Pumping concrete

Pumping wet concrete

By Robin Harold-Barry, BSc(CEng), MICE, MASCE, MICT

of Concrete Consultancy Ltd

NOTE: In any formula used * means multiplied by and / means divided by.

Wet concrete is a suspension of solids in water. It can be moved along a pipeline by applying pressure through the water. Other concretes, which may not contain enough water, can be transported through a pipeline in a stream of air. This article does not deal with that method of pumping.

Pumping wet concrete streamlines the job. It matches the rate at which ready mixed concrete can be delivered; it releases the crane for other work such as steel fixing and formwork erection so that these jobs are not interrupted and it helps to ensure a more uniform quality of concrete. If the mix varies excessively, through some accident, the pump operator will not accept the load for fear of it blocking or over straining his pump.

The contract should ensure that the supplier and pumper of the concrete are jointly responsible for it being delivered to the job in good time and in good condition. It does not help the client for one to be able to blame the other for a error rather than co-operating to sort it out.

Concrete can be delivered fast by pump: careful planning is important. In general, it is best to start furthest from the pump and keep the live face as small as possible. Not only must the mix be designed for the required strength and durability but also for workability and cohesiveness so that it can be pumped and flow into position without segregation. A simple check for pumpability is to squeeze a handful of the concrete; if it is mobile and grout escapes between the fingers it is pumpable. (Now wash your hands! Cement attacks skin.) An alternative is to use the 100 to 200 mm diameter by 300 mm high standard slump cone. If filled in the standard way and released the cone should rise at least 5 mm off the base plate. When the cone is gently removed the concrete should slump about 75 mm, though a slump of as low as 25 mm can give a pumpable concrete.

The aggregate grading is also important. All sizes above the maximum cement particle size, 150 µm, up to the maximum size suitable for the pour and for the pipeline, should be reasonably uniformly represented but with a preponderance of the maximum size. Not more than 1/3 of the concrete volume should be particles bigger than 1/8 of the pipe diameter, and they should all be smaller than 1/3 of the pipe diameter.

It is necessary to have sufficient fine particles to avoid water flowing through the mix under pressure, causing dry patches and blockages. It has been found that the total weight of solids below 300 µm, which is the dust, silt, cement, pfa and very fine sand, should weigh at least 380, 440 or 520 kg/m³ for 40, 20 or 10 mm aggregate respectively.

A laboratory aid towards designing a pumpable mix is the void meter. The proposed proportions of dry aggregate are mixed without cement or water. The mixture is added in layers and compacted in a glass cylinder. An air tight lid is bolted on and a partial vacuum is applied via a column of water in a graduated tube connected by a flexible tube to a reservoir which lowered a fixed amount to lower the pressure. The lowering of the water level in the graduated tube indicates the percentage of air voids between the aggregate particles. Several tests are carried out to find a combination which gives minimum voids. For a mix to be pumpable there should be enough cement, or similar fines, to fill these voids assuming a cement bulk density of 1440 kg/m³. For cement to be this densely packed the water / cement ratio would have to be 0.37 or some of the water would have to be replaced by micro silica. To give an excess of water for pumpability the w /c ratio has to be higher, usually above 0.4.

The greater the w / c ratio the lower the pipeline resistance. However, with a plain concrete, a w / c ratio greater than 0.6 can lead to difficulties if pumping has to stop for some reason because a quantity of water will readily bleed to the top of horizontal sections of pipeline. When pumping resumes this water is pushed along the top of the pipe causing scouring, segregation and a possible blockage. To minimize a blockage the concrete should be moved at intervals of a few minutes. To locate a blockage, note the point beyond which the couplings do not move when pressure is applied. A line full of concrete under pressure gives a thud when hit with a hammer, whereas beyond the blockage, where there is no pressure, the pipe rings under the hammer.

Lightweight concrete can be pumped provided account is taken of the high capacity of lightweight aggregate to absorb water. One approach is to vacuum soak the aggregate but this leads to a higher density concrete if it does not dry out. Alternatively, an admixture can be used to thicken the water so that it is not forced into the aggregate so easily. Air entrainment helps to reduce the migration of water further and slightly reduce the density of the paste. Pumping pressure should be applied smoothly. The manufacturers of Lytag seem to have done the most work on producing pumpable lightweight mixes and they recommend the following:

PIPE COUPLINGS must be kept clean not only to get a good seal but also to ensure that there is no build up of material that makes it difficult to close the couplings. A sledgehammer should never have to be taken to a coupling. A sledgehammer was used in the days when joints were closed by driving a couple of wedges to give a tight fit. A modern coupling is flexible and does not need such brute force, which could over strain it and lead to failure under pressure. The backlash from the sudden failure of a coupling could knock someone off the structure to his death. Blockages can also be caused by the loss of grout through poorly maintained leaking joints.

