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Abstract: This paper summarizes several common causes of pipe blockage and the preventive measures against them. In actual production processes, due to changes in external conditions, there are often other factors that can lead to pipe blockage. But as long as we follow the operating procedures strictly, take preventive measures, and continuously learn from experience and lessons gained from each blockage, we can certainly minimize the likelihood of blockages. Keywords: pumped concrete, pipe blockage; 1 Improper operation can easily lead to pipe blockage; 1.1 Inattention on the part of the operator – The operator of the pumping unit must remain focused during pumping operations, constantly monitoring the readings on the pressure gauge. If a sudden increase in pressure is detected, the pump should be reversed for 2-3 strokes before being operated in the forward direction again, thereby resolving the pipe blockage. If back-pumping (several operation cycles with the pump in forward mode) has not resolved the pipe blockage, the pipe should be removed and cleaned promptly; otherwise, the blockage will worsen. 1.2 Improper selection of pumping speed: When pumping, the choice of speed is crucial; operators should not prioritize speed at all costs, as rushing things sometimes leads to poor results. During the first pumping operation, due to high pipeline resistance, pumping should be carried out at a low speed; once pumping proceeds normally, the speed can be increased appropriately. When signs of pipe blockage appear or the slump of a batch of concrete is low, pumping at a low speed should be used to prevent blockages from developing. 1.3 Inappropriate control of the amount of residual material: During pumping, the operator must constantly monitor the residual material in the hopper; this material should not be below the level of the mixing shaft. If there is too little residual material, air can easily be drawn in, leading to pipe blockages. The material in the hopper should not be piled up too high; it should be below the protective barrier to facilitate the timely removal of coarse and oversized aggregates. When the slump of a batch of concrete is low, the remaining material can be kept below the mixing shaft, above the \"S\" tube or the inlet, in order to reduce mixing resistance, swinging resistance, and suction resistance. These regulations apply only to the “S” valve series of concrete pumps. 1.4 When the slump of concrete is too low and improper measures are taken, if it is found that a batch of concrete has such a low slump that it cannot be pumped, the concrete should be drained from the bottom of the hopper promptly. Attempting to force pumping in order to save time can easily lead to pipe blockages. Never add water and stir in the hopper. 1.5 Excessive downtime: During periods of shutdown, the pump should be started every 5–10 minutes (the exact interval depends on the temperature on that day, the slump of the concrete, and the initial setting time of the concrete) to prevent pipe blockage. For concrete that has begun to set due to prolonged downtime, it is not advisable to continue pumping it. 1.6 The pipes are not cleaned properly. If the pipes are not cleaned after the previous pumping operation, it will lead to blockages during the next pumping process. Therefore, after each pumping operation, the delivery pipeline must be cleaned thoroughly in accordance with the operating procedures. 2 Blockage of pipes due to pipe connection reasons: Incorrect pipe connection methods can easily lead to pipe blockage. The following principles should be followed during takeover: When arranging the pipes, they should be laid along the shortest path, with as few bends as possible and large bend radii, in order to minimize transportation resistance and thus reduce the likelihood of pipe blockages. At the pump outlet cone, a bend should not be connected directly; at least 5 mm of straight pipe must be connected first before introducing the bend. When connecting pipes during pumping, only one pipe can be added at a time; it is also necessary to lubricate the inner wall of the pipe with water and remove any air from it, otherwise blockages in the pipe are very likely to occur. For vertically downward pipes, an anti-segregation device should be installed at the outlet to prevent pipe blockage. When pumping at high heights, the length of the horizontal pipes should generally be no less than 15% of the length of the vertical pipes, and a pipe shut-off valve should be installed in the horizontal pipes. When the downtime exceeds 5 minutes, the shut-off valve should be closed to prevent concrete from flowing back and causing pipe blockage. For the 90-degree elbow when changing from horizontal to vertical, the bending radius should be greater than 500 mm. 3 Blockage of pipes caused by segregation of concrete or mortar. When concrete or mortar comes into contact with water, segregation easily occurs. Sometimes, pipe blockage occurs when pumping mortar; this is due to the separation of the mortar after it comes into direct contact with the water in the pipes. To prevent this, the pipes should be moistened with water before pumping, the pipe connections at the lowest point of the pipes should be loosened to allow all excess water to drain out, or a sponge ball can be placed inside the pipes after pumping water but before pumping the mortar, thereby separating the mortar from the water. When cleaning the pipes after pumping is complete, a sponge ball should also be placed to separate the water from the concrete; otherwise, it is very easy for the pipes to get clogged. 4 Blockage of pipes caused by local mortar leakage: As the mortar leaks, it affects the quality of the concrete; moreover, after the leakage, the workability of the concrete decreases and the pumping pressure is reduced, which in turn leads to pipe blockage. The main reasons for grout leakage are as follows: 4.1 Poor sealing at the joints of the delivery pipes – Grout leakage occurs when the seals at the joints of the delivery pipes are not tight, the pipe clamps are loose, or the sealing rings are damaged. At this point, the pipe clamp should be tightened or the seal ring replaced. 4.2 Excessive gap between the lens plate and the cutting ring: When the lens plate and the cutting ring are severely worn, the gap between them increases. When the gap is greater than % --, it is necessary to adjust the special-shaped bolts in order to reduce the gap between the lens plate and the cutting ring; if adjustment is no longer possible, the worn parts should be replaced immediately. These regulations apply only to the “S” valve series of concrete pumps. 