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CROSS Safety Report

Composition of new precast concrete blocks

Report ID: 380 Published: 1 July 2014 Region: CROSS-UK

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Overview

A UK firm has raised concerns over a new large precast concrete block range which seems to use a wide range of secondary recycled aggregates in its mix, including some potentially hazardous waste.

Key Learning Outcomes

For civil and structural design engineers:

  • It is essential that any new products specified have undergone sufficient testing backed up by third party certification, with adequate quality control, such that the chances of future failure are minimised

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A UK firm has come across a new large precast concrete block range which seems to use a wide range of secondary recycled aggregates in its mix, including some potentially hazardous waste. They are manufactured by a European precast concrete manufacturing company and contain, amongst other material, Cathode Ray Tube (CRT) glass without proper crushing and treatment to remove lead and other contaminants.

Tests were carried out on a sample and significant concerns over the applicability of the aggregates used in the blocks were found. A wide range of materials were found to have been used as fine and coarse aggregates including varying sizes of CRT, plastics, copper wire (with its plastic insulation), and fibrous and friable materials. The potentially reactive aggregate content may have affected the compressive strength of the blocks.

The blocks, which are supposed to be used in high capacity load-bearing retaining walls reaching six or seven metres in height, have a compressive strength of 10.5 N/mm2. This is compared to 18-35 N/mm2 compressive strength usually required for such blocks. There are also major concerns over their long term durability.

This is compared to 18-35 N/mm2 compressive strength usually required for such blocks. There are also major concerns over their long term durability.

The reactive aggregates may have connotations to Alkali-Silica Reaction (ASR). Microscopic testing also shows that some of the recycled material content appears to have undergone significant chemical and physical change from its original state (with some affecting the permeability of the product – see Figure 1 and 2).

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Figure 1: Microphotograph of typical inclusion in concrete
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Figure 2: Microphotograph of fibrous and unsound particle in concrete

There is also concern about bonding and evidence of general oxidation/ expansion issues and chemical attack on the hydrated cement paste which may have been caused by salts leaching from the waste materials used in the mix.  The reporter is extremely worried about the structural integrity of such blocks and the potential environmental and health and safety concerns associated with their use.

Expert Panel Comments

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The issues with the retaining wall blocks made from recycled materials is interesting (and worrying). There was industry interest a few years ago about the use of recycled materials being incorporated in structural concrete (at the higher value end of recycling). The Highways Agency undertook research to look into this, and indeed produced an outline specification (not published).

The underlying issue was the need to have traceability and provenance of materials to be used in recycling (RCA – recycled concrete aggregates were essential and not general recycled waste), and also to have plants available to ensure that only suitable materials are tracked and recycled.

WRAP were involved on the periphery of the work. Demonstrating long term durability was the main concern. There have been numerous other research projects looking at putting all manner of recycled and secondary materials into concrete. Experience tells us that many of the deterioration mechanisms that affect concrete take many years to develop and a 28 day strength may not represent the strength or condition after 10 years.

Deterioration can be by failure of the base materials, freeze/thaw, moisture instability, or by unforeseen internal reactions. It is essential that any new products have undergone sufficient testing backed up by third party certification, with adequate quality control, such that the chances of future failure are minimised.

The Environment Agency is responsible for regulating exports of any waste materials from England and Wales. CRT glass as mentioned in this report is classified as a hazardous waste and export from the UK must take place under a Transfrontier Shipment (TFS) of waste notification. The notification process ensures that the competent body in the receiving country is satisfied with the import of the CRT glass.

The notification process also allows the Environment Agency to be satisfied that the material is being sent for recovery to a correctly permitted facility overseas. It is understood that the Agency takes a tough line on illegal waste exports and works with a number of partners to tackle this issue using intelligence from the market place.

It is also understood that British Precast will produce a specification for the inclusion of waste and secondary products into precast concrete and how the End-of-Waste criteria should be looked at for such products. The dangers of a retaining wall collapse are illustrated by report 433.

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