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Thu 15 Jan 2009 04:00 AM

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Mobile batching plant

The batching plant has been put together with a truck, to create something new and versatile.

The batching plant has been put together with a truck, to create something new and versatile.

Sometimes things that already exist can be put together to make something entirely new and different. Think of fish and chips, or tea and biscuits for that matter.

However, in Dubai it is profit and concrete that matter most, and each of these has a close correlation with each other. Prices of raw materials have stabilised now, but don't forget that it was only five months ago that the cost of everything was going straight through the roof.

Previously, engineers have come up with mobile placing booms as well as truck-mounted concrete pumps. Until now though, there has been very little choice for getting concrete on site. Usually, it is mixed at one of the large plants at the edge of the city and brought onto site via a mixing truck. Alternatively, some projects have their own small batching facilities on site. However, there are drawbacks with both sorts of methods.

Concrete is sensitive to time and heat. Any more than a couple of hours on the road, then the quality starts to drop dramatically. This is a problem for most cities in the region, but Dubai in particular has truck tailbacks in many of the industrial areas where the plants are sited. Additionally, the heat through much of the year can cause problems that have to be solved by using chemical icing and other cooling methods.

So it is interesting to see that at last somebody has thought of making a mobile batching plant. To be fair, these devices have been seen for a few years in North America, but apart from two dozen working in Saudi Arabia, they are a novelty in the Middle East. This is all set to change though, with not one, but two manufacturers setting bringing these devices to the Big 5 recently.

Fresh

Unlike anything distributed by a conventional mixer truck, the concrete from the chute of a volumetric mixer is always fresh ‘At the point of delivery, it is only seven seconds old' said Bill McFarlane, a director of 21st Century Concrete, one of the firms bringing the new trucks in to the region. ‘There is almost no waste either, as only the amount needed has to be produced.

This is an important point as, apart from being a waste of money, unused concrete has to be disposed of, which causes an environmental nuisance. The flip side also, is that if a customer under ordered, a regular mixer truck would have to return to the batching plant.

All materials necessary for producing specification concrete and concrete-types of products are contained on board. Coarse aggregates, fine aggregate, cement, water and admixtures as well as special components can be proportioned and mixed according to weight and volume in any combination or ratio. This process conforms to the ASTM specification C 685-07 for proportioning and mixing.

Thoroughly homogenised concrete is discharged from an auger-style chute assembly ready for placement by conventional means. The rate of production is variable from 15 to 60 cubic meters per hour.

Sand and aggregate are carried in open topped bins with a rigid proportioning belt at the bottom. Blending is achieved by calibrated and adjustable strike off gates at the rear of the bins. Cement sufficient for full load of materials is contained behind the aggregates in a top loading, weather-tight bin with inspection windows, and metered through sequential auger vane feeders directly into the mixer throat. Water and admixtures are injected into the enclosed mixer throat at its bottom where the dry materials enter.

The blended concrete is then discharged from the end of a high-shear mixer into a chute at eyelevel. Chute extensions can be added depending on the distance from the machine required for placement. Hydraulics

The production system is powered hydraulically by a direct-drive transmission PTO with in-cab controls and a pre-set throttle located at the operator's control panel behind the bins. Manual levers turn the proportioning conveyor belt and mixer off independently as needed.

All liquids are injected under pressure supplied by their own pumps, and can be pre-set independently of production, but are infinitely variable to facilitate fine-tuning the mix. Admixes can be switched on or off, depending on need, but will operate automatically when that mode is selected. The mixer itself can be raised and lowered as well as rotated while the system is in operation.

The production system requires an independent air supply, drawn from the trucks air system, but it must be adequate enough to operate switches, valves and piston vibrators that keep materials flowing.

All production is governed and monitored through an anologue metering system linked directly to the main drive shaft enabling the operator to determine exactly how much concrete has been produced.

It is not necessary to over order with this type of equipment. Simply ‘pull the lever' until the correct amount of concrete has been produced or the project has been filled and the meter shows what has been used. This can occour in interrupted stages if desired, as in the filling of wheel barrows, or allowing a placement crew to catch up.

Any slump can be discharged by simply adjusting the water content - even whilst making concrete. Mix design can also be changed within the same load of materials.

Tasks

Different jobs require different types of concrete and these can be quickly accommodated adjusting the various gate settings, to allow recipes for whatever the project being worked on at the time is. McFarlane said; ‘We can produce any mix design required by the customer. We can also change very quickly. Because you are only batching at the point of placement, you can change in a few seconds just by adding to the aggregate mix.'

There's a gate setting we can adjust. So you can adjust all the parameters we can control the aggregates of sand, you can control the water to cement ratio. In essence you can use any mix. There's a few people in the UK that have BSI accreditation, it's that level of performance.

Another new entrant to the local mobile batching area are US-originated Cemen Tech. Marketing manager, Rick Eisiminger said "Our machine features a patented fibre-chopper which cuts, proportions and dispenses fibre into the mix per specifications to provide a better product.

The Cemen Tech vehicle is also available with a variety of options, some, such as the ‘maxle', a trailing axle which can be lowered when the vehicle is near its design weight. Legislation in the Middle East doesn't require this, but other features are of more use, such as a plexiglass window to see the level of the aggregates.

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