How RAAC Is Investigated: From Survey to Structural Evidence

Reinforced Autoclaved Aerated Concrete (RAAC) presents a uniquely complex challenge for building owners and engineers. Its condition cannot be reliably assessed through visual inspection alone, and its behaviour under load is not always predictable.

This means that for a RAAC investigation to be credible and be relied upon, it must go beyond basic surveys and be carried out by experienced, qualified professionals. A structured, evidence-led process that moves from initial identification through detailed structural assessment, testing, and ultimately leading to informed decision-making.

Why Visual Inspection Alone Is Not Enough

One of the most persistent misconceptions around RAAC is that its condition can be judged visually. But in reality, visible defects are only part of the picture. Some panels may appear serviceable while operating close to their structural limits, while others may show deterioration but retain adequate capacity. And without testing and analysis, it is difficult to distinguish between the two.

This is why a layered, structured investigation approach - survey, assessment, testing, and appraisal - is essential. Each stage reduces uncertainty and improves the quality of decision-making.

From Identification to Action

A proper RAAC investigation is not a one-off exercise. It follows a structured process, starting with identifying the material and how it has been used, then moving through structural assessment and, where necessary, intrusive investigations and testing. From there, more detailed analysis can be completed to feed into an appraisal of options that considers a range of criteria including safety, cost, and operational impact.

For building owners and other Responsible Persons, the issue is not simply confirming the presence of RAAC, but understanding what it means in the context of that particular building. That goes beyond a visual check. It requires informed engineering judgement, supported by evidence.

Why RAAC Investigation Requires a Structured Approach

RAAC is not a conventional material. It is lightweight, aerated, and often concealed within roof or floor structures. In many buildings it has been in place for decades, and there may be limited documentation of the structure's history.

Guidance from industry and government bodies is clear: where RAAC is suspected, it must be inspected to establish:

• Whether RAAC is present
• Its condition and structural behaviour
• The level of risk it presents
• What interventions, if any, are required

Each stage builds on the last. Skipping steps or relying on incomplete data introduces both technical and commercial risk; informed judgement is crucial.

Stage 1: RAAC Surveys: Identifying Presence and Configuration

The starting point for any RAAC investigation is a targeted survey. They are designed to confirm the presence of RAAC material and to understand exactly how it has been used within the structure. This often starts with desktop reviews of available drawings and records, then moves on to a site inspection to identify characteristic panel types. After that, opening up works may be necessary to confirm the construction of the RAAC.

In practice, RAAC is frequently used in roof planks, though it is important to remember that it is not limited to flat roofs and also exists in walls. Variations in panel geometry, support conditions in place, and the way the RAAC was installed and maintained mean that no two buildings are the same.

A robust survey will establish several factors, including:

• The panel type, shape and dimensions
• Its span and support conditions
• The fixings and bearing details
• Any signs of deterioration, water ingress, excessive deflection or distress

At this stage, the aim is not to reach conclusions about safety, but to create a reliable baseline of information for further assessment.

Stage 2: RAAC Structural Assessment: Understanding Risk

Once RAAC has been identified in a building, the next step is a structural assessment. This is where specialised engineering judgement becomes critical. The assessment considers not just what is visible, but how the structure is likely to behave under load. Crucial in this is a recognition that RAAC exhibits particular behaviours which are distinct from traditional concrete.

Key factors include the configuration and condition of the panels, including cracking, deflection, or moisture ingress; load paths and the building's current use; any historical alterations or additional loads that have occurred during the building's life; and, finally, any unique support conditions and bearing adequacy.

Importantly, RAAC does not always show clear warning signs before it fails. This means assessments must be robust and grounded in evidence, a deep understanding of the material, experience of its behaviour in relevant settings and, in some cases, tests to determine its structural capacity. Rather than producing a simple pass/fail outcome, a structured assessment will typically categorise areas by risk level, allowing building owners to prioritise action.

What Happens in a RAAC Survey?

A RAAC survey is a structured process that combines visual inspection with targeted investigation to confirm the presence of RAAC and understand how it has been used within a building. The starting point is a detailed site inspection, where the surveyor looks for evidence of RAAC and assesses its condition in situ.

• Identification
  The surveyor determines whether RAAC is present. It is most commonly found in roof planks, particularly in flat roofs, but can also occur in floors and walls.
• Visual Assessment
  Inspectors look for signs of structural movement or distress, such as sagging, bowing, or excessive deflection relative to the span.
• Surface Condition
  The exposed surface of the material is checked for cracking, spalling, water ingress, or staining, all of which may indicate deterioration. Particular attention is paid to areas where moisture could affect the embedded steel reinforcement.
• Support and Bearing
  The adequacy of end bearings is assessed to ensure panels are properly supported. Insufficient bearing can increase the risk of localised failure, although should be considered in the context of a broader assessment of condition.
• Tap Testing
  A simple but effective technique where the surface is tapped with a hammer to identify changes in sound that may indicate voids, delamination, or loss of integrity within the material.

