Deflection Ductility Index Research Articles

The impact of different length hooked‐end fibers on the structural performance of RC folded plates

AbstractIn this article, the effect of hooked‐end fibers with different lengths on the structural performance of RC‐FPs fabricated from hybrid fiber‐reinforced self‐compacting concrete (HFR‐SCC) was investigated. For this purpose, a total of 15 full‐scale test samples having different plate thicknesses (60, 70, and 80 mm) were produced and tested under bending after a 90‐day curing period. Subsequently, load‐carrying capacity (Pp), flexural toughness (Fth), and deflection ductility index (μu) of all RC‐FPs were found using load‐deflection curves obtained from bending tests, while crack patterns were drawn from the samples tested. Besides, high‐precision contour plots are proposed to estimate the structural performance values of RC‐FPs depending on plate thickness and fiber reinforcing index. As a result, the best structural performance in RC‐FPs was obtained from the use of a longer hooked‐end steel fiber together with micro steel fiber as a hybrid, followed by the lower length hooked‐end steel fibers as singles. Specifically, irrespective of the plate thickness, the hybrid use of longer hooked steel fibers in combination with micro fibers increased the Pp, Fth, and μu values of RC‐FPs on average 1.67, 1.76, and 1.57 times, respectively, compared to the control specimens. As for when using the lower length hooked‐end fiber as single, the values of Pp, Fth, and μu increased on average 1.57, 1.69, and 1.30 times. Lastly, whereas plate thickness has little effect on improving the structural performance of thin‐walled carrier elements such as RC‐FPs, adding fibers in different lengths, aspect ratios, and combinations is much more effective. The collective test results demonstrate that using RC‐FPs made of HFR‐SCC in the roof carrier system of large span structures could improve structural performance, aesthetics, erection time, and earthquake behavior thanks to reduced dead load.

Experimental study on the effects of external strengthening and elevated temperature on the shear behavior of ultra-high-performance fiber-reinforced concrete deep beams

The shear behavior of deep beams, especially of ultra-high-performance fiber-reinforced concrete (UHPFC), is rather intricate, as it depends on several factors, including its depth, concrete strength, amount of flexural and shear reinforcements, the ratio of shear span to depth, and the bearing area under the load and at the support. Moreover, these beams may be exposed to elevated temperatures requiring strengthening, which has not been adequately studied. This study examines experimentally the shear capacity of UHPFC deep beams strengthened using carbon fiber-reinforced polymer (CFRP) sheets for improving the shear strength of deep beams after exposure to an elevated temperature. Eight specimens were cast in four groups of two specimens each. Testing of the beams of the first group was performed at the ambient temperature, whereas the beams of the second group were tested after being exposed to a temperature of 450 °C. For the third group, the deep beams were strengthened by CFRP U-wrap strips and tested at the ambient temperature, whereas the beams of the fourth group were subjected to the elevated temperature of 450 °C, followed by strengthening using CFRP U-wraps and then testing to collapse. The test results indicated that the post-peak behavior was affected significantly by the elevated temperature. Moreover, the experimental results indicated the excellent performance of strengthening schemes in improving the post-cracking behavior of UHPFC deep beams, enhancing shear strength (up to 16 %), energy-based ductility index (39 % to 648 %), and deflection ductility index (68 % to 144 %). An analytical model is developed to predict the load-carrying capacity of UHPFC deep beams strengthened after exposure to elevated temperature.

