A certain trading core in Anqing city is located in the city center, with a frame-shear wall structure of 10 floors. The construction structure safety grade is level two, and the building site soil category is three types of site soil. The seismic fortification of the building is considered as class C. In order to obtain larger clear space and usable area, after structural design optimization and analysis, GBF high-strength lightweight thin-walled pipe cast-in-place hollow floor slab was adopted. A column grid of 8m x 8m large-column network was used, and hollow plates were arranged around with a certain torsional resistance capacity frame beam as the peripheral support of the hollow plate. Meanwhile, hidden beams with the same height as the board thickness were set between vertical frame columns to enhance the integrity of the floor slab. The layout of the pipes ensures that there are certain core areas left at the edges of openings, columns, and beams. Structural steel bars are configured in the upper part of the four-side supports and the four corners of the hollow plate, with lengths no less than 1/4 of the span.
The design basis for the cast-in-place hollow floor slab according to Document [1] is as follows: when the design parameters of the floor slab are as follows: slab thickness 350mm, ratio to span about 1:23, bottom and top slab thickness both 50mm, longitudinal rib width 60mm, vertical rib width 50~80mm, tube core inner diameter 250mm, tube core spacing 310mm, elevated work standards, the entire space within the pipe arrangement direction adopts a checkerboard pattern, as shown in Figure 1. According to the above parameters, the calculated void rate of the cast-in-place hollow slab is approximately 46%, the equivalent thickness of the hollow slab in the longitudinal pipe direction is 326mm, and thus the flexibility conversion factor of the hollow slab in the longitudinal direction is 0.93. When designing the cast-in-place hollow slab as an equivalent beam, the bearing conditions and internal force analysis and reinforcement coordination of the equivalent beam and edge-supported frame beam should be considered: if the ends of the equivalent beam are calculated as fixed connections, the edge-supporting beam will generate significant torque, which may lead to over-reinforcement of the section; if the ends of the equivalent beam are calculated as hinged connections, the reinforcement of each beam will be more reasonable, but to control the component cracks, the structural reinforcement of the equivalent beam and edge-supporting beam should be appropriately increased. The above analysis applies to the case where the edge-supporting beam is the edge support. If the edge-supporting beam is located at the end support of a cast-in-place hollow slab with equal or similar spans, and the usage live load on the floor is not large, since the bending moments of the two sides of the cast-in-place hollow slab balance each other, the edge-supporting beam can be considered as a fixed support. If the edge-supporting beam is an intermediate support, but the other side is a common cast-in-place slab with greater thickness, the constraint effect of the common cast-in-place slab can be appropriately considered, but the flexural carrying capacity of the equivalent beam and the torsional carrying capacity of the edge-supporting beam should be improved, while the thickness of the common cast-in-place slab should be increased and the reinforcement design strengthened [2].
Construction of GBF Pipe Cast-In-Place Concrete Floor Slab
The specific construction process and operation manual for GBF pipe cast-in-place concrete floor slabs need to be formulated based on the specific circumstances of the project to ensure smooth construction. The specific process flow is as follows: measurement and layout on the completed floor slab → installation of bottom formwork ribs and supports → installation of floor slab bottom formwork → layout below the template → marking and positioning of thick-walled pipes and embedded water and electrical line conduits and boxes → installation of hidden beam reinforcement, embedding of top slab electrical conduit boxes and vertical through-board sleeves → installation of bottom layer reinforcement, spacers, pads, and rib reinforcement mesh, inspection of bottom layer reinforcement → installation of core tubes with anti-floating technical measures → installation of top layer reinforcement → erection of walkways, laying of pump-delivered concrete pipelines → concrete pouring, simultaneous compaction and adjustment of thin-walled tubes → concrete curing → removal of forms before reaching strength requirements.
(1) Formwork Installation and Support
The requirements for installing and supporting the formwork of the cast-in-place hollow slab are the same as those for ordinary cast-in-place slabs. However, due to the larger span of the hollow slab, the mid-span should be arched at a slope of 3‰ during form removal. After the formwork installation is completed and inspected and found to be qualified, the lines for the hidden beams, GBF core tubes, embedded water and electrical conduits, reserved holes, etc., should be laid out and positioned. Once verified to be error-free, proceed to the next step of construction. Reserved holes should be checked and drilled in advance, and it is strictly prohibited to make holes later. The formwork must not be removed until the concrete strength reaches 100% of the design strength [3].
(2) Reinforcement Binding
After the hidden beams and bottom layer reinforcements are bound, install the concrete cover blocks immediately and fix them firmly. After the reinforcement binding is complete, embed the water pipes, electrical conduits, and electrical boxes immediately. The installation must be coordinated with the progress of civil construction. To minimize the weakening of the floor slab, the intersection points of the embedded conduits should be placed as much as possible in the rib positions between the pipes. Pipes adjacent to the core tubes should preferably be steel pipes, and then proceed with the laying and construction of the core tubes. The reinforcements between the core tubes should be welded into a mesh and installed on-site.
