Specifications often contain clauses dealing with the transport of concrete, requiring re-mixing after transport beyond a certain limit, limiting the height through which concrete can be dropped, and requiring no concrete be placed when more than a certain time has elapsed since mixing. In practice, problems of this sort seldom prove significant. Sometimes it may be necessary to insist that a contractor uses a closed chute to discharge concrete through a height in order to prevent segregation. Also it may be desirable to ensure mixed concrete is not left unplaced for over-long. A requirement often found in specifications is that concrete must not be placed after it reaches its ‘initial set’ which, for ordinary Portland cement concrete may take place 1–2 h after mixing, dependent on temperature, etc. However, a hardening on the outside due to surface drying can occur after about half-hour’s standing, especially in hot weather.
If this concrete is ‘knocked up again’ and shows it can be satisfactorily placed it need not be rejected. On the other hand, if a delay is so lengthy that the concrete hardens into lumps, such concrete must be discharged to waste.
Pumped concrete usually poses more problems for the contractor than it does for the resident engineer, since only well graded mixes relatively rich in cement are pumpable. Usually several mortar batches must be sent through the pipeline to ‘lubricate it’ before the first batch of concrete is pumped through, and pumping must thereafter be continuous. It is not easy to pump concrete more than 300–400 m. If a stoppage of the flow of concrete occurs for any reason, the contractor has to take swift action to prevent concrete solidifying in the pipeline. Compressed air is used to force the final concrete batch through the line, followed by water to clean the pipes. Plasticizers are frequently used in pumped concrete; these increase its workability without requiring increased cement or water. There are a wide variety based on different chemicals; BS 5075:1982 gives their main characteristics, but they should not be permitted by the resident engineer except to the extent allowed in the specification or sanctioned by the engineer.
Concrete can also be blown through a delivery pipe using a blower or compressed air. One batch at a time is blown through. The end of the delivery pipe must be directed into the area to be concreted, not against formwork which may be dislodged by the force of the ejected concrete. Proper warnings must be given to personnel before each ‘shot’ because aggregate can rebound and be dangerous, especially when blowing concrete into closed spaces such as the soffit to a tunnel lining.
The skip method of placing concrete is widely used. Skips can be either bottom-opening, or tip-over. In either case there can be a considerable bounce and sway of the skip when the concrete is discharged. The work should always be under the charge of an experienced ganger who keeps a continuous watch over the safety of his men.