Ultra-high Performance Concrete Layer Research Articles

Experimental and simulative analysis of flexural performance in UHPC-RC hybrid beams

With high strength, excellent durability, and cracking resistance, ultra-high-performance concrete (UHPC) has gradually become popular. In this study, a novel hybrid beam was designed to fully harness the cracking resistance and tensile strength of UHPC. A series of experiments were conducted to investigate the influence of UHPC layer depth and steel bar diameter on the flexural performance of UHPC-RC hybrid beams. The experimental results indicated that the use of UHPC in the tensile zone of UHPC-RC hybrid beams can significantly improve the bearing capacity, crack resistance, and ductility of the structure. Based on the application of cohesive zone model (CZM), a finite-element (FE) model was proposed to simulate the mechanical behavior of the UHPC-RC hybrid beams, and the cohesive elements were used to represent the potential fracture surfaces of UHPC (concrete) and the rebar-concrete interface. A constitutive model for potential fracture of UHPC, taking into account the bridging effect of fibers, has been proposed. This FE model can reflect the fracture behavior of UHPC-RC hybrid beams. The simulation results demonstrate that the proposed model can accurately simulates the mechanical response and fracture behavior during the whole fracture process. Based on the results obtained from the experiment and simulation, a theoretical method for predicting the bearing capacity of UHPC-RC hybrid beams was developed, which can provide the reference for the design of similar structures.

Flexural and Shear Strengthening of Reinforced-Concrete Beams with Ultra-High-Performance Concrete (UHPC)

Ultra-high-performance concrete (UHPC) is considered to be a promising material for the strengthening of damaged reinforced concrete (RC) members due to its high mechanical strength and low permeability. However, its high material cost, limited code provisions, and scattered material properties limit its wide application. There is a great need to review existing articles and create a database to assist different technical committees for future code provisions on UHPC. This study presents a comprehensive overview focusing on the effect of the UHPC layer on the flexural and shear strengthening of RC beams. From this review, it was evident that (1) different retrofitting configurations have a remarkable effect on the cracking moment compared to the maximum moment in the case of flexural strengthening; (2) the ratios of the shear span and UHPC layer thickness have a notable effect on shear strengthening and the failure mode; and (3) different bonding techniques have insignificant effects on shear strengthening but a positive impact on flexural strengthening. Overall, it can be concluded that three-side strengthening has a higher increment range for flexural (maximum, 81%–120%; cracking, 300%–500%) and shear (maximum, 51%–80%; cracking, 121%–180%) strengthening. From this literature review, an experimental database was established, and different failure modes were identified. Finally, this research highlights current issues with UHPC and recommends some future works.

Mechanical Characteristics of Cracked Lining Reinforced with Steel Plate–UHPC Subjected to Vertical Load

The steel plate reinforcement method is widely used for strengthening damaged linings. Nevertheless, low durability is one of the disadvantages of the steel plate reinforcement method, which uses epoxy resin as the interface binder. To enhance the load-bearing performance and strengthening effect of steel-plate-reinforced structures, this study introduced ultra-high performance concrete (UHPC) as the reinforcing bonding layer and proposed a novel method for steel plate–UHPC reinforcement of cracked linings. A mechanical performance model test was conducted on a 1/5 scale lining model using a loading test device to evaluate the load-bearing performance and stress deformation of both conventional steel plate and steel plate–UHPC reinforced cracked linings. The characteristics, mechanisms of failure, and impacts of strengthening of the steel plate reinforcement method and steel plate–UHPC reinforcement method for cracked linings were compared. A numerical simulation model was developed to investigate the reinforcement effect of cracked linings using steel plate–UHPC reinforcement. The analysis included examining the influence of steel plate thickness, UHPC bonding layer thickness, and reinforcement timing. Model test results show that the overall damage mode of the steel plate–UHPC-reinforced structure had good elastic–plastic behaviour, and the deformation and damage process under the vertical concentrated load can be divided into four typical phases. Compared with the traditional steel plate reinforcement, the ultimate load-carrying capacity and ductility of the steel plate–UHPC-reinforced structure were increased by 53% and 366%, respectively, showing significantly better load-carrying capacity and deformation performance. Numerical simulation results show that the reinforced structure’s load-carrying capacity and stiffness enhancement rate increased non-linearly with the increase in UHPC layer thickness and steel plate thickness. However, reasonable reinforcement timing exists for steel plate-UHPC reinforcement, and too late reinforcement timing leads to a decrease in structural load-carrying capacity and stiffness enhancement rate.

