Abstract

Terahertz (THz) and sub-terahertz (sub-THz) waves are unexplored waves between the infrared and microwave ranges. This range features unique characteristics of both light resolution and electromagnetic wave transmission. Sub-THz waves are part of attractive new diagnostic methods for inner objects because they are safer than normal non-destructive inspection methods involving high energy, such as X-rays. In this study, a novel non-contact and non-destructive inspection method for reinforced concrete (RC) structures using sub-THz reflection imaging is proposed to inspect the deterioration of inaccessible damaged RC structures. The results of this study confirmed that the proposed sub-THz imaging method can detect cracks and voids in concrete, as well as in metals inside or behind the concrete, based on differences in reflectance. In addition, the camera-based measurement system makes real-time on-site field measurements possible.

Introduction

In recent years, the deterioration of reinforced concrete (RC) structures has become an issue all over the world. Especially in Japan and other developed countries, large number of social infrastructures built during periods of rapid economic growth are aging significantly ( Alexander and Beushausen, 2019 ). The importance of appropriate maintenance management, including investigation and diagnosis, has been highlighted. In addition, from the perspective of a decarbonized society, it is desirable to avoid demolishing buildings and extend the service life of RC structures through necessary and affordable repairs. Thus, these considerations highlight a growing need for non-contact, non-destructive inspection methods to diagnose infrastructure deterioration without compromising performance. Furthermore, if remote inspection is possible, it is expected to improve measurement efficiency and expand the applicable areas. Therefore, a new inspection method that is not only non-destructive but also non-contact is required.

Several non-destructive and micro-destructive testing methods have been proposed for RC structures. For example, for internal defects that cannot be detected by visual inspection, some of the most widely used methods include percussion inspection ( Kazemi et al., 2019 ), ultrasonic flaw detection ( Lawson et al., 2011 ; Choi et al., 2016 ) and ground penetrating radar (GPR) ( Hugenschmidt and Mastrangelo, 2006 ; Chang et al., 2009 ; Laurens et al., 2005 ; Liu et al., 2020 ; Wiwatrojanagul et al., 2017 ). Table 1 shows a comparison between these methods. During percussion inspection, the concrete surface is knocked, which makes it possible to infer the state of internal deterioration based on the difference in the echo sound. In the case of ultrasonic waves, also a type of elastic wave, it is possible to measure the location of cracks and rebars inside concrete due to differences in acoustic impedance inside the material. Using GPR makes it possible to probe embedded rebars and voids inside concrete owing to its high penetration depth. However, these methods require contact with an object to be measured, which causes measurement problems at elevated or generally inaccessible places, thus highlighting the need for developing a method that can inspect the interior of concrete without contact.

Conclusion

In this study, we conducted measurements using a sub-THz camera to simulate embedded rebar and defects on the surface and inside concrete, assuming that a non-destructive inspection method using sub-THz waves will be applied to reinforced concrete structures. We then conducted actual measurements on real structures. The findings are as follows:

1) An aluminum tape placed on the back of the specimen to simulate the embedded rebar was detected up to 20 mm thickness of the specimen.

2) Field measurements at 10 GHz indicated the possibility of exploring rebars with a cover thickness of approximately 30 mm.

3) In the measurement of specimens with slits simulating cracks, detection was possible for slit widths greater than 1 mm.

4) Even when the slit was hidden by placing a 5-mm-thick mortar, detecting a slit up to 2.5 mm was possible.

5) The reflectance and transmittance varied sensitively with frequency. This fact indicates the potential for more detailed measurement with the selected appropriate frequencies. Further studies are needed to establish their relationship.

6) Camera-based measurement enables areal measurement and real-time monitoring.

7) Downsizing and weight reduction of the measurement system improved its transportability, confirming the possibility of measuring real structures.

Abstract

Composite slabs, consisting of a precast plank and a cast-in-situ concrete topping, are the most commonly used horizontal structural components in prefabricated buildings. In order to solve the problem that the precast plank of composite slab is easy to crack and a lot of vertical supports are needed in its construction, this study proposes a new type of composite slab with a joint. The precast plank of this novel composite slab is fully prefabricated in the midspan area, and concrete is poured on-site only at the surrounding joints. In this paper, the bending performance of three composite slabs with a joint in different shapes of concrete topping and one cast-in-situ slab was investigated. The test results showed that the novel composite slabs with a joint in T-shape and trapezoidal concrete topping had similar properties as the cast-in-situ slab, including bearing capacity and cracking load, which can meet practical requirement. The use of bent-up steel bars and roughening of the interface between the precast plank and topping concrete can ensure effective transmission of internal forces at joint. The bearing capacity of the composite slabs with a joint in T-shape and trapezoidal concrete topping was evaluated using the formula in GB50010–2010, and the experimental values were 1.06 and 1.1 times the theoretical values, respectively, which provides a foundation for the design of two types of composite slabs with a joint. Meanwhile, based on the FEA software ABAQUS, numerical simulation was conducted on the composite slab with a joint in T-shaped concrete topping. The numerical finding exhibit close agreement with experimental results in terms of load-midspan deflection curve and the crack distribution.

