【Daily Discussion】Utilities - Civil Engineering - 20210517

Channel grouting serves two purposes: one is to fix the rebar, and the other is to provide better protection for it against corrosion. There are no special requirements for grouting; it’s sufficient to fill the holes thoroughly

Method: After prestressing with steel strands, the ducts should be grouted as early and promptly as possible. Channel grouting involves using a grouting pump to force cement slurry into the channels of the prestressed tendons, thereby filling the gaps between the prestressed steel strands and these channels and ensuring a strong bond between the prestressed steel strands and the concrete, forming a single unit. The cavities should be grouted promptly. The grouting process involves the use of a piston-type grouting pump; the pressure applied during grouting is determined by the need to ensure that the cement slurry fills the cavities tightly. The maximum grouting pressure should generally be kept between 0.5 and 0.7 Mpa. When grouting is carried out in a single operation, the pressure can be increased slightly, but it should not exceed 1.0 Mpa. Grouting should be carried out slowly and evenly, without interruption. The initial pressure should be low, with it being increased gradually. Grouting must continue until the exhaust hole at the other end of the tunnel is filled and thick slurry (cement slurry with the specified consistency) emerges; thereafter, a bolted plug should be used to seal the outlet for the slurry. To ensure that the holes are filled with mortar, after grouting each hole to its maximum pressure, a stabilization period at a pressure of not less than 0.5 Mpa should be maintained, and this stabilization period must last at least 2 minutes. Close the slurry inlet valve; the grouting machine will return the slurry. Once the cement slurry has set, remove the connection joints and clean them immediately. To check the actual density of the grout in the ducts, it is necessary to sample and inspect the compactness of the grout through the inspection holes after grouting; if the density is not as required, appropriate action should be taken promptly, and re-grouting can be used as a solution. While mixing the cement paste, standard test blocks are prepared and cured under the same conditions as the beams. Compressive strength of cement paste: The strength of the cement paste shall be not less than M30 grade, and the cement paste test blocks are made using cubic mortar molds with an edge length of 70.7 mm. Requirements: (1) The grouting for the ducts shall be prepared by mixing ordinary Portland cement with water. The quality of cement shall comply with the requirements of \"General Silicate Cement\" (GB 175). (2) The water-cement ratio of the cement mortar used for grouting is generally not greater than 0.4 ； After mixing, the water separation rate should not exceed 1%, and all the separated water should be reabsorbed by the cement paste within 24 hours ； The free expansion rate should not exceed 10%. (3) High-performance admixtures should be added to the cement slurry. The addition of various chloride salts or admixtures that are corrosive to prestressed tendons is strictly prohibited. After adding admixtures, the water-cement ratio of the cement paste can be reduced to 0.35–0.38. (4) The pumpability of cement slurry is controlled by fluidity: when measured by the flow method, the diameter should be no less than 150 mm, and when measured by the flow cone method, it should be 12–18 s. (5) The cement slurry should be mechanically stirred to ensure that the grouting material is evenly mixed. Stirring should be continued during the grouting process to prevent water separation and sedimentation. When sedimentation and separation occur due to an excessive retention time of the cement slurry, secondary grouting should be carried out.

Method: After prestressing with steel strands, the ducts should be grouted as early and promptly as possible. Channel grouting involves using a grouting pump to force cement slurry into the channels of the prestressed tendons, thereby filling the gaps between the prestressed steel strands and these channels and ensuring a strong bond between the prestressed steel strands and the concrete, forming a single unit. The cavities should be grouted promptly. The grouting process involves the use of a piston-type grouting pump; the pressure applied during grouting is determined by the need to ensure that the cement slurry fills the cavities tightly. The maximum grouting pressure should generally be kept between 0.5 and 0.7 Mpa. When grouting is carried out in a single operation, the pressure can be increased slightly, but it should not exceed 1.0 Mpa. Grouting should be carried out slowly and evenly, without interruption. The initial pressure should be low, with it being increased gradually. Grouting must continue until the exhaust hole at the other end of the tunnel is filled and thick slurry (cement slurry with the specified consistency) emerges; thereafter, a bolted plug should be used to seal the outlet for the slurry. To ensure that the holes are filled with mortar, after grouting each hole to its maximum pressure, a stabilization period at a pressure of not less than 0.5 Mpa should be maintained, and this stabilization period must last at least 2 minutes. Close the slurry inlet valve; the grouting machine will return the slurry. Once the cement slurry has set, remove the connection joints and clean them immediately. To check the actual density of the grout in the ducts, it is necessary to sample and inspect the compactness of the grout through the inspection holes after grouting; if the density is not as required, appropriate action should be taken promptly, and re-grouting can be used as a solution. While mixing the cement paste, standard test blocks are prepared and cured under the same conditions as the beams. Compressive strength of cement paste: The strength of the cement paste shall be not less than M30 grade, and the cement paste test blocks are made using cubic mortar molds with an edge length of 70.7 mm. Requirements: (1) The grouting for the ducts shall be prepared by mixing ordinary Portland cement with water. The quality of cement shall comply with the requirements of \"General Silicate Cement\" (GB 175). (2) The water-cement ratio of the cement mortar used for grouting is generally not greater than 0.4 ； After mixing, the water separation rate should not exceed 1%, and all the separated water should be reabsorbed by the cement paste within 24 hours ； The free expansion rate should not exceed 10%. (3) High-performance admixtures should be added to the cement slurry. The addition of various chloride salts or admixtures that are corrosive to prestressed tendons is strictly prohibited. After adding admixtures, the water-cement ratio of the cement paste can be reduced to 0.35–0.38. (4) The pumpability of cement slurry is controlled by fluidity: when measured by the flow method, the diameter should be no less than 150 mm, and when measured by the flow cone method, it should be 12–18 s. (5) The cement slurry should be mechanically stirred to ensure that the grouting material is evenly mixed. Stirring should be continued during the grouting process to prevent water separation and sedimentation. When sedimentation and separation occur due to an excessive retention time of the cement slurry, secondary grouting should be carried out.

