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Placement of concrete construction joints and issue resolution Abstract: The problems that occur at construction joints during the building process are analyzed, the causes of these problems are explored, and reasonable suggestions are put forward on how to prevent and resolve issues related to construction joints during construction. Keywords: concrete construction; construction joint; placement of construction joints At present, it can be said that most buildings rely on concrete. In the casting process of cast-in-place concrete, construction joints are almost a problem that every concrete project has to face. The method of creating construction joints, the quality of these joints, and their location have a direct impact on the quality and safety of the entire building structure; therefore, it is essential to strictly control the quality of construction joints. Based on the current construction codes as well as the author’s experience in design and on-site work, some discussions are presented on the issues related to construction joints. Due to design, construction technology, and construction organization reasons, it is not possible to pour the entire structure in one continuous operation; moreover, when the interval between pours exceeds the allowable time for concrete transportation and pouring, the joint between the concretes poured at different times is referred to as a construction joint. 1 Types of problems that occur at construction joints during construction and their causes From various projects I have been involved in in the past, the construction quality issues arising from construction joints fall into two main categories: first, improper placement of the construction joints; second, inadequate handling of these joints during construction. 1.1 Improper placement of construction joints The construction joints were not placed in locations with less stress, as required by the specifications; for example, they were placed on the concrete slab or as vertical joints in the walls. Alternatively, they were located in places that were inconvenient or difficult to work with during construction. For instance, when a steel waterstop is used and the joint is placed right above the foundation beam, it is difficult to install it due to conflicts with the position of the beam’s rebar. 1.2 Improper handling during construction This is the main and most common construction quality issue caused by construction joints. The reasons for this include the following: (1) The concrete surface was not roughened, and the residual debris was not cleaned properly, resulting in a weak bond between the old and new concrete. (2) During the process of formwork installation and rebar binding, debris such as sawdust and nails fell into the gaps and was not removed in time; as a result, after the upper layer of concrete was poured, a layer was formed between the old and new concrete. (3) When pouring the upper layer of concrete, a layer of cement mortar was not applied first at the construction joint, resulting in poor adhesion between the upper and lower layers of concrete. (4) A water stop strip was not installed at the construction joint. (5) The improper method of material placement causes the aggregates to accumulate at the construction joints. (6) The concrete walls are thin, the rebar is too dense, vibration is difficult, and the concrete is not compact. (7) Compensating shrinkage concrete was not used, resulting in shrinkage cracks at the joints. (8) The joint type for construction joints was selected improperly. 2 Measures to prevent problems arising at construction joints during construction The placement of construction joints must be carried out in strict accordance with relevant specifications; proper attention should be paid to this aspect to avoid quality issues resulting from improper positioning or inadequate handling, thereby ensuring the safety and service life of the structure. The main approaches include the following: 2.1 Strict control over the location where construction joints are placed Construction joints should be located in areas of the structure where shear forces are low and where construction is easier to carry out. They must also comply with the following rules: horizontal joints should be used for columns, while vertical joints should be used for beams, slabs, and walls. (1) Construction joints should be placed on the top surface of the foundation, below the beams or the brackets of the crane beam, above the crane beam, and below the column caps of floor slabs without beams. (2) For large-section beams that are integrated with the floor slabs, construction joints should be placed 20 mm to 30 mm below the bottom surface of the slab. When there is a beam support under the slab, it is placed below the beam support. (3) For one-way slabs, construction joints should be placed at any position parallel to the short side of the slab. (4) For floor slabs with primary and secondary beams, pouring should be carried out in the direction of the secondary beams, and construction joints should be placed within 1/3 of the span of the secondary beams. (5) Construction joints on the wall should be located within 1/3 of the span of the lintel above the door opening, or they can also be placed at the junction of vertical and horizontal walls. (6) The construction joint on the stairs should be located at 1/3 of the tread height. (7) The construction joint in the pool wall should be located on the vertical wall at a height of 200 mm to 500 mm above the surface of the bottom slab. (8) For floor slabs subjected to bidirectional loads, mass concrete, arches, shells, tanks, equipment foundations, multi-story rigid frames, and other complex structures, the locations of construction joints shall be determined in accordance with the design requirements. (9) The location of the post-cast strip shall be determined in accordance with the relevant specifications and taking into account the specific requirements of the project; no further details are needed here. 2.2 Types of construction joints The joint types for construction joints include convex-concave joints, high-low joints, flat joints, and joints with water-stop strips, among others. Furthermore, for construction joints that require waterproofing, past experience has shown that several commonly used joint methods currently pose a risk of water leakage. The biggest drawback of using a \"concave-convex\" type construction joint is the high difficulty of construction, as well as the challenge of ensuring quality. When chiseling the concrete at the construction joint, it’s easy to damage some