With the volume of concrete needed across the world increasing each day, new technology is increasing the output of the batching plant and to get it on site.
Two months ago we looked at the new technology that is transforming the way quarries are run.
Now, we take a step along the path and see how the process of making crushed limestone in to cement and concrete is being forced into the twenty first century.
Time
Back in the mists of time, an engineer named John Smeaton was charged with building a lighthouse on a shard of rock some twenty miles of the south coast of England.
This was a problem as in 1756, nobody had succeeded in building anything offshore in this manner, in fact it was the third time that a lighthouse had been tried. and it was far from clear what substance would hold back the ravages of the tide. Smeaton came up with the idea of using a substance named ‘hydraulic lime’ which would set even underwater.
The plant technology that mixed these early mortars was, to say the least, crude, though somebody worked out very early on that a simple Archimedes screw in a drum could effectively mix cement. The mix, of course, is critical and early facilities lacked any real means of controlling the materials, resulting in variable quality and a lot of waste.
The product itself was also of variable quality, due to the all-natural formula of the cement.
Many years later, in 1824 another Englishman took out a patent on a synthetic formula of limestone and clay, which he named ‘Portland Stone’ on account of it’s resemblance to the rocks quarried on the island of Portland.
Later, the recipe was refined and is still used to this day.
Concrete made with Portland cement is superior to that made from natural cement because it is of more consistent quality, and is stronger and more durable.
Today, there are several types of concrete, defined by methods of installation. Ready or pre-mixed concrete is batched and mixed at a central plant before it is delivered to site.
Because this type of cement is generally delivered in a mixer truck it is also referred to as transit-mixed concrete. Shrink mixed concrete is partly mixed at the central plant, and partly in the truck later.
Process Control
As with new-style quarry crushing plants, the application of process control is perhaps the most crucial step in increasing plant efficiency.
This involves installing computer equipment with specially written software to automatically adjust the mix as it is pumped through the machine.
The computers are linked to various electronic valves and servos throughout the plant and an algorithm what is needed and where to produce the required perfect product.
Majed Chahla, Sales, HST said; “Process control is the modern way. It is many times more efficient than before.”
Measuring devices
The moisture in the concrete mix needs to be kept under careful control, otherwise the amount of aggregates going into the mix will be wrong as the weight of the water will give a false indication of how much volume there really is.
There are now devices available that measure the moisture content both in the aggregate hoppers as well as and in the finished product.
These devices detect the moisture by using microwaves.
One product of this type is the Hydronix system. Steve Cook, Applications and Support, Hydronix said.
“The digital microwave moisture sensing technique developed by Hydronix has proved to work very successfully, not only with concrete, but also with a wide variety of other bulk materials, such as Soya beans and corn.”
Increasingly our equipment is being used in applications such as animal feed plants for monitoring the moisture in the incoming raw materials and in addition it is for controlling the moisture at all of the critical points during the whole process.”
Of course the next step in the operation is to get the mix to site.
Mixer trucks
Once the concrete has been mixed, it needs to be loaded into a mixer truck and carted off to its destination. In the UAE, this is usually done in the dead of night, and for several reasons.
Firstly, and most importantly, it is only legal to drive a big truck on many of the major routes in Dubai during the hours of darkness.
This actually benefits the industry as it is far easier to deliver the mix when the air is slightly cooler and the reduced risk of being caught in a traffic jam means the risk of having to waste a spoiled batch is much reduced.
The biggest mixer any PMV manager could get his hands on is currently the Terex FDB7000 front-discharge transit mixer. This vast machine boasts a vast 11-cubic yard mixer and a gross permissible weight of 79,500 pounds, it is currently pretty much the biggest mixer in town.
A cheaper option is one of the many Chinese mixer truck bodies from companies like Sany. These can then be mounted on the back of a truck of the customer’s choice and then tailored to their individual needs.
Currently Swedish truck maker Scania is in the process of building a plant that can mount whatever body the client requires. The new facility in Jebel Ali will be able to handle around 1,400 trucks per year.
“We will be able to deliver high-quality fully-built vehicles that are adapted to the requirements and operating conditions that apply in the region,” said Per Halberg, Head of Production and Procurement, Scania.
Pumps and booms
The final step is to get the mixture to where it needs to go on site – and fast. Once on site, the concrete must be placed and compacted. These two operations are performed almost simultaneously.
Placing needs to be done so that full compaction – with all air bubbles eliminated – can be achieved. The rates of placing and of compaction should be equal; the latter is usually accomplished using internal or external vibrators.
An internal vibrator uses a poker housing a motor-driven shaft. When the poker is inserted into the concrete, controlled vibration occurs to compact the fresh concrete.
A concrete pump is required, but up until a couple of years ago, these were either hulking, cumbersome items of fixed plant, or they were small trailer-mounted items useful only on small sites.
Now though, a good compromise has been developed for use in the region.
The truck-mounted concrete pump delivers a high flow rate, with the added advantage of being extremely manoeuvrable.
Former director of Al Bakhit told us earlier in the year; “The flexibility is fantastic. Say a construction site has four towers together, one pillar can be filled and then it’s a quick disconnect and on to the next pillar.”
Of course, to get the concrete mix where it needs to be, the pump should be attached to a device known as a placing boom.
This is part hose, part folding crane, and it will deliver to the tallest of projects.
Currently, the tallest of booms is made by Sany who make one with a 66 metre reach. The displacement is up to 200m3/h as well, which is good for fast use before the mix turns to stone.
With the concrete delivered and placed on site, all that remains to do now is to sit back and watch it dry… Until the next batch arrives of course.