Pumping up the right pressure
Jurgen Kronenberg elaborates for why Putzmeister's truck-mounted pumps come with a high 85 bar pressure rating.
01 September 2003
Putzmeister truck-mounted concrete pumps have a pressure of 85 bar as standard, to provide vital performance reserve and to offer operators full use of compressive resistance of the delivery line.
As operators most often are unaware of how much boom reach is required until on site, Putzmeister's pumps - which offer on average 25 bar more pressure than other manufacturers - make them better equipped.
Different concrete consistency
Concrete consistencies can vary even for the same order, either due to the type of aggregate or because of the mixing method. If a readymix operator sends a truck filled with 2.5 cm slump instead of the ordered 6 cm, this 15 per cent stiffer mix will require 100 per cent more pumping pressure if it has to be delivered at 80 cu m/hr over a distance of 50 m.
Line extension and reduction
The same is true when concrete mix is pumped through 150-mm-diameter additional delivery lines laid on the ground. If a concrete mix of a slump of 8.5 cm is to be pumped at 55 cu m/hr through 125-mm-diameter delivery lines, which are laid and connected over a conveying distance of 200 m, then the pressure on the concrete is 43 bar. If the slump deviates by just 10 per cent (6 cm), then a pump with insufficient pressure with encounter problems. When connected to a vertical line, the problem is much worse, and an additional one bar pressure is required for every 4 m increment in height. If the delivery line is further reduced to a 80-mm diameter, the pump pressure required rises considerably.
For high-strength concrete
Things can become critical while conveying high-strength concrete, if insufficient conveying pressure is available because the delivery output must be reduced. A C60/75, for example, with a compressive strength of 70N/sq mm only has a W/C value of approximately 0.3. The pumpability only comes after adding plasticiser as well as a high percentage of finest material and cement. This results in a higher line resistance than while pumping a C35/45.
For self-compacting concrete
Higher pressures are required for self-compacting concrete - especially when the material has a high compressive strength - despite a spread figure of , say, 70 cm because of the extremely low water content, which is necessary to lubricate the line.
Also, concrete with a grading curve on the border of what is pumpable may require considerably higher pressure reserves than what had been assumed when the order was received. The same is also true when conveying heavy-duty concrete. In addition, every operator knows how valuable additional pressure reserve is when the concrete in the line 'staggers'.
Calculating real conveying current
PM truck-mounted concrete pumps' data on maximum conveying pressure is found on the model plate, in the technical documentation and on the respective data sheet. The effective pressure required can be arrived on the basis of the calculation:
P (Pressure on concrete) =
P1 (oil pressure):I (transmission ratio)
The hydraulic ratio is determined by the surface ratio between the delivery pistons and the hydraulic pistons. With pressurisation on the rod-side of the hydraulic system, the transmission is greater than for head-side pressurisation. This means a greater output but a lower pressure on the concrete. The easiest way to determine conveying pressure is to read the high pressure gauge on the machine and then divide the hydraulic pressure by four (the transmission ratio for PM pumps).
Sufficient output
85 bar concrete pumps have a further advantage with respect to the effective rate of placing as the performance diagram below shows. The performance restriction of the concrete pumps for a Putzmeister BRF, is 180 kW. Drive performance is important to enable larger outputs with corresponding high pressure reserve.