The impact of the welded wire mesh as internal reinforcement on the flexural behavior of RC beams exposed to elevated temperature

This paper investigates the efficiency of using the welded wire mesh (WWM) as internal reinforcement to regain the structural performance of heat-damaged reinforced concrete (RC) beams to achieve a 28-days average compressive cylinder strength of 40 MPa. These (150 × 200 × 1100 mm3) were prepared using ordinary-strength concrete, cured for 28 days in moist burlap, and heat-damaged at temperatures (23–750 Celsius degree (°C)) two hours after internal strengthening. The mechanical response of control and strengthened beams was evaluated under a four-point loading test setup. The reduction percentage is ranged from 7% to 44 %, 2% to 20%,2%–20%, 2 %–31 %, 17 %–65 %, 4 %–36 %, 17 %–40 %, and 17 %–65 % for 250 °C to 750 °C, respectively, in terms of the ultimate load, the ultimate deflection, the elastic stiffness, the energy absorption, the deflection ductility index, the energy ductility index, and the performance factor, respectively. Furthermore, cracking and failure modes were monitored and characterized during the loading process. The results show that exposing reinforced concrete beams (to the above-elevated temperatures) creates intense cracking on their surface and degrades their mechanical performance. The proposed schemes using internal WWM help avert/ reduce the tendency of concrete cover separation. Hence, the overall mechanical performance of heat-damaged beams is noticeably improved. The efficiency of the number of WWM layers is enhanced with further heat damage in which the enhancement percentage is ranged from 15 % to 97 %, 47% to 129%, 12% to 92%, 81 %–415 %, 43 %–123 %, 81 %–415 %, and 69 %–352 % for beams strengthened internally one to five layers of WWM for the ultimate load, the ultimate deflection, the elastic stiffness, the energy absorption, the deflection ductility index, the energy ductility index, and the performance factor, respectively. Finally, this study indicated that using WWM as internal reinforcement represents an acceptable technique for enhancing the loading capacity of flexural-deficient reinforced and thermally damaged RC beams than internal carbon fiber reinforced polymer (CFRP) after considering the profitability index issue. The contribution of one layer of FRP is equivalent to 5.00, 1.31, 1.52, 1.70, 1.10, 1.70, 1.68, and 0.50 times the contribution of one layer of WWM in terms of the ultimate load, the ultimate deflection, the elastic stiffness, the energy absorption, the deflection ductility index, the energy ductility index, the performance factor, and profitability index. Through demonstrating an outstanding structural performance with significant enhancement in the ultimate strength, ultimate deflection, stiffness, and energy absorption, the findings of this paper strongly attest that WWM can be effectively utilized as internal flexural reinforcement in reinforced concrete beams exposed to elevated temperatures. New guidelines are developed for predicting the damage level and flexural strength of heated damaged RC beams internally reinforced with WWM and considering the influence of the number of WWM layers and elevated temperature.

Span-to-depth ratio limits for RC continuous beams and slabs based on MC2010 and EC2 ductility and deflection requirements

In the design of reinforced concrete structures several limit states are usually verified using simplified methods instead of using complex direct calculation. Span-to-depth ratio limits are applied for deflection control. In the same way, a redistribution factor and the relative depth of the compression zone are applied for the required ductility, but this is not enough. In fact, specific span-to-depth ratio limits, which are developed in this paper, should also complement the conditions referred to here.A numerical study is developed to compute the deflection ductility index of a wide range of continuous beams (or one-way slabs) considering as ultimate point when the rotation capacity (given by MC2010 and EC2) is achieved. From the boundary between fragile and ductile failures, the new span-to-depth ratio limits for the required ductility are defined and compared with the existing similar limits for deflection control.The results indicate that: (i) for normal strength concrete (up to 50 MPa), the ductility limit is more restrictive than the deflection limit when the redistribution factor δ is lower than 0.9, being always more restrictive for high strength concrete; (ii) for the current building beams, the slenderness used (10 < l/d < 20) allows ductile failures, while for typical building slabs (low reinforcement ratio, but high redistribution), the slenderness should be l/d ≤ 20; (iii) the conditions of MC2010 and EC2 to use plastic analysis without any direct check of the rotation capacity are actually incomplete and need to be revised.The new span-to-depth ratio limits proposed in this paper, which satisfy both ductility and deflection requirements, depend on the redistribution factor, characteristic compressive strength of concrete and total mechanical reinforcement ratio.