Unlike a liquid, concrete slides through a pipeline as a plug except for an outer lubricating layer. The lubricating layer is about 3 mm thick and consists of water, cement and very fine sand particles. The coarse aggregate is pushed away from the pipe wall by the tumbling rolling action of the lubricating layer. The rolling action causes the lubricating layer to travel at only half the velocity of the plug. Therefore before concrete is pumped through a pipeline a lubricating mix must precede it to make up for the fine material left behind. Its w / c ratio should be 0.6 to 1.0 by weight. Sand is not usually necessary but on a line more than 60 m long a sand / cement ratio of up to 0.5 can be used. If the pipeline is not already damp, and especially if it is long or new, 10 litres of water should be pushed ahead of the lubrication mix. The water can, alternatively, be blown through the pipeline between two sponge balls, which also tests it for leaks while it is still clean.

The minimum number of 25 kg bags of cement required for the LUBRICATING MIX is the pipeline length (m) * the pipe diameter (mm) / 2500. This is equal to 1 kg of cement per metre of 100 mm pipe. For new or difficult conditions, it would be safer to use twice or even three times as much.

The PRESSURE REQUIRED to move concrete through a horizontal steel pipeline will be proportional to its length and to its circumference (or diameter) and inversely proportional to the cross sectional area (or diameter squared). The resistance of a flexible rubber hose is about two to six times that of a steel pipe. Since the concrete is not a liquid but a suspension of solids in liquid, a certain pressure will be required to start the concrete moving and above this the additional pressure will be proportional to velocity. Because the initial pressure is difficult to determine it will be assumed that the total pressure is proportional to velocity. This will lead to inaccuracies at very high or very low pressures. Velocity is proportional to the rate of placing and inversely proportional to the cross sectional area of the pipe (or diameter squared). Pressure is therefore proportional to d * (1/d)² * (1/d)² = (1/d)³ or inversely proportional to the cube of the diameter.

The pressure will also be greatly affected by the water content, workability or slump of the concrete. The slump is a very convenient and universal method of measuring workability but rather imprecise. However, it is such an important factor in relation to pressure that an attempt, however imprecise, has to be made to quantify it. From a study of the literature available to hand it is reasonable to say that the pressure halves for a 41% increase in slump (slump * 1.41). In other words, the pressure is inversely proportional to the square of the slump (1.41² = 2).

If the concrete is being pumped to a different level then an extra pressure will be added which is proportional to the density of the concrete and the height to which it is being pumped.

There are several other factors which could be taken into account in determining the pressure required to push the concrete through a pipeline if more was known about their effects. The following factors will affect the pressure: pipe smoothness, pipe hardness, aggregate hardness, aggregate texture, aggregate grading, aggregate size, aggregate shape, cement content, concrete temperature, any changes in pipe diameter and the number and radius of bends. If the length of the bend is included in the length of the pipeline, which it should be, then a large radius bend will cause less pressure loss than a small one. The literature does not agree on an allowance for bends, but a compromise of an extra 1 m of pipe per 90 degree bend will be used here.

Pumping experts approximately agree that the pressure required to push 100 mm slump concrete through 200 m of horizontal 100 mm diameter steel pipe at 25 m³/h is 33 bar when pumping continuously, which is 6 m per bar.

Putting all this together we get the following PRESSURE FORMULA:

P = (L+(6 * F) + B) / 6 * (100 / d)³ * R / 25 * (100 / S)² + H * D * 9.81 / 100 000 bar

where:

P bar = pressure required to push the concrete

L m = total length of steel pipeline including bends

F m = total length of flexible rubber hoses

B = the number of 90 degree bends

d mm = pipeline diameter

R m³/h = Rate of pumping concrete

S mm = slump of the concrete

H m = height to which the concrete is pumped

D kg/m³ = density of the concrete.

1 kg = 9.81 N (in earth's gravity)

1 bar = 100 000 N/m² (approximately 1 atmosphere)

The POWER REQUIRED to push the concrete, W kW, multiplied by a constant, c, to tie the units together, is equal to the pressure on the concrete, P bar, multiplied by the rate of pumping, R m³/h:

W kW * c = P bar * R m³/h

The value of c is found by converting the units used back to basic S.I. units.:

1 kW = 1000 W = 1000 J/s = 1000 Nm/s (watts, joules and Newton metres per second)

1 bar = 100 000 N/m² (Newtons per square metre)

1 m³/h = 1/3600 m³/s (3600 seconds = 1 hour)

So:

c = P * 100 000 N/m² * R / 3600 m³/s / W * 1000 Nm/s

c = 36

Therefore:

36 * W kW = P bar * R m³/h

To get the power required from the pump its efficiency must be taken into account. This can be assumed to be 0.7. In other words 30% of the power is lost within the pump. The constant we can therefore use in practice is 36 * 0.7 = 25. Instead of dividing by 25 it is easier to multiply by 0.04.

