RAAC concrete

How Is RAAC Concrete Made?

Reinforced Autoclaved Aerated Concrete (RAAC) is utilised in many different types of architectural developments. Despite being a kind of concrete, its manufacturing process sets it apart from conventional concrete. It has been estimated to have a 30-year useful life. However, the safety and structural integrity of RAAC structures is extensively questioned.

According to the Department for Education, over ten schools in London are among 147 schools that the collapsing concrete has impacted. Due to this, administrators at schools and colleges are now in a difficult position where they must tackle local conditions without assistance or guidance from the federal government. Further, the wellness of hundreds of students is jeopardised, which calls for the implementation of precautionary measures and an understanding of potential hazards. 

What is RAAC?

RAAC concrete is made of sand, cement, lime, and aluminium powder, it is a lightweight precast concrete construction material. In addition, it incorporates recycled aggregates, such as crushed concrete, bricks, and other construction waste, along with a binder and water. Because it is affordable, lightweight, and offers superior thermal insulation, it has been utilised widely in buildings. Nevertheless, the Building Research Establishment has already expressed concerns over its:

  • Durability
  • Excessive displacements
  • Breaking 

All due to its multiple structural flaws! From the 1950s until the middle of the 1990s, contractors and builders used RAAC concrete in the construction of schools, colleges, and other buildings. As a result, it can be located in any educational or ancillary building at a school or college that was established or remodelled during this time.

Read more: Why is Concrete a Popular Building Material

How Is RAAC Concrete Made?


Collection and Sorting

Gathering and arranging construction waste materials is the first step in making RAAC concrete. Then, to ensure that the recycled aggregates are of the highest calibre, these materials are meticulously processed to eliminate any impurities, such as:

  • Wood
  • Plastic
  • Metals

Crushing and Sieving

To achieve a consistent particle size distribution, the recycled aggregates are crushed and sieved. Further, this process guarantees that the final concrete mixture will have consistent qualities.


After that, a binder is added to the recycled aggregates. Usually, this is Portland cement or a combination of cement and supplementary cementitious materials (SCMs) such as slag or fly ash. 

As the aggregates’ glue, the binder keeps them togethe                                                                                                  r; water is mixed to achieve the desired workability and uniformity.

Check out: What Is Concrete Made Of?

Autoclave Curing

The RAAC concrete mixture is positioned in moulds or formwork and subjected to autoclave cure after it has been properly mixed. Calcium silicate hydrate (C-S-H) gel is formed more quickly when high-pressure steam is applied to concrete in autoclaves. Well, these are sizable vessels. This gel improves the overall resilience and longevity of the concrete.

Why Is RAAC Considered a Threat to Safety?

Building owners were notified in December 2018 by the Local Government Association (LGA) and the Department for Education (DfE) of a recent building component failure in a property built using RAAC. Then, SCOSS issued a warning in May 2019 to draw attention to the:

  • Possible dangers associated with this type of construction
  • Failure of an RAAC panel roof installation within an operational school

Since then, reports of multiple RAAC collapses have been generated, even though the material seemed to be in good shape. Thus, without taking appropriate engineering and design into account, its usage in structural components can cause structural instability. It may lead to disastrous building collapses, including severe injuries and casualties.

Besides, the following embedded systemic issues are present in RAAC:

Low Compressive Strength

Because panels only have 10–20% of the compressive strength of regular concrete, they have less shear and bending strength. Also, water saturation has an additional effect on its durability.

Decreased Density

It is less dense than traditional concrete, weighing around one-third less. This is because it is aerated and doesn’t include “coarse” aggregate.

High Porosity

It is extremely permeable and porous. Further, this indicates that compared to steel reinforcement in conventional concrete, the steel reinforcement inside the panels is less adequately shielded against corrosion or rusting.

Poor Stiffness

Because of RAAC’s decreased stiffness, there are more displacements, deflections, and sagging.

Weak Reinforcement

Moreover, the RAAC panels’ reinforcement has a weaker bond with the surrounding concrete. Secondary reinforcement, or transverse reinforcement, is the main method of connection.

Great Variation

There is a lot of variation across panels because there was little quality control throughout production and installation.

Insufficient Bearing

Planks frequently have inadequate bearing in relation to contemporary standards, which poses a serious concern.

So, if structural issues are found, it may be necessary to remove the problematic material and replace it with one that is more appropriate. For this purpose, you must take the area out of service in the meantime.

How Can RAAC Endanger Your Property and its Inhabitants?

If RAAC is not appropriately preserved or waterproofed, it can also be at risk of moisture-related issues, including mould growth and deterioration. Also, incorrect installation or errors can lead to:

  • Structural problems
  • Air leaks
  • Additional security concerns

They may not be immediately obvious but might become severe or challenging to deal with over time. Furthermore, the possible disintegration or degradation of RAAC may also cause hazardous materials such as asbestos to be released. In Britain, asbestos is found in the majority of schools. When asbestos particles are disturbed and discharged into the air, it becomes daunting to breathe in the dusty surroundings. Also, it can lead to serious health problems in the future.

4 Safety Measures to be Taken

We require prompt assurance regarding the protection of occupants residing in or using buildings made of RAAC concrete, particularly our school communities. Trade union representatives have the right to review any structural strength assessments conducted and should inquire with the employer to verify if a certain building contains radioactive asbestos cement (RAAC).

  1. When you recognize problems, fix them as quickly as feasible. In cases where a RAAC element in a public building has been acknowledged and judged to pose a risk to people, we need the government to help.
  2. When structural work is necessary, the government must give sufficient financing for both corrective action and interim fixes.
  3. If school administrators or their teams suspect or find evidence of Reinforced Autoclaved Aerated Concrete (RAAC) in their property, it is not appropriate for them to be tasked with determining the immediate risk of harm or damage. After all, making these kinds of choices is not their area of expertise.
  4. Additionally, the Department of Education needs to guarantee that structural engineering inspections are performed in each and every school where RAAC use is detected.

How to Find RAAC Concrete?

RAAC might be hard to spot visually since it resembles other building materials like regular concrete. But, it can be distinguished by a few traits. 

  • When the material is fractured or chopped, a distinctive, honeycomb-like structure is visible. 
  • Besides, there is no visible aggregate or coarse material on the smooth surface of RAAC.

The easiest way to confirm that a structure is constructed of RAAC is to depend on the documentation or have it professionally inspected. If RAAC was used during a building’s construction, it may be shown through:

  • Building plans 
  • construction records


Although RAAC is cheaper than most of the mix onsite concrete options available, it is a highly detrimental material that can culminate in substantial financial losses and catastrophic consequences. Hence, always contact a qualified inspector or construction engineer to schedule an inspection to see if RAAC is present on your property.

The Department of Education released revised guidelines on RAAC and requirements for duty bearers in August 2023. It offers valuable information on identifying RAAC in structures and aids in highlighting the dangers and procedures involved in mitigating the risks associated with RAAC.

At Pro-Mix Concrete, we offer the best quality mix onsite concrete and customise it in accordance with your specific project’s needs.