Pumped concrete moves as a cylinder riding on a thin lubricating film of grout/mortar on the internal diameter of the pipeline. Before pumping begins, the entire pipeline’s interior diameter must be coated with either grout or a specialized commercial primer using the methods for 100% coating of the pipe walls as recommended by the manufacturer. Once concrete flow through the pipeline is set up, the lubrication will be maintained as long as pumping continues with an appropriately proportioned & consistent mixture.
A steady supply of pumpable concrete, defined as a mixture that is necessary for satisfactory pumping. A pumpable concrete requires good quality control; that is, it is uniform, has appropriately graded aggregate, and its materials are uniformly batched and mixed thoroughly.
Site Planning and Precautions for Pumped Concrete
The following points must be taken care of by site personnel if successful pumping of concrete is to be achieved.
- The concrete mix must be so designed that it satisfies the pumpability requirements and also the specifications.
- The total quantity of good quality materials required must be readily available and delivered at a required rate.
- The pipeline layout must be fixed with a minimum of bends and should be installed on firm supports.
- An appropriate signaling system should be set up between the pump operator and the concrete placing crew at the discharge end of the pipeline.
- Pump location must be on the firm ground having proper drainage facilities. However, modern pumps do not need any water flushing on a permanent basis. Cleaning out water is still essential but not necessary at the pumping location.
- Access for truck mixers should be clear from all obstructions and on firm ground. Access must be made available on both sides of the pump for better efficiency and output.
- The flow of water for washing the pump and pipeline must be maintained.
- Maximum horizontal and vertical distances to be pumped must be calculated in order to fix the mix consistency and other parameters.
- The initial grouting for lubricating the pipeline must be done to avoid choking in the initial stages of work. Often a grout containing 50 kg cement along with 25 kg sand and having a good consistency is preferred for the 20m length of the pipeline. If the pipeline is inclined or vertically down it is essential to place a sponge ball in the pipeline before introducing the grout. This helps in lubricating the entire surface of the pipeline.
- Separate areas for concrete delivery trucks to be washed should be provided to avoid slush formation around the pump.
- The pumping rate should be compatible with both the production rate and the rate that the placing gang can cope with. Increased outputs can often be achieved by increasing the number of men in the placing gang, compacting gang can, and finishing gangs where applicable.
- An experienced person should direct the flexible hose at the placement point.
- Concreting should be started at the farthest point from the pump and work should proceed by detaching a length or two of the pipelines as concreting proceeds.
Additional Points to be Noted for Pumped Concrete
- Proper planning is required when concrete is to be pumped for small structures as wastage will offset the economy. Concreting must be planned for a number of columns at a time especially when it is to be done at high elevations. It is, at times, possible to coordinate the work in such a way that concrete from the emptied pipeline at ground level can be placed as plain concrete (leveling course) or used in reinforced concrete works such as small pile caps, pedestals, footings, or precast units. At times, the wastages may even be up to 1m3 to 2m3 depending upon the size and length of the pipeline.
- If pumping is to be done only horizontally, concrete wastage can be reduced to almost zero by correctly calculating the concrete requirements and volume of concrete in the pipe.
- If the concrete is to be placed in narrow sections such as thin beams, walls, fins, etc., the forms should be fitted with chutes to prevent wastage by spilling. The reinforcement must be planned and arranged in such a manner that it does not interfere with the lowering and lifting of the hose pipe. The concrete must fall freely to avoid segregation and honeycombing.
- If concrete is to be placed in deep, densely reinforced structural members, it should be placed through pipe chutes having rigid and strong funnel-type arrangement at the top. These chutes should be placed at preplanned locations and must allow concrete to fall freely without striking horizontal reinforcement bars.
- Kinks in flexible pipes must be avoided. A flexible hose should be used only at the end of the pipeline and not in between. Pipe joints should be perfectly sealed and made leakproof.
- The pipeline should have a slight slope towards the pump so that the lubrication mix covers the entire pipe surface.
- In case of high vertical rises, the distance between the concrete pump and the vertical pipeline should be about 10% to 15% of the maximum height, so that the concrete in the horizontal pipeline creates a counterbalance to the concrete in the vertical pipeline.
- If there is an unforeseen stoppage at the placing end or at the batching plant, a single stoke should be given to the pump every 5 minutes to 10 minutes to prevent choking within the pipeline. Depending on the ambient temperatures, concrete temperatures, and the mix consistency, stoppage up to even one hour can be taken care of in this manner without choking the pipeline.
- It is a must to design the formwork for the maximum expected concrete pressure as the concrete build-up is much faster than in the conventional method. To cater to pump concreting the formwork needs extreme care while designing, or else bulging, twisting, or even snapping of certain members may occur during concreting, resulting in stoppages, poor surface finish, and time-consuming repairs. The pumping rate should not be reduced because of poor formwork, but the design of the formwork should be done to cater to the pumping rates.