The pump power required is then:

W = P * R * 0.04 kW

The pressure formula can be inserted to give a PUMP POWER REQUIREMENT:

W = (F + (L+B)/6) * (100/d)³ * R² *0.04/25 * (100/S)² + H*D*9.81*R*0.04/100 000

W = (100 / d)³ * 2.7 ((6 * F) + L + B) * (R / S)² + H * D * R / 255 000 kW

This formula only works between limits: pumps have a maximum pressure no matter how low the output is, and they have a maximum output no matter how low the pressure is. It should also be noted that the peak rate of placing is used, not the average. On many sites the pump may only be working for 45 minutes in every hour.

The TEMPERATURE RISE of the concrete due to friction may sometimes have to be estimated. If no heat is gained from or lost to external sources, the thermal capacity of the concrete is 1.2 kJ/kgK and its density is 2380 kg/m³ then the temperature rise is:

i = (100 /d)³ * 2.7 ((6 *F) +L+B) * (R / S)² * 3600 / R / 1.2 / 2380

i = (100 /d)³ * 3.4 ((6 *F) +L+B) * R / S² degrees Kelvin

In hot climates pipelines can be painted white or draped with wet hessian to keep them cool.

Attention must be paid to safety:

A skid mounted pump should have a permanent notice indicating its overall weight.

Vehicle mounted pumps should have a notice in the driver's cab indicating the maximum traveling height of the unit. A notice near each outrigger should indicate the maximum load that could occur at the foot of the outrigger when it is fully extended.

A hinged grille on the hopper should have parallel bars not more than 60 mm apart capable of supporting a 75 kg man. Opening the grill or the valve chamber should dissipate all power to the operating mechanisms. Cleaning tools must not be passed through the grille when the agitator is operating. The placement boom should be designed to carry placement hoses 8 m long including the contained concrete, but check with the manufacturer that this is so for their pump. It is obviously important not to suspend more than the recommended length of hose from the boom. If abnormally heavy concrete is being placed by boom the maximum reach should be correspondingly reduced. The boom should never be used as a crane.

Hydraulic rams supporting booms must have lock valves so that the boom does not collapse should an hydraulic hose fail. Controls should automatically turn off when released.

Four separate gauges should indicate pressures to the concrete pumping mechanism, the valve mechanism, the agitator mechanism and the boom.

Pipe ends must be a shoulder type 17.5 mm wide and at least 6 mm high. A groove in the end of the pipe is not acceptable. There is little danger from pipe failure due to wearing through; but a coupling which slips off the end of a pipe could cause a very serious accident.

Worn pipes should be used towards the end of the pipeline where the pressure is lowest.

In a straight pipeline of constant diameter, the pressure will be proportional to the distance from the outlet. This means that, provided there is no blockage, the pipe thickness half way from the pump to the outlet needs only to be half the thickness required at the pump. The following rule of thumb is a useful guide, where t mm is the pipe thickness, P bar is the pressure and d mm is the pipe diameter: t = P * d * 3 / 10 000.

Regular turning of a static pipe will keep wear uniform. The greatest wear occurs on the invert of the pipe. When a pipe does wear through it shows as a small spurt of grout. The loss of grout causes extra friction further down the line, which increases the pressure and may eventually cause the pipe to gradually tear open at the worn hole. Pipes with dents or other irregularities will wear more rapidly and cause greater pressure loss.

PIPE SUPPORTS should cause minimum stress and movement at the joints. Each 3 m length of pipe should have one timber block or similar support 1 m from a coupling. Each vertical pipe should be supported by a chain, not a rope, or preferably a pipe clamp. The bottom bend should be adequately supported by a bracket or anchor block so that it can carry three times the static weight of the concrete above it, or alternatively, the full pressure capable of being generated at the pump, whichever is the least. It is useful to incorporate a shut off valve at the bottom of a large vertical rising pipeline to enable blockages to be cleaned out without having to empty the vertical section of pipeline. Blockages, if they occur, normally happen near the pump, usually in the valve mechanism or in the tapered section at the beginning of the pipeline. The shut off valve is a plate with a hole in one half, which slides across the pipe either under hydraulic power or by being hit with a sledgehammer.

Friction loops, a series of bends at the bottom of a descending section of pipeline, have been superseded by shut off valves as a method of preventing negative pressures and possible segregation in the pipeline. Where bends occur and the pipeline is clamped to falsework the thrust due to the surge of concrete in the following straight must be allowed for in the falsework design. The thrust, T N, can be taken as the length of the following straight, l m, divided by the length of the whole pipeline, L m, multiplied by the pressure available at the pump, P bar, divided by 10 to convert bar to N/mm², and multiplied by the cross sectional area of the pipeline: T = P * l / L * pi * d² / 40 Newtons.

