Research on the meaning of reinforcement ductility for a behavior of double-spans reinforced concrete beam.

Transcription

1 Research on the meaning of reinforcement ductility for a behavior of double-spans reinforced concrete beam. Prepared by:

2 Contents list Page 1. Purpose of the research 3 2. Test models and stand description Test models Stand 4 3. Materials 7 4. Description of the test 7 5. Test results Deflections Cracking Failure Conclusions 11 2

3 1. Purpose of the research. The research aim was to identify the differences between the behavior of reinforced concrete beam contains two different classes of steel: class A (low ductility) and class C (high ductility). 2. Test models and stand description 2.1 Test models As test models two sets of double-span reinforced concrete beams were used. Cross-sectional sectional dimensions of the beam were equal 0.4 m x 0.2 m and a length 8.8 m. Beams were propped up on three supports, with axial spacing 4.0 m. Eight forces were located on the beam, selected in such a way as to reproduce as closely as possible the distributed load. Schematic model of beam is shown in Figure 1. Fig. 1 Schematic model of the beam Reinforcement models consisted of longitudinal bars with diameter of 12 mm and stirrups with diameter of 10 mm. In each model the same percentages of longitudinal reinforcement ranging were used, ρ 1 = 0.46 % in spans (3 φ 12mm) and ρ 2 = 0.31 % (2 φ 12mm) above the middle support. Around the edge supports spacing of stirrups were equal s 1 = 0.1 m. Around the middle support spacing varied from s 1 = 0.1 m to s 1 = m. In the compression zone in the middle of spans, where deflections of stretched steel were measured, mounting reinforcement was not used. Concrete cover to all longitudinal reinforcing bars were the same in all beams and equal c nom = 25 mm, and cover to transverse reinforcement was equal c nom = 15 mm. Scheme of reinforcement model is shown in Figure 2. 3

4 Fig.2 Scheme of reinforcement model 2.1 Stand The research was performed on a specially made stand. It consisted of two steel frames located perpendicular to the axis of the beam in the span of 2.25 m. A girder was fastened to the frames, with 4.2 m length, serving as a support of two hydraulic cylinders. The load was transmitted by the hydraulic cylinders and separated into 8 equal concentrated forces. The beam was propped up on three supports, consisted of concrete blocks with dimensions of m. On the concrete blocks there was a cement layer and steel plates, on which the dynamometer or bolts rectification were placed. View of the stand is shown in Figure 3, 4 and 5. 4

5 Fig. 3 Stand scheme. View A-A Fig. 4 Stand scheme. View B-B 5

6 Fig. 5,6 Stand 6

7 3. Materials Following materials were used to perform test models: Concrete with strength class C16/20 Reinforcing steel with grades dependent on the beam series: Series BI Series BII Stees with high ducility EPSTAL (SI) Steel with low ducility (SII) Class C acc. EC2 Class A acc. EC2 ɛ uk > 7,5 % f tk /f yk > 1,15 2,5 % < ɛ uk < 5 % f tk /f yk > 1,05 4. Description of the test Before the actual test were performed, models were weighed and rectified at the supports, in order to obtain the value of support reactions close to the theoretical values. Test program included 3 cycles of loading and unloading. In the first two cycles load reached value of about 5 % of predicted ultimate load and measures about 8 kn. In the third cycle the destructive one models were loaded monotonically until rise of the force and simultaneously rise of spans deflections were noticed. The load was increased every 4 kn (on each dynamometer). Simultaneously an automatic registration of the following parameters were made: deflections of beams, stresses in the longitudinal reinforcement, support reactions. In addition, since first visible cracks appeared, the cracking measuring were made using glass Brinell. The scheme of test program is shown in Figure 6 5. Test results 5.1. Deflections Fig.6 Cycles of loading the beams Beam deflections, until the reinforcement reach the yield strength, were similar and measured about 15 mm. Further increase of load resulted with the destruction of beams reinforced with steel of class A (BII series). The maximum deflection in the middle of the span of this beams was about 20 mm. In beams reinforced with EPSTAL steel (BI series), as soon as stresses of the reinforcement in the support and span zone exceeded the yield strength, significant increase of deflection was noticed at a constant value of the load. Deflection in the middle of spans ranged between mm during failure. Graphs of deflections for beams BI and BII and the list of the average deflections in the middle of the spans are shown below. 7