Introduction

Precast concrete (PC) structures can be seen in buildings throughout the world due to its advantage of better construction quality, lower labor costs, and shorter construction times [1]. Composite slab, as one of the most commonly used elements in floor systems of PC residential buildings, is formed by bottom precast planks and a cast-in-situ concrete topping. In order to improve the flexural stiffness of the bottom precast plank and increase the bonding performance between the precast plank and the cast-in-situ concrete topping, lattice girders are usually installed in the precast plank. However, due to the limitation of the thickness of the composite slab with lattice girders used in China, the height of the lattice girder is usually small [2]. As a result, the improvement in stiffness of the precast plank by the lattice girder is limited, which requires the erection of vertical supports during the construction process, increasing the construction cost and prolonging the construction period [3]. Therefore, it is very necessary to propose a composite slab that can achieve unsupported construction.

Some improvement measures have been proposed to reduce the use of vertical supports in the construction process of composite slabs. Wu et al. [4] set a detachable truss rib on the bottom of the precast plank to replace the vertical support, realizing the construction without vertical support and improving the crack resistance of the slab. The truss rib that was placed at the bottom of the precast plank can be reused, but it is inconvenient to remove it. Moreover, based on the concept of a steel-concrete composite structure, measures such as concrete ribs and steel ribs were set on top of the precast plank can effectively improve its bending performance [5], [6], [7]. In addition to the above measures, good results have also been achieved by using grout steel tube, special materials, or components. Hou et al. [8] proposed a new type of composite slab with grouted-round-steel tube truss and conducted bending tests on it. The test results indicate that this type of composite slab has good ductility and can meet construction requirements. Ou et al. [9] investigated the flexural performance of composite slabs made of crumb rubber concrete or conventional concrete. They discovered that the bearing capacity and end-slippage of composite slabs made of crumb rubber were equivalent or even better than those made of the conventional concrete. Erfan et al. [10] conducted numerical simulations on concrete beams strength with FRP materials and found that using CFPR sheets to reinforce concrete beams can effectively improve their bearing capacity and prevent crack propagation. Maedeh et al. [11] conducted bending tests on concrete beams with steel reinforcement and a hydrid usage of GFRP reinforcement with varying fiber ratios, respectively. They concluded that concrete beams using 1.5% polypropylene fibers and GFRP rebars can reach the same bending strength as concrete beams using steel bars. Wang et al. [12] proposed a novel bamboo-concrete composite beam with precast lightweight concrete slab. Shear test results indicated that this new type composite slab had a similar shear strength but a higher stiffness that the cast-in-situ bamboo-lightweight concrete composite slab. However, the composite slabs mentioned above have a large amount of concrete poured on site and a low assembly rate.

Conclusions

In this study, the flexural behaviors of composite slabs and three different forms of cast in-situ slabs with a joint were compared, and the specimen SS-1 was taken as an example to conduct finite element analysis. the conclusions are as follow:

The failure mode of cast-in-place slabs and composite slabs with a joint in shape of T-shaped and rectangular concrete topping is that the crack width reaches the standard limit, while the failure mode of trapezoidal composite slabs is that the midspan deflection reaches the limit. Overall, all four slabs are ductile failure.

The ultimate bearing capacity of composite slabs with a joint in the shape of T-shaped and trapezoidal concrete topping are equivalent to or even higher than that of cast-in-situ slabs, and 12.9% and 19.7% higher than that of composite slab with a joint in shape of rectangular concrete topping.

According to the calculation formula for the bearing capacity of cast-in-situ slabs provided by GB50010–2010, the measured bearing capacity of composite slabs was evaluated. The ratio of actual measured values to theoretical values of CS, SS-1, SS-2, and SS-3 was 1.09, 1.09, 1.16, and 0.97 respectively. The reasonable agreement indicating a good potential for the code in the design of the composite slabs with a joint in shape of T-shaped and trapezoidal concrete topping.

