STABILISATION & HYDRAULICALLY BOUND MIXTURES FOR PAVEMENTS

Introduction

Large quantities of imported natural aggregates are consumed for use as foundation materials on construction projects. In many cases the addition of cement to the materials already in the ground or available nearby would render them suitable for use and avoid the need to bring in other material. This treatment process is known as soil stabilisation.

Soil stabilisation is a process that combines soil, cement and water to produce a hard, durable paving material that can be used for the foundation or base of road and airport pavements, parking and storage areas. Cement can be used to treat most soils. In addition, deteriorating or failed pavements and roads can also be reconstructed by stabilising (or recycling) the existing pavement using cement.

Materials treated by cement are described variously as soil cement, cement-treated materials, or cement-stabilised materials. Where aggregate is being treated, the resultant mixture may, depending on strength or national terminology, be referred to as cement-bound granular material, roller-compacted concrete (RCC), dry lean concrete, cement-bound granular base (CBGB) and cement-treated base (CTB). Whatever the terminology, they are all part of that family of paving materials known generically as cement-bound materials (CBM).

CBM are part of a larger family of paving materials known as hydraulically bound mixtures (Hydraulically Bound Mixtures). Hydraulically Bound Mixtures describe soil or aggregate mixtures that use binders made from the following:-

• cement
• lime
• gypsum
• granulated blast-furnace slag
• air-cooled steel slag
• coal fly ash (in the UK, fly ash is also known pulverised fuel ash or pfa).

Such binders are known as hydraulic binders since they set and harden in the presence of water. Hydraulically Bound Mixtures generally have a water content compatible with compaction by rolling. After compaction, the water is free to hydrate the binder or hydraulic combination and commence the setting and hardening process.

Why use hydraulically-bound mixtures?

Lower cost
Hydraulically Bound Mixtures made from in-situ material, or borrow-material taken from elsewhere on the site or nearby, is usually cheaper than the conventional approach of importing granular or other treated material. These savings arise from the use of existing resources and the resultant reduction in the cost of materials and transport. In addition, there is no need to remove and dispose of material off site – this may also avoid landfill tax liability.

Fast construction
Modern construction methods make Hydraulically Bound Mixtures quick and easy – thereby reducing contract duration.

Sustainable construction
Since Hydraulically Bound Mixtures can be made from site arisings, recycled material or artificial materials, primary aggregate extraction is avoided. Traffic between aggregate sources and site can therefore be eliminated. There may also be a reduction in lorry traffic needed to dispose of the site material.

Improved performance
Depending on the aggregate being treated, the properties of Hydraulically Bound Mixtures can be superior to those of unbound granular materials and often equivalent to those of bitumen-bound materials. Research has shown that Hydraulically Bound Mixtures sub-bases could be laid up to 60% thinner than unbound granular sub-base, while producing an equivalent or better performance (TRL 248). Alternatively and technically better however, it is often more advisable to keep the sub-base thickness the same and use the enhanced performance to reduce the thickness of the more expensive, overlying, pavement layers.

Proven long-term performance
Hydraulically Bound Mixtures in its various forms have been used in road, airfield, port and other pavement construction for over 50 years.
Among other stakeholders, Hydraulically Bound Mixtures is permitted and used by the Highways Agency, BAA and Associated British Ports.

More information

Information on the ‘nuts-&-bolts’ aspects of Hydraulically Bound Mixtures can be found in the Concrete Centre’s guide CCIP-009, ‘Hydraulically-bound mixtures for pavements’.

This guide covers the stabilisation of naturally occurring soils or other materials to improve their mechanical properties and performance for use in capping layers, sub-bases and bases. This document, which is compatible with European standards, covers treatment with cement and the full range of hydraulic combinations based on fly ash, granulated blast-furnace slag, gypsum and lime, and covers;

• binder selection
• soil/aggregate selection
• site investigation and preliminary assessment
• mixture design
• production and construction
• construction control.

Intentionally, the guide does not cover thickness design and specification, but should provide the background for the formulation of such application documents.