Abstract

     This paper presents a rapid machine learning-based damage detection framework for identifying the damage extent of concrete shear wall buildings. For this purpose, a parametric study was carried out to determine the most efficient machine learning algorithm in classifying the damage states of the building. According to this parametric study, the K-Nearest Neighbor (KNN) learner was selected as the reference prediction model because of the higher accuracy achieved by this algorithm. Bayesian Optimization (BO) algorithm was used to tune the hyperparameters affecting the accuracy of the model. The most efficient attributes were selected from the set of damage indicators through the BO algorithm to train the model. Three different benchmark buildings, including 7-,9-, and 13-story concrete shear wall buildings, were used to evaluate the robustness of the proposed framework. A suite of 111 pair motions, originally developed for the SAC project, were employed to create a generalized dataset. These motions were uniformly scaled from 0.05 g to 1.5 g to expand the intensity range of the events. All the acceleration signals were polluted to 10% noise using white Gaussian signals to simulate the field condition. Results reveal the efficiency of the proposed framework in identifying the extent of damage in concrete shear wall elements of the building. In addition, a parametric study was conducted to illustrate the reliability of two commonly used features, called Cumulative Absolute Velocity (CAV) and the energy ratio between the acceleration response and the input excitation, in determining the damage states of the shear walls under seismic motions.

Introduction

     Assessment of structural safety is essential for post-earthquake restoration. Generally, to evaluate the post-earthquake vitality of the exposed structures, a complete visual inspection is required [1]. Coordination and implementation of the manual visual inspection needs several dedicated teams and monetary resources. In this regard, considerable efforts have been carried out to automate the visual inspection process, e.g., image-based visual inspection [2]. However, such an engineering visual inspection is only able to detect the visible defects that occurred in the structures [3]. This means that some serious invisible damages may be left latent during the visual inspection. The process of identifying and tracking the structural damage is known as the Structural Health Monitoring (SHM) [4,5]. In SHM, damage detection is related to the methods developed for identifying the probable existence, severity, and location of the structural damage. Model-based and data-driven methods are two of the most commonly used strategies proposed for damage detection. Model-based methods generally involve a system identification algorithm paired with a finite element analysis to update the structural model [6]. The performance of the model-based approaches directly depends on the accuracy of the information about the physical properties of the under-study structure. In addition, updating the finite element model based on the physical properties of the structure is computationally expensive for large-scale structures, and rapid condition monitoring could be challenging in this condition [7,8]. On the other hand, data-driven methods apply statistical learning algorithms to the vibration data captured from the structure. This method uses learning algorithms to construct a classification or regression learner for predicting structural damage [9, 10]. Tsou and Shen [11] proposed the use of Neural Networks (NNs) for predicting the severity and location of the structural damage. They used the variations in the modal properties of the structure as the damage feature to identify the damage. Worden et al.

Conclusion

     This paper presented a rapid algorithm for identifying the severity of local damage in the concrete shear wall buildings. A total number of 1884 nonlinear response history analyses were conducted for each building using the SAC motions. A suite of damage indicators was extracted from the acceleration signals to construct the prediction models. A parametric study was carried out to determine the most efficient learner for classifying the damage states of the buildings. The KNN classifier was selected to construct the predictive models because of its maximum accuracy compared to the other algorithms. The Bayesian optimization algorithm implemented to tune the hyperparameters of the classification learners. The main conclusions derived from the study are as below:

Abstract

The marine environments reduce the working life of the concrete structures owing to the severe exposure to the chemical ingress, and continuous contact with the humid air, and saline soil conditions. A scientific investigation and the conditional assessment of the existing structures may be helpful to learn the damage mechanism. This article represents the scientific conditional assessment of some existing old concrete structures in the coastal areas of Jamnagar and Dwarka of Gujarat. The objective of the study was to collect information on the types and severity of the damagesin the existing structures and preparing an assessment rubric mentioning the significance of modes of marine environment resulting in a specific damage type. To evaluate the severity of the damages, the parametric approach and a root cause analysis including the experimental investigations on the collected samples has been utilized. The outcomes revealed several interesting observations those have been classified into three major attributes namely material limitations, structural design and third as the gross environmental impacts. The study encouraged the authors to dig deeper in the area of work and showed potential of the methodology to understand the damages caused by the marine environments to the on-shore concrete structures.

