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UNITED EQUIPMENT SALES, INC.
(800) 742-2070
Concrete Mix Guidelines - Small Line Concrete Pumps
Uniform gradation of concrete sand and aggregate is extremely important to good pumpability of all concrete mixes. (Refer to ASTM standard specifications for grading aggregates in your Mayco pump manual). In some areas, the sand may be basically coarse and lacking in fines in the #50 - #100 - #200 screen sizes. If this is the condition (called gap grading), it is likely that jamming or blocking due to separation will occur in the pump outlet. It may be necessary to add blending sand, blow sand, mortar sand, fly ash, extra cement, or some other material to make up for the deficiency of fines.
Separation and blockages are normally caused by poor gradation, assuming that sand/rock ratios and cement/water ratios are controlled per these recommendations.
Your local sand and rock supplier, or job engineer, will compute the weights required for thee sand/rock ratio shown above. They will also recommend the amount of cement and water necessary to meet your strength requirements. If ideal gradation of material exist, the above sand/rock ratios may be adjusted less sand to more rock. Sand and cement mixes (without rock) can be pumped by increasing the cement content; e.g., some wet-gunning applications, cellular foam concrete, cement slurries used for pressure grouting, tunnel liners and mud-jacking.
The Concrete Producer
What makes concrete pumpable?
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Question:
Contractors often request us to provide a pumpable mix. When the mix proportions aren't specified, what should we consider when designing a mix to be pumped?
Answer:
A pumpable mix must slide along the wall of the pipeline and be able to flow around bends. The mortar acts as the lubricant, so concrete mixes for pumping must be plastic, not harsh. Give particular attention to the mortar and the amount and size of coarse aggregate. Pumping is sensitive to variations in gradations and in batch proportions. After you develop a pumpable mix, quality assurance is needed to ensure repetition of all the factors that produce the mix. Coarse aggregate. The amount of coarse aggregate used in the mix depends on the shape and size of the aggregate. The maximum size of an angular coarse aggregate is limited to33% of the smallest inside diameter of the pump of the or pipeline. For well rounded coarse aggregate , the maximum size can be up to 40% of pipe diameter. As with most aggregates in concrete, coarse aggregate must meet ASTM 33. Preferably, the gradation will be as close to the middle of the range as possible. However, grading uniformity on a daily basis is most important. The maximum size of the coarse aggregate has a significant effect on th amount that can be used. The quantity of aggregate per unit volume must be reduced as the maximum size is reduced. For example, ACI 211.1, "Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete," recommends using 69% of 1-inch-diameter coarse aggregate versus only 48% of 3 _ 8-inch-diameter coarse aggregate when using fine aggregate with fineness modulus of 2.60. ACI 211 notes that this quantity of concrete, which my be required when pumping. When absorptive aggregates are used, especially lightweight materials, pumping pressure may force mixing water into aggregate pores. The resulting slump loss reduces pumpability. Presoak the aggregate stockpiles for at least three days to minimize this problem. Turning the pile over with a front end loader helps to ensure uniform presoaking. Fine aggregate. Fine aggregate, cement, and water create the paste that lubricates the coarse aggregate and holds it in suspension, making the mix pumpable. Fine aggregate properties have a greater effect on pumpability than do coarse aggregate properties. Fine aggregate gradation should also conform to ASTM C33 requirements. ACI 304.2R, "Placing Concrete by pumping Methods," suggests that particular attention should be given to those proportions passing the finer screen sizes.
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For pumplines less than 5 inches in diameter, 15% to 30% should pass the No. 50 screen and 5% to 10% should pass the No. 100. If the smaller sizes are missing, small quantities of materials such as crusher dust, wash pit settlings, fly ash, and beach or dune sand will often improve pumpability. To meet ASTM C 33, the fineness modulus of the sand should be between 2.30 and 3.10. To meet ASTM C 33, the fineness modulus of the sand should be between 2.30 and 3.10. Concretes made with fine sands (lower fineness modulus) usually pump increase the water and cement requirements and may produce sticky mixes that increase the pressure needed to pump. The gradation recommended in the graph is a guide to selecting a suitable fine aggregate. The upper and lower limits of ASTM C33 are shown with dotted lines, and the solid line represents the recommended gradation. This gradation meets the requirements of ASTM C 33 and the percentages passing the No. 50 and No. 100 screens are within the limits previously stated. In practice it may be difficult to duplicate this gradation; however, a gradation near the upper limit will be more desirable than one towards the lower. Low-cement-content concrete made with coarse sands, or concrete made with gap-graded sands, bleed excessively and are likely to cause line blockages. Water and slump. Establish the optimum slump and maintain this slump throughout the pumping portion of the job. Slumps below 2 inches and above 6 inches may cause pumping problems. Depending on the pump used, mixtures with low slumps may not feed well from the hopper into the pumping chamber. When high slumps are produced by adding water only, segregation may cause pumpline blockages. Cement content. Choosing the cement content for pumpable concrete follow the same principles used for any concrete. Don't fall into the trap of using extra cement as the only cure for pumping deficiencies. This is both uneconomical and shortsighted. Instead, pay close attention to the coarse and fine aggregate gradations. Admixtures. Many admixtures will improve pumpability. Some of the admixtures commonly used are:
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Water reducers, Including superplasticizers
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Air-entraining agents
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Pozzolans A full scale test to ensure pumpability is always recommended when designing a pump mix.
