Experimental Study of the Effectiveness of Strengthening Reinforced Concrete Slabs with Thermally Prestressed Reinforcement

Tests investigating the punching shear of a column-slab connection strengthened with non-prestressed or prestressed FRP plates

In this paper, the possibility of achieving a high level of ductility by flexural yielding in fully grouted reinforced concrete masonry shear walls is evaluated. Six full-scale walls were tested to failure under reversed cyclic lateral loading to investigate the effects of amount and distribution of vertical reinforcement and the level of axial compressive stress on the inelastic behavior and ductility. Results showed that yielding of the outermost vertical bars extended to a height equivalent to half the wall length. Also, the top wall displacement (drift) at the onset of yielding of the vertical reinforcement was highly dependent on the amount of reinforcement, but only minimally affected by the level of axial compressive load. However, at maximum loads, the displacements were less sensitive to either the amount of vertical reinforcement or the level of axial compression. Correspondingly, the displacement ductility was found to be very sensitive to the amount of vertical reinforcement, but was not dependent on the level of axial compression. In general, high levels of ductility and energy dissipation capabilities accompanied by relatively small strength degradation were observed for the test specimens.

ObjectiveThe aim of this study was to evaluate and compare the biomechanical effects of locking plate superior and anteroinferior positioning on the osteosynthesis of the clavicles osteotomized obliquely. Materials and methodsTen matched pairs of fresh cadaveric clavicles osteotomized through the mid-shaft obliquely were repaired with a titanium 7-hole 3.5-mm reconstruction locking plate in the superior or the anteroinferior position. The maximal failure loads and the displacement of the specimens at 166N, 183N, 203N loads were recorded by the machine in 3-point cantilever bending. Bending failure stiffness was calculated between 10–150N and 151N to maximal bending failure loads. ResultsThe mean maximal failure load was 396.2N (SD, 117.3) for superior constructs and 220.1N (SD, 51.1) for anteroinferior one (P<0.05). There was significant difference in displacement between superior and anteroinferior plated specimens at 183N (6.3 [SD, 2] vs. 9.9 [SD, 3.6]) and 203N (6.4 [SD, 0.6] vs. 11.7 [SD, 6.6]) loads; P<0.05). Mean bending failure stiffness between 151N and maximal loads was 22.6N/mm (SD, 13.2) for superior plates and 11N/mm (SD, 9) for anteroinferior plated clavicles (P<0.05). ConclusionsThe superior plating of obliquely osteotomized clavicles with the titanium 7-hole 3.5-mm locking reconstruction plate had a significantly greater biomechanical stability at fixed loads of 183N and 203N than the anteroinferior plating in the inferior directed cantilever bending. The superior plating osteosynthesis exhibited a significantly greater stiffness from 151N to maximal bending failure loads as well.

Many infrastructure buildings show deficits in their load‐bearing capacity and need to be strengthened in the short or medium term. However, the available methods are usually not effective for the self‐weight without temporary raising or prestressing the structure. To resolve this problem, thermal prestressing of slotted reinforcing bars is proposed here. The bars are embedded in a grouting material tempered from the outside and thermally expanded according to the temperature change. When the heat supply is stopped, the bond with the hardened grouting material restricts the re‐deformation of the bars during cooling. This creates prestress that counteracts the bending moment from the self‐weight and relieves the tensile zone of the cross‐section. Of course, it is important to limit the heat input to the slots when tempering. Heating the entire cross‐section would reduce the effectiveness of the strengthening. The paper shows the experimental and numerical development of an induction method to thermally prestress subsequently added slotted reinforcement. The strengthening method is first implemented experimentally in reference tests with varying slot shapes. The results are then used to calibrate a numerical computation that is used to investigate the influence of other boundary conditions on the effectiveness of the strengthening measure. The optimum result is derived therefrom and experimentally validated, taking into account practical construction needs. The results show that the numerical optimization of the induction method increases the effectiveness of the strengthening by up to 100%. Accordingly, the reinforcing bars must be installed in triangular, 10 cm wide slots filled with high‐strength concrete (HPC), which is tempered with the highest available thermal output and up to 95°C. In order to counteract the temperature increase in the concrete at the initial cross‐section, which reduces the effectiveness, it must be cooled in parallel. It is advisable to place dry ice at a distance of 6 cm from the edge of the slot over the largest possible width. For practical realization of the strengthening with usually several bars added in parallel, a minimum bar spacing of 40 cm must be maintained to ensure sufficient cooling between the slots. Alternatively, the bars should be attached and thermally prestressed in sequence.

