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  Breaching Operations on Hollow Core Concrete Floors
  Background    
  Part of the role of the USAR Technical Working Groups focuses on understanding how new developments may impact upon urban search and rescue operations. For some time the Rescue Operations Working Group have been aware of hollow core concrete materials being used within the construction industry, mainly as flooring elements in a wide range of modern buildings. Following lengthy negotiations, we have been able to carry out some basic operational tests of these hollow core concrete sections.
  Introduction    
  Introduced in the 1970’s, hollow core concrete floors are increasingly being used within the construction industry and form part of many commercial and residential buildings and multi-storey car parks. Therefore, hollow core concrete floors are likely to be found within collapsed or damaged buildings.
  The floor sections are nominally 1200mm wide and vary in depth from 150mm - 450mm. The are made in 110m lengths, so there is no standard length in which they are supplied to the construction industry. A typical 1200mm wide section will have 9 hollow cores and 21 x 3mm reinforcing wires tensioned and embedded towards the base of the section (Figure 1).
Figure 1: Typical hollow core floor section
For further information have a look at the Bison Concrete website
www.bison.co.uk/
 
Hollow core floor section on traditional block wall
 They are placed in position, supported at each end, butted together and then grouted and screeded. The floor sections are not generally fixed in place, relying on the grout and screed to remain in place. Due to the smooth screed floor, USAR personnel cannot easily identify this type of construction from above.
Hollow core floor section on steel frame
  The Tests
  The tests were conducted by Lee Giffard and Ray Cousins from the Working Group kindly assisted by the USAR team from Buckinghamshire. The venue and test materials were provided by Bison Concrete of Iver in Buckinghamshire. The tests involved the creation of both clean and dirty breaches through hollow core concrete sections using a variety of suitable USAR equipment.
  Dirty Breach  
Figure 2: Early failure of the section following dirty breach operation
  The dirty breach tests showed that the integrity of the floor failed soon after commencement of operations. For example, utilising a Stanley BR67 hydraulic breaker, the floor section failed along its length almost immediately the breaching operation was started (Figure 2).
  The tests also showed that the floor section could be dirty breached using just a sledge hammer but that the integrity of floor was compromised during this procedure. Due to the method of fixing the floor sections in place, once a section looses its integrity it could collapse completely onto the floor below injuring anyone on or beneath the section being breached (Figure 3).
  Clean Breach
  The clean breach operation was much more successful, the floor section retaining its integrity in most situations. For example, utilising a Stanley DS11 hydraulic saw, the tests showed that approximately 90% of the floor section can be cut before the section failed and collapsed (Figure 4).
Figure 3: Total failure of floor slab
  Conclusions  
 
  1. When tensioned, the hollow core floor sections are very strong in compression but weak in tension or shear.
  2. Dirty breach operations involving impaction of the floor section will often cause the section to fail.
  3. Clean breach operations are less likely to cause failure of the floor section, but only when the section is not cut across its whole width.
  Recommendations
 

All operational USAR personnel should be made aware of the content of the issues with hollow core floors. During training or at an operational incident, the initial risk assessment for breaching and breaking operations must establish if hollow core concrete sections are present within the structure and if they will be involved in the operations. Consequently, associated controls and systems of work must be implemented in order to safeguard against the risk of collapse of the concrete sections. Factors to consider should include…..
Is the breaching operation necessary?  
The selection of appropriate equipment and techniques (Clean Breach).
Figure 4: Clean breach extended to within 100mm of the edge of the section
Establishing the location of the floor sections; Which way do they run? Where are the edges? Where is the centre of each section? Consider coring the floor section and examining the underside of the floor where the joints are visible. Alternatively, using hand tools or a chipping hammer, remove the screed layer to expose the joints, taking care not to damage the floor section itself.
Ensuring that the Clean Breach encompasses two adjoining sections or is confined to the centre of one section.
Implement safety cordons to restrict access both onto and below the floor being breached.
The location of other personnel, above and below the working area.
Restricting the numbers of personnel within the working area.
The use of fall restraint equipment for those working on the floor section.  
  This is by no means an exhaustive list and other factors may need to be considered prior to commencing breaching operations.The information from the tests will be published in a New Dimension Information Note 071/08. If you want further details of the tests or the recommendations regarding the techniques to be adopted, please contact Lee Giffard at lee.giffard@hantsfire.gov.uk.
  Lee Giffard, Hampshire FRS  
 
Undertaking the operational tests
 Figure 5: Recommended method for breaching hollow core concrete floor Laying hollow core floor sections (courtesy of Bison Concrete Ltd)
 
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