Abstract

Two destructive earthquakes (Mw: 7.7 and Mw: 7.6) occurred in Kahramanmaraş, Türkiye, on February 6, 2023. The epicenters of these two earthquakes were very close to each other (approximately 90 km apart), and both took place on the same day. The first earthquake occured at 04:17 local time with a magnitude of 7.7 and a depth of 8.6 km, while the second earthquake occurred at 13:24 local time with a magnitude of 7.6 and a depth of 7.0 km. These two earthquakes caused extensive damage, particularly in 11 provinces, with Adıyaman being one of the hardest-hit areas. Most buildings in Adıyaman were severely affected or collapsed as a result of the earthquakes. Reports indicate that over 35,000 buildings collapsed in the aftermath. Furthermore, between February 6, 2023, and May 6, 2023, more than 30,000 aftershocks, ranging in magnitude from 0.2 to 6.6, were recorded. In this study, earthquake damages in reinforced concrete buildings in Adıyaman province were evaluated. The causes of damages in reinforced concrete buildings were evaluated in terms of material quality, design of building and reinforcement details. Field observations are also provided, along with accompanying pictures. The earthquake regulations in Türkiye, established in 1975, 1998, 2007, and 2018, which apply to a significant portion of the country’s buildings, are discussed. An evaluation was conducted to assess whether the damaged buildings complied with the earthquake regulations in Türkiye. Field observations revealed that the earthquake regulations, particularly in relation to reinforcement details, were overlooked. In addition, designers should avoid design mistakes such as to make discontinuous of frames, to place most of vertical element strong axis in one direction, insufficient support condition of stairs and adjacent building effects.

Introduction

Türkiye has active earthquake fault zones, including the North Anatolian Fault, East Anatolian Fault, and West Anatolian Fault zones, which have the potential to cause destructive earthquakes. Türkiye has often been exposed to destructive earthquakes in the past. After these earthquakes, many studies about structural failures have been conducted and documented in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. On 6 February 2023, two destructive earthquakes occurred in Kahramanmaraş provinces in Türkiye (Mw: 7.7 and Mw: 7.6). The first earthquake was occurred in Pazarcık (distinct of Kahramanmaraş) at 04:17 local time with a magnitude of Mw:7.7. The second earthquake was occurred in Elbistan (distinct of Kahramanmaraş) at 13:24 local time with a magnitude of Mw:7.6. These two earthquakes were occurred in same day (approximately nine hour apart) and there is approximately 90 km distance between epicenters of these earthquakes [13]. The pair of earthquakes resulted in significant structural damage, particularly impacting 11 provinces, including Kahramanmaraş, Hatay, Gaziantep, Malatya, Diyarbakır, Kilis, Şanlıurfa, Adıyaman, Osmaniye, Adana, and Elazığ, where many buildings either sustained severe damage or collapsed entirely.

According to official announcements from the Disaster and Emergency Management Authority (DEMA) on 2 June 2023, it was reported that over 35,000 buildings had collapsed after the earthquakes. Furthermore, during the period from February 6, 2023, to May 6, 2023, more than 30,000 earthquakes with magnitudes ranging from 0.2 to 6.6 were documented [13]. The Kahramanmaraş earthquakes had a profound impact on Türkiye, causing severe damage not only during the initial earthquakes but also due to subsequent aftershocks.

Conclusions

Two destructive earthquakes occurred in Pazarcık (Mw: 7.7) and Elbistan (Mw: 7.6), Kahramanmaras¸, Türkiye. These earthquakes severely affected 11 provinces (Kahramanmaras¸, Hatay, Gaziantep, Malatya, Diyarbakır, Kilis, S¸ anlıurfa, Adıyaman, Osmaniye, Adana,Elazıg). Adıyaman is one of the provinces most affected by the earthquakes. Due to these two earthquakes occurring on the same day ˘ (nearly 9 h apart), damaged buildings were only examined after the second earthquake. This study focuses on examining the damaged reinforced concrete buildings in Adıyaman. Structural damages and the reasons for these damages are examined in comparison with Türkiye Earthquake Codes.

Abstract

Concrete structures are widely used in modern constructions including energy storage system such as all concrete liquefied natural gas (ACLNG) storage tanks. In ACLNG tanks, concrete structure is exposed to cryogenic temperatures and freeze-thaw (FT) cycles. Cryogenic temperatures and FT cycles are recognized to influence the mechanical characteristics of concrete. To ensure safety of the critical energy infrastructure, it is crucial to explore the concrete structural response under the combined cryogenic FT cycles and accidental impact loading. This study aims to numerically examine the damage caused by impact loading on reinforced concrete panels after exposure to cryogenic FT cycles. A plasticity based continuous surface cap model was adopted to simulate concrete. The material modulus, uniaxial and triaxial strength surface as well as damage parameters were updated to incorporate the effect of cryogenic FT cycles. A numerical model was established to forecast the impact resistance of the reinforced concrete panels after various cryogenic FT cycles. Through the numerical simulation, it was evident that FT cycles exerted detrimental effects on the impact resistance of reinforced concrete panels. With an escalation in the number of FT cycles, there was a pronounced increase in the size of the crater formed on the top surface, accompanied by a corresponding rise in the penetration depth of the panel. The results of this research offer insights into the impact resistance of reinforced concrete structures following cryogenic FT cycles. Such insights are vital for the design and maintenance of critical structures like liquefied natural gas (LNG) storage tanks and other cryogenic facilities.

