Precautions for post cast strip construction of th

Precautions for the construction of post cast strip in high-rise buildings

post cast strip has been widely used as a technical measure to leave no temperature expansion joints in super long buildings and no settlement joints between high-rise parts of high-rise buildings and podiums. However, if some links are not noticed in the construction process, the post cast strip will often not play its due role, which may also bring inconvenience to the construction and cause hidden dangers to the structure. Combined with our practice of constructing post cast strip of high-rise building projects in recent ten years, this paper discusses some experiences on matters needing attention in construction

1. The formwork support system should be independent

as the temperature post pouring belt. Generally, the interval between concrete pouring is not less than 60 days; The post cast strip of settlement requires that the concrete be poured after the construction of the main structure of the high-rise part is completed, and the interval may be several months or even more. During this period, the follow-up works have to be constructed upwards, and the beam slab structure on both sides of the post cast strip that has been constructed has become a cantilever member, and it has to bear the self weight and construction load of other upper structural layers. It is far from enough to rely on the formwork support system of normal construction alone. Therefore, the formwork and support under the post cast strip and within 1m on both sides must be set independently through calculation when preparing the construction organization design to ensure stress and stability. Otherwise, it may cause quality accidents such as cracks on the upper part of the beam and slab or downward deflection of the post cast strip, uneven and sinking of the roof surface, etc. This part of support shall be adjustable up and down as much as possible. After the structural concrete strength meets the design requirements, it shall be unloaded one by one and then tightened and braced firmly to avoid the unreasonable situation that the original support layer is fully loaded and transmitted downward, resulting in excessive stress and deflection of the unsupported structure at the lowest layer

1.1 at least two rows of loaded supports shall be reserved on both sides of the post cast strip, and the row spacing shall not be greater than 1m (including beam and slab supports). The formwork support system of this part shall be relatively independent to facilitate the normal removal and turnover of other formwork and supports

1.2 the structural beam that is cut off by the post cast strip and forms a cantilever shall be reserved with early removal support, so that it can be used as a load bearing support after the removal of other supports (the removal time of other supports shall be subject to the concrete strength of the beam reaching 100% of the design value and the concrete strength of the upper beam ≥ 75% of the design value). The spacing of the early dismantling support shall be determined by calculation. During the calculation, the load that the cantilever beam can bear can be considered to be deducted. Other loads (including part of the load of this floor structure and the construction load of the previous floor structure) are also borne by the high-cost silver containing composite antibacterial agent material support of the antibacterial and mildew proof air conditioner hanging Level 2 energy efficiency KFR ⑵ 6gw/wxaa2@ (ceramic white) launched by early dismantling Midea. If general φ 48. If the ordinary steel pipe with 3mm wall thickness is not stressed enough (or the spacing is too dense), the support shall be replaced by 120 × 120 or 150 × 150 square timber

1.3 the support retained by the post cast strip shall be reliably tied horizontally to prevent instability

1.4 for the post cast strip of large volume concrete, such as the thick base plate and girder of the basement, special formwork and support must be set on both sides to prevent the bottom of the post cast strip from being disconnected due to concrete leakage. If the basement is required to be waterproof and impermeable, a waterstop or tongue and groove formwork shall be reserved to prevent water seepage at the post cast strip

2. The reinforcement in the post cast strip should be treated carefully according to the nature of the post cast strip

the post cast strip can be divided into temperature post cast strip and settlement post cast strip according to its use function. During construction, the nature of the post cast strip should be distinguished and different measures should be taken to deal with it

2.1 temperature post cast strip is mainly to solve the shrinkage deformation of structural concrete on both sides of the post cast strip before concrete pouring. Therefore, in order to ensure the free deformation of concrete during this period, the stressed reinforcement in the post cast strip perpendicular to the post cast strip must be disconnected; This is often ignored in the construction process, resulting in the post cast strip not playing its due role. The positions of the disconnected reinforcement shall be staggered on both sides of the post cast strip as far as possible, so as to facilitate the staggering of the joint positions during reconnection. In order to facilitate the construction, it is generally possible to pass through the post cast strip according to the normal reinforcement. Within 3 days after the construction of this layer of concrete, the main reinforcement perpendicular to the post cast strip is cut by gas cutting, and the gap is generally about 5mm. After meeting the interval time of the post cast strip, the horizontal reinforcement narrow gap arc welding process is used to weld the disconnected reinforcement