Flexural strength and toughness of steel fiber reinforced concrete beams

This study has proposed to evaluate the flexural response of reinforced concrete (RC) beam elements of size 150 mm × 200 mm × 1000 mm (with conventional steel rebars of reinforcement ratio of 0.6%) with and without steel fibers of varying reinforcement index (RI) (product of fiber aspect ratio, AR and volume fraction, Vf%). The mechanical strength (compressive, split tensile and prismatic flexural strength) of concrete are ascertained with hooked steel fibers (HSF) and crimped steel fibers (CSF) in RI of 0.25–1.50 in comparison to controlled concrete (CC) with no fibers. RC beams (with and without HSF and CSF of RI = 0.5–1.0) have been tested under flexure and the loads and deflection at first cracking, yielding, peak and ultimate failure points show that the performance of fibrous beams are higher than non-fibrous beams, and CSF is effective than HSF in RC beams. The RC beams with CSF has higher residual strength and toughness (at deflection point, L/150, L being the span), and total absorbed energy than the specimens with HSF. The residual strength and toughness (at L/150) and the total absorbed energy (TAE) of RC beams with HSF and CSF are higher than the control beams (without fibers). The experimental reserve flexural capacity (ERFC) of RC beams ≥ 1.30. The stiffness of the fibrous RC beams at initial pre-cracking, post-cracking and pre-peak stages are higher than the non-fibrous beams. The deflection ductility indices (yield and peak) have shown an increase when RI is increased from 0.5 to 1.0. The flexural test results have shown that addition of HSF and CSF has improved the first cracking, yielding and peak loads of RC beams (at RI = 0.5, 0.6 and 1.0). The crack spacing and width of flexural cracks in the RC beams have reduced due to the addition of steel fibers, and CSF has shown better cracking behavior than HSF in the beams. The equivalent flexural strength (toughness factor) and strength ratio of RC beams (with conventional reinforcement) with CSF are higher than the beams with HSF, and, therefore, crimped fibers are recommended for effective use in construction.

Precast segmental bamboo reinforced concrete beams with bolted connections subjected to flexural loads: an experimental investigation

Indonesia is one of the wealthiest botanically countries in the world. There are more than 130 species of Bamboo can be found in this country. Bamboo is a natural raw material which is one of the fastest growing plants that has many advantages as a construction material. We conducted an experimental investigation to study the performance of the precast segmental bamboo reinforced concrete beams with bolted connections subjected to flexural loads. Tests were carried out on three specimens with dimensions of 120 mm x 150 mm x 1000 mm, consisting of one control beams (BK), one precast segmental bamboo reinforced concrete beam with four bolted connections (BS4), and one precast segmental bamboo reinforced concrete beam with six bolted connections (BS6). We found that the load carrying capacity of precast segmental bamboo reinforced concrete beams with four bolted connections (BS4) and six bolted connections (BS6) had a decrease of 57.99% and 55.62% respectively compared to control beam (BK). Similarly, the deflection ductility index of the precast segmental bamboo reinforced concrete beams with four bolted connections (BS4) and six bolted connections (BS6) had a decrease of 49.78% and 46.59% respectively compared to control beam (BK). Also, we observed that the crack pattern developed in the control beam indicates a flexural failure mechanism while the crack pattern developed in the precast segmental bamboo reinforced concrete beams indicates a shear failure mechanism. As a conclusion, this study will significantly contribute to the growing development of research on the bolted connection system in bamboo reinforced concrete members, especially in Indonesia.

Performance of hybrid carbon and basalt FRP sheets in strengthening concrete beams in flexure

This study presents the bending performance of reinforced concrete (RC) beams strengthened with hybrid carbon and basalt fiber reinforced polymer (CFRP/BFRP) composite sheets. The proposed hybrid system is intended to improve the properties of the strengthening composite material, in which high strength CFRP sheets are combined with ductile BFRP sheets via epoxy adhesive. A total of 10 beams were tested, nine of which were strengthened with different combinations of CFRP and BFRP sheets, and the remaining beam was unstrengthened to serve as a control specimen. The test results indicated clear improvements in the load-carrying capacity and ductility of the strengthened specimens. The range of increase in the ultimate capacity of the hybrid-strengthened beams over that of the control beam specimen was between 66 and 75%. In addition, the deflection ductility indices of the hybrid beams presented an improvement up to 108% compared to those strengthened with CFRP laminates only. Finite element (FE) models were also developed to predict the response and load-carrying capacity of the tested specimens. The models precisely predicted the experimentally obtained load-carrying capacity and associated mid-span deflections at yield and ultimate loads, with a deviation that did not exceed 12%.