(3) Installation and Fixation of Core Tubes
The core tubes use 250mm GBF high-strength lightweight thick-walled pipes. Since the forming material of the core tubes is sealed GBF lightweight material, their buoyancy in concrete is very large. Therefore, before the concrete solidifies, the floating of the hollow tubes cannot be avoided subjectively, and effective measures must be taken to ensure that the position of the hollow tubes does not change; otherwise, it will affect the quality of the concrete and the safety of the structure. There are many ways to fix the tubes currently. Initially, coarse iron wire was used to fix the thin-walled tubes to the templates at 1/4 of the pipe length from each end of each segment of the tube. Then, a method of adding pressure reinforcement was adopted to press the tubes onto welded reinforcement grids. With the widespread application of hollow tube technology, many labor-saving methods have been discovered in construction. For example, steel pipes with welded rebars spaced according to the pipe spacing dimensions can be used. During binding, the ends of the steel pipes can be fixed to the beam reinforcement or tied to the templates, and the steel pipes can be removed after the concrete begins to set, allowing for repeated use of these fixing rebars. During the installation of the internal formwork, the installation of reserved and embedded facilities (water and electricity pipelines, electrical boxes, etc.) should be inserted in a timely manner. The arrangement of wiring and embedded conduits for the scattered distribution of the board should be comprehensively planned, and the method of lateral connection plus junction boxes can be adopted to avoid burying conduits in thin concrete layers. It is better to place the conduits in the concrete ribs. If the number of conduits is small, centralized placement of conduits can be adopted, and hollow tubes of larger diameters can be used in this position to reduce the thickness of the concrete. If the reserved and embedded facilities cannot avoid the internal formwork, the internal formwork can be interrupted or sawn together with the remaining core, but it must be sealed beforehand. The internal formwork is lightweight and high-strength, but it belongs to brittle materials. During transportation, stacking, and loading/unloading processes, handle with care, and if necessary, special hoisting baskets can be used for transportation [4].
(4) Binding of Top Layer Reinforcement
Before the thick-walled pipes are installed and fixed, bind the top layer reinforcement of the hollow slab. When binding the top layer reinforcement or pouring concrete, overhead walkways must be erected. It is strictly prohibited to place construction equipment directly under the internal formwork, and construction personnel must not step directly on the internal formwork.
(5) Concrete Pouring
Before pouring, in addition to checking and inspecting the installation quality of the reinforcement and reserved/embedded facilities, the installation of the internal formwork should also be inspected according to the specified inspection items, methods, and quality requirements [1]. If any non-conformities are found, they should be corrected promptly. During the concrete pouring process, the internal formwork should be carefully observed and maintained at all times. If any abnormalities are detected, they should be handled promptly. Before the concrete is poured, avoid stepping directly on the tubes to prevent the hollow tubes from breaking and reducing the void rate of the hollow slab. Although the hollow tubes are made of forming materials and have some strength to support human loads, when laying pipes over a large area, a long-term bridgeboard laying method should be adopted to avoid construction personnel stepping on the tubes. During concrete pouring, because the distances between the pipes, between the pipes and the slab surface, and between the pipes and the slab bottom are all relatively large, usually about 50mm, and there are reinforcing bars in between, the concrete falls relatively easily. If the vibration is not thorough, it is extremely difficult to produce honeycombing, pitting, exposed reinforcement, and other quality defects on the top of the slab. Therefore, during construction, the size and grading of the coarse aggregate must be controlled, and effective measures must be taken to ensure the quality of the concrete pouring: ① Before concrete pouring, all GBF pipes should be fully watered and moistened to prevent the GBF pipes from absorbing too much water and lowering the workability of the concrete, resulting in honeycombing, pitting, etc. However, the amount of water should not be excessive, and there should be no standing water on the top of the formwork. ② During concrete pouring, it should proceed along the longitudinal pipe direction, and vertical pouring should be done twice so that the gaps between the pipes can be clearly seen, avoiding missed vibrations. However, the second pouring must be completed before the first pouring of concrete has lost its plasticity. Use a 30mm diameter vibrator combined with a traditional flat plate vibrator for resonance vibration, but do not let the vibrator directly contact the GBF pipes. The vibration spacing should not exceed 0.30m, and the one-time pouring range should not exceed 3.00m. ③ It is preferable to use pump delivery for concrete pouring and strictly control the concrete gradation. The maximum aggregate size should not be less than 25mm, and the slump of the concrete should not exceed 160mm.
(6) Strengthen the secondary finishing work of the concrete, covering the surface with plastic film after a section of the concrete has lost its plasticity. For large-scale concrete pouring, the curing time is 14 days after the concrete has lost its plasticity.
Conclusion
When the project was fully completed in June 2005, the acceptance was qualified. Especially for the cast-in-place hollow floor slab section, the entire space is spacious and bright, with unobstructed sightlines. After measurement and testing, the cast-in-place hollow slab has visible cracks to the naked eye, and the deformation meets the standard requirements. After the project was put into use, it satisfactorily met the requirements of the client, achieving good social and economic benefits. Related theme articles: Water tower slipform design and water tower construction, Water-based environmental healthy wood coatings formula revealed