Precast concrete industry at the forefront of SCMs

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Immense strides have been made by leading role players in the global precast-concrete industry to manufacture high quality elements for the construction of more robust final structures.

This mitigates the need for maintenance and repair and, in so doing, negating the use of finite resources and construction materials to significantly reduce the overall carbon footprint throughout the service life of the structure or building.

Noticeably, these innovative concrete mixes that are used to manufacture precast-concrete elements also contain less cement content.

The cement industry accounts for about five percent of man-made carbon dioxide (CO2) emissions, of which about 40% is as a result of burning coal and 60% from the calcination of limestone.

It is also an energy-intensive industry, consuming about three percent of total international electricity generation capacity, or accounting for almost five percent of global industrial demand for power.

Meanwhile, South Africa’s cement producers are all reliant upon mainly fossil fuels-based electricity generation and are, therefore, expected to incur significant penalties when the proposed carbon tax is implemented.

This is despite the immense strides they continue to take to optimise their processes in line with what is considered to be best international practice.

One of the preferred supplementary cementitious materials (SCMs) used by the global precast-concrete industry is silica fume, or “micro-silica”.

Considering the material’s long association with high performance concrete applications and the stringent quality controls in place during all stages of its procurement, it is also favoured by CoreSlab for the design of its concrete mixes.

Silica fume is used by the leading South African precast-concrete specialist in the manufacture of all of its precast-concrete systems that have been used to build durable structures for government bodies and private sector clients.

Shaun Hadkinson, marketing manager of CoreSlab, says that silica fume is a recyclable by-product of the production of silicon alloys in electric arc furnaces.

“It can generally be used to supplement between five percent and 10% of the total mass of cementitious material in the concrete mix. Highly reactive, the smallest of the particles speeds up the reaction with cement hydration. The very small particles range between 0,1 and 0.2 μm and are, therefore, able to pass through the void between the particles of cement to improve packing,” he says.

This material has enabled CoreSlab and its international counterparts to design and produce concrete that has an increased tensile and compressive strength, while providing compressive and flexural modulus, as well as tensile ductility.

It also enhances vibration damping capacity, abrasion resistance and bond strength with steel rebar, in addition to significantly improving the chemical and corrosion resistance traits of the precast-concrete mixes.

Leading global precast-concrete companies are also harnessing their extensive experience in proportioning with silica fume to achieve viscosity modification in highly flowable concrete mixes and to significantly accelerate production cycles to achieve compressive strengths of up to 68,94 MPa in as little as a day.

Moreover, silica fume has been used by these participants for more than 30 years to design concrete mixes with compressive strengths that exceed 137 MPa specifically for high-strength and high-performance applications.

Hadkinson lauds the global precast-concrete sector for its ongoing work in the field of SCMs which, he maintains, is key to producing more sustainable concrete, the single most widely used construction material in the world.

“The use of industrial residues, including granulated blast furnace slag and fly ash, to supplement cement content in concrete needs to remain high on the agenda. This will help push existing limitations, including the significant retardation of concrete strength due to the large scale use of SCMs. Experienced operators have been able to strike a fine balance between the benefits and drawbacks associated with each SCM to carefully proportion and modify any mix design according to recommendations, governing standards and specifications,” he says.

However, he concludes that building durable structures remain(s) the most important means available to ensure sustainable development.

This focus starts at the design phase when an initial decision is taken to use materials, such as precast concrete, to ensure the construction of more durable final structures and spans the actual construction phases by guaranteeing the highest possible standards in workmanship are maintained throughout.

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