Explore the world of precast concrete

Builders who want to put up a structure fast as well as cut on costs can consider this technology, writes JAMES WANZALA

Precast concrete technology (PCT), commonly known as waffles, is gaining popularity as a cheaper building alternative.

Precast concrete is a construction product produced by casting concrete in a reusable mold or “form”, which is then cured in a controlled environment, transported to the construction site and lifted into place.

In Kenya, many companies are in the business of making precast concrete slabs or waffles. Kizuri Waffles, started in 2008 by Nick Evans from the United Kingdom and located in Rongai on Mea Road, is the pioneer company. Today, it has about ten branches countrywide dealing in waffles.

Other notable ones are Isinya Concrete Products, Gomu Industries and Premium Construction Waffles. The word waffle is derived from the shape of the precast concrete technology. It originated from the UK.

In contrast, standard concrete is poured into site-specific forms and cured on site.

Precast stone is distinguished from precast concrete by using a fine aggregate in the mixture, so the final product approaches the appearance of naturally occurring rock or stone.

Precast concrete refers to concrete that has been poured into moulds and specially designed to give a specific size and shape with definite lengths and dimensions.

Origins

It is a technology that is old, having been practised in Egypt during the building of the mighty pyramids and in ancient Rome, where it was applied in many projects like building aqueducts that still exist to date; mold culverts and tunnels.

This technology is applied in production of several superstructures all over the world, Kenya included. It is being used in the Kenya Ports Authority mega port expansion project in Berth 20 and 21 at the Coast.

In Kisumu city, it is being used to construct the Kisumu Superhighway that connects Nyamasaria and Kondele, through to the Airport Road along Riat.

“It is a technology that saves us the time of waiting until the cast concrete dries up, and thus the name precast. If a bridge was to be constructed to connect Mombasa Island and the Likoni mainland today, this technology would be used to hasten the process since this path is used continuously by ships coming and leaving the country,” says Awala Lazarus Asewe, a civil and structural engineer with Southern Engineering Company.

Asewe says that although this technology has the potential to provide better results than we get in our usual ways of construction.

Roofing is done using concrete hollow-pots for suspended slabs, waffle slabs and ribbed slabs.

The waffles can also be used in walling door and door lintels, window sills, sun shading devices, laundry fittings like basins and dhobi sinks, as well as foundation columns.

For external works, they can be used for storm water drainage, water reticulation, manhole covers and garden furniture like seats and chairs.

There is usually substantial wastage when standard concrete is used and pre-casting building elements like floor slabs, walling, beams and windowsills can afford one great cost savings.

The technology provides room for recyclable materials since broken concrete pieces can be crushed and recycled for making other concrete forms.

This reclaiming of aggregates and incorporating them into future forms being pre-cast is an added advantage, hence reduction in the amount of construction wastes, both at the plant and construction sites.

Laying material

According to Peter Muchina, a contractor at Kizuri Limited, laying of the material is easy compared to other conventional materials. “You use a 4 metres by 2 metres timber, unlike traditional casting where you use 6 metres by 1 metre drapers,” he says.

He adds that using this technology saves on labour since a 6.9 metres by 6 metres room can be laid by two fundis in a day.

“Precast concrete technology optimises on mass production of members such as the roof, wall panels, columns and beams together with the slabs. Once these have been pre-produced, they can be simply assembled on site using a tower crane or a simple crane, depending on the height of the structure,” supports Asewe.

He adds that time of completion of a project is a key factor in engineering world. That is the only point the client can easily be on your neck. This in return reduces community disruptions.

“The main advantage is that it saves on construction costs since it costs less concrete and labour,” says Nick Evans of Kizuri Waffles. He says so far, he has a pool of about 800 customers of his material. When making the material, little cement is applied to the ribs where the waffle slabs meet one another.

“The Kizuri prides itself in having waffles that also absorb sound. That is why it has been used to build Kenyatta University’s lecture hall, administration block, Olelai Primary, Meru University, Balozi Estate in Thika and most of ACK churches,” said Evans, who has used the technology for more than 20 years.

The process of using waffles involves fixing the timber props, then rails measuring four metres by two metres, and then waffles. You then put reinforcing steel bars between the waffles. Then you place electrical conduits or power pipes, then BRC, then concrete 25mm from the centre of a waffle to the rear.

Kizuri designs and draws the slabs, which, involves calculations of the number of slabs, cement, sand and ballast to be used. This enables the house owner to estimate the cost the house.