Research on the Seismic Performance of Concrete Compressed‐Flexural Members After Cumulative Damage From 100 Years of Design Usage

Wang Y., Li Y., Lu L., Wang F., Wang L., Liu Z., Jiang J.

Durairaj R., Varatharajan T., Srinivasan S.K., Gurupatham B.G., Roy K.

This paper deals with an experimental study of the flexural behavior of sustainable reinforced cement concrete (RCC) beams with a smart mortar layer attached to the concrete mixture. In total, nine RCC beams were cast and tested. Two types of reinforced concrete beams were cast, and three different beams of sizes 1000 × 150 × 200 mm and six different beams of sizes 1500 × 100 × 250 mm were considered. The flexural behavior of these RCC beams was studied in detail. The electrical resistivity of these beams was also calculated, which was derived from the smart mortar layer. Research on the application of smart mortars within structural members is limited. The experimental results showed that the smart mortar layer could sense the damage in the RCC beams and infer the damage through the electrical measurement values, making the beam more sustainable. It was also observed that the relationship between the load and the fractional change in electrical resistance was linear. The fractional change in electrical resistivity was found to steadily increase with the increase in initial loading. A significant decrease in the fractional change in electrical resistivity was seen as the load approached failure. When a layer of mortar with brass fiber was added to the mortar paste, the ultimate load at failure was observed and compared with the reference beam specimen using Araldite paste. Compared to the hybrid brass-carbon fiber-added mortar layer, the brass fiber-added mortar layer increased the fractional change in the electrical resistivity values by 14–18%. Similarly, the ultimate load at failure was increased by 3–8% in the brass fiber-added mortar layer when compared to the hybrid brass-carbon fiber-added mortar layer. Failure of the beam was indicated by a sudden drop in the fractional change in electrical resistivity values.

Madan C.S., Panchapakesan K., Anil Reddy P.V., Joanna P.S., Rooby J., Gurupatham B.G., Roy K.

Concrete structures provided with steel bars may undergo deterioration due to fatigue and corrosion, which leads to an increase in repair and maintenance costs. An innovative approach to eliminating these drawbacks lies in the utilisation of glass-fibre-reinforced polymer (GFRP) sheets as reinforcement in concrete structures instead of steel bars. This article relates to the investigation of the flexural behaviour of ordinary portland cement (OPC) concrete slabs and high-volume fly ash (HVFA) concrete slabs reinforced with bi-directional GFRP sheets. Slab specimens were cast with 60% fly ash as a replacement for cement and provided with a 1 mm-thick GFRP sheet in 2, 3 and 4 layers. The flexural behaviour of slabs reinforced with GFRP sheets was compared with that of the slabs reinforced with steel bars. Experiment results such as cracking behaviour, failure modes and load–deflection, load–strain and moment–curvature relationships of the slab specimens are presented. Subsequently, the nonlinear finite-element method (NLFEM) using ANSYS Workbench 2022-R1 was carried out and compared with the experimental results. The results obtained from the numerical investigation correlated with the experimental results. The experimental investigation showed that the HVFA concrete slabs reinforced with GFRP sheet provided a better alternative compared to the steel reinforcement, which led to sustainable construction.

Sivanantham P., Gurupatham B.G., Roy K., Rajendiran K., Pugazhlendi D.

The plastic hinge is the most critical damaging part of a structural element, where the highest inelastic rotation would occur. In particular, flexural members develop maximum bending abilities at that point. The current paper experimentally investigates the influence of steel fiber reinforcement at the plastic hinge length of the concrete slab under repeated loading, something which has not been reported by any researcher. Mechanical properties such as compressive strength and tensile strength of M20-grade concrete that are used for casting specimens are tested through the compressive strength test and the split tensile strength test. Six different parameters are considered in the slab while carrying out this study. First, the conventional concrete slab and then the steel-fiber-reinforced slab were cast. The plastic hinge length of the slab was calculated through different empirical expressions taken from methods by Baker, Sawyer, Corley, Mattock, Paulay, Priestley and Park. Finally, the steel fiber was added as per methods detailed by Paulay, Priestley and Park in the plastic hinge length mechanism in the concrete slab at 70 mm and 150 mm separately. The results arrived through experimental investigation by applying repeated loads to the slab, indicating that steel fibers used at critical sections of plastic hinge length provide similar strength, displacement, and performance as that of the conventional RCC slab and fully steel-fiber-reinforced concrete slabs. Steel fiber at a plastic hinge length of slab has a better advantage over a conventional slab.

