By Eamonn Ryan
Cement and brick production and consumption is a reliable indicator of a nation’s growth, as its infrastructure is a reflection of development.
Although precast hollow-core flooring has been available locally for the past 30 years, the potential for further growth, especially in affordable housing, is considerable. One of the main reasons for this is that hollow-core flooring provides for large, open spans and flexibility of application. It also allows for structures which are safe and quicker to erect.
The technology is particularly suited to residential and apartment buildings. Speed of construction and a reduced formwork requirement are great benefits and the advantage in terms of overall cost savings is a major consideration. Another benefit is that by the time such structures reach the fifth floor, services and other trades are already working on the first and second floors.
Infrastructure is impossible without concrete
According to Bryan Perrie, Managing Director of The Concrete Institute (TCI), “Without infrastructure, development is impossible, and without concrete, infrastructure is unachievable. Looking at future trends of the concrete sector, there are five interlinked focus areas: jobs, training, infrastructure, transformation, and sustainability.
“The cement sector has a long history of building and developing the country. The government needs to make good on its commitment to development, including the support and recognition of the manufacturing industries.
Cheap imported cement affects the demand for locally-produced material to such an extent that South African manufacturers are considering mothballing plants and retrenching staff.
“Despite stagnation in the construction industry over recent years which has seen civil engineers being retrenched and engineering graduates struggling to find jobs, there are signs the sector is recovering and will continue to do so in 2020,” says Perrie.
The real threat of using sub-standard cement
The cement industry is seen as a good indicator of a country’s economic growth as it is the key input material in infrastructure, development and much like the rest of the modern world, the foundation of South Africa is built on this premise. Yet, according to Njombo Lekula, Managing Director of PPC RSA, sub-standard cement products are threatening the built environment industry and placing South African lives at risk. Usage of sub-standard cement has various implications that may negatively affect the sustainability of buildings and structures thereby leading to increased repair or maintenance costs, injuries and fatalities due to structural failures or collapse.
During a normal market surveillance exercise whereby competitors’ products were all tested for comparison, PPC found that some products supplied by cement producers were sub-standard and warranted further investigation.
In an effort to protect the greater South African cement industry and consumer, PPC appointed Beton-Lab, a South African National Accreditation System (SANAS) accredited independent laboratory in September 2017 to physically purchase the bags themselves in order to maintain the chain of custody and assure no interference from any outside party. Part of the process was to take photographs of each bag (front, side, back panels and bag weights) to verify Letter of Authority (LOA) numbers, cement type and strength class. The weights of the bags were checked and the EN strength testing in accordance with SANS 50197 for two, seven and 28 days was performed. The South African Bureau of Standards (SABS) prescribed ‘uncertainty of measurement allowance’ of 2.5% was applied when analysing the resultant data. The results show the inability to produce a consistent quality product and thwarting of standards. Alan De Kock, MD of Beton-Lab says, “As an independent laboratory our work is tightly controlled, ensuring accurate data that is in no way influenced by outside parties.”
Various elements are transported to site where they are lifted directly from truck trailers and placed on top of the load-bearing walls using a mobile crane. Credit: Coreslab
HCS are reinforced or pre-stressed concrete slabs, comprising cores that extend the full length of the elements. Credit: Coreslab
The report was recently released, and the findings were shared with the SABS and the National Regulator for Compulsory Specifications (NRCS), showing continued non-performance of the cements tested. According to Lekula, non-conformity of strength and weights of some products ranged from 11% to 73% of the sample set. “This failure to conform to local standards not only has an impact on the structural integrity of buildings, but also poses a threat to possible damage of property and even loss of life should the walls come tumbling down.”
It was also found that most of the sub-standard cement products carry the SABS mark. The SABS stamp is a mark of regulatory approval, instilling trust in the product being sold and, if used in accordance with the instructions, will result in a structure that is robust and safe. Local cement producers have stringent internal quality regulations in place to ensure compliance of their products. However, with cement producers supplying the market with sub-standard extended products, Lekula questions the long-term effect on the South African built environment as well as the sustainability and impact on our infrastructure. With non-compliance of quality and durability standards, consumers are unaware of the risk they face.
It takes up to 28 days for cement to develop strength and fly ash does not start developing strength before 28 days. The durability of mortar or concrete is primarily dictated by the amount and the strength performance of the cement that is used.
Retailers, builders and construction companies are also at risk of future legal action or loss of income as selling and using these sub-standard products can have a negative effect on the perception consumers have of their businesses and standards.
Lukela says, “The use of sub-standard cement products has been identified as one of the main causes of building collapse globally. PPC has decided to take a stand against sub-standard cement products to ensure the safety of consumers and longevity of structures.”
The floor system comprises rectangular-shaped reinforced or
pre-stressed precast-concrete ribs supporting rebated filler blocks. Credit: Coreslab
Suspended slab technologies – horses for courses
There are a number of factors that need to be considered when selecting the most suitable suspended slab technology for a building project. Jaco de Bruin, Managing Director of CoreSlab, a leading South African manufacturer of hollow-core slabs (HCS) and rib-and-block (RB) technologies, discusses some of these considerations with SA Affordable Housing.
“Both HCS and RB technologies provide a faster and more cost-effective means of constructing floor slabs than conventional cast-in-place methods. However, each system has distinct advantages that make them better suited to specific construction projects and teams,” says De Bruin.
HCS are reinforced or pre-stressed concrete slabs, comprising cores that extend the full length of the elements, which generally vary in thickness from 120mm to 250mm. They span up to 12m depending upon loading and are between 900mm and 1200mm wide. These precast-concrete elements are placed on a 30mm to 45mm-thick levelling screed on masonry walls with a minimum bearing of 100mm.
