Concrete floors, on the ground, account for the majority of problems dealt with by the technical staff at The Concrete Institute, says Bryan Perrie, MD of the Institute.
The problems encountered can be categorised into surface defects, joint defects and structural defects, with numerous defects within each category. An analysis of these identified six primary reasons for the occurrence of these defects:
- Design by structural engineers
- Inadequate detailing and/or specifications
- Split responsibility on site
- Lack of skills or knowledge
- Inadequate knowledge of materials and their behaviour
- Inadequate appreciation of construction techniques
Design by structural engineers
Concrete floors on the ground are effectively concrete pavements and should be designed, detailed and constructed as such by pavement engineers. The use of structural concepts such as the use of reinforcement to increase load carrying capacity and reinforcement through construction joints results in unacceptable cracking and in some cases overstressing of the floor and structural failure. Often there is a lack of understanding of restrained drying shrinkage in floors on the ground. “Often engineers are not aware that there are design guides and software available for the design of concrete floors on the ground,” Perrie states.
Inadequate detailing and/or specifications
Joint detailing, location and layout are all critical factors in the behaviour of concrete floors on the ground. There is a tendency to mix and match joint types and details from different design technologies which often results in poor joint performance and also joint failure.
To date there is no standard specification which deals with floors on the ground. There is however a SANS Standard for the design, detailing and surface finishes, namely SANS 10109 Parts 1 and 2, as well as a book entitled Concrete Industrial Floors on the Ground, available from The Concrete Institute. These documents provide guidance on the detailing and specifying of floors on the ground.
The best specification for tolerances is contained in the British Concrete Society’s Technical Report No 34 (TR34). This often results in the need for special equipment or a specialised contractor to measure the floor to ensure compliance with the specification.
Split responsibility on site
Typical issues here include:
- The earthwork contractor’s tolerances not being compatible with those for the concrete floor;
- The main contractor supplying to, or purchasing inappropriate concrete for, a flooring subcontractor; and
- The flooring subcontractor being only responsible for placing and finishing the concrete, but not responsible for installation of shutters, joint cutting or curing.
Lack of skills or knowledge
It may well be that the client does not know what he wants, the engineer not understanding the requirements of the client, the main contractor not understanding the risks for the subcontractor working under certain conditions, the main contractors doing specialised flooring contractors’ work and subcontractors not understanding joint and tolerance details. Good communication amongst all parties is essential.
Inadequate knowledge of materials and their behaviour
“There is a distinct lack of knowledge among all parties regarding concrete materials and their effect on both plastic and hardened concrete. For example the effect of cement type, water content, cement-water ratio on the behaviour in concrete and specifically on concrete floors, which may be largely unreinforced and have a very large surface-area-to-volume ratio. These all affect the rate of moisture loss, drying and therefore the risk of cracking,” Perrie observes.
Inadequate appreciation of construction techniques
The need for adequate compaction and the effect of different types of equipment on compaction, the use of incorrect terminology with respect to power floating and power trowelling, and the effect of different equipment on the finish obtained, are all due to a lack of appreciation of different construction techniques. “By far the biggest lack of appreciation is with respect to the need for adequate protection and curing of floors on the ground. This protection should start as soon as the concrete is discharged and should continue until the required finish is achieved after which effective curing measures should be implemented.” Perrie says addressing these issues will result in better quality floors with fewer defects.
To assist the industry, The Concrete Institute runs a one-day training course on the design and construction of industrial floors at which all of the above issues are covered.
