Under The hollow-core slabs are estimated to cut building costs by over a third, while reducing demand for sand and ballast, two items at the centre of great environmental degradation in areas near and far from major urban building hot spots in the country.
Concrete waffles, as an alternative to concrete slabs, are steadily gaining traction in the multiple-storey building industry, largely due to the emphasis on sustainable buildings at reduced cost. Also known as ribbed slab construction procedure, it is preferred for strengthening slabs, whether a foundation or storey ones, and for cutting down the inputs.
Last year (2020), the Kenya Bureau of Standards (KEBS) and multiple players, in recognition of the rising use and advantages of the technology, for the first time developed local standards for the construction industry.
The hollow-core slabs are estimated to cut building costs by over a third, while reducing demand for sand and ballast, two items at the centre of great environmental degradation in areas near and far from major urban building hot spots in the country. They can be used on both ceiling and floor construction, although the former has been more prevalent in Kenya’s construction industry.
The technology reduces the carbon footprint and energy consumption that is estimated at 60 percent of the total construction cost in East Africa, according to UN-Habitat. Often, it is a flat slab at the top with a bottomed out grid-like downside that is put together as a system to form a floor in a storeyed structure. The thin top with ribbed bottom slab are laid perpendicular to each other and have equal depth. Its ribs and double reinforcement gives it more stability, making it ideal for large foundations and slabs.
30% – Concrete savings made with waffles, which also save 20% more concrete than pontoon chunks. They give low floor avoidance and have great vibration control.
There are a number of clear advantages that make waffles suitable for construction, especially in flat areas that are common around Nairobi and major population centres like Mombasa, Nakuru and Eldoret, targeted for the Affordable Housing Scheme. With cement and aggregate being a significant cost component in conventional construction, the hollowed bottom helps cut the input costs.
This is because the recommended diameter of a waffle slab is normally between 85 and 100 millimetres (mm), compared with that of a normal slab that is anywhere between 300 and 600mm. On top, the width of the beams or ribs take 100mm to 200mm that, in normal circumstances, would be filled with concrete and bars. It is estimated that waffle slabs require 70 percent of concrete and two-fifths of steel used in conventional or stiffened slabs. In the Kenyan context, being plant-produced, waffle slabs are ideal for guarding against loss of materials at the construction site, largely through pilferage.
Reinforcement has been a major selling point too. The waffle slabs are reinforced with wire mesh or individual bars.
At the construction stage, the support-beam excavation is done together with the installation of the waffle slabs.
There are three ways in which the waffles can be applied. First, the concrete can be poured at the site, including by contracted formal companies or small machinery operators. Here, it is poured into precast cardboards or other structures in a process technically referred to as in situ. The reinforcement framework is done at the site as the waffle slabs are installed.
The framework or form for installation of waffles requires horizontal and vertical supports first, which are fixed together with wall connectors. The latter join the walls and the slabs. Connectors are also required to join the horizontal beam.
You will also need waffle pods placed in the connectors, whose size and shape are available in plastic and depend on the beam size. Hole plates, cube junctions and, very importantly, steel bars, are also required. They are much favoured for construction of large public facilities. These include car parks, airports, bridges, and industrial warehouses and plants, as well as residential developments. The technology, in its varied forms, is as old as the construction industry, having first been recorded in ancient Egypt where it was employed in putting up the famous pyramids. It was also used in the ancient Roman Empire in building the city of Rome.
Locally, it is widely used in construction of commercial flats around Nairobi and other major towns. The Kenya Ports Authority deployed the technology in the construction of Berth 20 and 21, one of the largest expansion by the agency in many years.
In other public works, it was used in Kisumu town to construct a highway through Nyamasaria to Kondele to Kisumu International Airport.
Elsewhere, a number of iconic buildings globally have utilised this technology. They include the Washington Metro Building in USA, the Royal National Theatre in the United Kingdom (UK) Logistics, the Telecommunications SL in Madrid, Spain, and the Galbraith Hall in San Diego, California, in USA.
Kenya has recently established standards for precast concrete building waffle units. The KS 2932:2020, now in its first edition, by the Kenya Bureau of Standards, was arrived at after intense consultations with multiple stakeholders, including the built-environment sector.
Notable among them were Bamburi Cement Plc., East African Portland Cement Plc., Institution of Engineers of Kenya, Institution of Quantity Surveyors of Kenya, Jomo Kenyatta University of Agriculture and Technology, Kenya Electricity Transmission Company, Materials Testing and Research Division, Ministry of Transport, Infrastructure, Housing, Urban Development and Public Works, Ministry of Water and Sanitation, Nairobi Western Bypass Project, and National Housing Corporation of Kenya. The elaborate nine-page draft document lays down the specifications and conformities of the waffle units, including cement, aggregates, water and steel reinforcements, as well as the dimensions. There are downsides to the waffle slab, however. The slabs are thicker from top to bottom, meaning the length between floors must be higher; that translates to more usage of materials. This applies mainly to the form work.
The slabs are not very suitable for sloping areas and are more recommended for flat topography. The technology is locally prone to low uptake from masons, not used to adopting unfamiliar technology far removed from the brick-and-mortar norm.
Locally, there are many suppliers molding the waffles in various places using a number of technologies. They include Rifloin Industries, Gomu Industries, Bimtech Manufacturers, Kizuri Waffles, Boleyn Magic Wall Panel Limited and Premium Construction Waffles.