Research on mining-induced surface soil cracking mechanism and development depth of downward fracture

The flexural behavior of slender (1/40) long‐span precast high‐strength concrete girders was experimentally investigated as a competing solution to steel alternatives for highway overpasses. Half‐scale specimens were produced using an economical and high‐strength (120 MPa) concrete mixture, non‐fiber reinforced, designed only with conventional raw materials, currently available at Portuguese precast companies. Two girder specimens were subjected to quasi‐static short‐term loading up to structural failure in bending. Results have shown the typical behavior of conventional reinforced concrete members in bending, despite the brittle behavior of non‐fiber‐reinforced high strength concretes. Results also showed acceptable ductility (2.6) and the ability of the girder specimens to experience large deformation without significant softening. An analytical approach, based on plane sections and short‐term stress–strain constitutive model recommended by the Eurocode 2 for concrete up to the C90/105 strength class, showed to predict accurately the behavior of girders in bending, both to ultimate and serviceability limit states, including cracking, yielding, maximum loads, and ultimate deflections.

Many achievements in recent studies in the field of RC structures are related to high performance phenomenon. This phenomenon can have structural, technological or economical aspects, but it is not always emphasized. Moreover, each structure’s improvement cannot be defined as high performance – it can be just upgrading some of its definite properties. Hence, a problem of a strong definition of high performance structural element arises. This paper deals with definition, related to the structural aspect only taking into account that a concept of high performance RC structure is mainly related to bending elements. Considering concrete behavior in these elements, it is important to achieve high performance properties separately for tensile and compressed zones of a bending section. For this reason it is logical to use different concrete classes, i.e. high and normal strength concretes in the compressed and tensile zones, respectively. It is necessary to provide suitable section ductility in the compressed zone and necessary cracking resistance in the tensile zone. A high strength concrete with elastic-plastic properties should be used in the bending element section compressed zone in order to withstand both static and dynamic loads. With this aim fibered concrete is used and the fiber quantity should be calculated according to the required ductility. Using pre-stressed reinforcement in the tensile section zone allows for improvement of the cracking resistance and reduces the bending elements’ deflections. Thus, a two-layer RC beam represents an effective bending element having fibered high strength concrete in its section compressed zone and pre-stressed normal strength concrete in the tensile zone. Such a beam can be defined as a high performance bending element, because, on one hand, the concrete compression properties are maximally used for a given ductility level, and, on the other hand, the section’s design stiffness requirements are provided by pre-stressing the tensile zone.

Deployable load-bearing structures are useful for confined or temporary settings due to their stowability and deployability. Designing these structures involves balancing package dimensions with load-bearing capacity - defined as the maximum load under which a structure can maintain rigidity or the load above which a structure deforms. This challenge is further complicated when considering load bearing capacities in different directions. Tendon constrained inflatables (TCIs) offer a promising solution with its design flexibility using internal tendon configurations. TCIs feature a deployable bladder held between rigid end caps which are connected internally by inextensible tendons. A TCI maintains rigidity up to its rigid load-bearing (RLB) capacity, at which some tendons become slack. The RLB capacities can be customized in different directions depending on the tendon configuration. This paper introduces a model-based design approach of a TCI's tendon configuration to trade-off RLB capacities in different directions and against package dimensions. A 3D kineto-static equilibrium model is developed to relate tendon configuration to six-dimensional RLB capacities. Visual design strategies for planar tendon configurations guide the customization of TCIs to specific package dimensions and loading scenarios. Experimental validation of the model enables TCIs, an emerging adaptive structure, to be useful for a broad range of applications.

In lightly reinforced concrete (RC) structures, the area of steel cannot be lower than a minimum value, so that the ultimate limit state can be reached under a yielding moment higher than the cracking moment. Also in the serviceability stage, a minimum amount of reinforcement should be provided in tensile zones, in order to reduce crack widths. In fiber-reinforced concrete (FRC) members, due to the presence of structural fibers in the cementitious matrix, the minimum amount of steel area can be significantly reduced. Fiber can guarantee tensile stresses in a cement-based matrix even in the presence of wide cracks. Therefore, for the same cross-section of steel, a reinforced FRC member in bending can show higher bending moments, and reduced crack widths, than those measured in classical RC beams. This is particularly true in case of massive members, like the structures of tunnel linings. For such elements, and starting from the constitutive relationships recommended by Rilem TC 162-TDF, a new approach for the evaluation of minimum reinforcement area is proposed in this paper. By means of this nonlinear model, it is possible to calculate a reinforcement area lower than that calculated according to Eurocode 2 and Rilem TC 162-TDF prescriptions.