Introduction

The marine exposure condition imposes a challenging environment on concrete as a material, structures and member. The long-term performance of concrete in marine environment is observed to be subjected on a complex integrated physical and chemical mechanisms [1]. The marine environment is not always regarded as the sea water, but it is the combination of the marine sand /ground, air and temperature at and around the coastal regions and the shallow as well as deep sea water together. In such cases, the damage occurred on the concrete structures needs to be evaluated or assessed for the damage with a systematic approach. Therefore, conditional assessment of the marine structures is merely not similar to the other structures. In fact, how to assess the damages of concrete in marine environments is a challenge because the nature of degradation and skill of damage varies with the location of marine structure. The concrete in the marine environment along with the reinforcements of steel bars shows damages namely corrosion, efflorescence, staining, erosion, cracking, mortar spalling and surface losses, lump mass losses, and many others. Regarding reinforced concrete in particular, chloride ingress by diffusion is probably the most important mechanism of deterioration, leading to corrosion of the embedded reinforcing steel. However, the overall situation is complicated by concomitant deterioration of the material itself by sulfate attack and physical effects such as salt crystallization and mechanical action of the waves.There are references available to refer on the topic of assessment of damage in marine environments. Use of common methods of assessment includes use of rebound hammers, porosity meters, carbonation tests on the samples [1] providing preliminary damage information, use of non-destructive tests methods and visual inspections [2], [3], on site assessment of the strength parameters of the concrete [4], use of permeability apparatus for fluid ingress, destructive tests on samples at the site and chemical analysis of the samples in laboratories [5], [6], assessment of the degree of the distresses in the concrete due to marine environment by visual and analytical inspections of the sites [7], applications of electromagnetic methods on the existing structures [8], the corrosion detection of the damages structures in the repaired structures by.

Conclusion

From the case study and the proposed systematic assessment methodology, the authors have observed that the conditional assessment of a marine structure requires a panoramic perspective on the investigations. The site survey remains the key factor for obtaining the exact details of the structural conditions, however the pre-assessment survey and related inquiries supports the process significantly. The laboratory tests and results contributed to a significant extent in ascertaining the scientific reasons for the damage. The study encouraged the authors to propose an overall severity rubric based on the systematic assessment process suggested in the present study. Table 4 indicates the overall rating to be utilized for declaring the damage scenario of the structures impacted by the marine environment.

Abstract

Concrete exposed to splash zone in partially submerged concrete structures deteriorates rapidly due to reasons such as wetting-drying cycles, the impact of waterborne materials, bio-deterioration, chloride and sulphate attacks. Existing condition assessment and rating systems do not adhere to the interrelation between different performance attributes of splash zones and these models are extremely subjective to the performance of the evaluator. The objectives of this study are to evaluate the effectiveness of parameters that deteriorate marine concrete structures and to develop a new model to predict the rate of deterioration of splash zone in marine concrete structures. The fuzzy Analytical Hierarchy Process (FAHP) was used to develop the model to predict the rate of deterioration. Here, local factors were grouped under five primary parameters as visual inspection, water quality, casual factors, Non-Destructive Tests (NDTs) and crack details. Data relevant to 15 structures were gathered by visual inspection, laboratory and field experiments to validate the proposed model. Attributes of water quality achieved the highest priority for the deterioration of the slash zone of marine concrete structures. Further, numerical values were assigned to this rating model to avoid ambiguity and vagueness of linguistic descriptors in existing rating systems. Finally, the model developed in this study can be used to evaluate the deterioration percentage of the splash zone in structures and according to that appropriate retrofitting and repair work can be done.