Flexural Strengthening and Rehabilitation of Reinforced Concrete Beam Using BFRP Composites: Finite Element Approach

Basalt fiber‐reinforced polymer (BFRP) is adopted widely in recent years in many countries to rehabilitate or strengthen structural elements such as reinforced concrete (RC) beams because it is cheap and it has stellar mechanical performance. By activating the finite element (FE) simulation, the present research submits an extensive study on the strengthening and rehabilitation of damaged full‐scale RC beams due to corrosions in the main reinforcement caused by BFRP sheets. Different parameters were taken into consideration such as corrosion grade, BFRP wrapping schemes, and the number of layers. The flexural performance of the models that build up as the control model and the damaged and the repaired methodologies by BFRP that are adopted and tested by others under the effects of four‐point static loadings were also underwent examination. The full interaction at BFRP‐concrete interface and the full bonding between sheets presupposed were investigated for all models. The numerical analysis findings were compared with the experimental measurements and found to be in good agreement. The current numerical analysis proved that the ultimate load rised by 14.8% in spite of 20% corrosion in the flexural steel rebar under eight layers of BFRP composite and bottom wrapping mode. In addition, under all strategies of wrapping schemes, the findings also indicated that the deflection ductility index noticeably reduced for RC beams with BFRP composites compared to the control beam. Finally, all the results of midspan deflection, crack patterns, and strain response of the composite system were analysed and discussed briefly.

Deflection Ductility of Slabs Made of Hybrid Mesh-And-Fibre-Reinforced Cementbased Composite

Abstract Steel Mesh-Reinforced Cementitious Composites (SMRCC) (traditionally known as ferrocement) have been in existence for few decades, but have some limitations set on element thickness and number of reinforcing mesh layers and the resulting deflection ductility. Therefore, the author has made an attempt to explore whether deflection ductility will improve in mesh-reinforced cementitious composites (25 mm thick) if discontinuous fibres are added to slab elements. For this purpose, thin slab elements of dimensions 700 mm (length) × 200 mm (width) × 25 mm (thickness) were cast and subjected to four point bending tests. Based on the flexural tests conducted on SMRCC (Control Slab Elements, cast with Steel Mesh Volume of reinforcement, MVr=0.78, 0.94, and 1.23%) and Hybrid Mesh-and-Fibre-Reinforced Cement Based Composite (HMFRCBC) (Test Slab Elements, combining MVr=0.78, 0.94 and 1.23% and Polyolefin Fibre Volume fraction, PO-FVf=0.5-2.5% of volume of specimens, with 0.5% interval), load-deflection and the deflection ductility index were analyzed. From the flexural load-deflection curves it has been observed that HMFRCBC slabs demonstrate higher flexural load-carrying capacity and deflection ductility when compared to SMRCC slabs. This study shows that higher the polyolefin fibre volume fraction (PO-FVr) from 0.5 to 2.5% (with a 0.5% interval) in HMFRCBC slabs, the higher the flexural deflection ductility. The Deflection Ductility Index (DDI) of HMFRCBC (with 5 layers of mesh and PO-FVr=2.5%) is 4.5 times that of SMRCC. This study recommends that HMFRCBC can be used as an innovative construction material due to its higher flexural ductility characteristics.

