Shear Capacity of Prestressed FRC Beams with Sparse Stirrup Spacing

S t r e s z c z e n i e Wykonanie tradycyjnego zbrojenia poprzecznego belek żelbetowych jest zazwyczaj pracochłonnym procesem, który może obniżyć efektywność masowej produkcji elementów prefabrykowanych. Nośność na ścinanie belek żelbetowych może być zwiększona przez zastosowanie zbrojenia rozproszonego włóknami, jako uzupełniającego w stosunku do zbrojenia strzemionami. Celem przedstawionych badań było zbadanie, czy, przez użycie właściwego zbrojenia rozproszonego włóknami, można częściowo bądź w pełni zastąpić tradycyjne zbrojenie na ścinanie w belkach prefabrykowanych. W pracy zbadano nośność na ścinanie kilku prefabrykowanych belek żelbetowych zbrojonych włóknami stalowymi i przy rozrzedzonym rozstawie strzemion. Wykonano również obliczenia analityczne i analizy numeryczne belek porównawczych w celu określenia wpływu ilości dodatku włókien na nośność na ścinanie oraz w celu określenia ilości zbrojenia w postaci włókien stalowych, które umożliwia zastąpienie tradycyjnego zbrojenia na ścinanie dla zbadanych belek. K e y w o r d s : Prefabrication, prestressing; Steel fibre reinforced concrete; Shear strength; Beam. 1/2018 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 81 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 1/2018 d o i : 1 0 . 2 1 3 0 7 / A C E E 2 0 1 8 0 0 8 K . K o r i s , I . B ó d i even completely neglected by the application of proper fibre reinforcement. The objective of our research was to find out whether the use of fibre reinforced concrete mixture could partially or fully replace the conventional shear reinforcement in prefabricated beams for industrial halls. The concept of using fibres as reinforcement is not new, they have been used as reinforcement since ancient times. Fibres are usually used to increase ductile behaviour of concrete, control cracking due to plastic and drying shrinkage, reduce bleeding of water and permeability of concrete, and produce a better resistance against dynamic impacts. Generally, fibres do not increase the flexural strength of concrete beams, however, shear strength can be significantly improved as tensile strength of the concrete is increased by the application of fibre reinforcement [2, 3, 4, 5, 6]. The present research was preceded by previous studies. To find the fibre type that is best suited to the objective set in terms of performance, workability and efficiency an extensive experimental program was carried out at the Laboratory of Materials and Structures, Budapest University of Technology and Economics [7]. Another experimental program was also carried out at the Structural Laboratory of Budapest University of Technology and Economics between 2014 and 2015. In this experimental program the shear strength of small span (6 m), prefabricated, prestressed beams without stirrups, but with variable fibre type and content were tested [8]. During the current research, we focused on the shear capacity of larger span (12–25 m) prefabricated, prestressed FRC beams with sparse stirrup spacing (applying stirrups at the ends of the beams only) or without shear reinforcement. Within the frames of the research, the shear strength of several beam specimens was tested, and the analytical and numerical analyses of these beams were also performed. 2. INTRODUCTION OF THE TESTS 2.1. Tested beam specimens In frames of the research four different prefabricated, prestressed FRC floor beams were studied. Beam types T70 and T140 have variable height T sections, beam type T90 has constant height T section and beam type R70 has constant height rectangular crosssection that works together with an in-situ reinforced concrete slab in the final state. Shear reinforcement was completely neglected for beam type T90 while other beams included stirrups with partially sparse spacing (compared to the requirements of EC2) at their support region only. In each beam, Dramix steel fibre reinforcement was applied. Typical shapes, cross-sections and main dimensions of analysed beams are illustrated in Fig. 1. Applied concrete grade, longitudinal reinforcement and fibre dosage of different beams are shown in Tab. 1. 82 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 1/2018 Figure 1. Side view and cross-section of tested beams SH E A R C A PA C I T Y O F P R E S T R E S S E D F R C B E AMS W I T H S PARS E S T I R R U P S PA C I N G During the tests, fork supports were used at both ends of the beams according to the usual construction solution. For beam types T70, R70 and T140 the load was applied step-by-step using verified weights that were put on the beams in predetermined points. In each load step, the deflection of the examined beam was measured by mechanical dial indicator placed in the middle of the span. To avoid the complete destruction of the beams they were loaded only up to ~80% of their load carrying capacity calculated on the mean level. Beam type T90 was tested on a test stand. In this case the asymmetrically applied concentrated force (Fig. 2) was produced by a hydraulic jack and the deflection of the beam was measured in the middle of the span as well as under the acting force. This beam was also loaded up to ~80% of its calculated carrying capacity only to avoid the collapse. Before performing the load test on the beam, its concrete strength was determined by conducting non-destructive Schmidt hammer tests in 10 locations along the web and in 5 locations along the flange. The measured concrete compressive strength values were considered for the numerical analysis of the beam [9]. 2.2. Results of the tests Measured shear force – displacement diagrams are presented in Fig. 3. The shear force acting at the support was always calculated from the actual load arrangement and intensity in each load step. In case of the tested beams the measured shear capacities were higher than the corresponding design shear forces. However, it does not automatically mean that the shear strength of FRC beams is satisfactory on the same reliability level as the beams with conventional shear reinforcement. According to our previous studies [8], FRC beams with even higher fibre content (35 kg/m3) have a higher probability of shear failure than the control beam containing stirrups as shear reinforcement. Because of the insufficient number of specimens, the shear reliability of the tested beams could not be directly determined. It is planned to test further beams in the future so statistical evaluation of the results can be also carried out. The results of current tests were nevertheless used, on the one hand, to verify the results of analytical calculations and, on the other hand, to calibrate the numerical model. C I V I L E N G I N E E R I N G e 1/2018 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 83 Figure 2. Shear loading arrangement of beam T90 and picture of the T90 beam on the test stand Table 1. Dimensions, material grades and longitudinal reinforcement of tested beams