significance of recent research on specifications and standards for soil-lime pavement layers --the need to reationalise the design procedure, The

Road & Transport Research, Jun 2000 by Holt, C C, Freer-Hewish, R J

Abstract

The treatment of materials for road pavement layers with quicklime or hydrated lime has generally been successfully used for many years. A review of thirty seven specifications throughout the world indicates that there are considerable variations in the construction processes and testing standards. The need to review and attempt to rationalise these procedures is particularly urgent as some notable failures have been recorded and there is evidence to suggest that more failures have occurred.

The international review is reported in this paper in relation to the research findings on soil improvement at the University of Birmingham, UK over the last ten years and whilst the criteria used are based on those used in British practice the significance of the findings are of wider interest. The research shows that construction 'mellowing' periods, the period between mixing and compacting, are considerably longer than those used in standard laboratory testing. The results indicate that this variation in time can significantly affect the performance of the lime-treated material. The paper contains recommendations for a laboratory design procedure to replicate expected site conditions and construction.

INTRODUCTION

The use of lime as an agent for modifying and/or improving the performance of certain soils goes back to Roman times. However, the products of the treatment have not been fully understood and this limits the full potential of the process.

Lime is generally in the form of either quicklime or hydrated lime for road construction. When it is added to a suitable material, generally a clay soil with a plasticity index and a clay content greater than 10%, two reactions take place. Firstly, flocculation and subsequent aggregation of clay particles occur due to ion exchange. This exothermic reaction is very rapid, drying excessively wet soil, and causes immediate alterations to the properties of the treated soil. These are a reduction in plasticity and, swell/shrinkage potential, and an increase in workability. This phase is commonly known as modification. The second reaction phase starts generally within 1 to 3 days - Hydroxyl ions from the lime increase the pH of the lime-soil mix and cause a dissolution of alumina and silica from the clay particles. Calcium from the lime then reacts with the alumina and silica from the clay to produce calcium silicate hydrate and calcium aluminate hydrate gels that cement the clay particles together. This phase is more time dependent and is commonly known as stabilisation of the material. The purpose of stabilisation is to generate an increase in strength and durability. Durability is more difficult to measure and predict, and the need for long-term performance is often overlooked by designers.

The lime treatment of natural materials is generally carried out in an in situ condition. The treatment process generally comprises the following three basic stages:

* mixing the lime with the soil;

* leaving the soil-lime mix to stand for a specified period;

* re-mixing the treated layer and compacting fully to specified density targets with moisture criteria in some cases.

The second stage of the treatment process is often referred to as the 'rotting period', 'aging period', ,maturing period', or the 'mellowing period'. The term 'mellowing period' is used in this paper.

The considerable variations in international practices of this second stage are particularly addressed in this paper and some background on the early research of the 1960s which influenced these variations is provided.

The recent research at Birmingham by Holt (1996) and Holt and Freer-Hewish (1996,1998) highlights how the choice of mellowing period, the time between mixing and compacting, is dependent on long and short-term performance criteria needs and this is explained below.

AN INTERNATIONAL REVIEW OF CONSTRUCTION PROCEDURES FOR LIME TREATMENT

An extensive review by Holt (1996) of thirty seven specifications used worldwide revealed small, but possibly significant, variations in the lime treatment procedure. Figure 1 demonstrates the variations in the construction procedure stages for lime treatment in various specifications.

A generalised step by step treatment procedure drawn from these is given below.

The time and soil should be mixed with sufficient water, generally ranging from optimum moisture content (OMC) -2% to OMC 10%, to facilitate the necessary reaction between the soil and the lime. (Note: It is unclear in the majority of specifications whether the OMC is that of the treated or untreated material.)

Mixing should then continue until sufficient pulverisation requirements are met, typically 98-100% passing a 25 mm (1 inch) sieve and 60% passing a 4.76 mm (No. 4) sieve. The treated layer should then be given a light roll to seal the surface and prevent moisture loss and/or carbonation.

Construction operations are then often ceased and the treated layer allowed to mellow. This period can vary between 0 hours and 21 days, although mellowing periods usually vary between 1 and 3 days. During mellowing, the surface of the treated layer is generally kept moist. Only five of the specifications reviewed allow for the mellowing period to be waived, or reduced, if pulverisation requirements were achieved after initial mixing. These were the specifications from the States of Illinois, Kansas, Maryland, Mississippi and Nebraska in the USA.