Before grouting, the concrete ducts should be promptly sealed. For duct grouting, pressurized water should be used to clean the ducts and moisten their walls. The grouting process should proceed from bottom to top to prevent the upper ducts from blocking those below. An electric grout pump can be used for grouting; the process must be carried out slowly and evenly, without any interruptions. After filling the ducts and sealing the vent holes, it is advisable to maintain a pressure of 0.5–0.6 MPa for a certain period of time to ensure proper compaction of the grout. For cement that does not contain additives, a second grouting step can be employed to improve the density of the grout. Once the strength of the cement paste and mortar in the ducts reaches 15 MPa, the prestressed concrete components can be lifted, transported, or installed.

Piston-type grouting pumps are generally used for grouting, and the pressure applied should be such that the cement slurry is compacted firmly within the holes. The maximum grouting pressure is usually recommended to be between 0.5 and 0.7 Mpa; when single-shot grouting is employed, the pressure can be increased slightly, but it should not exceed 1.0 Mpa. (1) Pore grouting is prepared by mixing ordinary Portland cement with water. The quality of cement shall comply with the requirements of \"General Silicate Cement\" (GB 175).

Before grouting, the concrete ducts should be promptly sealed. For duct grouting, pressurized water should be used to clean the ducts and moisten their walls. The grouting process should proceed from bottom to top to prevent the upper ducts from blocking those below. An electric grout pump can be used for grouting; the process must be carried out slowly and evenly, without interruptions. After grouting the ducts and sealing the vent holes, it is advisable to maintain a pressure of 0.5–0.6 MPa for a certain period of time to ensure proper compaction of the grout. For cement that does not contain admixtures, a second grouting step can be employed to improve the density of the grout. Once the strength of the cement paste and mortar in the ducts reaches 15 MPa, the prestressed concrete components can be lifted, transported, or installed.

High-strength grouting materials are generally used; for bolt holes, etc., a frame can be placed on top and then grouting is sufficient.

Before grouting, the concrete ducts should be promptly sealed. For duct grouting, pressurized water should be used to clean the ducts and moisten their walls. The grouting process should proceed from bottom to top to prevent the upper ducts from blocking those below. An electric grout pump can be used for duct grouting; the grouting process must be carried out slowly and evenly, without any interruptions

After the prestressing tendons are tensioned and anchored, the ducts should be grouted promptly to prevent corrosion of the tendons and to enhance the integrity and durability of the structure; however, when the electrothermal method is used, grouting of the ducts should be carried out after the rebar has cooled down. Before grouting, the concrete ducts should be promptly sealed. For duct grouting, pressurized water should be used to clean the ducts and moisten their walls. The grouting process should proceed from bottom to top to prevent the upper ducts from blocking those below. An electric grout pump can be used for grouting; the process must be carried out slowly and evenly, without interruptions. After grouting the ducts and sealing the vent holes, it is advisable to maintain a pressure of 0.5–0.6 MPa for a certain period of time to ensure proper compaction of the grout. For cement that does not contain additives, a second grouting step can be employed to improve the density of the grout. Once the strength of the cement paste and mortar in the ducts reaches 15 MPa, the prestressed concrete components can be lifted, transported, or installed.