A New Way Of Thinking

Hanson Aggregates launch new material to meet with the needs of the present without compromising the needs of the future

The author, Josephine Ford, a recent graduate of Doncaster College, works for Hanson Aggregates. Her paper received The Institute of Quarrying’s 2003 Marston Award for the best paper presented to a branch meeting

A restricted amount of old bituminous surfacing has been reclaimed and reused in the UK for a number of years now. This material is removed from roads prior to resurfacing and at present is only widely used in base or binder courses or as fill for lightly trafficked roads — generally low-return products.

During the past few decades the general population has gradually become more aware of the impact on the environ-ment resulting from industrial activity. The Government and local authorities have started to be affected by this rising concern and have begun to implement a broad spectrum of recycling initiatives. One of the more recent additions to this is a request to include recycled material in the construction of new roads.

Savings

From the producer’s viewpoint there is also the benefit of saving money by taking advantage of the recently launched aggregates tax. As is well known, the Government placed a charge of £1.60 per tonne on all raw aggregate and this was immediately passed on to the coating plants. By using recycled material, operations should make significant savings on virgin aggregate.

Landfill tax is another area where savings are possible. With the recycling of coated materials within modern roads the amount of material being disposed of in landfills can be reduced. This does not greatly affect most batch-heater coating plants although the amount of waste produced during the start-up/shutdown of continuous and drum-mix plants is considerable.

Should this project go ahead, either at Hanson’s Leeds operation or another site in the area, it will be the first Hanson hot-mix recycling operation in the Yorkshire region. This would close a rapidly growing gap in the market which is swiftly being filled by competing companies.

If all companies continue to develop the recycling arms of their businesses, the chances are that, in future, local authorities will opt only to receive surfacing materials from those companies who are able to provide them with these ‘green’ materials in order for them to satisfy their own recycling quotas. This would mean that any company wishing to get involved in such contracts would need to implement some form of recycling scheme in the not to distant future.

Incentives

Other reasons for implementing recycling operations are being brought about through government policy with its stated aims to ‘encourage prudent stewardship of mineral resources’ and to ‘reduce environmental impacts during mineral extraction and restoration’. MPG6 itself placed an emphasis on sustainability and set targets for 40 million tonnes of recycled material to be used by 2001, with a further target of 55 million tonnes by 2006. The Royal Commission on Environmental Pollution also set targets to double the use of recycled material in roads by 2005 and to double this use again by 2015.

How close are we to meeting these targets today?

Bitsand

Various products have been developed by Hanson for use in road construction. They utilize road planings, recycled aggregates or incinerator bottom ash (IBA) and are known commercially as Roadfalt. These products are designed to reduce the environ-mental impact of the industry as a whole while supporting local authority and government objectives for sustainability.

In December 1999 Hanson entered into an agreement with a contractor who took motorway planings and crushed and screened them to produce a bitumen sand (‘Bitsand’). This was then recycled through the drum-mix coating plant at Golborne, near Manchester. By May 2001 Hanson had ‘acquired’ the intellectual property rights of the process. The Bitsand process has been granted approval under The Patents Act 1977 and the Patent Rules 1995.

Bitsand addition

The operation studied in this project was a Braham Millar single batch-heater-type plant located in Leeds. This meant that the addition of Bitsand would be directly into the mixer, which is different to most other Hanson recycling operations in the UK.

Recycling: the facts

There are several points that need to be considered when contemplating off-site recycling:

• The need for significantly greater storage space than that required for a conventional mixing plant in order to accommodate additional stockpiles of both reclaimed blacktop arriving from site and re-crushed material prior to remixing.
• The further the distance between the site and the mixing plant the less attractive the economic benefits become due to transport costs. Separate transport is also desirable for reclaimed material to prevent contamination and to avoid logistical problems.
• The addition of more than about 40% reclaimed material in a mix can, in some circumstances, result in unacceptable blue fume emissions when passed through the flame. As Bitsand is usually delivered straight into the mixer, bypassing the burner, no unacceptable fume emissions should be produced.
• In some cases coagulation of planings in stockpiles can occur, particularly in hot summer weather, resulting in the need to hire in additional plant to break the stockpile. For this reason under-cover storage is highly recommended to maintain the consistency of the material, together with regular remixing to prevent any coagulation.
• In 1992 the Department of Transport gave permission for bituminous mixes to contain up to 10% recycled material, with greater quantities permitted with certain controls. In 2000 the amount of recycled material permissible in hot-mix asphalt was increased from 30% to 50%.
• Much has been published about ‘secondary’, ‘alternative’, ‘reclaimed’, ‘waste’ etc materials. All these titles suggest that recycled components may be in some way inferior to new components, but this is not necessarily the case. However, it is essential that valid comparisons be made in an engineering sense if recycled materials are to be considered on an equivalent basis to primary materials.

Trials

Trials on the input of recycled material into a continuous drum-mix plant had taken place at other Hanson sites and produced highly successful results. These led to the following trials being carried out using the batch-heater plant at Leeds.

The method of converting planings to Bitsand and coarse aggregate feed at the various locations around Hanson is being reviewed in terms of cost and best practice. In order to deter-mine the necessary additional heating times, the possible percentages of Bitsand material and any extra mixing time that may be required, trials needed to be carried out.

