This article provides information on two different applications for legacy asbestos containing materials (ACMs) used in reinforced concrete (RC) from the 1960’s through to the 1980’s and even potentially into the 1990’s. It is essential before undertaking demolition or structural refurbishment of buildings with insitu or precast RC from this period to undertake an intrusive Asbestos Survey. look for visual indicators (see photos), take samples for analysis to confirm if ACMs are identified.
The demolition industry is familiar with asbestos sheeting used as shuttering for concrete, mastic expansion joints and bituminous floor sealing compounds etc.
ACMs may have been used as asbestos cement spacers for in-situ concrete, but also asbestos insulating board fixing blocks in precast post tensioned beams, columns, and panels.
These fixing blocks could be friable Millboard or packed thermal Insulation
Asbestos Cement Spacers (insitu RC)
Asbestos has been used in concrete since the 1960’s through to the late 1980’s where legacy asbestos cement spacers had been manufactured and used for insitu reinforced concrete to ensure sufficient concrete coverage over the steel reinforcing cage to prevent corrosion, spalling and failure. One manufacturer was Creteco Limited who was founded in 1974; in a 1977 trade journal they advertised ‘Frank Asbestos Cement Bar Spacers and Distance Tubes – Bond naturally with concrete – stronger than the concrete itself – and fireproof’!
A retired Concrete Manager who worked in the construction industry from the 1950’s, described how cement spacer blocks used to be manufactured on site until work practices changed. The process used a strong mix of sand and cement only in prefabricated moulds with depths ranging from circa 15mm to 75mm, allowed to set, scored, and then broken the following day into rough square sections. It wasn’t until quality controls required contractors to purchase manufactured uniform dog-bone shaped extruded asbestos cement spacer blocks (circa 40mm to 50mm square) in 50kg sacks. These asbestos cement spacer blocks containing circa 10% Chrysotile (white) asbestos were installed at approximately 1m centres and wired onto the metal cages, however, the majority would be dropped or become loose from walking on the cages, fall off and sink down onto the formwork below leaving a visible indicator impression shaped like a dog-bone or butterfly.
Estimated Asbestos concentrations and possible solutions
I have estimated there are a minimum of 4 spacer blocks installed per square or lineal metre of reinforced concrete floors, beams, and columns and invariably you can see the evidence on exposed soffits or beams, its often difficult to see these in columns for obvious reasons.
The total concentration of asbestos in the concrete is well below the accepted hazardous waste threshold of 0.1% w/w, however, if you are demolishing or part demolishing the structure you have a duty of care to manage the health and safety of persons undertaking the works and anyone affected e.g. to prevent exposure and spread of asbestos. Following good industry practice by adopting appropriate methodologies and dust controls e.g. wetting to reduce airborne respirable crystalline silica (RCS) you would also reduce the airborne asbestos risk.
Not all concrete spacers contain asbestos
1000’s of tonnes of these spacer products were manufactured with asbestos from the 1960’s, later they contained other reinforcing materials e.g granulated blast furnace slag (GBFS), therefore don’t despair even if your structure is within the significant age range your RC may not necessarily contain asbestos, however, the only way to be certain is to take representative samples and test using polarised light microscopy at modest cost.
Possible solutions for managing or removing asbestos cement spacers
Asbestos is only a risk when it’s disturbed so where these are found in structures to remain in use it’s simply a case of identifying these, recording the findings, and provide this information to anyone liable to disturb them for maintenance, repair of refurbishment etc. It may be necessary to protect exposed spacers that have been damaged or in poor condition using a proprietary encapsulating material (thixotropic paint) and label if appropriate.
Where it become tricky is when you need to demolish or remove sections of concrete containing these asbestos cement spacers.
Subject to the law and regulations that apply in your country and the regulators position you may be able to avoid expensive landfill of all the arisings. This could potentially involve some pre-treatment and control measures to allow limited re-use of the arisings in low-risk applications on site e.g. under floor slabs or car parks as sub-base aggregate where future disturbance is unlikely, irrespective of what you do records must be kept for the future.
Asbestos Insulating Board blocks (precast Concrete)
Another application was asbestos insulating board (or compressed thermal insulation) fixing blocks containing 15% to 40% Chrysotile (white), and, or Amosite/Grunnerite (brown) asbestos in prefabricated reinforced concrete elements. These were developed and manufactured in the 1960’s as post-tensioned reinforced concrete beams, columns, and prefabricated system-built concrete panels for residential (typically high rise), commercial and industrial buildings and structures. They were designed to provide easy fixings by having small asbestos blocks (circa 70mm x 30mm) installed at regular intervals typically at 500mm and 900mm centres for fitting e.g. windows, door frames and suspended ceiling systems etc.
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