During a test, duplicate job conditions as closely as possible. Reference ACI 304.2R, "Placing Concrete by Pumping Methods," American Concrete Institute, Farmington Hills, Michigan.
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SAFETY BULLETIN:
BLOCKAGES IN HOSE & PIPELINE
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Blockages in a pipeline can occur in many places and for many reasons. Blockages will occur (by order of likelihood) in reducers, hoses, elbows and pipe. A blockage can create a dangerous situation if not dealt with correctly. Certain procedures must be followed when blockages occur:
- When a blockage occurs, immediately stop the pump, then pump in reverse at least two strokes to relieve pressure.
- Clear the immediate area of the pipeline of all non-essential personnel.
- Locate the blockage by tapping the system lightly with a hammer. A dull thud usually indicates the location of the blockage.
-Striking the system too hard with a hammer will cause damage and make the system wear faster.
- Before removing any couplings, reconfirm that you're wearing all necessary personal protective gear.
- When removing couplings, make sure to look away, since some pressure may fill still exist inside the pipeline
- Remove the coupling just ahead of the blockage with a shovel.
- Then remove the coupling behind the blockage with a shovel.
- Clear the blockage by holding in blocked section vertically, with the end that was closest to the pump downward. Tap lightly on a reducer or pipe. With a hose, tap lightly while the hose is suspended vertically until emptied. Pipe will ring and hose will sound hollow when emptied. Look through the hose, pipe or reducer to ensure it is clean before reattaching.
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- When the blockage is removed, reattach the empty section. Then slowly pump a small amount of concrete through.
- Reattach the rest of the system, remind co-workers to stay a prudent distance from the discharge, and resume pumping slowly, watching the pressure gauge for signs of more blockages.
- When a section or two are removed and emptied, air is introduced into the system and can be expelled with considerable force. Coworkers can return to the discharge hose only after all air has been removed, concrete is flowing smoothly and you have given them permission.
GENERAL RULES:
1. If pump pressure builds gradually, then goes to relief, you probably have a mix or priming problem. (Segregation, concrete too dry, insufficient price, etc.) Tap on the system to find how far the concrete traveled.
2. If pump pressure suddenly goes to relief, you probably have a foreign object. Look in reducers and elbows first.
3. NEVER TRY TO REMOVE A BLOCKAGE WITH PUMP PRESSURE. Try "rocking" the concrete. Turn the pump on at low volume in reverse, then stroke forward slowly and watch the pressure gauge. Do not allow the pump pressure to go to relief. If pressure starts to build again, stop pumping, reverse pump at least two strokes, and repeat the procedure. If it still won't go, locate and remove the blockage manually.
4. Many blockages occur while priming. Always use proper prime and in sufficient quantities. Blockages are costly and annoying to
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the customer, and potentially dangerous. Sometimes they result in back charges to the pumping company. It is easier, safer and cheaper to deal with excess prime than to deal with a blockage. Ask your supervisor about how to prime properly in various circumstances. Sometimes it is necessary to have prime delivered by the ready mix supplier.
5. Blockages are stressful situations. Remain calm and take control of the situation. YOU are the expert. Keep unnecessary persons away from the area. Make sure that all assisting personnel agree to follow your orders. You may be held responsible if you could have prevented someone's injury.
No attempt has been made to cover all situations with the guidelines shown above. Concrete pumping can be dangerous if not performed with reasonable care. The use of experienced personnel, a thorough understanding of safety rules, and common sense are necessary for a safe operation.
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7606 NE M.L.K. JR. BLVD., PORTLAND OR 97211
EMAIL:
(800) 742-2070 or (503) 283-2105
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