contractor to do the following: inspect the granular base elevations, form the bulkheads, supply the concrete, place, finish and cure the floor including all jointing and sealing. This has repeatedly proven to produce well constructed, durable concrete floor surfaces.
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