RAAC in the UK: a structural strengthening approach using aluminium telescopic beams

For many years, Reinforced Autoclaved Aerated Concrete
(RAAC) has represented a significant structural challenge
within the UK’s public building estate. Schools, hospitals and other
essential facilities have been forced to close, restrict use or rely on
extensive temporary propping due to concerns over sudden structural
failure.

Although the risks associated with RAAC are now widely recognised, a
key question remains:
how can structural safety be ensured quickly, reliably and with
minimal disruption, particularly where full replacement is not feasible
in the short term?

This article outlines a structural strengthening
approach based on aluminium telescopic beams, developed to act
as a reinforcement — or, where required, a functional replacement — of
deteriorated RAAC planks.

Understanding the real structural challenge of
RAAC

RAAC roof and floor planks were widely used between the 1950s and the
1980s. Many of these elements have now reached or exceeded their
intended service life and may exhibit:

• significantly reduced load-bearing capacity,

• brittle failure modes, and

• limited or no visible warning prior to collapse.

In practice, this means that assumptions regarding residual
structural capacity cannot always be relied upon. Any
strengthening solution must therefore be capable of ensuring
stability even in scenarios where the RAAC element provides
little or no structural contribution.

What an effective strengthening solution must
deliver

From both an engineering and asset-management perspective, solutions
for RAAC-affected buildings should:

• guarantee structural safety independently of the RAAC
  plank,

• add minimal additional weight to the existing structure,

• be installable in confined spaces, often while buildings remain
  operational,

• avoid extensive demolition or highly intrusive interventions,
  and

• align with sustainability and low-carbon objectives increasingly
  embedded in public-sector procurement.

These requirements are particularly relevant for hospitals
and schools, where prolonged closures or disruptive works are
rarely acceptable.

The structural concept: reinforcement by functional
substitution

The approach presented here is based on the installation
of EXTEND aluminium telescopic beams on the underside
of affected RAAC planks.

From a structural standpoint, the system operates as follows:

• The aluminium beam is positioned beneath the RAAC
  element.

• The gap between both elements is fully infilled with a non-shrink
  structural mortar, enabling effective load transfer.

• Loads are transferred directly from the beam to the existing
  load-bearing structure (walls, primary beams or edge beams) through
  aluminium support brackets and mechanical or chemical anchors.

Depending on the adopted design strategy, the system may:

• partially consider the structural contribution of the existing
  RAAC plank, or

• be designed as a complete functional substitute,
  ensuring stability even in the event of total loss of RAAC load-bearing
  capacity.

This flexibility is particularly valuable given the highly variable
condition of RAAC elements across different buildings.

Why aluminium telescopic beams?

The use of structural aluminium provides several
technical and practical advantages:

• Low self-weight, minimising additional loads on
  columns and foundations.

• High strength-to-weight ratio, suitable for
  medium spans and significant imposed loads.

• Telescopic configuration, allowing adaptation to
  different spans and structural geometries, including confined or
  restricted spaces.

• Rapid installation, reducing programme duration
  and disruption to building users.

From a sustainability perspective, aluminium is fully recyclable and
offers a comparatively low carbon footprint over its life cycle — an
increasingly important consideration for UK public-sector projects.

Installation in operational buildings

One of the key advantages of this approach is its suitability
for occupied or partially operational buildings.

The beams are supplied in modular segments, with
lengths adapted to the structural and logistical requirements of each
project. This modularity facilitates transport, handling and on-site
installation, and in many cases allows strengthening works to be carried
out without full building closure, avoiding the heavy temporary propping
systems typically associated with steel solutions.

A solution aligned with UK regulatory
expectations

The structural design of the EXTEND system is carried out in
accordance with Eurocode 9 (Design of aluminium
structures). The beams are modelled as simply supported
elements with variable inertia, following classical structural analysis
principles and verified against applicable serviceability limits.

The system is CE-marked under EN 1090 and benefits
from a long track record of laboratory testing and technical approvals
for structural applications. Since 2023, CE marking has been confirmed
as indefinitely accepted in the UK, with equivalence to UKCA for
construction products.

Looking ahead

RAAC will remain part of the UK building stock for the foreseeable
future. While full replacement will be unavoidable in certain
cases, well-engineered structural strengthening
solutions can play a critical role in risk management,
service-life extension and the continued operation of essential
buildings.

Aluminium telescopic beam systems offer a lightweight,
adaptable and technically proven response to one of the UK’s
most pressing structural challenges.

In the next article, we will examine the EXTEND system in greater
technical detail, focusing on materials, structural behaviour, design
principles and regulatory compliance in RAAC-affected
buildings.

JOSEP M. VULART, ARCHITECT | EXTEND SYSTEM