On many jobs several different methods, or even a combination of methods may be required. In all cases the concrete must be moved to the point of placing as quickly and economically as possible without allowing segregation, loss of any constituents, contamination with water or any other material after it has left the mixer.
Pumping
Pumps were first used to transport and place concrete in this country in the 1930s, and their use has grown to the extent that some 20% of concrete is now placed in this way. Many pumps are capable of moving up to 100 m3 per hour, depending on the pump type, the horizontal and vertical length of the pipeline, the number of bends, and the consistence of the concrete. In practice, the output is usually about 30 m3 per hour due to supply and organizational limitations. Most pumps can transport concrete more than 60 m vertically or 300 m horizontally (shorter distances when pumping both horizontally and vertically). Some high- pressure pumps have achieved heights in excess of 400 m and horizontal distances greater than 1000 m.
Most mobile pumps can place concrete directly to where it is required (Figure 15), removing the need for other forms of transport and pumping is particularly beneficial when access is difficult or restricted. Standard boom sizes are 16 m, 22 m, 24 m, 32 m, 40 m and 52 m whilst greater distances require a static pipeline which bypasses the boom altogether. There is usually little or no waste. Labour costs are generally minimized since only one person is usually needed to place the concrete, with others compacting and finishing, although the workforce must be adequate to cope with the fast rate of placing. For high-rise construction, pumping permits placing rates to be maintained regardless of the height, without any increase in labour costs. In order to make best use of the pump, the concrete mix design may have to be adjusted to make it suitable for pumping without using excess pressure. Essentially the concrete should not be prone to segregation or excessive bleeding and should have a low enough frictional resistance for the pump to be able to push the concrete along the delivery line. It is sometimes necessary to increase the sand content by a few per cent, along with an increase in the cement content, in order to provide sufficient fine material and to achieve an overall aggregate grading that is continuous and without gaps.
In this context the consistence of the concrete is an important factor, since a very low consistence class may result in increased resistance to pumping. A target slump of 70 - 90 mm is generally considered to be about the right level and the addition of a plasticizing admixture avoids a higher free water/cement ratio.
Discussion at an early stage with the ready-mixed concrete supplier and/or the concrete pumping subcontractor is therefore recommended in order to ensure that a satisfactory pumpable concrete is obtained.
For trouble-free pumping the concrete must be consistent, since minor variations in the mix proportions are sufficient to make an otherwise pumpable concrete difficult to pump or even completely unpumpable. If only a small part of a load proves to be unpumpable the pump may become blocked, leading to a time- consuming and expensive delay while the pump and/or line is stripped down and the blockage removed.
In order to make the most economical and efficient use of pumping, it is important to understand that the decision to utilize a concrete pump ought to be made at the planning stage of a project. Bringing the pump onto site to solve placement problems, when other alternatives have proved unsuccessful, is likely to result in additional costs.
Each site where pumping is proposed will require careful individual planning but a number of considerations, likely to be common to all sites, can be identified. The concrete placing gangs must be able to cope with the pump output for the duration of the pour, without skimping on compaction, finishing or curing. The pump should not be allowed to stand idle waiting for concrete to be delivered and a steady supply of concrete to the pump should be planned, consistent with the rate of placing which can be accommodated. For large or important pours, standby pumps should be arranged. The siting of the pump should be such that delivery vehicles have easy access and two vehicles can be accommodated at the pumps so that the second one can start discharging as soon as the first one finishes, maintaining a continuous flow of concrete. The choice of pump location should also take into account the need to keep pipelines as short and as straight as possible. Good communication between the pump operator and the placing gang is essential. Most mobile concrete pumps are fitted with folding booms, which are an advantage for placing concrete in difficult situations. However, a boom is not necessarily the best method of placing. There are circumstances where a static pipeline is better, this system having less resistance to flow than a delivery boom of the same diameter.
The pour should be planned so that pipes may be removed as it progresses - pipes should never be added. All couplings should be completely free from leakage, otherwise loss of fine material from joints is likely to result in problems due to blockage. In hot, sunny weather, it may be necessary to protect the pipeline from overheating. In such conditions, concrete in the pipeline must be kept moving.
Concrete in pump pipelines is often under considerable pressure, so that the safety of site staff must be considered. The pump should be stopped, and if possible reversed, while pipes are being disconnected. Flexible end sections of pipes may move violently when a cleaning plug is passed through and operatives should be kept well clear. Falsework should be designed to accommodate the vibration and additional loading caused by pipelines resting on it.
Crane and skip
The crane is still probably the most common method of handling concrete for combined vertical and horizontal movement. A crane is frequently needed on site for handling formwork and reinforcement, and its further use in transporting concrete may be both economic and convenient. However, when concreting requirements dictate the choice of crane capacity, or if the crane is likely to be fully occupied with other tasks, it may be more economic to transport the concrete by other means.
Skips generally range in capacity from 0.2 to 1.0 m3 (but there are many variations in detail), and are of two broad types (Figures 16 and 1 7):
n Lay-back or roll-over skips
n Constant attitude skips.
The openings of skips should be large enough to allow easy discharge and the consistence class needs to be adequate to allow for controlled discharge into difficult sections. When concrete with a low consistence class is placed by skip, poker vibrators may have to be used to assist discharge.
Skips should be properly maintained if they are to function efficiently. After a day's concreting the skip should be thoroughly cleaned and washed down and the gate operating mechanism should be oiled and greased. A build-up of hardened concrete on the outside of the skip may be prevented by rubbing it over, before it is used, with a light coating of oil or chemical release agent to prevent adhesion.
Dumpers
Dumpers, generally of about 0.5 m3 capacity, are a common form of transport on many construction sites. They may be discharged either forward or sideways and are best when hydraulically operated so that the discharge can be controlled. The main disadvantage of gravity discharge is the sudden uncontrolled surge of concrete - the heavy impact can displace reinforcement and other objects in the formwork. For small sections it may be necessary to discharge onto a banker board first and then shovel in the concrete by hand.
If the haul routes are so long that segregation becomes a problem, agitator trucks, or lorry-mounted transporters fitted with screws or paddles to remix the concrete as it is discharged, may be preferred.
Other methods
There are several other less common methods of transporting concrete, including pneumatic placers, monorails and the railcars sometimes used in tunnel work, which are not covered in this publication.