Introduction

At the present, reinforced concrete has been widely used in coastal and underwater structures in order to construct bridges, jetties, breakwaters and floating structures. However, concrete is more prone to deteriorate due to the aggressive environmental conditions in the atmospheric and splash zone due to chloride content in water, sulphate attacks, abrasion erosion caused by slit, sand and gravel, wetting and drying cycles and due to presence of waste and chemicals in water [13], [5]. These environmental parameters will cause cracking, spalling and corrosion which leads to incurring an enormous amount of money on repairing purposes and also this will reduce the structural health and safety of structures [23].

According to researchers, deterioration of concrete structures due to chloride induced reinforcement corrosion is a major problem around the world and reinforcement in partially submerged concrete structures is mainly corroded due to the ingress of chloride ions [18], [39]. According to Neville [20], a protective oxide layer that consists of γ-Fe2O3 is formed on steel which is generated soon after the hydration of concrete has started. Chloride ions that ingress inside concrete can be dissolved in pore solution (free chloride) or bound to the cement hydrates (bounded chloride) and only free chloride contributes to breaking this passive layer which results in corrosion [17]. When the concentration of chloride ions in the pore solution around the steel surface reaches the threshold value this film will be destroyed [4].

Conclusions

In this study, a rating model was developed to predict the rate of deterioration of the splash zone of marine concrete structures and the model was validated by assessing the current condition of 15 existing marine concrete structures. The model was developed based on FAHP and numerical definitions were used to avoid subjectivity of evaluators. According to the model, water quality has the highest priority for the deterioration of the splash zone of the structure and visual inspection, causal factors, crack detail and NDT have priority weights in the decreasing order. The proposed method can be used to rate the deterioration percentage of existing marine concrete structures in tropical countries and according to the results obtained from the model, proper remediation can be done. As future research work in this domain, freeze-thaw action can be taken into consideration in order to modify this model.

Abstract

     As a key component in composite beams, the studs are prone to fatigue failure under reciprocating load. Fatigue problem is more prominent especially under circumstances of concrete cracking, stud corrosion or with initial damage. Therefore, static and fatigue tests are carried out to study the fatigue properties of push-out specimens with initial defects, including stud damage and concrete cracking. The test results show that the damage of stud has a great influence on fatigue life, while the crack of concrete has a little influence on fatigue life. The damage of stud has a greater influence on fatigue performances compared to static properties. In addition, based on the dissipative energy theory and experimental phenomenon, the roles of stud and concrete in the process of stud fatigue failure and their interaction are summarized. The dissipative energy mainly includes the shear plastic strain energy of the stud, the plastic strain energy of concrete under compression after the stud, and the heat energy generated by the friction between the stud and concrete.

Introduction

     Steel-concrete composite beams have become popular in recent years for use in bridge engineering [1,2]. Under the complicated environment and load, there may be some damage to the composite beam. There might be some damage, such as corrosion, fractures of weld leg and longitudinal or transverse cracks in the concrete deck. The static and fatigue behavior of stud connection is sophisticated and affects the design resistance and stiffness of composite beams [3,4]. The damage of the stud and concrete may have a great influence on the shear fatigue performance of the interface.

      Welding defects significantly lower the shear capacity of studs, as is widely known. There are many types of welding defects that might appear, such as inadequate penetration, absence of fusion, and gas inclusion [5]. To determine how the damage location and degree affected the static behavior and shear capacity of stud shear connections, six specimens with identical geometrical dimensions were investigated by Qi et al [6]. The test findings showed that, in comparison to the shear capacity of normal specimen, a loss of up to 36.6 % and 62.9 % of the shank area might cause a falling shear capacity of 7.9 % and 57.2 %, respectively. The push-out test was simulated using mathematics, and the results of the test were used to validate the study. It was shown that even though the area of the stud had significantly decreased, the shear capacity was not affected by the severity of the damage when the damaged part was placed 0.5d (where d is the shank diameter) from the stud root, based on the numerical model.

