Floor screeds crack as contractors cut corners

The dropping or lowering of standards in the construction industry has been under the media spotlight for the past five years or more; and for good reason. Incidents of failure, some of them major and incurring injury and loss of life, have led to huge wastage in human and material resources. It is a dangerous development which has set alarm bells ringing among reputable manufacturers and construction-related institutions, and if ignored, will haunt South Africans for generations.

In a recent example of what happens when standards are side-stepped in the attempt to win contracts at any cost, the floor screeds of a large mixed-use retail and residential development south west of Johannesburg’s central business district have been subject to extensive cracking.

In this instance the main contractor opted to exclude steel mesh, an essential component which should always be used in throwing screeds on precast flooring. Apparently the contractor was informed by the precast flooring supplier that its precast hollow-core slabs were produced in a manner which precluded the need for mesh in the screed topping. The particular slab manufacturer even went as far as issuing a written guarantee that the screeds would not crack.

The manufacturer’s claim proved false, however, and the screeds began cracking soon after they were laid. Cracked screeds might appear as little more than a superficial nuisance, however, the consequences are anything but that. The main contractor has been replaced and a painstaking process of removing the flawed screeds and replacing them using approved building practice has begun. Moreover, the cost of replacing the screeds goes far beyond that of labour and materials, as the completion of the whole development has been delayed by several months, thereby resulting in a loss of rental income.

Commenting on this incident and the flaunting of standards in general, Bryan Perrie, managing director of C&CI (Cement and Concrete Institute), says that the advantages of taking short-cuts in the construction industry are always short-lived.

“There is a price to pay and often it’s the client who pays it, as proving culpability is an expensive, time-consuming and often fruitless exercise. Therefore, getting it right the first time using tried and trusted standards endorsed by institutions such as C&CI, the CMA (Concrete Manufacturers Association), the NHBRC, the CSIR, and the SABS, among others, is in everyone’s interest. Of course, industry professionals, such as architects, quantity surveyors and civil engineers should insist that proper standards are followed.

“Nonetheless, as this screed failure amply demonstrates, the contributions made by professionals are sometimes found wanting. Therefore it is in the interest of property developers to become acquainted with building standards and insist on their implementation. Alternatively, they should se to it that the professionals they employ will do so on their behalf. If not, they are behaving irresponsibly, both to themselves and their prospective tenants,” says Perrie.

These views are supported by Echo Prestress, South Africa’s largest manufacturer of precast hollow slabs and a member of C&CI and the CMA.

Commenting on the case of the cracked screeds, Echo sales and marketing manager, Melinda Esterhuizen, says any precast slab supplier who claims that mesh is not a requirement when throwing screeds on floor slabs, no matter who supplied it, is being deliberately misleading.

“The construction industry in general and contractors in particularly should be extremely wary of such claims which ultimately are prejudicial to the reputation of the precast slab industry as a whole. We therefore encourage any user, be they a contractor, engineer or property developer, to contact us about any aspect on the proper installation of precast flooring, especially when confronted with performance claims which appear to flout good building practice. Our screeding procedures leaflet, which includes details on screeds for balconies, roofs, walkways and car parks, is available at no charge and offers sound technical advice. Moreover, more detailed and technical screed information is available from C&CI which has published a brochure on the subject entitled “Sand-cement screeds and concrete toppings for floors.”

In most instances, especially when Echo Prestress flooring slabs are used, a simple 40mm screed is all that is required. However, in buildings with large floor areas, movement jointing should be specified by a consulting engineer to avoid cracking.

Esterhuizen advises that before a screed is laid all loose material should be removed from the slab surface.

“In addition, once the steel mesh is in place, slabs should be made thoroughly wet and the screed mix applied immediately thereafter. The screed should comprise a 1:4 mix by volume of cement and clean river sand, and should achieve a consistency which ensures a smooth steel-floated finish. In some areas the screed may need to be thicker than 40mm to level out camber. After floating the screed should be kept wet for 48 hours to prevent shrinking and cracking,” says Esterhuizen.

Examples of Cracked Floor Screed