Introduction

Reinforced concrete structures play a vital role in various engineering applications, providing strength, durability, and resilience. However, when subjected to extreme environmental conditions, like cryogenic freeze-thaw (FT) cycles, the structural safety of concrete elements can be remarkably affected. Moreover, of specific concern is the accidental impact resistance of reinforced concrete structures while enduring the challenges posed by cryogenic FT cycles. The demand for efficient and reliable energy storage systems, particularly All Concrete LNG (ACLNG) storage tank, has been rapidly increasing in recent years [1], [2]. Liquid natural gas storage tanks commonly maintain a storage temperature of approximately − 165 °C, therefore, concrete is under ultra-low temperature condition. These storage tanks are subjected to both cryogenic temperatures, FT cycles and potential impact loading scenarios, rendering it crucial to assess their structural response and safety under such conditions.

Numerous researchers have investigated the concrete’s mechanical properties at low and super-low temperatures to facilitate the scientific, rational, and cost-effective design of structures while ensuring their performance and safety in extreme cold environments [3], [4], [5], [6], [7]. Van de Veen [8] had a comprehensive review on the properties of concrete such as the compressive strength, tensile strength, modulus of elasticity at low and cryogenic temperature and after FT cycles. It revealed that the mechanical properties of concrete could be affected by the type of aggregate, cooling condition, water to cement ratio (w/c) and moisture content (MC). It is commonly reported that the strength of concrete was improved at low or super-low temperatures [1], [8], [9], [10]. This improvement can be attributed to several factors. Firstly, as water within the concrete freezes at lower temperatures, it crystallizes into a solid reticulated pattern that imparts prestress to the concrete. This prestressing phenomenon contributes to an enhancement in the compressive strength due to the existence of multiaxial stress [6]. Moreover, the complete filling of capillary pores with ice helps eliminate stress concentrations within the concrete and inhibits the formation of microcracks at low temperatures. For a more comprehensive understanding of the reinforcement impacts of low temperatures on concrete, researchers have employed micro-scale models to explore the formation of ice within concrete pores and subsequent cracks [11], [12].

Conclusions

The impact resistance of reinforced NSC panels after FT cycles was investigated using a refined finite element model. The CSCM model was adopted and modified to incorporate various parameters such as shear modulus (G), bulk modulus (K), uniaxial and triaxial compression surface, as well as damage parameters. The subsequent conclusions can be drawn:
• After FT cycles, the elastic modulus, splitting tensile, and compressive strength as well as fracture energy of concrete decrease. As the number of FT cycles increases, the performance of concrete also continuously decreases.

• The modified CSCM model effectively captures the impact behaviour of reinforced concrete panels after FT cycles, considering various mechanical parameters.

• The predictive capability of the numerical model was validated through experimental verification, demonstrating its reliability in assessing the impact resistance of reinforced concrete structures.

• FT cycles were observed to adversely affect the impact resistance of the concrete structures. As the number of FT cycles increased, there was an observable escalation in the dimensions of craters formed on the surface as well as an increase in the penetration depth of the panels. These trends collectively suggested a decline in the impact resistance of the panels with the progression of FT cycles.

Abstract

This study aims to the practical detection of sub-meter scale voids located inside reinforced concrete structures with different diameters and depths using different frequency antennas of 800, 1000, 1200, and 1600 MHz. The experiment was performed on two concrete blocks with medium-sized voids between 70 and 100 mm diameter, and small-sized voids between 10 and 25 mm diameter. A GPR survey was conducted by dividing each block into two grids of profiles with 50- and 100-mm profile spacing. Then, GPR data was collected along both the horizontal and vertical directions of each block with multi-frequency antennas. The gathered data was processed in 2D and 3D to detect the exact location, dimension, and brightness of these voids. The results showed that GPR has the potential to be quite effective in automating the identification and location of embedded voids within concrete blocks. The accuracy with which the system was able to identify void locations depended mainly on the frequency of the antenna used and the diameter of the void, while the depth of penetration was inversely proportional to the frequency of the used antenna, with estimated depths ranging from 25 cm (using a 1600 MHz antenna) to 1.5 m (using 800 MHz antenna). Moreover, a GPR survey was conducted to evaluate two sites inside residential buildings before and after the rehabilitation process. The radar scan exhibited a notable proficiency in identifying the positions of rebar sites, whether they were situated at a single level or two levels, inside certain areas of construction. The study revealed the ability of GPR to identify the depth of the reinforcing steel within the cement material, particularly in areas where the soil has experienced subsidence.

