Thailand is one of Southeast Asia's economic success stories. It has the highest levels of prosperity since the regional economic crisis of 1997. According to the World Bank, Thailand's Gross National Income (GNI) per capita of US$1980 is more than double that of the East Asia and Pacific average of US$950 in 2002. Like any other developing country the efficiency of its logistics and transportation system is a very important consideration. At the heart of this are ambitious plans to expand the country's motorway network from 225km to 4150km. With such a significant expansion on the horizon, Sitthisak Wittayangkoon argues for the use of EAF slag as an additive in road construction. He believes that making use of EAF slag will add value to a substance that has long been perceived as a problem.
At present over 90% of highway paving in Thailand is constructed from asphalt concrete. Asphalt concrete (also referred to as hot mix asphalt) is a mixture of fine and coarse aggregate with an asphalt cement binder. The mixture is compacted at high temperatures at a central plant and then placed on the road using an asphalt spreader.
The aggregate used in asphalt mixtures (hot mix asphalt, cold mix asphalt and surface treatments) comprises approximately 95% of the total mix by mass. Moreover, the stability of the mixture depends on control of aggregate grading, strength, toughness and shape. The asphalt binder, used to coat and bind the aggregate particles together, typically makes up about 5–6% of the total asphalt mixture.
Asphalt cement is also used in hot mix asphalt. Liquid asphalt, which is asphalt cement dispersed in water with the aid of an emulsifying agent or solvent, is used as the binder in surface treatments and cold mix asphalt pavements. The properties of binders are often improved or enhanced by using additives or modifiers to improve adhesion (stripping resistance), flow, oxidation characteristics, and elasticity. Modifiers include oil, filler, powders, fibers, wax, solvents, emulsifiers, wetting agents, as well as other proprietary additives.
Slag Asphalt Concrete
Crushed steel furnace slag is characterised by highly angular shapes that is sometimes used as an aggregate for asphalt concrete paving. This asphalt concrete is called 'slag asphalt concrete'. The crushed slag aggregate provides for excellent stone-on-stone contact and particle-particle interlock when used in bituminous mixtures. This produces a highly stable, high frictional concrete, with rut resistant properties that are ideal for the pavement.
EAF Slag Aggregate in Thailand
In recent years the demand for steel products in Thailand has increased rapidly resulting in healthy growth of the Thai steel industry, with regular construction of new steel furnaces being the norm. This expansion has however generated more waste, creating environmental and social problems the country must face up to.
The principal waste of the Thai steel industry is EAF slag. Until recently there was negligable interest in utilising EAF slag in any commercial way. More often than not it has been dumped in the vicinity of the factory with little or no thought given to the environment.
However, in 1995 Siam Cementhai Group, the biggest steel producer in Thailand, embarked on a joint venture with the US building firm Edw. C. Levy Company. Together they formed the new company 'Siam Steel Mill Services Ltd.' to manage the recycling of EAF slag as a viable and commercially useful material. Specifically, Siam Steel Mill Services Ltd. provides the management of EAF slag in terms of handling, metal recovery and crucially the production of a quality slag aggregate for use in road construction.
Processing
Slag is typically received in pots from steel furnace refining vessels contaminated with metal that separates to the bottom. The separation of slag and metal can never be precise since slag always floats on iron or steel. Of the two options available, it is preferable from a metallurgical point of view to leave some metal in the slag, rather than to leave some slag in the metal.
When solidified, this large chunk of metal is called a skull, and needs to be broken down into useful sizes and the adhering slag removed. This is usually done by impacting the skull with a large drop-ball suspended on top of a crane. The drop-ball may either be captive (attached to a lifting cable) or a free ball (lifted by a cable hoisted by an electromagnet that releases the ball by de-energising the magnet).
The slag is fed through a grid by a front loader or truck. Another magnet crane may be used to pick up the metal which is too large to pass through the grid, and to assist with pushing the slag pieces through the grid. The smaller pieces of metal are removed by two or three smaller magnets during the crushing and screening stages.
Separated slag is processed in a manner similar to the production of aggregate at any standard quarry. Slag is fed into crushers and after reduction to the required size it is separated into the required grades. In terms of road base material the required grade is 0-50mm and for asphalt or thin bituminous surfacing, aggregate sizes are usually 0-5mm, 5-12mm and 12-20mm.
Dust control during this process is especially important. This is achieved by the addition of water before the slag is fed to the plant and during processing. The process described above has an added advantage: after conditioning and weathering, various types of aggregate mixtures can be produced conforming to the needs of individual customers.
Applications in Thailand
Siam Steel Mill Services Ltd and the Road Research and Development Center, part of the Department of Highways, Ministry of Transport and Communication of Thailand has taken a few years to study slag asphalt concrete by comparing how it performs vis a vis other types of natural stone. The results are shown in Figure 1.
Summary
We have shown that with proper handling and processing EAF slag is an excellent choice as an aggregate for asphalt concrete. The paving made with EAF slag has higher skid resistance and durability than paving made from natural aggregate. This is a vital element when considering the high volume of traffic that is common on Thai roads. Since 2000 EAF slag asphalt pavements have been adopted on several projects for major highway construction projects, including:
- Chonburi motorway.
- Highway number 1 and highway number 32.
- Expressway stage I and II.
- Surface material for the steel bridge over the Chao
- Praya river.
- Highway between Laem Chabang Deep Sea Port and highway number 331.
References
1. Asian Development Bank, Philippines, "A fact sheet: Thailand and ADB" December 2005. http://www.adb.org/Documents/Fact_Sheets/THA.asp?p=ctrytha.
2. Inter-City Transportation, Department of Highways, "Project and Master Plan", 2005. http://www.motorwaythailand.com/project_plan_02.html.
3. Road Research and Development Centre, Department of Highways, Ministry of Transportation and Communication, "Slag Asphalt Concrete", Thailand, 2002.
4. Production for Siam Steel Mill Services Limited, "Slag Processing" Chonburi, Thailand, 2000.