8 Beam Load F [kn] Average deflection in the middle of the span [mm] BI-1 156,896 53,413 BI-2 151,342 41,781 BII-1 155,333 19,009 BII-2 158,714 19,193 Tab. 1 List of the average deflections in the middle of the span. Przemieszczenie [mm] Odkształcona belki BI-1-50 F=139,428 kn F=160,955 kn -60 F=161,058 kn F=155,896 kn-zniszczenie F=139,428 kn - geodezyjnie -70 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 Rzędna [m] Fig. 7 Deflections of the BI beam (steel EPSTAL). 0 Odkształcona belki BII-1 Przemieszczenie [mm] F=139,660 kn F=147,428 kn -60 F=151,338 kn F=155,333 kn-zniszczenie F=139,660 kn - geodezyjnie -70 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 Rzędna [m] Fig. 8 Deflections of the BII beam (steel of class A) 8

9 5.2. Cracking The width of cracks and strength that caused their appearance was also substantially different in case of beams reinforced with EPSTAL steel (SI) and with steel Class A (SII). While loading beams from BI series, cracks appeared at a load of 20 kn, and for beams from BII series first cracks appeared at a load of 24 kn. The maximum width of cracks in beams reinforced with SI steel reached even 8.5 mm on a support, with a load of 164 kn. Whereas in case of beams reinforced with SII steel, the width of cracks obtained maximum 3 mm on the support at a load of 140 kn. The following table summarizes the results of cracking measured in each beam. Symbol of the measuring point (support) Beam B 1-1 Load [kn] ,05 0,05 0,05 0,15-0, ,05 0,05 0,10 0,20-0,3-0,05 0,3 0,5 0,7 2,2 3,5 4,4 5,4 Tab.2 Results of the cracking measurement. Symbol of the measuring point (support) Beam B 1-2 Load [kn] ,1 0,2 0,2 0,2-0,3-3,0 3,7-0,1 0,1 0,2 0,2-0,3-1,2 5,0 0,05 0,1 0,2 0,4 0,8 1,6 2,0 4,2 6,0 8,5 Tab.3 Results of the cracking measurement. Symbol of the measuring point (support) Beam BI 1-1 Load [kn] ,05 0,2 0,25 0,3 0,4-0,05 0,05 0,1 0,2 0,3 0,8-0,05 0,1 0,2 0,4 0,8 2,5 4 Tab.4 Results of the cracking measurement. Symbol of the measuring point (support) Beam BI 1-2 Load [kn] ,05 0,1 0,1 0,1 0,2-0,1 0,2 0,3 0,5 0,5 0,1 0,2 0,5 0,6 0,8 3,0 Tab.5 Results of the cracking measurement. 9

10 5.3. Failure Depending on the type of steel used for the reinforcement there were significant differences in terms of bearing capacity loss. In case of BI beams (reinforced with EPSTAL steel) formation of three joints was observed. First joint appeared above the middle support and then the rest in the spans. The process of collapse proceeded in a mild way, significant deflections in spans were observed at a constant load. In case of BII beams (reinforced with steel class A) no plastic joint on the middle support were noticed. Destruction of the model proceeded in a violent manner. The list of ultimate load and photos of damaged beams are shown below. Beam left span Ultimate load Middle support right span BI-1 BI-2 BII-1 BII kn 160 kn 160 kn 160 kn kn kn Tab.6 Ultimate loads 168 kn 164 kn - - Fig. 9 Damaged BI-1 beam Fig. 10 Damaged BI-2 beam Fig. 11 Damaged BII-1 beam 10

11 Fig. 12 Damaged BII-2 beam 6. Conclusions A significant difference was observed between the behavior of concrete beams reinforced with two different classes of steel: steel of high ductility (EPSTAL) and steel of low ductility (class A). Before stresses in a reinforcing steel reached the yield strength both types of beams behaved similarly, obtained similar values of deflections and cracks. However, after exceeding the yield strength, beams reinforced with EPSTAL steel obtained significant deformations without the appreciable increase of load. At the same time beams reinforced with steel class A obtained slight values of deflections and cracks, and after that got destroyed in a sudden way. The behavior of the element after exceeding by stresses the yield strength of the reinforcement has a great importance for the safety of the construction. An element which reaches a significant deflections and cracks in a plastic zone is safer, because those visible warnings helps to predict the destruction of the construction. At the same time elements which obtained very small deformations in the plastic zone are very dangerous for users of constructions contained such elements, as they do not indicate collapse of the structure. Based on research: Badanie wpływu plastyczności zbrojenia na zachowanie się dwuprzęsłowej belki żelbetowej Authors: Prof. dr hab. inż. Włodzimierz STAROSOLSKI* Dr inż. Radosław JASIŃSKI* Dr inż. Adam PIEKARCZYK* * Department of Building Structures, Faculty of Civil Engineering, Silesian University of Technology 11

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