Introduction

Ground-penetrating radar (GPR) is a well-established characterization and analysis tool in non-destructive test applications ( Brian et al., 2018 ). GPR is now a widely accepted field of scanning technology for surveying and imaging subsurface conditions (Annan, 1992, Tzanis, 2017 , Yang et al., 2020 ). GPR is an effective non-destructive testing method that is characterized by its rapidity, continuity, high detection accuracy, and ability to overcome the shortcomings of the core sampling and impact echo methods. The unique advantage of GPR causes concern for many scholars. Recently, GPR has been used in a wide range of civil engineering and geotechnical applications ( Li et al., 2003 , Rao et al., 2007 , Wu and Chang, 2007 , Rister and Graves, 2008 , Alhasanat et al., 2013 , Tian et al., 2018 ), and in reinforced concrete tests applications, due to its capabilities, benefits and fast performance in the non-destructive exploration of all defects that may be found inside the cement layers.

GPR can also be used to conduct a real-time concrete inspection survey using radar to locate rebar, pipes, tension bars, dowels, and plastic and fiber-optic conduits ( Bungey and Millard, 1993 , Muldoon et al., 2007 , Bala et al., 2015 ). There are few studies on concrete inspection surveys to detect voids/cavities within and under concrete slabs ( Watanabe et al., 2004 , Pollock et al., 2008 , Roger et al., 2011 , Abdul Razak et al., 2015 , Nobes, 2017 ). Radar surveys can be conducted on airport runways, tunnels, abutments, dams, pavements, and garages ( Millard et al., 2002 , Loizos and Plati, 2007 , Evans, 2009 , Lee, 2011 ). Asphalt roads and bridges can also be scanned ( Parry and Davis, 1983 , Maser and Scullion, 1992 , Hugenschmidt and Mastrangelo, 2006 , Loken, 2007 , Hoegh et al., 2015 ). The concrete slabs can be inspected for issues including relative thickness, deterioration, and structural problems ( Roddis et al., 1992 , Loizos and Plati, 2007 )

Results

These two blocks have been surveyed along the same lines to detect the internal voids by different GPR devices such as CX11 (with antenna of 1200 and 1600 MHz), Conquest (with antenna of 1000 MHz), Ramac (with an antenna of 800 MHz), and HandyScan with antenna of 1500 MHz) and the results are then compared.

4.1. Medium-scale void-size (70 mm and 100 mm)

For block No.1, using an 800 MHz antenna, the location of both voids could be detected at a depth of 280 mm for the first void and 200 mm for the second one ( Fig. 3 a). When a 1000 MHz antenna was used the accuracy level of the data increased in the second step for both 2D and 3D, due to the good coverage of the survey concerning the 28-grid lines with a spacing of 50 mm. The first void was represented more accurately in the second step ( Fig. 3 b & 3c) while the level of accuracy decreased significantly concerning 14-grid radar profiles for the same void ( Fig. 3 d & 3e). When a 1200 MHz antenna was used, the depth of penetration increased up to 500 mm ( Fig. 3 f). The degree of brightness decreased in the upper part of block No.1, however, due to the decrease in the frequency range compared to what is considered to be the optimum value in the case of deeper targets. When a 1600 MHz antenna was used, it was noticed that the buried voids could be detected to a depth of 450 mm ( Fig. 3 g). The accuracy level of data decreased with the increasing depth (e.g., see void No.2 which was represented with low accuracy at a depth of 280 mm, and compared with void No.1 at a depth of 200 mm).

Abstract

The Alcántara Bridge is one of the most important Roman bridges in the world and has been a Spanish National Monument since 1924. Since its construction, almost two thousand years ago, it has suffered numerous damages due to the passage of time and human action, which has led to different interventions. The most important was that carried out by Millán (1856–1860), for the reconstruction of the arch demolished in 1810. During a visit to the bridge, areas with reinforced concrete were observed. Given the areas where it appears, the geometry of the reinforcement and its configuration, is possible to think that it is a very old intervention, which could date from the same period as that carried out by Millán. If so, it would be the first application of reinforced concrete in Spain and one of the first in the world. The aim of this study is to date the intervention and validate the initial hypothesis. To this end, based on a 3D survey of the rehabilitated areas using a drone, a comparative analysis has been carried out with the plans of Millán’s project, as well as sheets and photographs from different periods to establish the date of the interventions. Research was also carried out in the archives of the Real Academia de Bellas Artes de San Fernando to gather information on the interventions on the bridge throughout the 19th century. Finally, the geometry and layout of the reinforcement is analysed in relation to the knowledge of reinforced concrete from its first patents worldwide to its widespread application in Spain. These analyses allow us to conclude that the date of the reinforced concrete repairs on the bridge is undoubtedly 1857, which would be the first application in Spain and the first in Europe on bridges.