4.3 If the concrete piston is severely worn, operators should regularly check whether the water in the tank is cloudy and whether there is any mortar present. If the water becomes cloudy or contains mortar, it indicates that the concrete piston is worn out; in such cases, the piston should be replaced promptly. Otherwise, leakage of mortar and pressure loss can lead to pipe blockages, and it will also accelerate the wear of both the piston and the delivery cylinder. 4.4 Grout leakage caused by severe wear of the concrete delivery cylinder: If the water in the tank becomes cloudy quickly after each piston replacement, and the piston is in good condition, it indicates that the delivery cylinder is worn out, and it needs to be replaced at this point. 5 Pipe blockage caused by substandard pumped concrete: The concrete used for pumping must meet the requirements for pumped concrete; not all concrete is suitable for pumping. Substandard pumped concrete will accelerate the wear and tear of pumping equipment, and often leads to problems such as pipe blockage and pipe rupture. 5.1 Excessively high or low concrete slump The degree of concrete slump directly reflects the fluidity of the concrete, and the resistance to transporting the concrete decreases as the slump increases. The slump of pumped concrete is generally in the range of 8 to 18 cm; for pumping over long distances and at great heights, it needs to be strictly controlled at around 15 cm. Too low a slump degree will increase conveying pressure, accelerate equipment wear, and cause pipe blockages. If the slump is too high, the concrete tends to segregate under high pressure, leading to pipe blockage. 5.2 Too low sand content and unreasonable grading of coarse aggregates. Fine aggregates can be classified by source into river sand, artificial sand (i.e., manufactured sand), sea sand, and mountain sand; among these, river sand has the best pumpability, while manufactured sand has the worst pumpability. Fine aggregates can be classified by particle size into coarse sand, medium sand, and fine sand, among which medium sand has the best pumpability. Coarse aggregates can be classified by shape into: pebbles and crushed stone. Gravel has better pumpability than crushed stone. The maximum particle size of the aggregate is also related to the minimum diameter of the conveying pipeline; the maximum particle size of gravel should be less than 1/3 of the pipeline diameter, and that of crushed stone should be less than 1/4 of the diameter, otherwise blockages in the pipeline are likely to occur. Due to differences in materials, there is an optimal value for the content of fine aggregate (i.e., the sand content) as well as for the gradation of coarse aggregate. Generally, the sand content should not be too low; it should be greater than 40%. The proportion of coarse aggregates with large particle sizes should not be too high either. Choosing the sand content appropriately and determining the aggregate gradation are crucial for improving the pumping properties of concrete and preventing pipe blockages. 5.3 Too little or too much cement usage: In pumped concrete, cement serves a binding and lubricating function; it also has good water-retaining properties, which prevent water from seeping out during pumping. There is an optimal amount of cement to use. If too little cement is used, it will severely affect the concrete’s ability to be pumped, increase the pumping resistance, reduce the concrete’s water-retaining capacity, and lead to water seepage, segregation, and pipe blockages. Under normal circumstances, the cement content in each cubic meter of concrete should be greater than 320 Kg; however, it should not be too high either. Excessive use of cement increases the viscosity of the concrete, which in turn leads to increased transportation difficulties. Additionally, the amount of cement used is also related to the shape of the aggregates; the greater the surface area of the aggregates, the more cement paste is needed to cover them, and consequently, the higher the cement content will be. Therefore, determining the amount of cement appropriately is also important for improving the pumpability of concrete and preventing pipe blockages. 5.4 Improper selection of admixtures: There are many types of admixtures, such as air-entraining agents, water reducers, superplasticizers, retarding agents, and pumping agents. Selecting the appropriate admixtures based on the required strength of the concrete and the type of cement plays a crucial role in improving the pumping properties of the concrete. Unsuitable admixtures will degrade the pumpability and fluidity of concrete, thereby causing pipe blockages. 6 Blockage of pipes caused by insufficient amount of mortar or improper mix ratio 6.1 Insufficient amount of mortar: During the first pumping operation, the mixing unit, the mixer tank of the concrete delivery vehicle, the hoppers, and the pipes all absorb a certain amount of mortar. If too little mortar is used, some of the delivery pipes will not be lubricated, which leads to pipe blockage. The correct amount of mortar required should be calculated at approximately 0.5 m3 per 200 meters of pipeline; the mixing machine, hoppers, and mixing tanks of concrete transport vehicles require around 0.2 m3 of mortar. Therefore, it is essential to calculate the amount of mortar required before pumping. Too little mortar can lead to pipe blockages, while too much mortar can affect the quality of the concrete or result in unnecessary waste. 6.2 Unqualified mortar mix ratio – The mortar mix ratio is also very important. When the pipe length is less than 150m, use 1:2 cement mortar (1 part cement/2 parts sand) ； When the pipe length is greater than 150m, use 1:1 cement mortar (1 part cement/1 part sand); too little cement can also cause pipe blockage. 7 Blockages caused by temperature changes: In summer, when temperatures are high, the concrete in pipes can lose water under intense sunlight, which leads to blockages. Therefore, wet straw bags or other cooling materials should be placed over the pipes. In winter, insulation measures should be taken to maintain the temperature of the concrete. 8 Conclusion. The above summarizes the several common causes of pipe blockage and the preventive measures against them. In actual production processes, due to changes in external conditions, there are often more reasons for pipe blockage than just these. But as long as we follow the operating procedures strictly, take preventive measures, and continuously learn from experience and lessons gained from each blockage, we can certainly minimize the likelihood of blockages.