RAAC Investigation and Structural Strengthening

Background

Beta Design Consultants were commissioned by the client, University for the Creative Arts (UCA), to provide structural investigation, structural assessment and consulting services regarding the structural integrity of reinforced autoclaved concrete (RAAC) roof panels, their condition, and to report on how to strengthen the RAAC roof in line with the IStructE Guidance and Further Guidance on RAAC.

The campus was constructed around the 1960s and some RAAC roof panels were discovered within the Farnham campus during a surveyor inspection, following the BBC news on the collapse of a RAAC panel in a school.

The activities included:

Visual Inspection of existing roof and confirmation of the presence of reinforced autoclaved concrete (RAAC) panels within the roof.
Structural Investigation (intrusive and NDT) and Structural Assessment of the RAAC panels capacity for the existing condition and for the proposed condition as the client required to upgrade the roof. The structural investigation included:
Schmidt Hammer Test to determine the compressive strength of the panels
Ground Penetration Radar survey to confirm the rebar layout.
Ferro-scanning and intrusive inspection of the panels to determine the rebar layout, depth, diameter and cover, especially the transverse rebars above the supports.
Core drilling of roof panels for Laboratory testing to determine the compressive strength of RAAC panels.

Figure 2: Castellated steel beams supporting the RAAC panels with diagonal braces

Figure 3: Side view of the beam supporting the RAAC panels

Figure 4: Profoscope cover meter to determine the rebar cover

Figure 5: GPR scan for rebars layout and spacing

Figure 6: 75mm core drilled for laboratory testing

Beta Solution

We carried out an intrusive inspection, including core drilling of the roof slab and exposing the waterproofing locally, to determine the compressive strength of the concrete and to identify the bearing length of the panels. This enabled us to establish both the bearing length of the roof panels and their compressive strength.

Due to the impact of upgrading the roof insultation (resulting in increased self-weight) and the roof panels having a bearing length of 60-65mm, which is less than the recommended bearing length for the RAAC panels, we recommended increasing the panels' bearings width. We proposed to achieve this by adding steel brackets that we designed to be fixed to the existing supporting steel beams. WE recommended installing these brackets before adding the additional overlay to the roof.

As per the advice of the IStructE RAAC Guidance and Further Guidance, we organized with the client and their maintenance team, Mitie, a presentation on RAAC risks to explain to the client’s management team ways to manage risks of this roof and how it can be managed on site.

Figure 7: Beta Team explaining to the client maintenance team how to manage RAAC risks

One of the challenges was to establish the RAAC panel capacity for bending and shear under the additional loading. The solution was to develop a rigorous analysis that reflects the RAAC characteristics and to carry out assessments for the panels in both their existing condition and the proposed condition. Another challenge was to ensure the solution was buildable considering fire risks, services and need to install solution during the university break.