Introduction

Reinforced concrete is the most widely used material in the construction of bridges and buildings during the last 150 years. This new material emerged as the result of the combination of concrete and steel bars strategically placed inside the section of the structural element, which allows it to acquire the advantages of both compressive and tensile strength respectively [1].

On the other hand, concrete is the result of combining a binder, fine aggregate and coarse aggregate. The most common binders used in the manufacture of concrete have been aerated lime and hydraulic lime, giving rise to common and hydraulic concretes, respectively [2].

The first scientific studies of hydraulic binders were carried out by Smeathon for the construction of Eddystone’s Third Lighthouse (1756–1759) in search of a material that would harden under water [3]. But it was Vicat, at the beginning of the 19th century, who deepened his knowledge of the hydraulicity of limes, laying the foundations of hydraulic cement [4], the basis of concrete, and therefore of reinforced concrete.

Throughout the first half of the 19th century, natural cement was developed in Europe, which began to be manufactured in Spain in 1835 [4]. Artificial cement, also known as Portland cement, began to be manufactured in France and Great Britain in the second quarter of the 19th century, but its manufacture in Spain did not begin until 1898, more than half a century late [5].

Conclusions

The Alcántara Bridge is one of the most important Roman bridges in the world due to its size and state of conservation. Since its construction almost two thousand years ago, it has suffered damage because of the passage of time and human action, and therefore, it has required restoration work. During an inspection of the bridge, areas with reinforced concrete were observed, which, due of its typology, suggested that it was a very old intervention. In order to date this intervention, a comparative analysis was carried out between the current situation, the survey plans that Millán made for the repair of the collapsed arch, the sheets from different periods and photographs from different dates. In addition, research has been carried out in the archives of the Real Academia de Bellas Artes de San Fernando on other possible restoration works on the bridge in the 19th century.

Following this research, it can be concluded that the areas of the bridge with reinforced concrete were built during the restoration work carried out by the engineer Millán (1856–1860). Specifically, the application of reinforced concrete was carried out during the year 1857.

Abstract

Corrosivity category of the atmosphere in coastal regions is usually very high. Reinforced structures frequently show short service life if adequate measures are not applied. Determination of time-to-corrosion-initiation (ti) and time to corrosion with induced cracking (tcc) to calculate the service life in buildings located in coastal cities is not usually performed. Changes in ti and tcc, depending on the w/c ratio, and concrete covering thickness, were determined in extremely aggressive coastal outdoor exposure site located within 10 m of the shoreline in Havana, one of the most aggressive coastal sites in the world. The electrochemical corrosion rate was determined in the reinforced concrete specimens. It is a very important parameter to follow environmental degradation of concrete. Environmental parameters were determined at the site. Calculated Chloride deposition rate is over the maximum level established in ISO 9223 standard (S3). Effective capillary porosity, compressive strength and ultrasonic pulse velocity were determined for all specimens. An acceptable service life was obtained for w/c ratio 0.4. However, effective capillary porosity is an important parameter to determine the concrete quality assessment before building reinforced structures exposed to extreme corrosivity. It is recommended to consider this parameter to predict a service life in very aggressive coastal sites.

Introduction

Many reinforced concrete (RC) structures located in coastal cities and exposed to the direct influence of marine aerosols present generally a severe deterioration, indicative of short service life (Sl), caused mainly by atmospheric corrosion of the reinforcement steel [1–4]. The RC structures are often built using very permeable concretes with low quality.

Different softwares (Life-365, Eucon, and Duracon/fib MC SLD) have already been developed to calculate the Sl of RC structures in coastal cities [5]. Some input basic parameters as cement type, concrete constituents, water/cement (w/c) ratio, additions, air content, concrete covering thicknesses, and protective systems, play a fundamental role in the determination of Sl. However, some input basic parameters depending on concrete quality as effective capillarity porosity, compressive strength, ultrasonic pulse velocity, and behavior of electrochemical corrosion rate in the reinforcement steel are not included in the software. Atmospheric transitory processes such as chloride deposition rate, relative humidity, and wind speed influencing in the corrosivity category of the atmosphere, and in the electrochemical corrosion rate in the reinforcement steel, neither are included in the software as input basic parameters [6, 7].

Conclusions

The Sl of RC structures in extremely coastal site of a city like Havana, considered one of the most aggressive environments of the Ibero-American region, depends on concrete quality, and atmospheric corrosion of reinforcement steel. Changes in the w/c ratio and concrete cover thickness, determined significant changes in the atmospheric corrosion rate of reinforcement steel in RC specimens exposed to extreme (CX) corrosivity categories of the atmosphere. The role of concrete cover thickness with respect to the Sl in RC structures become important for 0.4 w/c ratio.

