4.3 Concrete Composite Floor Technology
4.3.1 Technical Overview
Concrete composite floor technology involves dividing a floor slab along its thickness into two distinct layers: a prefabricated bottom plate and a post-poured concrete composite layer on top. The prefabricated bottom plate, reinforced at the bottom, functions as a part of the floor slab and serves as formwork during construction to support the freshly poured concrete layer, resulting in an integrated composite concrete element.
Based on their stress characteristics, concrete composite floor slabs are categorized as either unidirectional or bidirectional composite slabs. Prefabricated bottom plates are classified as “bearded bars” or “unshaped bars,” depending on whether they have protruding steel bars. According to their jointing method, these plates can have separate joints—meaning tight joints without separation—or integral joints, where post-poured concrete strips connect the bottom plates.
Prefabricated bottom plates fall into two types depending on their reinforcement: precast concrete bottom plates and precast prestressed concrete bottom plates. Non-prestressed precast concrete bottom plates commonly use truss reinforcement to enhance stiffness, while prestressed concrete bottom plates may take the form of flat plates, ribbed plates, or hollow plates.
When spans exceed 3 meters, truss reinforced concrete bottom plates or prestressed concrete flat plates are recommended. For spans over 6 meters, prestressed concrete ribbed bottom plates or hollow slabs are preferred. If the composite floor slab thickness surpasses 180mm, prestressed concrete hollow composite slabs should be used.
The crucial aspect in designing precast concrete composite floor slabs is ensuring both sides of the composite surface bear and coordinate loads effectively. This is typically achieved through a roughened composite surface and shear-resistant structural steel bars embedded at the interface.
The presence or absence of reliable supports during construction determines the design calculation approach. When supported properly, prefabricated components should resist deformation under the weight of the post-poured concrete and construction loads, and are designed based on overall bending behavior. Conversely, unsupported composite panels must account for the influence of the additional weight and construction loads on internal forces and deformations, requiring a two-stage stress design method.
4.3.2 Technical Standards
(1) The design and construction of precast concrete composite floor slabs must comply with current national standards, including the “Code for Design of Concrete Structures” (GB50010), “Technical Specification for Prefabricated Concrete Structures” (JGJ 1), and “Technical Standard for Prefabricated Concrete Buildings” (GB/T 51231). Production, construction, and temporary design of prefabricated bottom plates should follow the “Code for Construction of Concrete Structures” (GB 50066), and construction acceptance must adhere to the “Code for Acceptance of Construction Quality of Concrete Structures” (GB 50204).
(2) Relevant national standard design drawings include “Reinforced Concrete Composite Plate with Truss (60mm Thick Bottom Plate)” (15G366-1), “Prefabricated Ribbed Bottom Plate Concrete Composite Plate” (14G443), and “Prestressed Concrete Composite Plate (50mm, 60mm Solid Bottom Plate)” (06SG439-1).
(3) The concrete strength grade for prefabricated concrete bottom plates should be at least C30. For prefabricated prestressed concrete bottom plates, the minimum strength is C40, while prefabricated buildings require no less than C30. The post-poured concrete composite layer must have a minimum strength grade of C25.
(4) The prefabricated bottom plate thickness should not be less than 60mm, and the post-poured concrete composite layer should also have a minimum thickness of 60mm.
(5) The joint surface between the prefabricated bottom plate and the post-poured concrete layer must be roughened, covering at least 80% of the surface area with a depth of irregularities no less than 4mm. For bottom plates reinforced with truss steel, a naturally rough surface is required.
(6) When the span of the prefabricated bottom plate exceeds 4 meters, or when the longitudinal steel bars on the upper part of cantilever plates and adjacent cantilever plates anchor into the cantilever layer, truss steel bars or other shear-resistant structural steel bars must be installed.
(7) Reliable measures should be taken to control reverse arch effects when using prefabricated prestressed bottom plates.
4.3.3 Application Scope
This technology is particularly suitable for floor structures in various building types, especially residential and public buildings.
4.3.4 Project Examples
Notable projects utilizing this technology include Beijing Investment Vanke New Mileage, Jinyu Huafu, Baoye Wanhua City, Shanghai Urban Construction Pujiang Base Phase V Affordable Housing, Hefei Shushan Public Rental Housing, Shenyang Metro Huisheng New City, and Shenzhen Hong Kong New City Industrialized Housing.
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