Introduction

For a sprayed seal to perform, the aggregate must be retained securely by the binder. This is strongly influenced by:

a) the quantity of binder

b) achieving an adequate bond between the binder and each aggregate particle, at the time of applying the seal, and for the life of the seal.

The binder rate of application needs to be carefully designed for the conditions, the aim is to have sufficient binder to envelop the bottom half to two thirds of each aggregate particle. Too much binder may cause bleeding, and too little binder may simply not be able to retain the aggregate against the traffic stresses, leading to stripping.

There are certain conditions under which the binder may be expected to fail to form or maintain an adequate bond with the aggregate even with the appropriate binder application rate. These are:

the aggregate surface chemistry

the presence of dust

the presence of water

the viscosity of the binder (may be too high for "wetability" of the aggregate).

This can generally be improved by:

selecting the most suitable available aggregate

careful screening and/or washing to ensure that the aggregate is clean

careful heating and proper cutting back of the binder and

Where possible, restricting sealing to favourable weather conditions.

Almost all aggregates contain some dust, often as a result of loading and transporting, and being stockpiled. To minimise the risk of stripping, it is strongly recommended that ALL AGGREGATES BE PRECOATED.

What is precoating?

Precoating is the pretreatment of the aggregate with a thin film of bitumen compatible material, generally a petroleum based product, to:

neutralise the effect of dust

to aid the initial "wetting" of the aggregate by the binder, and allow the use of a more viscous binder (less cutter) which will set up more quickly and improve the early retention of the aggregate

improve the bond of the binder to the aggregate.

Types of precoat

The common precoating materials used are:

oil based - generally distillate, but a 50:50 mixture of distillate/ kerosene, or kerosene only, may be used in cooler weather conditions.

bitumen based ~ generally bitumen mixed with distillate or kerosene. Bitumen content varies between 25 and 40% by volume of the mixture.

bitumen emulsion - generally, diluted cationic emulsion.

Less common are water based proprietary materials consisting of a mixture of water and adhesion agent.

Laboratory testing and field experience will assist the practitioner to make the choice of the preferred precoating material and adhesion agent for the conditions. Field trails have shown that bitumen based precoats perform generally better in severe circumstances in the long term than oil based or water based precoats. Bitumen emulsion has the advantage of containing little or no oil which might reduce the early strength of the bond.

Use of adhesion agents

In Precoating

The oil based precoating materials are sometimes used alone just to "kill' the dust. When the aggregate is damp, or the weather conditions are unfavourable with rain

AUSTROADS PAVEMENT RESEARCH GROUP

SPRAYED SEALING AGGREGATE PRECOATING

November 1996

APRG TECHNICAL NOTE 6

Transport Research

For further information please contact

Name Organisation Phone Fax Wally Holtrop AAPA (03) 9819 4999 (03) 9819 5278 Ted Booth Department of Transport SA (08) 260 0531 (08) 260 0454 Reg Leach Main Roads WA (09) 350 1444 (09) 451 1400 Ray Gaughan RTA Technology, NSW (02) 9662 5891 (02) 9662 5123 lan Cossens VicRoads (03) 9881 8915 (03) 98818900 Steve Hogan Queensland Main Roads (07) 3834 3081 (07) 3834 3055

threatening or humid conditions, adhesion agent should be added to the oil based precoating materials.

A bitumen based precoat should always contain an adhesion agent.

In cationic bitumen emulsion, the emulsifier is similar in action to an adhesion agent, and therefore use of an adhesion agent is not required.

Adhesion agents may be supplied as a paste, in granular form or as a fluid. They are chemicals designed to improve the adhesion of the binder to the aggregate, particularly in the presence of water. They often have a nauseating smell, and can irritate the skin and eyes. Therefore, the manufacturer's instructions given in the Material Safety Data Sheets (MSDS) should be observed at all times.

In the Binder

Adhesion agents are most effective and economical when added to the precoating material, but they may also be added to the binder. This is usually necessary only when acidic (silicate/ quartz rich) aggregates are used, or when using a polymer modified binder. Dosing binders with adhesion agents is particularly beneficial when siliceous aggregates are used.

Concentration

It is important to measure accurately the quantity of adhesion agent added in the field to the precoat or binder. Too much adhesion agent can actually make things worse, and result in the aggregate stripping. The typical concentrations, (by volume) of commonly available and used adhesion agents are:

in the precoat: 1 to 2 parts per 100 parts of precoating material (approx. 1 to 2%)

in the binder: 1/2 to 1 part per 100 parts of binder (approx. 0.5 to 1%)

Precoating practice

When using oil based materials, aggregate should be precoated in a manner to ensure a uniform film of precoating agent is applied to all faces of the aggregate. Aggregate may be precoated and stockpiled for a short period just prior to use, provided precautions are taken to protect the precoated aggregate from moisture.

When using bitumen-based or emulsion precoats, it is preferable that the aggregate is precoated 1 to 4 weeks in advance. This is to let the material set up and avoid pick up by the traffic and/or softening of the binder. Precoating of this nature may be done at the quarry prior to delivery, or at the stockpile site. When precoated at the stockpile site, it should be restacked neatly to allow economic and efficient loading at the time of use. The precoated aggregate will require protection from dust, and this may be done with plastic sheeting or similar.

Ensure a uniform rate of precoating. The quantity of precoating material applied should be just sufficient to coat each aggregate particle uniformly.

A dusty, dry aggregate will require a heavier rate of precoating than a clean, dry aggregate of the same type and size. As a general indication only, the rate of application of oil based precoating agent on dry aggregate should be approximately as given in Table 1. For porous aggregates, the rate may need to be increased by up to 2 L/m3, and for smooth, hard aggregates the rate may be reduced by as much as 2 L/m3. Generally, bitumen based materials and emulsion precoats require a heavier rate.

Reminder

Precoating is just one step in the chain of accepted and well established practices, and attention to detail is required to achieve a consistent high quality of sprayed sealing. Precoating is not a substitute for proper workmanship or specifying and accepting only clean aggregates.