Ribs are placed on the masonry walls with a minimum bearing of 100mm at approximate centres. Credit: Coreslab
To accommodate thermal movement on roofs or exposed balconies, bitumised soft-board or other similar materials are used. This makes allowance for changes in camber or deflection, particularly where light parapet walls are built on pre-stressed HCS. Light mesh reinforcement is also placed in the finishing screed or topping in these circumstances.
Notably, HCS are significantly lighter than a large solid concrete floor slab of equal thickness or strength due to the use of fewer raw materials. This reduces the cost of manufacture and transportation of the ‘planks’ to site where they are lifted and placed, levelled and grouted to complete the floor structure.
However, one of the biggest advantages of HCS is that they are manufactured in a controlled factory environment where the non-modular sections are also cut to size according to the building dimensions immediately after the concrete has reached the required strength.
The various elements are then transported to site where they are lifted directly from truck trailers and placed on top of the load-bearing walls using a mobile crane. Once they have been installed, building contractors and trades are able to work on the structure almost immediately to significantly accelerate the building project.
CoreSlab is able to install an element every six minutes to complete a floor structure in record time, and the company is maintaining a similar track record on a large affordable housing project in Limpopo where it is working alongside a leading black-owned building contractor. Another major advantage is the extremely high quality outcome that is possible the first time round by outsourcing the manufacture and installation of the floor structure to a specialist while the main contractor builds the load-bearing walls and prepares the mortar bedding.
De Bruin says that this attribute remains strong marketing and sales points for HCS, especially in the residential construction market where building capabilities and capacities are often limited. “The growing popularity of HCS in this market is largely being driven by the severe workload in a competitive and volatile industry. These factors are being exacerbated by widespread skills shortages in the construction sector. Home builders and their professional teams want peace of mind that the installation will be done correctly the first time round, while smaller architectural and structural-engineering firms are also increasingly specifying HCS in the early design stages of the project to mitigate risk and reduce construction costs,” De Bruin says.
Accuracy also reduces wastage, another notable benefit that is complemented by the significant cost-savings in concrete, propping and labour that is associated with the use of both in-situ construction methods and RB systems. However, there are still many skilled and experienced building contractors that prefer RB suspended floor systems, despite it being more labour intensive and therefore time consuming than HCS and CoreSlab continues to supply this system to this market.
Unlike HCS, these suspended slab technologies are installed by the contractor which also invests in the propping and raw material, such as the concrete, required for their installation. Generally, the floor system comprises rectangular-shaped reinforced or pre-stressed precast-concrete ribs supporting rebated filler blocks. In-situ concrete is then poured between and over the blocks to form a single monolithic slab.
Slab depths vary between 170mm and 380mm with clear span up to 10m. Beams with a width of between 100mm and 200mm and a minimum depth of 60mm are used with infill blocks of between 200mm and 250mm in length, 440 to 485mm in width and 100mm to 355mm deep. Ribs are placed on the masonry walls with a minimum bearing of 100mm at approximate centres.
Their position is finally adjusted according to the width of the filler block with a 35mm minimum bearing of block on rib. Closed end filler blocks are then placed at the end of each line and beams that do not exceed 1 800mm centres are temporarily propped to a suitable level and camber. Transverse stiffener ribs are also sometimes specified instead of blocks to accommodate reinforcement and concrete. Services are installed over the blocks and the specified mesh placed throughout.
All rubble is removed and the blocks thoroughly wet, and a minimum grade 25 concrete continuously cast. The temporary propping is removed once the compressive strength of the in-situ concrete reaches 17MPa to avoid long-term deflections. While there is still a strong market for RB, demand for HCS continues to grow. This trend has gained momentum since 2008 when CoreSlab initially established its operation in Polokwane to supply the domestic home-building market with quality precast concrete technologies.
“We mainly started off by servicing the high demand for RB systems in the area. However, over time, we increasingly fielded more enquiries concerning our HCS offering. More than ten years later, HCS have become our main product offering, complementing our extensive range of other precast concrete systems for an array of applications,” De Bruin says.
Clay bricks look good in affordable housing
Musa Shangase, Corobrik Commercial Director, says, “We are able to supply the CoroJem as well as the traditional clay face brick and non-face brick to the affordable housing space.
“When it comes to building homes, Corobrik understands that this is more than just a place to sleep and store one’s belongings. A home is an extension of the resident, a visual representation of their place in society. This is why so many developers are turning to Corobrik’s face brick range for the affordable housing market. Clay bricks are available in a range of earth tones and types, retaining their colour-fastness despite intensive weathering. In addition to this aesthetic appeal, there is no need for future maintenance related to painting and plastering, which suits the budgetary constraints of this particular sector.
“Joshco (Johannesburg Social Housing Company) is the City of Johannesburg’s entity mandated to provide and manage quality for affordable rental housing for the lower income market. Committed to building ‘places people love to live’, they prioritise quality, affordability and overall appeal, which is why they have incorporated Corobrik’s face brick range for use in many of the rental developments,” says Shangase.
Clay bricks have considerable advantages in terms of long-term maintenance for residents, he says. “Fired clay brick is robust and extremely durable which means it will not require refurbishment and replacement at a later stage. In addition, the colourfast properties of clay mitigate any need for plastering and painting, making face brick one of the biggest cost-savers for long-term maintenance. The clay from which Corobrik’s face bricks are made offers a distinctive range of colours, because the bricks are fired under high temperatures. This means that they will not fade over time or deteriorate in severe weather or harsh environments.”