Conclusions

     Static and fatigue tests were carried out on studs in steel–concrete composite study the effects of initial damage in stud and concrete on fatigue performance. The following conclusions are drawn:

     1.  Compared with nondestructive specimens, the fatigue life of transverse crack and longitudinal crack specimens were reduced by 22 % and 4.9 %, respectively, which is determined by the stress characteristics of the stud. Transverse crack makes the stud easily in a state of bending and shear composite, while longitudinal crack does not

     2.  Compared with the preset cracks of concrete, the damage of stud and weld collar had more influence on fatigue performance, in which the loss of stud section of 62.9 % leaded to only about 10 % of fatigue life. The initial damage of stud accelerates the damage accumulation in the stable fatigue development stage, while the concrete cracking leads to the increase of residual slip in the early stage.

     3. The existing calculation methods of residual slip are not completely applicable to the test results of different stresses, and most of the calculation results are too small, which requires some modification of their coefficients.

Reinforced concrete (RC) is one of the most widely used construction materials in the world. RC structures are usually regarded as permanent structures that can be free of severe degradation problems for a long period time owing to their good service performance. Although steel bars embedded in concrete are normally protected against corrosion during the service life of a structure, the concrete cover can deteriorate under the effects of some aggressive agents, leading to corrosion of the steel bars in concrete. For RC structures, corrosion of steel bars is one of the most important factors causing durability problems [1]. On the one hand, corrosion can lead to the cross-sectional loss of steel bars and degradation of their mechanical properties thereby reducing the bearing capacity of a structural member. On the other hand, steel bars corrosion can also degrade the bonding behavior between the concrete and steel, thereby inducing stiffness reduction, which can increase the deflection during the service life. Much research has been done on behavior of corroded RC beams. However, the most previous work has focused on the short-term behavior of corroded RC beams where the beam specimens were pre-corroded before loading to failure. Cabrera [2], Mangat et al. [3] and Torres-Acosta et al. [4] found that transverse crack width and crack spacing increased with the corrosion degree, but the quantity of cracks decreased. Actually, corrosion of steel bars normally takes place while a structure is subjected to a sustained load. The sustained load may lead to an accelerating effect on the corrosion process [5,6]. Furthermore, beam behavior under the combined effects of loading and corrosion is time-dependent. To predict the service life of corroded RC structures, it is important to clarify the time-dependent behavior of the corroded beam. Combined action of loading and corrosion was considered in the experiments by many researchers, and different current densities have been applied in the accelerated corrosion process. Experimental study by Ballim et al. [5] showed that the sustained load effect on the deflection of a RC beam could not be ignored during the corrosion process. The current density applied in the experiment carried out by Ballim et al. was 400 lA/cm2 , which could get an ideal corrosion degree in a short period of time. However, the common current density that governs the corrosion rate of actual inservice corroding concrete structures normally ranges between 0.1 and 100 lA/cm2 [7,8]. Additionally, the nominal current density applied in the accelerated corrosion testing should be less than 200 lA/cm2 [9]. The experiment carried out by El Maaddawy et al. [10] and Malumbela et al. [11,12] used a relatively low current density. EI Maaddawy et al. monitored the corrosion crack development and the deflection of the testing beams under the current density of approximately 150 lA/cm2 during the testing process; moreover, results showed that the sustained load significantly reduced the initiation time of corrosion-induced cracking and slightly increased the corrosion crack’s width. The results also indicated that the flexural crack induced by the sustained load could initially increase the corrosion rate of the steel bars. Malumbela et al. did a series of experiments on the beams under the current density of 189 lA/cm2 and recorded the deflection, corrosion crack width, crack pattern, and so on. The results also suggested that effects of the sustained load could not be ignored.

