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


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Q0012 - 'Limestone in Surface Course'

Date Submitted: Wednesday, 21st July 2009

Question:

We have received high numbers of core failures where the failure is listed as (unacceptable level of limestone present in surface course). There is no indication as to the level of Limestone just that there is some present. There appears to be nothing wrong with Limestone in asphalt mixtures and BS EN 13108, PD6691 and the SROH all allow Limestone, however specified PSV rules seem to rule out Limestone in most surface course materials.

The presence of small amounts of limestone may cast doubt over the compliance of the material to reach the required PSV value but that is not the same as it failing. Wearing course materials will have been purchased with a given PSV value, is it not up to the testing body to prove that this has not been achieved by testing it. If a material passes a PSV test then is level of limestone irrelevant. Having spoken to tarmac manufactures it would seem to be unrealistic for them to exclude every partial of limestone from a hard stone mixture manufactured at a limestone quarry.

Answer:

Generally, limestone in a Surface Course mix will not provide the minimum PSV required by Table 2.7 of the SROH CoP. We believe the query probably relates to instances of Surface Course materials mixed using other suitable aggregates (providing the requisite PSV), but which has some limestone present, most probably through some contamination.

Contamination of mixes is known to occur through: (a) Contamination at the batching plant - contamination of the stockpiles, ground feed hoppers mixer, bins or delivery vehicle - all down to poor control & not acceptable in any case (b) Contamination of the collect load vehicles/hotboxes – this is more prevalent when split loads are picked up, which is common in utility work. Both instances are substantially avoidable through improved handling/control practices.  However, it is recognised that some contamination will still be a possibility.  The issue therefore is whether a permitted level of limestone contamination is actually detrimental to the key Surface Course property driver – PSV, being a key safety matter for road users.

The Working Party consider that very small quantities of limestone present in the Surface Course (due to contamination) are unlikely to materially affect the intended PSV.  In this regard, where it is clear that the Surface Course mix demonstrably contains the intended aggregate (needed to give the required PSV), that the occasional presence of limestone (at levels of approximately 5% or less) should not constitute a defect under the PSV requirements of S2.6.3 of the Code.

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