Testing and Investigation Methods

When visual inspection alone is insufficient, additional investigative techniques are used to build a clearer picture of the structure.

• Non-Intrusive Testing
  This includes visual inspection supported by tools such as cover meters to locate embedded steel reinforcement, helping to understand panel construction without damaging finishes
  
• Intrusive Investigation
  Where access is limited, or uncertainty remains, opening-up works may be required. This can involve removing ceiling tiles, finishes, or small sections of material to inspect the structure directly. This is often required at the bearings to confirm the full, effective bearing width as well as the presence of a transverse reinforcement bar, critical to prevent uncontrolled shear failure.
• Material Sampling
  In some cases, samples of the concrete are taken for laboratory testing. This can provide additional information on material properties, including strength, density, corrosion, moisture content & condition.

Stage 3: Off-Site RAAC Testing: Moving from Assumption to Evidence

Where uncertainty remains - and it often does - testing becomes essential. It provides the data needed to move beyond assumptions and into measurable performance.

A notable example is the work carried out at Medway Council’s Gun Wharf building, where a programme of full-scale testing was undertaken in collaboration with industry partners, including the BRE and an academic team from the University of Loughborough.

Rather than relying solely on guidance developed for flat roof systems, the testing programme examined pitched RAAC panels with trapezoidal geometry, reflecting the building’s actual construction and produced several important outcomes that gave the RCS team a clearer understanding of load capacity in real-world conditions. This evidence helped the team challenge overly conservative assumptions that the entire roof would have to be replaced, with detailed data to back up engineering recommendations. This science-backed evidence led to a targeted remediation solution that saved Medway Council millions of pounds.

Testing does not remove risk, but it significantly reduces uncertainty and allows decisions to be based on how a structure performs, not just how it is expected to perform.

Stage 4: RAAC Options Appraisal: Turning Evidence into Decisions

The final stage of the process is the options appraisal. This is where the focus shifts from analysis to decisions. The question moves from “what is happening?” to “what should be done?” A structured RAAC options appraisal looks at a range of factors, including safety and risk, cost (both capital and operational), programme and disruption, building use, and longer-term considerations such as sustainability.

It isn’t just a technical exercise. It brings together input from building owners, facilities teams, and commercial stakeholders to reach a balanced, workable outcome.

At Medway, this approach led to a zone-by-zone strategy. Higher-risk areas were managed with temporary works, while other parts of the building were monitored and reopened in phases. This approach maintained safety and provided long term structural integrity without defaulting to wholesale demolition or unnecessary cost.

What to do if you have RAAC in your Building

If RAAC is suspected in your building, the priority is to move from uncertainty to evidence. A structured investigation provides a robust foundation for informed, proportionate decisions.

RCS supports building owners, Responsible Persons, and project teams at every stage, from initial RAAC surveys through to structural assessment, testing, options appraisal and implementation of remedial solutions.

To discuss your building or project, or to book a RAAC survey, contact the RCS team.

This article is part of a wider series exploring RAAC from an engineering perspective. Each piece examines a different stage of the process, from initial surveys and structural assessment to testing, risk management, and decision-making. Together, they set out what a proportionate, evidence-led approach to RAAC looks like in practice.

Frequently Asked Questions

Below are answers to some of the most common questions about RAAC investigations, structural assessments, and testing. If you can't find the information you're looking for, please get in touch with our team.

1. How do I know if my building has RAAC?

RAAC is often hidden within the structure, so it is not always easy to identify through a simple visual check. The only reliable way to confirm the presence of RAAC is through a targeted structural survey, which may include reviewing original drawings, site inspection, and, where necessary, opening up the structure to verify the material.

2. What is a RAAC investigation?

A RAAC investigation is a structured process that includes surveys, structural assessment, testing, and options appraisal to determine the condition, risk, and required actions for buildings containing RAAC.

3. What happens in a RAAC survey?

A RAAC survey involves a site inspection to confirm whether RAAC is present and assess its condition. Surveyors identify where it has been used, check for signs of deflection, cracking or water damage, and assess support and bearing. Simple techniques such as tap testing can be used to detect internal defects. Where needed, further investigation includes non-intrusive methods (such as locating reinforcement) or intrusive opening-up works and material sampling to understand the structure better.

4. Can RAAC be assessed without testing?

In some cases, assessments can be made without testing, but testing is often required to reduce uncertainty and understand actual structural performance.

5. What is a RAAC structural assessment?

A structural assessment evaluates the condition and behaviour of RAAC elements, considering factors such as loading, support conditions, and signs of deterioration.

6. Why is RAAC testing important?

Testing provides real-world data on how RAAC panels perform under load, helping engineers make informed, evidence-based decisions rather than relying on assumptions.