These trials were conducted using 10 tonnes of material shipped in from across the Pennines. The amount of virgin material was calculated using the standard mix design for 20mm DBM 190p binder course 6.5LL and the analysis of the Bitsand to be used in the trials.

Five trials were carried out, altering either the percentage recycled or the mixing time for each. Each of the trials was done using a 4-tonne total mix, which meant that 2.6 tonnes of Bitsand would be required in total. Due to haulage rates 10 tonnes of Bitsand were transported to Leeds, 2.6 tonnes were used in the trials and the rest was stored under cover for further trials.

The main purpose of the trials was to establish whether the addition of recycled material would be possible at the Leeds operation; this was mainly due to the high temperatures that were required at other Hanson sites to keep the material within specification.

Expectations

The factor of most interest was the amount of extra heating time needed to bring the final product temperature within the specification limits of 120–150°C. The addition of cold Bitsand into the mixer makes the temperature drop dramatically which means that the virgin aggregate needs to be heated to a greater extent in order to maintain the necessary temperature. It had been seen at other sites that this temperature drop could be quite large, with some sites showing a fall from 200°C to 140°C in a matter of seconds.

Another aspect was the amount of steam produced when cold Bitsand meets the hot material. This would be greater the higher the moisture content of the Bitsand and may reduce the life of the filters in the bag house.

Results

From these trials it was found that it was possible to add Bitsand into basecourse mixes in a single batch-heater plant. The temperatures required could be reached and (under normal circumstances) the dust-extraction system could cope with the amount of extra steam produced.

The following recommended heating times, in relation to the amount of Bitsand in the mix at 3.1% moisture content, were concluded:

• 10% – 120s
• 15% – 130s
• 20% – 140s

It was also found that the gradings, bitumen content and moisture content of all three products were well within specification leading to a perfectly saleable recycled product.

Owing to the increase in cycle time at a batch plant, it was decided that, for maximum savings, the Bitsand material should be added to every base and binder course mix using a ‘little and often’ philosophy, ie 10% per mix. This would create a market of 4,000 tonnes of Bitsand per year at each site adopting the project.

Engineering

To achieve Bitsand addition several alterations are required, including civil engineering as well as alterations to the plant itself. One of the main problems with urban coating plants is the limited amount of space that may be available, thus reducing the amount of alterations that are possible. With the aid of drawings and physical measurements of the current plant it was possible to redesign the system at Leeds to create enough space for the proposed alterations.

The first area to be examined was that of Bitsand processing. This would require a hardstanding for the plant under consideration as well as a secure area for planings storage and at least two storage areas for the products (–6mm and +6mm). The equipment selected to screen the Bitsand was a two-deck mobile screen similar to the Finlay 390 Hydrascreen currently used for this process at other Hanson sites. This offered many advantages including manoeuvrability and the possibility of re-screening the +6mm material.

Once produced the Bitsand needed to be fed into the plant and accurately weighed and batched. To do this the designs created included the use of a low-level ground feed hopper to reduce the loss of space around the plant, a feeder unit mounted on load-cells for initial weighing of the material, a receiving batch weigh hopper capable of holding up to one tonne of Bitsand, and a discharge weigh hopper to release the small amount of material required for addition into each batch.

Investment

Once it has been decided that a scheme is required, finance is needed to bring it to life. In order to do this Hanson concentrated their efforts on one particular site and centred their research on the introduction of recycled material into their products at this site with the aim of meeting any new recycling targets introduced in the future, and replacing as much of the virgin materials as possible to create a saving for the company.

The various areas investigated included possible sources of material for creating Bitsand, alternatives to on-site Bitsand production, whether the remainder of the planings material should be added into mixes, which products should incorporate Bitsand, different methods of Bitsand production and, finally, whether the plant itself would be able to cope with the additional pressure placed on the system.

Payback

With regard to the financial implications of the project, the main advantages were the replacement of stone and bitumen by the Bitsand and the possibility of government funding for the operation. Bitsand is capable of replacing hard stone fines and a percentage of binder in the mix to create large saving for the company. The Bitsand production costs were looked at and compared to off-site production and transport. Plant waste imported from other sites in the area was also investigated and the conclusions showed that payback for the whole project could be achieved within three years of it being set up.

Control

There are several restrictions to be considered when recycling blacktop including those put in force by the SHW, with certain materials, such as concrete, being completely banned. It has been agreed that the key to the success of recycling is control at all levels. The need to meet strict requirements in terms of recovered binder properties and other technical constraints cannot be stressed enough. Therefore, strict requirements must be met in order for the quality of the products to remain at their current standard. ‘Best practice’ should clearly be adopted on all sites importing RAP around the UK.

The following points represent current best practice:

• All planings must be visually checked before acceptance and, if necessary, core samples taken.
• No variable planings are to be accepted unless these can be stocked separately.
• Plant waste is to be added consistently to normal planings in order to maintain consistency.

Conclusions

The key points revealed by this project are that the market for recycled material is increasing and that, if the new recycled products are proved to be equivalent to ‘all virgin’ materials, their inclusion may become a mandatory condition in the future of pavement contracts. The Government and the public are becoming more aware of the condition and care of the environment and the industry as a whole should be following their example, effectively meeting the needs of today without compromising the needs of forthcoming generations and the future mineral-extraction industry. Hanson have the right idea with forward-thinking attitudes together with cutting-edge research and technology. It is to be hoped that the entire industry can put its ideas together to meet these future problems head-on and to look to the future for generations to come.