Madan C.S., Munuswamy S., Joanna P.S., Gurupatham B.G., Roy K.

Fiber-reinforced polymer (FRP) rods are advanced composite materials with high strength, light weight, non-corrosive properties, and superior durability properties. Under severe environmental conditions, for concrete structures, the use of glass-fiber-reinforced polymer (GFRP) rods is a cost-effective alternative to traditional steel reinforcement. This study compared the flexural behavior of an OPC concrete slab with a high-volume fly ash (HVFA) concrete slab reinforced with GFRP rods/steel rods. In the fly ash concrete slabs, 60% of the cement used for casting the slab elements was replaced with class F fly ash, which is emerging as an eco-friendly and inexpensive replacement for ordinary Portland cement (OPC). The data presented include the crack pattern, load–deflection behavior, load–strain behavior, moment–curvature behavior, and ductility of the slab specimens. Additionally, good agreement was obtained between the experimental and nonlinear finite element analysis results using ANSYS 2022-R1. The study also compared the experimental moment capacity with the most commonly used design standard ACI 440.1R-15. This investigation reveals that there is a huge potential for the utilization of GFRP rods as reinforcement in fly ash concrete slabs.

Tang Y., Xiao J., Zhang H., Duan Z., Xia B.

• 100% recycled coarse and fine aggregates were selected to prepare full recycled aggregate concrete with partial recycled powder. • Effects of recycled aggregates and recycled powder contents on the mechanical and stress–strain behaviors were studied. • An empirical constitutive relationship for full recycled aggregate concrete was established and discussed. This paper proposed fully recycled aggregate concrete (FRAC), which was prepared by completely replacing natural aggregates (NA) with recycled coarse and fine aggregates (RCA and RFA) and partially replacing cement with recycled powder (RP). Four different aggregate systems (i.e., full-NA, full-RCA, full-RFA and full-RAs) and three different contents of the RP (i.e., 10%, 20% and 30% of the total binder) were taken as parameter variables. The mechanical properties (compressive and tensile strength) and uniaxial compressive stress–strain relationship of FRAC were investigated. The experimental results show that the addition of recycled materials has an adverse effect on the concrete strength. Incorporating RAs or RP has little effect on the ascending branch of the normalized stress–strain curves, but the former makes the descending branch of the curves steeper, while the latter makes them flatter. The elastic modulus is reduced due to the incorporation of RAs and RP. Based on the test results, an empirical stress–strain model was proposed and discussed.

Zhou Z., Xu H., Gardoni P., Lu D., Yu X.

• A bivariable function considering both mainshock and aftershock intensity measures is used to develop demand model. • A Bayesian approach is used to calibrate demand models. • The Gardoni bounds is used to quantify the variability in the fragility estimates. • The uncertainties in the seismic fragility estimates are quantified. This paper presents a formulation for developing seismic demand models and estimating fragilities for reinforced concrete (RC) structures under mainshocks (MSs) only and mainshock-aftershock (MS-AS) sequences. The demand model for the MS only is a function of the MS intensity, while the model for the MS-AS sequence is a function of two variables, one describing the MS and one describing the AS. A Bayesian method is used to calibrate the demand models. The demand models are then used to estimate the fragility of RC structures. Predictive fragility curves are developed to include the uncertainties in the model parameters estimated by Bayesian approach, and confidence bounds of the fragility curves are developed to separate the effects of the uncertainty in the model parameters from the other sources of uncertainties. The proposed formulation is illustrated using a typical five-story RC frame building. The results show that the fragility curves for MS only tend to significantly underestimate the fragility curves for MS-AS sequences. Also, the uncertainty in the fragilities for the MS-AS sequence is, as expected, larger than that considering only the MS.