of the \"convex\" protrusions, which reduces the slope and distance over which water can flow, leading to leakage. Additionally, the cement mortar powder in the grooves is difficult to clean completely; as a result, after new concrete is poured, a layer of debris forms in those grooves, affecting the adhesion between the old and new concrete and creating a risk of leakage. Using rubber water stop strips for waterproofing presents problems: since these strips are flexible, they are difficult to fix in place during installation, and they can easily be deformed or displaced due to pressure from the poured concrete, which can lead to local leakage. Moreover, rubber water stop strips tend to age and become ineffective over time, which is not conducive to the long-term durability of the structure. Based on numerous construction examples, it has been found that using steel plates 400 mm wide and 2 mm thick as water-stop strips at construction joints provides excellent waterproofing performance. Firstly, it is convenient for construction: the steel plate water stop strips are cut to the required length, and after being installed at the construction site, they can be welded together. Secondly, they are not prone to deformation and are easy to fix – the lower part of the water stop strip can be supported by tie bolts, while the upper part is fixed to the formwork support systems on both sides of the tank wall using spot welding with rebar. Thirdly, both the upper and lower water stop strips at the construction joint have a height of 200 mm; the slope for water flow is steep and the height is considerable, which provides an excellent leak prevention effect. Therefore, it is recommended to use steel plate water stops when conditions permit. Specific method: Metal water stops are generally made from thin steel plates with a thickness of 2 mm to 2.5 mm; the joints must be fully welded without any gaps. Fixed at the hidden columns in the wall; holes are often cut in the waterstop strip and stirrups are inserted, and welding should be done again before sealing the formwork. The BW water-stop strip is a long, soft solid with dimensions of 5,000 mm × 30 mm × 20 mm; its expansion rate after 7 days should not exceed 60% of its final expansion rate. When immersed in water, its maximum expansion ratio is 150% to 300%. Tests have shown that it can block the leakage of water at a pressure of 1.5 MPa. When using BW water-stop strips, the concrete surface to which they are to be attached must be leveled and cleaned thoroughly. A layer of cement slurry is then applied to level and smooth the surface, after which the strips are attached directly to the concrete surface thanks to their own adhesive properties. Steel nails are used to secure them at the joints. 2.3 Treatment of construction joints When continuing to pour concrete at construction joints, the following requirements shall be met: (1) The compressive strength of the already poured concrete shall not be less than 1.2 MPa. (2) On the hardened concrete surface, the cement film, loose gravel, and weak concrete layers should be removed; the surface must be thoroughly moistened and rinsed clean, with no water accumulation allowed. That is, it is necessary to remove the milk skin, leave a slight rough texture visible, and make the surface rough. (3) Before pouring, a layer of cement mortar with a thickness of 10 mm to 15 mm should be applied first to the horizontal construction joint; the mix ratio of this mortar is the same as that of the mortar used in the concrete. (4) The concrete should be vibrated thoroughly to ensure a tight bond between the old and new concrete. (5) In the design of waterproof concrete structures, the arrangement of reinforcement and the thickness of walls should be such as to facilitate construction and ensure good construction quality. (6) Waterproof concrete should be poured continuously, with as few construction joints as possible. When it is necessary to leave construction joints, the following rules should be followed: First, construction joints should not be left in the floor slabs or roof slabs, nor in the bottom arch or top arch. Second, vertical construction joints should not be left in the walls. Horizontal construction joints should not be placed at the points where shear and bending forces are greatest, or at the junction of the base slab and the side walls; instead, they should be located on the wall surface at a height of at least 300 mm above the base slab surface. When the wall has holes, the distance between construction joints and the edge of the holes should be no less than 300 mm. For the horizontal construction joint at the junction of arch walls, it is advisable to locate it 150 mm to 300 mm below the joint line of the arch (slab) wall. In the case of construction where the arch is built first and then the wall, the construction joint can be placed at the arching line, but it is essential to take extra measures to enhance waterproofing. On the water-facing side of the seam, a waterproof sealant is applied externally, along with waterproof coatings and mortar. Third, the equipment foundation subjected to dynamic forces should not have construction joints. (7) For walls with a height of more than 2 m, it is advisable to use a tube or a vibrating chute for feeding materials. 3 Measures to address problems that occur at construction joints during construction 3.1 Symptoms of problems The problems that arise at construction joints during construction mainly manifest in the following ways: accumulation of concrete aggregates at the joint, loose concrete, a distinct boundary between old and new concrete, and water leakage along the joint. 3.2 Measures for addressing problems (1) The areas where problems occur most frequently are usually the underground parts. Since these areas generally require waterproofing, depending on the degree of leakage at the construction joints and the water pressure, gel-forming grouts or cyanide-based grouts are used to seal leaks; details of these methods can be found in the section on \"Leak Sealing Techniques for Underground Waterproofing Projects\". (2) In the case of defects in construction joints that do not leak water, a V-shaped groove can be carved along the joint. Where there are loose particles, these must be removed; after cleaning the area, a high-strength cement paste should be used as a base, followed by applying 1:2 cement mortar to level and compact the surface. 4 Conclusion Leakage at construction joints is a common problem in engineering, especially in the exterior concrete retaining walls of basements; however, this can be prevented by using appropriate joint designs.