Dividing on-site flooring responsibility between main contractors and subcontractors leads to poor-quality concrete floors, says cement and concrete industry body The Concrete Institute.
The institute is, therefore, encouraging main contractors and subcontractors to engage better with one another to eliminate the risk of a poor-quality product, says The Concrete Institute MD Bryan Perrie.
“Typical issues here include the earthwork contractors’ tolerances not being compatible with those of the concrete floor; the main contractor supplying inappropriate concrete to a flooring subcontractor; and the flooring subcontractor being responsible for only the placing and finishing of the concrete, but not for the installation of shutters curing or joint cutting,” elaborates Perrie.
He tells Engineering News that most of the problems handled by the technical staff at The Concrete Institute pertain to concrete flooring and can be categorised as surface defects, joint defects and structural defects, with “several other defects within each of these categories”.
Perrie says, ideally, pavement engineers should do the design and detailing of concrete floors on the ground because they are, effectively, concrete pavements.
“The floors should be designed, detailed and constructed by pavement engineers and not structural engineers, as using structural concepts – such as reinforcement to increase load-carrying capacity and reinforcement through construction joints – results in unacceptable cracking and, in some cases, the overstressing of the floor and structural failure.”
Mixing joint types and details from different design technologies is also not desirable, as it leads to poor joint performance and failure, he adds.
As an example of the undesirable use of different design technologies, Perrie cites the use by some companies of reinforcement in floor panels to control cracking when panel sizes larger than 4 m are used. However, these companies still use sawn contraction joints or keyed construction joints, which cannot provide adequate load transfer, owing to the larger-than-normal opening of the joints, he adds.Perrie
says joint detailing, location and layout are all critical factors in the behaviour of concrete floors on the ground.
He adds that, to date, there is no standard specification that guides the laying of concrete floors with floors on the ground. However, contractors can use the South African National Standards model for the design, detailing and surface finishes (SANS 10109 Parts 1 and 2), as well as the book, titled Concrete Industrial Floors on the Ground, published by The Concrete Institute.
“They provide guidance on the detailing and specifying of floors on the ground. Often, a standard structural specification like SANS 2001 CC1 is used, but it does not cover specific requirements for floors on the ground,” he explains.
Failure to achieve the appropriate tolerances that are compatible with concrete-floor tolerances is also often caused by the use of outdated tolerance specifications, such as straight-edge measurements, says Perrie.
Although he advises that the best specification for tolerances can be found in the British Concrete Society’s Technical Report (TR) No 34, Perrie says “. . . this often results in the need for special equipment or a specialised contractor to measure the floor to ensure compliance with the specification”.
He adds that care should be taken to ensure reference to the particular edition of TR 34 as there are significant differences between the third and fourth editions.
Skills and Knowledge
Perrie tells Engineering News that, with regard to the overall project, a lack of skills or knowledge could include the client not knowing what he or she wants, the engineer not understanding the requirements of the client or not being able to design and specify accordingly, the main contractor not understanding the risks for the subcontractor when working under certain conditions, the main contractor doing specialised flooring contractors’ work and subcontractors not understanding joint and tolerance details.
“To ensure a good floor, all parties need to be involved at all stages of the project and there needs to be open communication among all stakeholders,” he advises.
Perrie adds that there is a lack of knowledge among most parties involved in a project regarding concrete materials and their effect on plastic and hardened concrete.
“This includes the effect of cement type, water content and the cement:water ratio on the behaviour of concrete and specifically on concrete floors, which might largely be unreinforced and have a very large surface-area-to-volume ratio.”
These aspects, in turn, influence factors, such as the rate of moisture loss and, therefore, increase the risk of cracking, he says.
Perrie highlights that a lack of appreciation for different construction techniques leads to inappropriate equipment use and, subsequently, inadequate compaction; incorrect terminology use regarding power floating and power trowelling; and a lack of understanding the effects of using different equipment on the finish obtained.
“The biggest lack of appreciation is 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 concludes that, as an industry association that aims to assist the industry, The Concrete Institute runs a one-day training course that covers the most pertinent issues regarding the design and construction of industrial floors.