Glass as unbound aggregate
Crushed glass is, in many respects, highly suitable for use in applications requiring volumes of unbound aggregate. In many of these applications, the performance of the material is of only minor importance, although these modes of use are by no means exploiting the material's full enegineering value. Nonetheless, higher economic value can be realised where the attractive appearance of glass is capitalized on, such as use as a loose surfacing material.
Probably the highest value route to using loose aggregate is as a granular sub-base material in road construction. In the UK, requirements for these materials is defined in the Highways Agency document, 'Specification for Highway Works' [1]. The document defines two types of granular material - Types 1 and 2. Type 1 is the coarser of the two types suitable for use as a sub-base in most sub-base application, whereas Type 2 is finer and not typically used in the constructon of heavily trafficked pavements.
The main requirements for Type 1 and Type 2 sub-base materials are as follows:
- Grading
Grading envelopes are provided in 'Specification for Highway Works' for both Types 1 and 2 material. Clearly grading can be controlled by appropriate processing of material prior to use, and therefore does not present any problem to those wishing to use glass. - Frost Heave
Frost heave is a measure of the volume change displayed by a compacted granular material containing moisture under freezing conditions, with a large change in volume considered as undesirable. Research carried out in the USA has found that glass aggregates display a very low susceptibility to frost heave [2], although it should be stressed that the test used was the ASTM test rather than the British Standard method. - California Bearing Ratio (CBR)
The California Bearing Ratio test involves the penetration of a cyclinder plunger into a granular material and relting the force applied to the depth of penetration.
Only Type 2 granular sub-base materials have a CBR requirement.
Many variables define how a material will perform in the CBR test, including strength and grading. Testing of combinations of 50% glass and 50% crushed rock have consistently given values exceeding those required by 'Specification for Highway Works'. In most cases, glass on its own performs less well. Given that grading plays an influential role, testing for compliance with the CBR requirement testing of glass aggregate prior to use is clearly necessary. - Soundness
The soundness of an aggregate material is a measure of its resistance to weathering, and is determined by assessing the reduction in particle size after repeated saturation with a solution of magnesium sulfate and subsequent drying. On drying, this compound is precipitated in pores within the material which creates stresses that may lead to fracture.
A pre-requisite for poor performance in the soundness test requires that water can permeate into the material. Water absorption of glass (being completely non-porous) is negligible, and so the material fares well under soundness testing.
Whilst glass, or mixtures containing glass, are likely to satisfy the requirements of 'Specification for Highway Works', the largest obstacle to its use is that the document currently only permits materials named in the text (crushed rock; crushed slag; crushed concrete; recycled aggregate; and well-burnt, non-plastic shale).
[1] Highways Agency, Manual of Contract Documents for Highway Works, Specification for Highway Works, Vol.1, 1998
[2] Henry, K.S. AND Morin, S.H., Frost susceptibility of crushed glass used as construction aggregate, Journal of Cold Regions Engineering, Vol.11, 1997, pp326-33