This work deals with the flexural performance of recycled concrete subjected to increasing loads up to failure. For this purpose, eight reinforced concrete beams were made with recycled coarse aggregates using two different water to cement ratios (0.50 and 0.65) and four replacement percentages (0%, 20%, 50% and 100%). Firstly, the basic concrete properties were determined (mechanical strengths and modulus of elasticity) and then, beam specimens were loaded up to failure using a four-point bending test at 28 days. As a result, bending moments, deflections, strains and curvatures were obtained at different load levels (cracking, service, yielding and ultimate state conditions), and also, the crack pattern. On the basis of these results, it can be noted that service, yielding and ultimate state of recycled concrete exhibits, in general, a similar trend to that of conventional concrete. However, the cracking behaviour shows differences between recycled and conventional concrete. Finally, code-based expressions were used to calculate bending moments and deflections under flexural load, taking into account the different content of recycled coarse aggregate.

Introduction

In order to promote the sustainability of concrete, efforts have been made to address some of the environmental problems associated with concrete waste. In line with this, numerous researches have been conducted to analyse the structural performance of recycled concrete [1–13]. However, its use in real building and civil engineering applications requires more full scale studies, to assess the load-deformation response of recycled concrete that leads to good agreement on structural design. Regarding flexural performance of structural recycled concrete, different investigations have been carried out [4,8,9,12]. However, the number of studies on concretes with high replacement percentages is scarce and additionally, some contradicting conclusions have been detected. While some authors [1,9,10,13] have found that recycled concrete beams show higher deflections, and cracking moments lower or similar to those of the conventional, others [6,8] have reported no significant difference between recycled and conventional concrete in terms of flexural performance. On the basis of the literature review, it can be seen that the codebased procedures for conventional concrete can also be applied to recycled concrete predictions for flexural behaviour [2,6,9], although some differences have been detected. In line with this, the increase in recycled coarse aggregate results in the reduction of concrete stiffness [9], which is consistent with the increased beam deflections of recycled concrete. Most authors [2,8–10] point out similar yielding and ultimate behaviour, but some [7] have found little differences in cracking behaviour, in terms of crack spacing and pattern. In general, it can be said that recycled concretes are able to fulfill strength and serviceability requirements similarly to conventional reinforced concrete. However, in order to encourage its use as structural concrete, it is important to be able to design reinforced concrete members made with recycled coarse aggregate, using existing design methods [6]. Accordingly, recent publications [2,9] have indicated that more full-scale research should be done to stablish a good agreement on its flexural performance and increase the results database for structural recycled concrete. This would lead to concrete properties, service-load and ultimate–load behaviour being predicted with the same approximation degree as conventional concrete. Furthermore, with the aim of carrying out an accurate structural design, some concrete parameters have to be considered such as, tension stiffening, strength capacity or cracking behaviour. Tension stiffening is known as the concrete contribution after cracking and has significant importance in structural design. This parameter is required to accurately design concrete structures as customary serviceability conditions occur after cracking.

