بررسي تحليلي و آزمايشگاهي رفتار تيرهاي بتن مسلح تقويت شده در برش به روش NSM همراه با مطالعات موردي

مقاله ي حاضر، عمل كرد تيرهاي بتن مسلح تقويت شده در برش را به روش آزمايشگاهي مورد بررسي قرار داده و به كمك تحليل اجزاي محدود شبيه سازي مي­كند. سپس با انجام تحليل هاي موردي، تاثير استفاده از بتن با مقادير متفاوت مقاومت فشاري و نسبت هاي متفاوت آرماتورهاي عرضي مورد بررسي قرار خواهد گرفت. در بخش آزمايشگاهي 4 نمونه ي تير بتن مسلح به دو گروه با و بدون خاموت تقسيم شده و در آن اثر استفاده از نوارهاي پليمري تقويت شده با الياف كربن به روش نصب در نزديك سطح تحقيق مي­شود. براي اين منظور، تيرهايي با ابعاد 200 300 2000 ميلي متر ساخته شده و تحت بار استاتيكي قرار گرفتند و منحني هاي بار- تغيير مكان وسط دهانه و نحوه ي گسيختگي آن ها مورد مقايسه قرار گرفتند. شيارهاي مورد استفاده در تكنيك NSM، مجموعه هايي از دو شيار هستند كه در فواصل منظم در دهانه هاي برشي ايجاد شده اند. نتايج آزمايشگاهي نشان مي دهد كه استفاده از روش NSM باعث افزايش ظرفيت برشي به ميزان 41 درصد و 69 درصد به ترتيب در تيرهاي با و بدون خاموت مي شود. به علاوه، شبيه سازي نمونه هاي آزمايشگاهي با مدل كردن احتمال جدا شدگي نوارهاي كامپوزيتي با استفاده از المان چسبنده نشان مي دهد كه نتايج حاصل از روش پيشنهادي مدل سازي، انطباق خوبي با نتايج آزمايشگاهي دارد. هم چنين تحليل هاي اجزاي محدود موردي نشان ميدهد كه با كاهش مقاومت فشاري بتن، احتمال جدا شدگي نوارهاي CFRP بيش تر است و با افزايش درصد آرماتورهاي برشي، كرنش محوري نوارهاي CFRP كاهش پيدا مي كند.

This paper examines the structural behavior of the reinforced concrete beams strengthened in shear experimentally and simulates using finite element analysis. Then, the effect of employing concrete with different compressive strengths and different ratios of transverse reinforcements is studied using the case analyses. In the experimental part, four reinforced concrete beams are divided into two series of with and without internal steel reinforcements and the effect of carbon-fiber-reinforced polymer (CFRP) laminates is investigated by near-surface mounted (NSM) technique as the shear strengthening method. For this purpose, rectangular beams with the dimensions 2000×300×200 mm are designed and monolithically tested in four point loading test up to failure and the load-displacement curves of the mid-span as well as their failure modes are compared with each other. All the beams were reinforced with 3 steel tension bars of 20 mm at the bottom and 2 steel compression bars of 12 mm at the top with end hooks. If stirrups are applicable, 6 mm diameter steel closed hoops spaced at designated distances, are applied. For strengthening using the NSM method, thin slots with 8 mm width and 10 mm depth are made on lateral faces of concrete cover. In order to install composite laminates, the CFRP strips after impregnating with strong epoxy resin are folded and embedded in these grooves. After curing the specimens, all the beams are subjected to a 2000 kN capacity hydraulic jack with the loading rate of 2.5 kN/Min. The ready-mix commercially concrete was delivered to the structural laboratory for casting the specimens with 28-day concrete strength of 30 MPa. The ACI code formulations were used for calculating the shear capacity of the beams before their casting and a suitable span to depth ratio was selected to inhibit deep beam failure. The experimental results indicate that using NSM technique enhances the shear capacity up to 41% and 69% in the beams with and without stirrups, respectively. Test results show that the NSM shear strengthened specimens failed by CFRP laminate rupture. Moreover, simulation of the test specimens by modeling the probability of FRP de-bonding using interface element and orthotropic behavior of laminates shows that the results of the proposed model are consistent with experimental results. In the numerical part, two case studies are carried out; in the first case analysis, three concrete compressive strengths of 20, 30 and 50 MPa are selected and in the second one, three steel stirrup spacing of 65, 130 and 190 mm are applied. Numerical case analyses show that as the compressive strength of concrete decreases, the failure mode the probability of de-bonding increases and as the stirrup percentage increases, the axial strain of CFRP laminates decreases. Numerical case analysis clarifies that by decreasing the distance of internal shear reinforcements from 195 mm to 65 mm, the maximum axial strain of CFRP laminate decreases about 45%. Load-deflection curves in the case analysis also show that by increasing the transverse steel ratio, ultimate displacement enhances and deformability capacity improves.