India dumps 15 metric tons (Mt) of plastic waste a day, but local builders are short of sand. Dr John Orr of Cambridge University reports on Anglo-Indian research into using plastic waste as a partial replacement for sand in structural concrete.
Demand for concrete is increasing fast in India due to a growing economy and rising living standards. However, mass extraction of sand for construction, often via river dredging, has been a problem for a number of years, leading to a high court ruling in 2010 which virtually eliminated sand dredging - with obvious impacts on supply.
At the same time, the Indian Central Pollution Control Board reports that around 15Mt of plastic waste is dumped every day across India, giving rise to long-term environmental concerns.
A solution to both problems was recently proposed by researchers in Goa, who initiated a project to substitute fine sand in concrete mixes with processed waste plastic. The idea would not only encourage the collection and use of waste but would also provide alternative sources of fine material in place of sand in concrete mixes.
However, volumetric substitution of waste plastic for sand in concrete reduces its density and compressive strength, with higher replacement ratios leading to greater losses in compressive strength.
The issue arises from a combination of poor bond between the plastic and surrounding matrix, excess water due to the hydrophobic plastic surface causing an increase in voids, and failure of the plastic in tension. The goal of the research reported by Thorneycroft et al was therefore to limit to an acceptable degree the immediate loss in compressive strength.
The research project tested 11 new concrete mixes to study five plastic material compositions, three groups of particle sizes, three different aspect ratios and two chemical treatments. The aim was to establish an appropriate choice of material to act as partial replacement for sand.
The results showed that a replacement of 10% sand by volume with recycled plastic could be a viable proposition, potentially saving up to 820Mt of sand every year – and a similar amount of plastic waste. However, further investigations are still needed to understand the underlying bond between matrix, plastic and reinforcement.
Methods to improve the bond through treatment are also needed, along with greater understanding of the impact of cement types and the effect plastic has on durability, workability, fire performance and construction cost.
A further avenue of work to support reducing sand consumption is simply to use less concrete. Research has shown that structural materials are often used inefficiently, with savings of 30–50% being feasible in many cases.
Reducing embodied energy
Another route forward is to consider embodied energy efficiency. This is currently not a high priority for many designers, despite the environmental impacts of material production.
If the incentives of clients, policy makers, architects, engineers and contractors can be aligned such that minimum embodied energy structures were the preferred outcome on all projects, then dramatic reductions in material use may be possible, with multiple positive environmental impacts.
Civil engineers should aim to meet clients’ performance requirements using only the material that’s needed, and no more.
This article is based on the authors’ briefing article in the latest issue (172 CE1) of the ICE Civil Engineering journal.