Xiao F., Jiang D., Wu F., Zou Q., Chen J., Chen B., Sun Z.

To investigate the effects of prior cyclic loading damage in rocks on subsequent unloading failure characteristics under true-triaxial conditions, a series of complicated unloading tests incorporating the damage-controlled cyclic loading path and stress σ 3 unloading path was conducted using a true-triaxial test system. The experimental results reveal that the prior cyclic loading damage has an impact on the strength and deformation characteristics , energy conversion and failure mode. As the number of prior cyclic loads increases, the unloading strength and Young's modulus increase firstly and then decrease, while the peak unloading strain, as well as the ratio ( η ) of crack damage stress to peak unloading stress, exhibits a descending trend. The energy storage capacity of rock samples is dramatically reduced as cycle number increases to 10 and enlarged slightly with a further increase in cycle number. Both shear fracture and tensile fracture appear in each rock sample under this unloading condition, as the prior cyclic loading number increases, the dominant failure mode of rock samples changes from tensile failure to mixed tensile-shear failure, then to shear failure, while the failure angle ranging from 65° to 80° deceases firstly and then turns to rise.

Lv J., Zhou T., Du Q., Li K.

In view of the excellent deformation capacity and energy absorption capacity, rubber particles had potential possibility to improve the fatigue properties of lightweight aggregate concrete (LC). However, the fatigue properties of LC containing rubber particles were not adequately understood. This paper focused on the effect of rubber particles on the uniaxial compressive fatigue properties of self-compacting rubber lightweight aggregate concrete (SCRLC). The results of uniaxial compressive fatigue tests indicated that fatigue life and fatigue strain of SCRLC added generally with increase of rubber particles substitution percentage, fatigue strain of SCRLC also raised with number of cycles increased. Analyses results indicated that the fatigue life of SCRLC conformed to two-parameter Weibull distribution. Based on the experimental results, fatigue equation of SCRLC was established by double logarithmic fatigue equation. From the curves of lgS-lgNf of SCRLC, it could be discovered that increasing of rubber particles substitution percentage in SCRLC resulted in a decrease of fatigue limit strength, but the stress level at fatigue life of 2 × 106 increased firstly and then decreased. The highest stress level of SCRLC would be obtained as rubber particles substitution percentage was 30%. By comprehensive consideration, under the same strength level, the fatigue properties of SCRLC was better than that of LC.

Amiri S., Bojórquez E.

Residual displacement demand is a key indicator of seismic damage and plays a pivotal role in post-earthquake decision making on damaged structures. This parameter can be effective as an estimator of the residual capacity of structural systems damaged under seismic sequences. Residual displacement ratio (Cr) is a conventional approach to predict the residual displacement demand from maximum linear displacement. This study investigates the residual displacement ratios of structures as bi-linear SDOF systems subjected to mainshock-aftershock sequences. Constant-strength spectra based on Cr are developed taking into account the effects of ground motion parameters (such as site condition, magnitude, epicentral distance, and duration), and of structural characteristics (such as strength modification factor and post-yield stiffness ratio). Several analytical equations are proposed to predict the residual displacement ratio as function of the elastic vibration period and the strength modification factor for different levels of the post-yield stiffness ratio and also various aftershock intensities. The proposed equations can be employed for seismic assessment of structures against mainshock-aftershock sequences.

Ji D., Wen W., Zhai C., Katsanos E.I.

120 earthquake ground motions recorded on soft soil sites were employed to assess, through response history analysis of simplified systems, the residual displacement demand, Cr, defined as the ratio of the residual displacement to the maximum elastic displacement. Single degree of freedom systems were considered and the lateral strength ratio was parametrized to account for varying structural inelasticity. Four hysteretic laws were chosen to represent degrading and non-degrading performance while variation in the post-yield stiffness was considered. The analysis scheme enabled assessing the relationship of the residual displacement with the aforementioned structural characteristics. The residual displacement demand was found to be sensitive in the post-yield stiffness ratio. An equation was finally introduced to accommodate the reliable estimation of residual displacement ratio, the latter being beneficial for evaluating the seismic performance of existing structures built on soft soil sites.