NEAR SURFACE MOUNTED BAMBOO REINFORCEMENT FOR FLEXURAL STRENGTHENING OF REINFORCED CONCRETE BEAMS

Near Surface Mounted (NSM) is a technique performed for the installation of strengthening material into grooves cut into the concrete cover of reinforced concrete (RC) beams bonded using a bonding agent. This technique is becoming more widely recognized because of its efficiency, effectiveness, and ease of application. We investigated flexural strengthening of RC beams with the NSM technique using bamboo reinforcements, through both experimental tests and a finite element analysis (FEA). The experimental tests were carried out on three RC beams, one consisting of a control beam, and the other two strengthened by the NSM technique with two steel reinforcements, and four bamboo reinforcements. From the experimental tests, we found that the flexural strength of the beam with NSM bamboo reinforcements was increased by 41.7% and the deflection ductility index was reduced by 21.55%. The mode of failure observed in all the strengthened beams was a flexural failure. Finally, the result of FEA behaved similarly to the results of the experimental test.

Flexural Strength and Ductility of CFRP Strengthened Reinforced Concrete Beams

A total of fourteen beams, 100×150 mm in cross-section were tested in the laboratory over an effective span of 2000 mm. Two of them were used as reference beams. Twelve fiber reinforced concrete beams were provided with externally bonded CFRP laminates at the soffit of the beam. The variables considered included number of CFRP layers, yield strength of steel reinforcement (fy) and steel reinforcement ratio (). All the beams were tested until failure. The test results showed that the ultimate load carrying capacity increased by 56% as average by increasing of the ratio of steel reinforcement from (0.0127 to 0.0324). The deflection ductility index DDI values averaged (1.80) and (1.75) for one-layer strengthened beams and two-layer ones, respectively. The corresponding energy ductility index EDI values averaged (1.75) and (1.73), respectively. The DDI and EDI for the control beams were 4.61 and 6.24, respectively. With the exception of the control beams, all of the beams exhibited poor ductility. Failures in all strengthened beams were accompanied by the release of large amounts of energy (known as elastic energy) relative to inelastic energy. Therefore, a reasonable factor of safety should be used in the design of FRP strengthened reinforced concrete members.

Ductility enhancement of 60-m long span prestressed concrete girder using new high strength prestressed strands (2,160 and 2,400 MPa)

In this study, quantitative evaluation on the enhancement in the ductility of 60-m long span prestressed concrete (PSC) girder to which the high-strength prestressed (PS) strands were applied will be made through the finite-element analysis. The extent of enhancement in the ductility was quantitatively evaluated by computing the deflection ductility index in accordance with the types of the PS strands. As the results of comparison with the deflection ductility index of PSC girder in which PS strands with a tensile strength of 1,860 MPa were used, the ductility was improved by about 1.38 folds when the PS strands with a tensile strength of 2,160 MPa are applied while there was approximately 1.71 folds enhancement in ductility when the PS strands with a tensile strength of 2,400 MPa were applied. In addition, the presence of large quantity of tendons can induce brittle failure of the girder due to the concrete fracturing in the compression region without the yielding of the PS strands. Therefore, it is necessary to reduce the quantity of tendons in order to prevent brittle failure and to secure the ductility, and, for this purpose, it is deemed appropriate to use high-strength PS strands.

Flexural performance of an innovative Hybrid Composite Floor Plate System comprising Glass–fibre Reinforced Cement, Polyurethane and steel laminate

This study explored the flexural performance of an innovative Hybrid Composite Floor Plate System (HCFPS), comprised of Polyurethane (PU) core, outer layers of Glass–fibre Reinforced Cement (GRC) and steel laminates at tensile regions, using experimental testing and Finite Element (FE) modelling. Bending and cyclic loading tests for the HCFPS panels and a comprehensive material testing program for component materials were carried out. HCFPS test panel exhibited ductile behaviour and flexural failure with a deflection ductility index of 4. FE models of HCFPS were developed using the program ABAQUS and validated with experimental results. The governing criteria of stiffness and flexural performance of HCFPS can be improved by enhancing the properties of component materials. HCFPS is 50–70% lighter in weight when compared to conventional floor systems. This study shows that HCFPS can be used for floor structures in commercial and residential buildings as an alternative to conventional steel concrete composite systems.