In Hong Kong, engineers turned waste glass into concrete blocks; 15 years later, field checks found no cracking from the reaction that can damage glass concrete

For years, discarded glass bottles have represented a difficult recycling challenge. While many glass containers can be reused, large amounts still end up as waste. However, engineers from Hong Kong decided to use this waste glass by crushing it and putting it into concrete pavers. The idea came with a major concern. Because glass contains silica, it could cause an alkali-silica reaction (ASR) in the concrete. This can cause the concrete to expand and crack over time. The engineers would need to find out if the recycled glass could withstand being outside for years. A team from Hong Kong examined concrete paving blocks made with recycled waste glass after they had remained in service for 15 years, following their installation around 2005. The researchers found that the blocks showed no cracking caused by alkali-silica reaction (ASR), according to their long-term evaluation of recycled glass in dry-mixed concrete blocks.

A long wait for a real-world answer

Laboratory tests might show how materials could behave, but they cannot replicate years of rain, heat, traffic, and the elements. This is why the researchers studied blocks that had experienced actual outdoor conditions. The team took a look at dry-mixed concrete paving blocks that were installed 15 years ago. The concrete blocks contained recycled waste glass particles that replaced part of the traditional aggregate materials used in the mix. The blocks are stable as per field checks done. Internal cracking normally linked with ASR was not found by the researchers. The glass particles didn’t produce the needed expansion in reaction products to damage the concrete.

This finding is important for cities looking for ways to manage waste. Landfills are crowded with glass waste, and stone, sand, and other natural resources are used abundantly by construction industries. Utilising discarded glass in concrete could tackle both issues.

Why glass has worried engineers

The issue of glass in concrete is not because of glass being weak. The problem happens when glass particles interact with the chemicals and moisture in the concrete from inside. During the alkali-silica reaction, certain types of silica can react with alkaline substances in cement. This creates a gel-like material that can absorb water and expand. If the pressure becomes high enough, cracks can develop inside the concrete. Given this risk, engineers have been historically cautious about using recycled glass as a substitute for natural aggregates.

The Hong Kong research showed that the dry-mixed concrete block method helped control this problem. The products of the reaction were primarily formed around the glass particle surfaces, preventing any harmful expansive development within the blocks.

What the researchers learned from testing

Before the long-term field inspection, laboratory tests were performed by the researchers to understand the material behaviour under accelerated ASR conditions. Those tests made clear why the blocks remained highly effective after exposure to the elements. The laboratory results matched the field observations, showing that the reaction occurring inside the blocks did not cause any structural damage.

The study further suggested that finely ground waste glass powder could behave differently from larger glass particles and may help reduce the harmful effects associated with ASR. This also suggested additional possibilities for using recycled glass in construction materials.

Turning old bottles into future building materials

The outcomes do not suggest that all types of concrete with glass will behave similarly. Different types of glass, methods of production, and construction will all have to be carefully tested. However, the 15-year observation provides engineers with data and evidence from a real-life case study. While many cities are trying to find ways of decreasing landfill waste and becoming more sustainable, the Hong Kong authorities show how waste can be put into practice. What people currently throw away is going to be the basis of the pavement they walk on tomorrow.