Numerical Analysis on Tensile Properties of Grout-filled Splice Sleeve Rebars under ISO 834 Standard Fire

: This paper presents some numerical simulation results of tensile properties of reinforcing bars spliced by grout-filled coupling sleeves under fire conditions to identify the effect of load ratio on fire resistance time of spliced reinforcing bars, which provide a useful base for predicting structural behaviors of pre-cast reinforced concrete buildings in fires. A spliced rebar system investigated in this paper consists of two equal-diameter steel reinforcing bars with 25mm diameter and a straight coupling sleeve with 50mm outer and 45mm inner diameters. As a result, the thickness of grout between steel bars and sleeves are 20mm. Firstly, the temperature distributions in steel bars connected by grout- filled coupling sleeves exposed to ISO 834 standard fire were calculated utilizing finite element analysis software ANSYS. Secondly, the stress changes in heated steel bars connected by grout-filled coupling sleeves under different constant tensile loads were calculated step by step until the rebar system failed due to fire. Thus, the fire resistant time of rebar spliced by grout-filled coupling sleeves under different axial tensile loads can be determined, further, the relationship between fire resistance time and axial tensile loads ratio can could be obtained. Finally, the fire resistant times versus axial tensile load ratios curve of grout-filled splice sleeve rebars exposed to ISO 834 standard fire is presented.


Introduction
In the late 1960s, Dr. Alfred Alphonse YEE invented grout-filled splice sleeve (vide Figure 1) in Hawaii and got some patents successively. In 1973, grout-filled splice sleeves were first used in precast concrete column-tree connections for 38 stories high Ala Moana Hotel, Honolulu, Hawaii. From then on, the use of grout-filled splice sleeve has been significantly increasing in precast concrete components worldwide due to their reliability, quality, and durability. The connections between two precast components are key positions for ensuring the integrity of the completed structure under such hazard conditions as earthquake, fire, exploration, and flood, et al, confronted during whole life cycle. Grout-filled sleeves are currently being used widely to splice reinforcing bars of the adjacent two precast components (vide Figure 2). Up to the present, a large number of studies focus on structural behavior of precast concrete structures splicing reinforcing bars in adjacent two precast components with grout-filled sleeves [1,2]. In March 2011, all buildings using NMB Splice Sleeves in the Tohoku area withstood the 9.0 Richter scale earthquake without any structural damage, that demonstrated the high reliability of splice sleeves. It is well known that fires could damage concrete components severely (vide Figure 3) even result in total collapse of whole structures (vide Figure 4) [3][4][5]. Relatively, very few studies on fire behavior of precast reinforced concrete structures were conducted all over the world. It is urgent needed to investigate the fire behavior of precast reinforced concrete structures using splice sleeves. As a small pilot study, this paper presents some numerical simulation results of tensile properties of reinforcing bars spliced by grout-filled coupling sleeves under fire conditions.  The investigated spliced sleeves system consists of three kinds of materials, namely, nodular graphite, steel bar, and pouring mortar, the materials thermal and mechanical properties of which are displayed in Table 1 to Table 3.

Finite Element Mesh
Finite element models for analyzing temperature distribution in splice sleeve system were developed by utilizing the commercial finite element software ANSYS, in which all parts of the system were divided into 62834 Solid70 elements connecting at 12436 nodes (vide Figure  6). It is worth to note that the rebar were cut and the bottom surfaces of sleeve, mortar, and rebar lie within the same plane for convince.

Thermal Boundary and Initial Conditions
Generally, there are at least two approaches to determine the nominal temperature-time curves of a fire and they incorporate various factors, such as fuel types and ventilation conditions [6]. For simplicity and to facilitate comparison of analysis results from different researchers, standard time-temperature relationship according to ISO 834 (vide Figure 7) is employed in numerous studies [7]. The temperature-time curve of ISO 834 fire can be described as  The heat flow across the fire-exposed surface is caused by both convection and radiation. Convection coefficient is taken as 28(W/(m 2 ·K)), and resultant emissivity is taken as 0.9. The initial temperature of the spliced rebar system equals to 20 o C.

Finite Element Analysis Results of Temperature Distribution
Temperature history of nodes in finite element mesh of splice sleeve system under ISO 834 were calculated until the time arrived 180 minutes, which would be used to determine temperature-dependent materials properties of splice sleeve system in structural behavior analysis. Contour plots of temperature distribution on four typical sections at t=7200s are shown in Figure 8.

Finite Element Model for Structural Analysis
After temperature history in splice sleeve system under ISO 834 have been obtained, the structural behavior should be analyzed to determine the fire resistant time of splice sleeve system under different load ratios. The same finite element mesh (vide Figure 6) are used, and Solid45 element were adopted instead of Solid70 element to simulate structural behavior of splice sleeve system. It is assumed that interfaces between sleeve and motor and between motor and rebar are perfectly bonded and no slips are considered. At room temperature, two identical axial tension loads are applied on the two ends of the sleeve connected rebars, the loads are increased step by step and increment of each step is 20kN, until the splice sleeve system fracture, the total loads are referred to as ultimate loads under room temperature, expressed as Fu. For the splice sleeve system investigated in this paper, the F u equal to 221.00 kN.

Results of Structural Analysis
If two identical loads F P , which are less than ultimate load under room temperature, is applied on two ends of rebar in splice sleeve system, then the value of F p over F u is called as load ratio, expressed as L r . Obviously, L r will influence the fire resistance time, R t , of splice sleeve system. Generally, fire resistance time, R t , should decrease with the increase of L r . The main purpose of this paper is to find the relationship between R t and L r .
Under several load ratios, the fire resistance time, R t , of splice sleeve system is calculated using software ANSYS. Figure 9 and Figure 10 show the Von Mises stress contour and first principal tensile strain at t=4200s with L r =80%. Figure 11 shows the relationship curve between R t and L r . Ones can obtain the fire resistant time of a splice sleeve system under ISO 834 standard fire according to the load ratio via linear interpolation.

Conclusions
This paper presents some numerical simulation results of tensile properties of reinforcing bars spliced by grout-filled coupling sleeves under fire conditions to identify the effect of load ratio on fire resistant time of spliced reinforcing bars, with the emphasis on relationship curve of between fire resistance times and load ratios. In addition to the spliced rebar system with straight coupling sleeve with 50mm outer and 45mm inner diameters investigated in this paper, a plenty of spliced rebar system with different sizes have been simulated using software ANSYS. Numerical simulation results indicate that the load ratio has significant effects on fire resistance time of reinforcing bars spliced by grout-filled coupling sleeves. Excessively higher load ratio should be avoided to prevent collapse of precast reinforced concrete buildings in fire conditions. Some complementary tests on fire behavior of reinforcing bars spliced by grout-filled coupling sleeves have been conducted, and results of which will be presented in subsequent papers.