- For slabs and beams the falsework or supporting systems have to be rigid enough to withstand the impact of concrete falling from the placing boom onto the formwork. Live load generally assumed has to be doubled in case or pumped concrete or impact factor of 1 t/m2 of the slab has to be taken into account while designing slab formwork.
- Reinforcement layout must be planned at the design stage itself and openings big enough for the entry of the concrete hose must be kept at predetermined locations. The concrete must have a free fall and not an obstructed fall. In deep beams, concrete should be dropped at one end and vibrated in such a manner that it flows below the main bars at the bottom. As the speed of placing is extremely fast, any obstruction against the flow can cause segregation of concrete. Clear cover to reinforcement for structures like columns and deep beams should preferably be 50mm.
- The compaction rate should match the rate of concrete pouring. Vibrator needles and motors to be used must be checked before concreting. This equipment must be available in large numbers, depending on the rate of the pour, type of structure, reinforcement layout, and total time or quantity of the concrete pour.
- It is often observed that the controlling factors for the speed of concreting are not the mechanized equipment but the problems at placing points and the compaction speed. Hence, it is desirable to have more vibrators than in the case of conventional concreting; or else the speed of concrete pumping will have to be reduced and the pumps would be used below their rated capacities.
- Using a truck-mounted concrete pump with placing boom requires very little break-out or set-up time. The boom can be moved up and out, quickly, delivering concrete to most hard-to-reach spots. Shorter placing booms up to 24m horizontal reaches are mounted on a two-axle truck which is less expensive and more fuel-efficient than tandem-axle models.
- Longer booms up to 60m vertical reaches are also available on 6 axle trucks. Articulating or telescoping designs allow the boom to unfold and place concrete straight out, straight up, or downward. Remote control of the boom is an important feature of many models wherein boom movement can be controlled from the placing point itself.
- The reach of boom pumps can be further extended by attaching the boom to the pipeline system set up at the job site. However, the boom should not be moved until the extension is removed. The pump pressure then determines the maximum horizontal and vertical distances that can be achieved.
- There is a tendency to over vibrate pumped concrete or push the concrete into its desired position using a vibrator. This causes segregation and mortar surfaces at the top later causing plastic shrinkage cracks.
- These shrinkage cracks on slabs, especially in warm/hot weather, can be prevented by the following methods.
- Use concrete mixes of good cohesiveness and at the same time minimize the cement content by appropriate admixtures.
- Use the largest possible aggregates which can be permitted in the structure. The maximum aggregate size should be either one-third of the slab thickness or three-fourths minimum clear spacing between reinforcing bars whichever is smaller.
- Use aggregates that are well graded.
- The use of retarders will help in easy placement and compaction of concrete but will increase the risk of plastic shrinkage cracking so good curing must follow soon after concrete finished surface shows signs of the initial set.
- Avoid concrete placement in a hot, dry, and windy atmosphere.
- Avoid over compaction as over compaction causes fine materials to rise up and heavier material to settle. Fine materials at the top are more susceptible to shrinkage.
- The use of chilled water will greatly help the concrete compaction, finishing, and reduce shrinkage.
- Revibration and refinishing will reduce plastic shrinkage.
- Curing is the key to the reduction of plastic as well as drying shrinkage. In Indian conditions, it can be best carried out by covering the slab with a plastic cover and flooding water between the slab and the cover. Protecting the surface against windy conditions and humidity reduces shrinkage. Curing in hot weather must start within few hours of finishing.
Blockages While Pumping Concrete
There is always a fear at the site that blockages will occur in the pipeline and work will be thrown out of gear. Blockages seldom occur due to defective machines. They most often occur due to defective mix, defective pipeline, or due to human failure.
The main reasons for blockages are:
- Defective concrete mixes
- Failure of the pump
- Defective pipelines
- Inadequate mixing
Pump Operator’s Check list for Pumped Concrete
The pump operator’s check list includes the following:
- Arrive at the site on time.
- Set the pump as near the job or construction site as possible.
- Ensure the two truck mixers can be positioned at the pump on either side.
- Ensure that all outriggers are on firm ground and are locked.
- Take care of overhead obstructions, scaffolding, power cables, and telephone wires while operating the boom.
- Ensure that the entire pipework is firmly supported, allowing for vibration and movement which may occur during pumping.
- Ensure that the hopper grill is in position.
- Take proper care when truck mixer back-up towards the pump.
- Have a visual check on the concrete before allowing the truck mixer to unload into the hopper. If the mix isn’t all right get it adjusted before unloading.
- Check whether lubricating grout is first placed & is sufficient before concrete is unloaded.
- Ensure that the persons handling the hose pipe are experienced.
- In case of difficulties or delay, inform the engineer concerned or the foreman on the construction site.
- Do not move the pump without having the boom folded together and resting on the boom supports and bring back the outrigger to its original positions.
- During the “blow-out” operation make sure that no person is standing in from of the pipe. Use safety devices available or blow out.
- At the end of the job, ensure proper cleaning of the pump, pipeline, and placing boom.
- Ensure that all power points are properly protected from water/rains. When working on the pump, ensure that power is isolated.