Menu



Provision of Access

The route(s) from the site entrance to the point(s) of discharge need to be planned in advance. A fully loaded six-wheeled truck mixer weighs 26 tonnes and eight-wheelers weigh 32 tonnes, so access roads must be strong enough to carry the load, even in wet conditions. In many cases truck-mixers have to reverse into position to discharge so an adequate turning space on firm ground may be needed near to the discharge point; a turning circle of about 18m is necessary for a typical truck. To avoid contamination of the site, an area should be designated for hosing down chutes and cleaning wheels.

Delivery

Before discharging any batch of concrete the delivery ticket should be checked to confirm that the concrete is of the correct class and conforms to what was ordered. This checking is best done by the contractor's authorised and nominated representative who also is responsible for the ordering. If all the details are correct the driver should be instructed to remix the load to ensure uniformity - for at least three minutes when the concrete has been plant-mixed, or for at least 4 - 5 minutes (40 - 60 drum revolutions) when any water is added on site.
 
Transit Mixer
Transit Mixer
Concrete mixed at a depot, either in a central mixer or in a truck mixer, should arrive on site with the ordered consistence, and no extra water should need to be added. Some suppliers using dry batching plants add a quantity of water when the truck arrives on site. It is then the driver's responsibility to add only the amount of water as instructed, to achieve the specified consistence as shown on the delivery ticket.
 
When the site asks for additional water to make the concrete more workable, this will have to be signed for on the driver's copy of the delivery ticket and, in such a case, the supplier cannot be held responsible for the concrete failing to meet the specified strength. The slump of concrete delivered in a truck can be measured using a spot sample obtained from the initial discharge, but note that such a sample is not representative for cube making. After allowing a discharge of about 0.3 m3, six standard scoopsful should be collected from the moving stream to provide a sample of about 20 kg, the scoopsful being taken as quickly as possible and
preferably from the next 0.2 cum of the discharge. The discharge should then be stopped, the slump measured, and if it is within the specified consistence class, the remaining part of the load may
then be discharged.
 
If the concrete does not conform to the requirements either by slump, ticket details or visual inspection, it must be refused and may be returned to the depot. The reasons for return should be written on the delivery ticket and the truck number and time of rejection be recorded. However, under certain circumstances, it
may be permissible for water to be added to a load of stiff ready mixed concrete in order to achieve the specified target consistence class. This is in accordance with BS 5328 and BS 8500 and applies when:
  • The slump is less than the lower limit of the consistence class
  • The quantity of added water is controlled by being measured accurately and recorded
  • The stiffness is not due to an excessive delay since batching.
Discharge

The truck-mixer can discharge at a rate of about 0.5 cum per minute.While it may not always be possible to handle the concrete as fast as this due to limitations of placing and compaction rates, it is to the advantage of the site and the supplier for the concrete to be discharged as quickly as possible - delays longer than 30 minutes from arrival on site to completion of discharge may be charged for.

For construction at or below ground level, the quickest and most efficient way to discharge concrete is directly from the truck. The maximum discharge height for the chute is about 1.5 m above the ground and, with extensions, chutes can cover a radius of about 3 m from the back of the truck. If discharging into trenches or pits, it is essential that the excavation sides are properly shored to prevent collapse from the weight of the vehicle. If the concrete is to be placed by crane and skips, a lot of time can be saved by using two skips; the empty one can be filled while the other is in use.

Samples of ready-mixed concrete for compressive strength tests should be representative of the whole load, with increments being taken from different parts of the discharge. Cubes must be made from incremental samples, not from soot samples.

References/Further Reading

1. The essential ingredient - Quality. 1992, British Cement Association, Crowthorne. Ref. 97.323. 23 pp.

2. The essential ingredient - Production and transport. 1993, British Cement Association, Crowthorne. Ref. 97.326. 20 pp.
 
3. Dewar, J D and Anderson, R. Manual of ready-mixed concrete. (2nd edition). 1992, London, Blackie. 245 pp.








0 comments:

Post a Comment

Share your views or discuss.

 
Top