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1.1 Provision of joints at the interfaces between concrete elements of different grades: In reinforced concrete structures, the beam-column joints in high-rise building frame structures are quite complex. As a result of load combinations and stress calculations, it is required that the concrete strength grade of the vertical elements (columns, walls) in the same floor be higher than that of the horizontal elements (beams, slabs). In reinforced concrete frame structures, horizontal construction joints are usually placed at the base of columns; if a horizontal construction joint is to be used at the top of columns, it should be located at the bottom of the beams. If the concrete of the vertical and horizontal members in the same floor is poured simultaneously, no construction joint will remain at the top of the column. ? 1.2 Construction of pouring concrete with different strength grades for beams and columns: Given that commercial concrete or concrete pumped from on-site mixing plants is commonly used in high-rise buildings, the method for pouring concrete in the core area of column joints is as follows: As shown in Figure 1, whether or not a construction joint is left at the top of the column, concrete of the appropriate grade should first be delivered to the site using a tower crane bucket or a concrete pump, and then vibrated in layers, with a 45° slope being created at the locations where the floor beams and slabs meet. Before the concrete begins to set, pump in the concrete for the floor beams and slabs. When using this method to pour concrete for floor columns, walls, beams, and slabs, it is essential to ensure that no cold joints form at the interfaces between concrete of different strength grades. Therefore, it is advisable to leave construction joints at the top of columns and the bottom of beams, in order to reduce the time required to pour concrete of higher strength grade in the core area of these joints, and thus prevent the formation of cold joints at the interfaces between concrete of different strength grades. At the same time, the core area of beam-column joints where the rebar is densely packed should be vibrated more thoroughly using small insertion vibrators to eliminate any areas that may not receive sufficient vibration. In cases where the rebar is indeed too densely packed, it is necessary to consult the design team in advance to implement appropriate technical measures, ensuring the compactness and design strength of the concrete in the core area of the joint. 2 Should the beam-column joints be poured together with the floor slabs? Using a separate pouring method for concrete of different strength grades at the beam-column joints poses difficulties in construction and can easily lead to cold joints at the adjacent surfaces. Therefore, when the strength grade of the column concrete is no more than two grades higher than that of the beam and slab concrete (10 N/mm²), it is possible to consider pouring the concrete at the beam-column joints together with the beam and slab concrete. It should be noted, however, that using the concrete strength of the beam slabs for the concrete at the beam-column joints at this time will result in insufficient bearing capacity of the columns under vertical loads, as well as insufficient shear resistance in the core area of the joints under seismic forces; therefore, this approach is generally not advisable. ? 3 Specific measures for controlling and eliminating cracks at beam-column joints? 3.1 Reasons for cracks at beam-column joints where concrete of different strength grades is used. Based on our company’s experience in constructing high-rise buildings in Ningbo, Hangzhou, Shanghai and other locations, concrete of different strength grades at beam-column joints is poured in the order of columns first and then beams; however, minor cracks have been observed near the interface between concrete of different strength grades at beam-column joints in a few floors. Based on on-site inspection and discussion analysis, it is determined that these cracks are not structural cracks caused by loads, and they do not affect the safe use of the structure. Although micro-cracks are difficult to avoid in concrete, strict requirements should be applied, the causes should be analyzed, and effective measures taken to control and eliminate such cracks as much as possible, thereby further improving the quality of the project. The specific reasons are: (1) At the beam-column joints, there is a significant difference in the strength grades of the concrete; when there is a difference of two grade levels, the concrete of different strength grades has varying amounts of cement used, water-cement ratios, and water content. Columns have a larger volume, which requires more cement, resulting in higher heat generation during hydration. Moreover, the shrinkage rates of concrete with different strength grades vary, so cracks are likely to form in the area where these grades meet. ? (2) The columns have a large cross-section and high stiffness, while the beams have a relatively smaller cross-section; due to the strong restraint imposed by the columns, the shrinkage of the beam concrete is restricted, which also makes cracks more likely to occur. ? (3) In the mix proportions of ready-mixed concrete, high-strength grades require more cement; higher water-cement ratios, sand contents, and slump levels can also lead to cracks in the area where high- and low-strength grades meet. (4) In cast-in-place beam-slab structures, the beams are located beneath the slabs; the water applied to the upper surface is fully absorbed by the slabs, leaving the beams without sufficient moisture for proper curing. This results in uneven contraction of the beams on the inside and outside, and it can also lead to cracks appearing on the two sides of the beams. ? (5) In some beams, there are too few structural reinforcement bars in the horizontal direction on the sides, which is not favorable for preventing shrinkage cracks in the beams. ? 3.2 Measures to prevent cracks at beam-column joints Based on the analysis of the above reasons, the specific improvement measures are as follows: ? (1) The concrete mixing plant is required to adjust the mix design; under the conditions of meeting the required strength grade and pumpability, for column concrete, the amount of cement used should be reduced, the sand content decreased, the stone content increased, the slump reduced, and the water usage lowered. Corresponding adjustments also need to be made to the amounts of fly ash and admixtures used. ? At node (2), the concrete pouring should follow the principle of \"pouring higher areas first and then lower areas\", that is, high-strength concrete should be poured first, followed by low-strength concrete. It is essential to ensure that pouring of concrete for beams and slabs does not start until the concrete in the columns has initially set; proper technical instructions and preparatory work must be carried out in advance. ? (3) For the concrete of beams and slabs, a secondary vibration method is used, that is, vibration is applied once more before the concrete begins to set, in order to improve the density at the interface between concrete with different strength grades and to reduce shrinkage. ? (4) On the sides of beams where cracks occur relatively frequently, add horizontal structural reinforcement to improve the beam’s crack resistance. ? (5) Strictly control the slump of the concrete mixture; for the concrete in the core area of the joints and columns, tower cranes are used for delivery in order to reduce the slump. On-site, the slump of each batch of concrete should be tested. ? (6) Improve the curing of concrete, especially for beams; in addition to watering the surface of the slabs, water should also be applied to the sides of the beams beneath them. Until the full-span load-bearing scaffolding is removed, high-pressure water guns can be used to water and cure the beams, and the timing for removing the formwork from the sides of the beams can be delayed. ? 4 Conclusion? At the joints of the frame structures in high-rise buildings, it is common for the concrete strength grade of the columns to be higher than that of the beams and slabs in the same floor. The standard construction method involves pouring the core portion at the joint first, which has a higher concrete strength grade, and then pouring the concrete for the beams and slabs before it sets. As long as the appropriate targeted measures are taken and construction is carried out carefully, cracks near the junctions where concrete of different strength grades is used in beam-column joints can be completely avoided.