The electrochemical phenomenon of corrosion is a global source of deterioration of reinforced 35 concrete structures. In environments lacking chloride ions (chlorides), low-carbon steel reinforcing bars (rebar) embedded within concrete form a passive film due to the high pH (ca. 13 to 13.5) of the concrete pore solution and are generally not expected to exhibit any significant corrosion activity during the designed service life of a structure. However, external chloride ions, which originate from sources such as ocean saltwater or deicing salts placed on roadways, can permeate porous concrete cover layers and depassivate the steel rebar after a critical threshold of local chloride concentration has been reached at the rebar surface [1, 2]. After depassivation, corrosion activity of the steel rebar readily increases. As the volume of corrosion products is greater than the sum of the reactant volumes, internal expansion of these products causes concrete cracking [3], subsequently increasing the chloride solution permeability of the concrete 45 [4, 5] and reducing the mechanical performance of the composite [6, 7]. Premature replacement of damaged structures and associated mass consumption of additional construction materials is undesirable due in part to the high energy demands required for concrete production [8]. Because reinforced concrete degradation is dependent on the cracked state of the 49 composite matrix, research of novel crack-resistant construction materials has gained popularity in recent decades [9, 10]. In particular, hybrid fiber-reinforced concrete (HyFRC) is a candidate to reduce corrosion-induced cracking damage due to the mechanical toughening provided by the inclusion of different types of short (e.g., 8-mm to 60-mm long), discontinuous 53 fibers dispersed throughout its cementitious matrix [11, 12]. A crack-resistant concrete such as HyFRC is further advantageous considering that a primary functional purpose of reinforced concrete is to resist mechanical loads, requiring any new implemented construction material to be damage-tolerant against not only corrosion-induced cracking but also structural loading [13, 14]. An increase in the time to corrosion initiation was observed for reinforced HyFRC (i.e., HyFRC composite with embedded steel rebar) compared to reinforced concrete after subjecting samples to flexural stress [15, 16]. While a lower corrosion current density icorr of reinforced HyFRC was also measured compared to reinforced concrete, indicating favorable durability performance after active corrosion had begun, electrochemical characteristics of samples were limited to corrosion potential, polarization resistance, and corrosion current density based on assumed Stern-Geary coefficients. Several corrosion-related studies with different types of fiber reinforced concrete have utilized similar techniques [17-20], making detailed information from certain other electrochemical tests, such as Tafel polarization and electrochemical impedance 66 spectroscopy (EIS), rarely available for fiber-reinforced concrete composites.

In field, concrete structures may be subjected to different forms of attack; the most common of which is chloride induced corrosion; especially in coastal environments or whenever deicing agent are used. Once chloride concentration at steel location surpasses a certain threshold value, corrosion initiates and progresses towards cracking of concrete; aggravating the physical states of exposed structural elements [1]. Beams and slabs (or bridge decks) are the most susceptible because their horizontal alignment allow easy intrusion of chloride and hence corrosion initiation and progression. Corrosion damage accompanied with loss in reinforcing steel cross-sectional area as well as undesirable changes in steel surface characteristics and mechanical properties result in sever reductions in the mechanical performance of flexural members [2–6]. Thus, repair becomes essential to preserve structural integrity and prevent catastrophic failures; especially in bridge decks. Repair of corroded flexural elements goes through several steps represented in removal of concrete cover, cleaning of rust, protecting steel, replacing concrete cover with new one, and finally providing additional reinforcement using either concrete jackets, steel plates, or fiber reinforced polymer composites (FRP) [7–10]. Even if existing concrete cover in corroded elements is replaced as a standard measure before strengthening, the probability of cracks’ reappearance in newly cast concrete cover remains high. As well stipulated, cracking of concrete cover in beams or slabs impact negatively their bond with carbon fiber reinforced polymer (CFRP), attached to regain flexural load capacity; undermining the significance of the proposed repair process [11–13]. The intensity, distribution, and size of cracking are key factors that determine the degradation extent in bond between damaged concrete and attached FRP composites [11–14]. Presence of openings in concrete structural elements was stipulated to impact negatively their performance unless efficient repair techniques were applied [15]. Recent published works indicated that cracks, produced by heating of concrete to temperatures of 600 °C, had resulted in significant degradation in bond strength between heat-damaged concrete and CFRP sheets reaching as high 64% [11,12]. Pre and post-repair sodium sulfate attack on flexural elements had a less negative impact on bond behavior between concrete and attached CFRP sheets represented in loss of bond strength by as much as 40% [13]. Load induced cracks are wider yet are more localized than durability induced ones; hence their effect on bond behavior would be different [14]. Thermal incompatibility between FRP and concrete induces additional concrete cover cracking hence aggravates bond performance further [16].