Zhou M., Liao J., An L., Deng W., Hassanein M.F., Yu Z.

A series tests on concrete and mortar cylinders under cyclic uniaxial loading were carried in this study to investigate the effect of internal stress-induced cracks on their mechanical properties. It was found that the radial strain cannot be changed continuously because the internal circular cracks appear firstly inside the specimens. Moreover, a unified approach for calculating the apparent volumetric strain of the cylindrical specimens at both elastic and inelastic stages was proposed. Additionally, the Acoustic emission (AE) experimental result shows that the AE hits rate can well reflect the crack formation and propagations of the concrete specimen under the cyclic axial compression.

Chitrala S., Jadaprolu G.J., Chundupalli S.

The present investigation is mainly focused on studying the complete stress-strain characteristics of geopolymer concrete (GPC) with different fine aggregate blending. In this study, granite fines (GF) were used as a partial replacement of fine aggregate. Sand and GF were used as fine aggregates blended in different proportions (100:0, 80:20, 60:40 and 40:60) (sand:GF) by weight. GPC cylindrical specimens were tested under compression and the results obtained from the tested data were analyzed to determine the compressive strength (fcm), stress-strain relationship, peak strain (ep), linearity of the stress-strain curve, ultimate strain (eu), various modulus of elasticity (MOE) values, and Poisson’s ratio (μ) of GPC after a period of 7, 28 and 90 days respectively. From the results, it is concluded that the increasing trend was observed in the properties till 40% (60:40) of GF replacement and then these values were decreased. So, optimum fine aggregate was blended at 60:40. Based on the test results, new models were developed for predicting the stress-strain characteristics of GPC under compression by using regression analysis. The results of proposed models were then compared with the experimental values and the predicted equations by various codes and past research.

Del Zoppo M., Di Ludovico M., Balsamo A., Prota A., Manfredi G.

The high vulnerability of existing Reinforced Concrete (RC) structures, even to moderate seismic events, has been confirmed from recent post-earthquake surveys. Short and wall-like RC columns are particularly prone to brittle failures, governed by concrete crushing. To reduce the vulnerability of existing RC columns, the use of externally bonded Fibre Reinforced Polymer (FRP) reinforcement has been recognized as an effective method for preventing the aforementioned brittle failure and, hence, increasing members' lateral capacity and ductility. In the first part of this study, the results of an observational analysis on columns shear failures in RC buildings severely damaged after the L'Aquila earthquake are presented. The second part of the study presents and discusses the results of an experimental program carried out on seven short RC columns governed by shear failure under load reversal and compressive axial load. Both columns in “as built” configuration and strengthened in shear with discontinuous carbon FRP (CFRP) strips have been tested. Two classes of concrete have been used, in order to simulate structures with medium or poor material quality, and different external reinforcement ratios have been investigated. The specimens' responses have been analysed in terms of failure modes, strength/deformation capacity and strain distribution in CFRP strips.

Ruiz-García J., Guerrero H.

Seismic demands computed from earthquake ground motions recorded at soft soil sites have shown different spectral trends with respect to those computed from earthquake ground motions gathered from accelerographic stations placed on firm sites. Therefore, an examination of the spectral trend of residual displacement ratios, C r , which allow the direct estimation of lateral residual displacement demands from their maximum linear elastic demands, for elastoplastic single-degree-of-freedom systems subjected to earthquake ground motions recorded at soft soil sites of the lake-bed of Mexico City and the San Francisco Bay Area is presented in this technical note. A functional form to obtain estimates of mean residual displacement ratios for this type of sites is introduced and its ability to capture the empirical spectral trend of C r is illustrated in this paper.