Good Practice Guide

Code of Practice - Specification for the Reinstatement of Openings in Highways. (Second Edition, 2002)

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Q0013 - 'Air Void testing in Small Excavations'

Date Submitted: Tuesday 21st July 2009


Should utility small excavation be subjected to air void testing. As A8.3 states that these should be compacted to the method specified in Table A8.3 and these would need to comply with the performance requirement of the SROH S2 performance requirements. All excavations outside A8.3 are to be compacted to in-situ air void requirement determined in accordance with SHW 937 sub-clause 32 & 33. SHW 937 only covers the testing of SMA binder course and regulating course, sub-clause 32-33 no longer exist following the revision of SHW 937 (08-08). SHW 937 refers to BS594987 for control by monitoring density and air void content. The following two quotes are taken from BS 594987.

9.1 note 2 End result compaction (core tests) is more appropriate for machine-laid work on major road contracts. Note, This method (end result compaction) is applicable for works intended to carry heavy traffic. The scale of the works should be such as to justify the cost of testing and control.

Most core test carried out on utility works are on small excavations that are hand laid the, apx cost of reinstatement is £40 per m2 but the cost of testing is with some Authorities over £150 per core. Is it not more appropriate to use the performance requirement of SROH S2 to manage the performance of small reinstatements.


For Small Excavations (generally trenches <300mm width and other openings of surface area <2m2) Air Voids limits set down in Table S10.1 should be achieved through the Minimum Passes method indicated in Table A8.3. Utilities should be testing their own works to meet these standards, and it is indeed appropriate and necessary for HAs to carry out their own tests.  From the HAs perspective, without this level of control of placement quality, there would be a dis-service to the public, as compaction is fundamentally a contributory control of durability (the effects of ageing and weathering, as well as on performance through deformation).  By implication, durability is also an essential component of sustainability (returning to repair defects).

In terms of guidance on the method of testing air voids, under S10.2.3(2), it is known that the current SHW Clause 937 has changed since the publication of the 2nd Edition SROH CoP in June 2002.  This has been addressed in the proposed 3rd Edition, thus:

“S10.2.3(2) The in-situ air voids content for all bituminous materials as permitted in Appendix A2 shall comply with the requirements shown in Table S10.1. The in-situ air voids content shall be calculated as the average from all results obtained. The maximum density shall be determined in accordance with EN 12697 – 5 Procedure A, in water. For reference purposes and in the event of dispute the bulk density shall be determined in accordance with EN12697 – Procedure C sealed specimen. The maximum density and core bulk density shall be used to determine air void content in accordance with EN12697-8.”

The Working Party recommends that the above is used for testing Air Voids.

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