Báo cáo nghiên cứu khoa học: Analysis of Early-age cracking in restrained ring specimens of high performance concrete using finite element method

Giới thiệu bê tông hiệu suất cao (HPC) được định nghĩa là bê tông mà luôn luôn cung cấp các lợi thế thực hiện cụ thể về sức mạnh và độ bền để so sánh với bê tông thường.
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Báo cáo nghiên cứu khoa học: "Analysis of Early-age cracking in restrained ring specimens of high performance concrete using finite element method" Analysis of Early-age cracking in restrained ring specimens of high performance concrete using finite element methodDr. Nguyen Quang Phu, F aculty of Hydraulic Engineering of Water Resources University, VietnamMEng. Do Viet Thang, Hydraulic Construction Institute, VietnamProf. Jiang lin huaCollege of Materials Science and Engineering, Hohai University, Nanjing 210098, Chinaliu jiapingJiangsu Institute of Building Science, Nanjing 210008, China1. Introduction High-performance concrete (HPC) was defined as concrete which always provides specific performanceadvantages in terms of strength and durability to compare with the conventional concrete. R ecently, the use ofhigh-performance concrete has i ncreased. High- performance concrete mixtures are usuall y produced withwater/binder (W/B) ratios in the range of 0.2 - 0.4, and incorporate with highly-active pozzolans such as silicafume (SF), fly ash (FA), and slag; so it cannot avoid the volume changes occur in concrete as a result of drying,self-desiccation, chemical reactions, and temperature change (ACI Committee 209)[1]. Hence, the problems withearly age cracking become prominent phenomenon. This paper used a numerical analytical study with finiteelement method (FEM) , to predict cracking of high performance concrete in restrained ring specimens. There were a lot of methods to determine the early age behaviour of concrete under restrained shrinkage(bar specimens, plate specimens, and ring specimens) ; perhaps the ring test is the most common methodapplied to assess early age cracking in HPC’s. When the concrete exposed to dry, the shrinkage of concrete isprevented by the steel ri ng, as a result for the tensile stress in the concrete develop. At the first days, when theresidual stress exceeded the tensile strength of the concrete that reason, the early age cracking was occurred. The ring test method is simple in use, the fact that is limited with respect to providing quantitative informationon early age stress development and the exact conditions under which cracking is taking place is referred as adisadvantage of the method. This paper is presented t he finite element method (FEM) t hat was used i n order toacquire quantitative information on early age stress development and early age cracking, using experimentaldata acquired from the ring test. The restraint from the steel ring can be simulated by separating the steel and concrete ring and using a“shrink-fit” approach. The concrete ring is permitted to shrink amount (USH) that is equal to that caused bydrying and autogenous shrinkage. The composite cylinder can be considered to have a fictitious pressure that isapplied on the outer surface of the steel ring that is equal to the pressure on the internal surface of the concretering (Hossain and Wei ss 2003a)[2]. The pressure is adjusted until the steel ring is compressed ( USteel) and theconcrete ring is expanded (Uconcrete) to compensate for the shrinkage as shown in Fig 1. (a) (b) ( a) Before concrete shrinkage occurs (b)Removing constraint and allowing the concrete to shrink Figure 1. Geometry of the ring to determine the elastic response Dally and Riley (1991)[3] provided the solution for the radial displacement of a hollow cylinder (ring) exposedto uniform external pressure which can be used to describe the steel ring. Since the circumferential strain in thering can be computed by dividing the radial displacement by the radius (Weiss et al. 2000)[4], the actual residualinterface pressure (pResidual) can be computed as the pressure required to cause a strain that is equivalent to themeasured strain in the steel (εSteel) as shown in Equation (1) (Weiss et al. 2000; Weiss and Furgeson 2001)[4, 5]. R 2 OS  R 2 IS presidual (t) = -steel(t) . ES . (1) 2 R 2 OS Where: steel(t) is the strain in the steel can be obtained experimentally using strain gage on the inner surface of thesteel ring; ES is t he elastic modulus of the steel; ROS and RIS are the outer and inner radius of the steel ring respectively. The pressure that can be thought to act on the steel ring could be related to an internal fictitious pressurethat acts on the concrete ring and as a result the stress distribution in the concrete ring can be determined asshown in Equation (2) and (3) (Hossain and Weiss 2003a; Timoshenko and Goodier 1987)[2, 6]. R 2 OS  R 2 OC  1  2   , rest.-ring (r) = presidual . (2)  r R 2 OC  R 2 OS   2 2 R 2 OC R OS  R IS   1  2    , rest.-ring (r) = -steel(t) . ES . (3) 2( R 2 OC  R 2 OS )  ...