Prediction and determination of the Ic, as indicator of atmospheric corrosion of reinforcement steel bars in RC specimens exposed in a coastal outdoor exposure site, can be used to obtain ti plus tcc. The importance of obtaining ti from the prediction and determination of Ic in the reinforcement steel is confirmed in the research work.

Highlights

Abstract

Keywords

1. Introduction

2. Research significance and objectives

3. Experimental program

3.1. Concrete specimens

3.2. Test setup and instrumentation

3.3. Concrete properties and test parameters

4. Results and discussion

4.1. Concrete deformations: Strains and deflections

4.2. Recovery strains

4.3. Deflection recovery

4.4. Analysis of uncracked and cracked concrete beams

5. Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

References

Abstract

The purpose of this research is to analyse the long-term deformations of reinforced concrete beams made of recycled coarse aggregates (RCA), using four different replacement ratios, 0%, 20%, 50% and 100%. For said purpose, three different loading stages were carried out: firstly, loading and sustained load for 1300 days, secondly, unloading and recovery where the deformations were registered over a one year period, and lastly, testing the concrete beams up to failure in order to analyse the effect of recycled aggregates on pre-cracked concrete members. The results led to the analysis of deformations under sustained load, as well as those produced after removing the load and the performance of pre-cracked members, while identifying the effect of recycled coarse aggregates on concrete performance in terms of plastic deformation, recoverability and concrete stiffness, and also on structural design.

1. Introduction

In recent years, the use of construction and demolition waste as aggregates for structural concrete has been widely studied. Numerous authors have contributed to the increasing knowledge in this field, analysing the structural performance of recycled concrete and its properties, such as mechanical strength, modulus of elasticity, creep and shrinkage or durability [1], [2], [3]. As a result, several expressions and methods have been assessed for the design of structural concrete using recycled aggregates [4], [5], [6], [7], [8], [9], [10], [11], [12]. Most authors deal with the short and long-term behaviour of concrete while considering only the influence of creep and shrinkage [13], [14], [15] and very few include the ageing effect of concrete, its different cracking behaviour or its ability to recover from deformations after a load is varied or removed [16].

As common concrete structures are usually submitted to load variations during their service life, it is important to consider the effect of creep over time, including not only loading periods but also unloading the stages and the recoverability of concrete. As mentioned by Rossi et al. [17] understanding the behaviour of concrete structures is greatly simplified by the numerical modelling of their mechanical behaviour. However, progress needs to be made to predict the long-term behaviour of concrete with sufficient accuracy. In order to determine the deformational mechanism, it is also important to analyse the effect of cracking and residual strain i.e. the irreversible part of strain. Many factors influence deformation and the ability of concrete to recover after unloading. Once the hysteretic recovery has finished, the residual deformation can be defined as plastic deformation. Additionally, it can be assumed that concrete deformations under loading are mainly due to creep, especially for long periods of loading and therefore, the ability of deformations to recover after unloading is closely linked to creep recovery.

Abstract

     This paper has proposed a new formula relating the damage state and effective state of the flexural member, this formula enables studying different concrete healing variables. A new relation between the effective reinforced concrete healing/damage variable and the effective healing/ damage variables of concrete and steel has been introduced to relate concrete and steel healing/ damage and the reinforced concrete healing/ damage. The proposed formula has been verified with experimental results of full-healed flexural members, it has achieved good agreement. The flexural member stiffness has been studied considering different parameters like concrete healing variables, concrete cover and percentage of steel reinforcement. The flexural member stiffness has increased as the concrete healing variable has increased, it has increased as the concrete cover has increased and the steel reinforcement percentage of 1% has given the higher flexural member stiffness.

Introduction

     Self-healing is one of the most attractive research topics in the last decades, reinforced concrete is a heterogeneous material that needs the self-healing to protect the reinforcement during serviceability life. Robins et al. (2001) studied cracks progression of reinforced concrete beam with steel fibers to obtain response in the form of a load deflection. The nonlocal formulation through concepts of continuum damage mechanics was proposed by Jirásek (2004), the standard continuum theory with a stress-strain law were shown and he increased efficiency of the non-local simulation. The analytical results of the concrete behaviour under plane strain conditions were introduced by Bobinski and Tejchman (2005) by using a simple damage continuum isotropic model which was enriched by non-local terms to avoid a pathological mesh sensitivity and to get a well-posed rate boundary value problem. Voyiadjis and Kattan (2009) investigated the damage tensor that was used to link the damage state of the material with effective undamaged configuration using different paths including fabric tensors to connect the two configurations.