SAFETY needs particular attention. Avoid sharp bends, especially kinks, in the placement hose. If a blockage occurs, reverse the pump to drop the pressure before a coupling is released. Final cleaning out of a pipeline should be done with a sponge ball and a cylinder made from old saturated paper cement bags, which are pushed through the pipeline by high-pressure water. Compressed air is potentially hazardous and should not be used. However, if, in exceptional circumstances, there is no practical alternative it should only be used under the very close supervision of a competent person. Personnel should then wear protective clothing, safety helmets and eye protectors. The placement hose and all other flexible hoses must be removed and emptied separately. A catch basket should be fitted to the end of the pipeline, which should remain properly supported. The dimensions of the saturated used paper cement bag should be such that it prevents the following sponge ball from leaving the pipe.

The possibility of concrete emerging at very high velocity due the expansion of compressed air must be considered. The direction of discharge must be such as to minimize damage. Personnel must stand well clear of the whole pipeline and they must be in effective communication with the operator. Compressed air can then be applied to the concrete via a purpose made washout gun which must have an air release cock whose diameter is about half the diameter of the pipeline. If the speed of the emerging concrete becomes excessive air must be dumped immediately to bring the velocity down to a safe level. It is important to ensure that there is no pressure in the pipeline before any couplings are released.

If a boom is used to pump concrete upwards or the pump is near the bottom of a vertical pipeline, a wet sponge ball can be inserted into the end and sucked back by putting the pump into reverse. Atmospheric air pressure pushes the sponge and the concrete ahead of it back down to the hopper, which can be discharged onto the ground or into wheelbarrows. If a boom is not being used a pour should be started at a point farthest from the pump. Pipes are then uncoupled individually and then upended to empty their contents into the pour as it progresses. Each pipe is cleaned ready for the next pour. The couplings and rubber gaskets must be thoroughly washed to remove all traces of grout to ensure a proper fit without strain.

No one but the authorized pump operator should operate or move the pump or its placement boom. Good communication between the operator and the placement gang is essential so that the pump can be stopped without delay. Pumps have been subject to many improvements since the first mechanical patents were registered in 1913. Manufacturers have now turned to hydraulics to drive concrete pumps though there are some mechanical mortar pumps. Hydraulic oil has the advantage of being able to push the piston at relatively constant velocity from the beginning to the end of each stroke. Power from the hydraulic pump is switched at the end of each stroke to the valve operating mechanism and then to the other cylinder. The short interval between strokes temporarily causes the pressure to drop in the pipeline. The pulsations that this causes in the pipeline can sometimes lead to problems with falsework, so make sure that all timber wedges are secured and that nothing can work loose. The elasticity of the pipeline and any compressed air in the concrete coupled with the inertia of the moving concrete tends to smooth out the pulsations. Difficulty has sometimes been experienced with the valve changeover when pumping air entrained concrete because the pressure in the pipeline tends to be maintained, and more power is needed to make the changeover sufficiently quickly for the concrete not to reverse its flow through the valve mechanism. Modern pumps are able to cope with this.

If a pump is to be fed by trucks with more than about 25 m³/h of concrete, access to the hopper will have to be provided for two trucks at a time because of limitations on the discharge rate of a truck. It is also important to have enough manpower to place and compact the concrete. It will depend on the workability of the concrete and the complexity of the pour, but an initial estimate is to have one vibrator for every 10 m³/h of concrete.

Pumps can be diesel or electric powered and static or sledge, trailer, lorry or truck mixer mounted. Each manufacturer has his own valve design, and each proclaims special advantages for their own make. Placement booms can be attached to lorry mounted pumps or mounted on a tower in the center of a structure to give increased cover. Hand operated distributors are also available which are attached to the end of a pipeline and consist of two vertically hinged counterbalanced pipes. They enable concrete to be placed where required more easily by one man than by two men manhandling a rubber placement hose.

Summary checkpoints: CONTRACT - tie up between the supplier and pumper. FALSEWORK- design for pulsations, secure wedges, clamp vertical pipes, chock horizontal pipes, anchor bends. FORMWORK - design for high pouring rate, grout tight. PIPELINE - laid to far end of pour, couplings clean and secure. PUMP - adequate capacity for the pour, located for easy access by two trucks, in good working order. BOOM - adequate reach, extend outriggers onto strong support. CONCRETE - check for pumpability, enough cement for the lubrication mix. clean up - water under suitable pressure, air if necessary. ACCESSORIES - wash out gun, wash out balls, saturated used cement bags, ball catch basket, shut off valve, first aid. SUPPLY - start time, rate of delivery, communication links, compaction equipment, fuel for pump and vibrators.