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

The load-bearing capacity analysis of pre-stressed concrete in bridge engineering is a core technology for structural safety evaluation. It has long faced challenges in insufficient detection accuracy under complex stress environments and low efficiency in multi-source data fusion. Traditional analysis methods rely on a single mechanical model or empirical experience, making it difficult to accurately capture the nonlinear relationship between crack development and load-bearing capacity degradation. Therefore, this study proposes a prestressed concrete load-bearing capacity analysis model based on a dual-threshold edge detection algorithm. Experimental results show that the accuracy of the improved edge detection algorithm reaches a maximum of 89.5% after iteration, with the misdetection rate of bridge cracks under various noise influences being as high as 9%. Evaluation of the fusion analysis model shows that the Mean Square Error (MSE) of its load-bearing capacity is only 0.015 kN·m2 , and the coefficient of determination R2 is 0.98. These results indicate that the proposed prestressed concrete load-bearing capacity analysis model can effectively improve the prediction accuracy of load-bearing capacity under complex stress environments and accurately capture the nonlinear relationship between crack development and loadbearing capacity degradation. Compared with existing research, the core contributions of this study are reflected in three aspects: 1. A collaborative analysis framework for crack characteristics and section loss was constructed, quantifying the coupling influence mechanism of the two on bearing capacity and breaking through the limitations of traditional single-factor analysis; 2. A prestressed concrete bearing capacity analysis model was proposed. Through algorithms, the core characteristics of crack-section loss were precisely screened, and the problem of dynamic bearing capacity prediction under small samples was solved, filling the technical gap of nonlinear mapping in complex stress environments; 3. The experiment verified the quantitative correlation between crack size and steel bar damage (for every 0.1mm increase in crack width, the steel bar corrosion rate increases by approximately 15%), providing an operational quantitative method for inferring internal structural damage from surface cracks. This study provides a new technical approach for bridge structural safety assessment and contributes to the development of intelligent monitoring and full-life-cycle maintenance technologies for prestressed concrete structures. Keywords: Bridge engineering, LSTM, Otsu, PSO, Crack characteristics, GAN.

This paper presents experimental results of a test campaign that examined the strength of shear critical RC pile caps in the ultimate limit state. The campaign comprised test specimens of different sizes - all scaled with identical geometrical ratios. The specimens simulated quadratic four-pile Pile Caps with concentric loading. This unique scaling allowed for an investigation of the effect of increased dimensions on the capacity of the solid RC structure in the ultimate limit state. The specimens scaled from overall dimensions of 450 × 450 × 230 mm3 (L × L × h) to 1188 × 1188 × 575 mm3 and the load carrying capacity increased from around 0.6 MN to 3.8 MN. Nevertheless, the average ultimate shear stress decreased with increasing depth. A novel two-camera setup for independent 2D Digital Image Correlation on two sides of the specimens was established to relate the crack development with characteristic points on the load-displacement relationship. This led to the identification of the first bending cracks and eventually development of shear cracks that contributed to the failure mode in the ultimate limit state. All specimens failed in a shear/punching-shear like manner, with several of the specimens failed in a mechanism involving shearing of one corner of the square pile cap. Based on the experimental findings, the paper investigates applicability of a rigid plastic upper bound solution - including the empirical account for size effect - to predict the strength and compare to the experimentally observed collapse mechanisms.

Mechanical properties of the composite sandwich structures with cold formed profiled steel plate and balsa wood core

The paper presents the application of methodology for the assessment of reliability and structural load bearing capacity using advanced methods of probabilistic FEM analysis in combination with mathematical modelling of degradation processes of concrete and reinforcement. The durability, serviceability and ultimate limit states are assessed. The authors deal with the validation of load bearing capacity of the bridge on the 3rd class road built in the 1st quarter of 20th century in the Czech Republic. Regarding the reliability requirements according to Standards and, considering the current state of the structure, the actual value of load bearing capacity is assessed. By reason of the time-consuming FEM analyses the stratified Latin Hypercube Sampling (LHS) method is used for generation of input random variables.

The paper presents problems related to the load-bearing capacity analysis of the building partitions of the cold storage facility. The subject of the case study are two different buildings built in the early fifties of the last century. Despite the similar cubature and the construction period, the structures of these buildings are different. The analysed cold store in Warsaw (Poland) is a steel and brick structure, while the second example concerns a reinforced concrete slab-pillar structure located in Wloclawek (Poland). In both cases, the issue related to the assessment of the current technical condition of the structural elements, including the safety of the load-bearing structure and the safety of its use was considered. Moreover, the permissible load for inter-storey slabs in both cases was determined. In order to properly determine the current load-bearing capacity of inter-story slabs, the archival technical and operational documentation of buildings was firstly analysed. Their technical condition was also taken into account in the assessment process. In-situ inspections of both buildings had been carried out. This allowed the determination of the scope of necessary tests and the selection of test and check points for each structural element. Such actions allowed to identify cross-sections of the structural elements and the parameters of built-in materials. The performed diagnostics also allowed to properly determine the technical condition of each structural element, the degree of degradation of the structure and to correctly determine its current load-bearing capacity while simultaneously satisfying both limit states - the Ultimate Limit State (ULS) and the Serviceability Limit State (SLS).