2.2 the settlement post cast strip is mainly set considering the settlement difference between the high-rise structure and the podium. The width of post cast strip is generally 0.8 ~ 1.0m. In this section, the structural settlement difference on both sides reflects the reinforcement stress and strain generated on the reinforcement, which is relatively small and will not affect the settlement of the structure. Therefore, users who need to buy high and low temperature impact testing machine after settlement hope to find a reliable high and low temperature impact testing machine manufacturer. Generally, the cast strip reinforcement can not be disconnected. However, during the construction, it should be noted that the concrete of the lower cushion of the post cast strip of the foundation or basement floor and ground beam should not be poured too thick (it should be thinned by about 3cm). The lower reinforcement must be padded with thickened cushion blocks to lift up, and the distance from the cushion is not less than 5cm, so that the structures on both sides can settle freely, and the reinforcement still has sufficient concrete protective layer thickness after settlement

3. Strengthen the protective treatment before the concrete pouring of the post cast strip

3.1 for the post cast strip of mass concrete, after the concrete pouring on both sides, the concrete on the inner side of the post cast strip should be the focus of the maintenance and crack prevention of mass concrete, because the maximum temperature difference often appears on the concrete surface on both sides of the post cast strip, and once there is a crack in the concrete on both sides, it is a deep crack in the structure, which sometimes runs through the whole structural surface. Therefore, for the mass concrete post cast strip, manholes should be reserved at the appropriate parts where the reinforcement on the side or top of the strip end is disconnected, and the construction personnel can directly enter the post cast strip to support the formwork, dismantle, clean and maintain. During the curing period, it shall be covered with plastic film and rock wool quilt (or straw cover). If necessary, iodine tungsten lamp shall be used to irradiate the side concrete for heating, so as to ensure that the curing temperature difference of mass concrete does not exceed 25 ℃

3.2 when pouring the concrete on both sides of the post cast strip, leave a 5 ~ 10cm wide groove on the top surface of the concrete. The post cast strip is covered with colored plastic cloth. The joint is pressed and leveled with 1:3 cement mortar within the width of the tongue and groove. A 120 wide berm is built on it, 120 ~ 180mm high, and the outer side is painted as reinforcement protection and safety protection of the construction site. In order to prevent sundries from entering the post cast strip and it is difficult to clean it, and prevent rainwater or construction maintenance water from entering the post cast strip. Especially in the basement, the water seepage foundation soil will affect the structural performance such as the bearing capacity of the foundation, and this protection must be carefully handled. The passage shall be paved with 6cm thick wood passage board, and the berm shall be properly lowered

3.3 for the end of the horizontal post cast strip of the basement floor and the vertical post cast strip of the outer wall, 240 brick walls should be built to block, the masonry mortar grade should not be less than M5.0, and painted with waterproof mortar; Damp proof or waterproof treatment shall be carried out on the outer wall of the basement. The normal load is 2n, which is to prevent rainwater or construction water from entering the post cast strip; Second, the wall can be used as a retaining wall, which does not affect the timely backfilling of the foundation pit and is conducive to the acceleration of construction; Third, it can be used as the permanent external formwork of post cast strip to reduce construction links

3.4 before the concrete pouring of the post cast strip, it is necessary to clean the post cast strip, chisel away the pumice and laitance, and adjust the reinforcement, etc

4. Post cast strip concrete configuration, pouring and curing

4.1 the concrete in the post cast strip is generally required to increase the strength grade by one to two levels compared with the poured concrete on both sides (determined by the Design Institute). Generally, non shrinkage or micro expansion concrete is selected, and the mix proportion design should be subject to calculation and trial mixing test. Common measures mainly include three kinds of benefits: high integration, stable function, convenient adjustment and so on:

4.1.1 reduce the water cement ratio and control the concrete slump. Add high-efficiency early strength water reducer such as fdn-2000 (water reduction rate is generally about 15%), control the water cement ratio not greater than 0.4, and control the slump at 1 ~ 3mm (pumping concrete can be controlled at 10 ~ 14)

4.1.2 add UEA expansion agent and configure shrinkage self compensating concrete

4.1.3 configure micro expansive concrete with the mix ratio of water: micro expansive cement: Sand: Stone =0.55:1:1.98:2.65, wherein the micro expansive cement is composed of ordinary cement and expansive components, and the expansive components are composed of vanadium cement and gypsum in 1:1. Adjust the mixing amount of expansion components during construction to obtain an ideal expansion rate of 0.1% - 0.2%

4.2 the mixing, pouring and tamping of concrete must be careful, the proportion measurement must be strict, and the mixing time should be appropriately extended for 1 ~ 3 minutes. The pouring sequence should be driven from one end to the other end by layered slope (because the influence of the lateral pressure of concrete vibration can be ignored). Pumping concrete is better, which is conducive to the combination of water precipitation and concrete, and no construction joint is left

4.3 the early curing of concrete is mainly the timely surface finishing. For large volume post cast strip concrete, attention should be paid to covering plastic film and straw mulch for thermal insulation and curing after finishing, so as to reduce the temperature difference and prevent cracks. When pouring the post cast strip with micro expansive concrete, pay attention to water storage and curing for 3 ~ 7 days to obtain the ideal expansion rate. (Zhao Jingwen)