     Harries et al. (2012) developed the current ACI and AASHTO crack control provisions using high-strength reinforcing steel to anticipate the higher service level stresses. Allam et al. (2012) investigated and verified codes of practice provisions beside some equations calculating the crack width of reinforced concrete beams, five reinforced concrete models were studied theoretically. Voyiadjis and Kattan (2012) introduced and verified new damage variables, by using higherorder strain energy to lay the theoretical results for the design of undamageable materials. Darabi et al. (2012) proposed a novel continuum damage mechanics approach to model the micro-damage/ healing state in the materials that perform self-heal. Several presented examples were used to show the powerful of the proposed model to treat microdamage healing.

Conclusion

     A new nonlinear healing/ damage formula was introduced to interrelate the damage state and effective state that enables determining the effective state geometric properties to help in finding the response of the flexural member in healing / damage state. In addition, a new relation between the effective reinforced concrete healing/damage variable and the effective healing/ damage variable for concrete and steel was introduced. These new formulas enable controlling the damage of flexural members and design the healing of these members where the reinforcement, concrete dimensions, healing variables, damage variables are correlated in these formulas. Good agreement between the new proposed formula and experimental results was achieved. The proposed model’s efficiency increases as the spacing of healing points decreases, the proposed model becomes very close to experimental results when the healing becomes continuous. The beam’s stiffness increased by increasing the concrete healing variables (hc) and the concrete healing variable hc ¼ 2 3 was the reasonable healing variable that achieved the majority of the beam stiffness. Effective healing can be obtained by increasing concrete cover to increase the beam stiffness. Also, the proposed model shows that the percentage of steel reinforcement affects the behavior of reinforced concrete in healing, where the percentage of steel reinforcement affected the efficiency of the healing, where 1% gave high beam stiffness, but 2% of steel reinforcement reduced beam stiffness and 3% increased beam stiffness again.

Abstract

     Weft-knitted textiles made from high-strength fibrous materials offer great potential for use as a flexible stay-in-place formwork and reinforcement system since they allow creating complex geometries (i.e. doubly curved or folded) and introducing spatial features such as ribs within the fabric. However, the closed surface of the textile and its placement at the bottom edge of the concrete element present major challenges regarding bond, which may lead to premature failure due to delamination of the reinforcement initiated by a substantial opening of a governing crack. This study investigates the influence of short integral glass fibres and spatial bond ribs on the mechanical behaviour of weft-knitted textile reinforced concrete elements subjected to bending and their potential to increase the shear resistance. To this end, an experimental campaign consisting of 14 four-point-bending tests was conducted, where the specimens were examined regarding their load-deformation behaviour, crack kinematics and failure modes. The contribution of the short glass fibres to the load-bearing mechanism was estimated with Pfyl’s fibre engagement model, based on the material characterisation results from prism tests on fibre reinforced concrete members.

Introduction

     The urgent need for the construction industry to reduce its ecological footprint has led to numerous advances to decrease the volume of concrete consumption. Thereby, the use of conventional steel reinforcing bars presents an intrinsic limitation for lightweight concrete structures due to the minimum concrete cover needed to protect the reinforcement from corrosion (between 20 and 50 mm depending on the regional building codes and exposure). Textile reinforcement allows minimising the thickness of the concrete elements to a few millimetres when using non-corrosive materials such as aramid, carbon or glass fibres [1]. The mechanical behaviour of such fibres is usually linear elastic with high strength (3000–4000 MPa) at a moderate to high stiffness (glass fibre: ca. 70 GPa; carbon fibre: ca. 240 GPa). Textile reinforcement typically consists of rovings formed from multiple filaments, which exhibit lower strength and stiffness than the individual fibres due to several reasons: (i) the failure mode is brittle and does not allow for redistribution of stresses within the roving cross-section once the weakest fibre reaches its tensile strength [2], [3], [4]; (ii) commonly used fibres are sensitive to lateral loading and may be damaged due to deviations at the crack edges [5], [6]; (iii) the loads are transferred via inter-fibre friction within the rovings, which leads to an inhomogeneous stress distribution over the cross-section [7]; (iv) while the ribs in conventional deformed steel bars provide a proper force transfer between reinforcement and concrete [8], the bond of textile reinforcement mainly relies on adhesion and friction [9], [10], [11]. Coating or even fully impregnating the rovings (e.g. with epoxy resin) can attenuate these effects and may significantly improve the mechanical performance of the textile reinforcement [12], [13], [14].

Conclusions

     This study investigated the influence of short integral glass fibres and spatial features to improve the structural performance of concrete beams with weft-knitted textile reinforcement attached to the bottom edge of the specimens, especially regarding their shear strength and their resistance against delamination of the reinforcement. To this end, an experimental campaign consisting of 14 four-point-bending tests was carried out. The mechanical behaviour was examined regarding the load–deflection curves, the stress–strain relationship of the textile reinforcement and the crack kinematics based on the digital image correlation measurements, following the methodology presented in [20] to assess the response of the tension chord for concrete members subjected to bending./