Shear performance of small-rib-height perfobond strip connector groups embedded in UHPC: Experimental investigations and design recommendations

Employing ultra-high-performance concrete (UHPC) with perfobond strip connector (PBL) groups for strengthening steel columns has emerged as a promising solution for the rehabilitation of steel structures. To reduce the cross-sectional area of strengthened members, PBL groups with small rib heights are adopted in thin UHPC layers. However, there are few studies focusing on the shear behaviours of small-rib-height PBL (SRHPBL) groups embedded in UHPC. Therefore, 20 push-out tests were conducted with the variables of SRHPBLs arrangements. The experimental results demonstrated the grouped effect of SRHPBLs embedded in UHPC: (1) the overall shear capacity of twin-rib SRHPBL groups were about 1.67 times higher than that of their single-rib counterparts; (2) the overall shear capacity of SRHPBL groups decreased as the rib spacing of SRHPBLs increased; (3) the overall shear capacity of both the single- and twin-rib SRHPBL groups increased with the row numbers of SRHPBLs, whereas the average shear capacity of an individual SRHPBL increased gradually as the row numbers of SRHPBLs decreased; and (4) the row spacing of SRHPBLs slightly affected the shear performance of single- and twin-rib SRHPBL groups. Finally, theoretical models were developed to predict the shear capacity of SRHPBL groups encased in UHPC.

Parametric study for structural performance of spliced hybrid deck beams with UHPC cast in place joints

Splice beams are considered a solution for some difficults in bridge construction and a method for repairing damaged beams. One of the most challenging aspects of this technique is avoiding bond failure between the spliced members. In this study, ultra-high performance concrete (UHPC) was used to splice three beam segments due to its superior performance. This paper investigate the effects of some parameters on the spliced beams by Finite Element Method using computer program ABAQUS. The study have two approaches, first one, a validation study to generate model agree with experimental test results of seven specimens were tested under three bending point load. The main variables being the depth of the joint, the presence of a shear key in the joint, the hybridization of reinforcement, and the addition of dowels in the joint. The model was simulated and achieved good convergence when compared with experimental results, considering the load-deflection response and failure mode. The second approach that investigated various parametric studies, such as reverse cyclic load, repeated cyclic load, thickness of the deck, type of interface joint, reinforcement ratio, strengthening by UHPC layer, and the loading pattern.

Enhancing fire resistance of masonry structures: The potential of ultra high performance concrete (UHPC)

The potential of ultra high performance concrete (UHPC) for strengthening masonry structures is evaluated in this work, both under normal service conditions and after exposure to high temperatures, as a significant gap has been identified on this topic at the present time. The mechanical properties of UHPC exposed up to 800 °C are analyzed by means of destructive and non-destructive tests, and the UHPC-to-masonry bond capacity is tested, depending on temperature and substrate preparation. The strengthening of masonry columns by confinement is studied, proving that UHPC jackets can double the compressive strength and provide ductility, even after exposure to high temperatures. Strengthening of masonry walls to in-plane loads is also evaluated, finding that UHPC layers can increase the shear strength and prevent brittle failures. Experimental results are compared with the predictions of theoretical models from some design guides, slightly adapted to the context of this research, obtaining a good degree of accuracy.

Utilizing ensemble learning in the classifications of ductile and brittle failure modes of UHPC strengthened RC members

This study aims to achieve the swift and precise classification of ductile and brittle failure modes in flexural reinforced concrete (RC) members, specifically those with tension sides strengthened by ultrahigh performance concrete (UHPC). Employing six ensemble learning techniques—Bagging, Random Forest, AdaBoost, Gradient Boosting, XGBoost, and LightGBM—the authors utilize a comprehensive dataset comprising 14 features, which include manually labeled failure modes obtain from load–deflection curves. The model training spans four scenarios, varying in the inclusion or exclusion of features describing the cross-sectional area of RC members and moment resistance. XGBoost emerges as the most effective classifier, achieving an impressive 84% accuracy with high confidence. Additionally, the study employs the Shapley Additive Explanation (SHAP) technique on the best-performing model to illuminate the significance and impacts of various features in UHPC-strengthened flexural members’ failure modes. Notably, moment resistance and UHPC tensile strength surface as the most influential factors in predicting failure modes. Increased rebar yield strength, UHPC compressive strength, UHPC reinforcement ratio, and steel fiber volume in UHPC contribute to enhanced ductility in flexural members, while heightened moment resistance and UHPC layer thickness, along with a robust RC-UHPC interface, tend to induce brittleness. The introduction of such an effective failure modes classification model, coupled with the model’s explainability, instills trust in its predictions and facilitates seamless integration into real-world applications, particularly in seismic areas. The model’s ability to operate without the need for pre-experimental tests marks a significant advancement in the field.

Bending performance of wet joints in negative moment zone of prefabricated small-box girder bridges: Experimental and numerical study

The prefabricated small-box girder (PSBG) structure adopts the construction method of simple supporting beam transforming to continuous beam, which results in negative bending moment on the box girder supports and large tensile stress on the top of the longitudinal wet joints. Due to the relatively low tensile strength of concrete, the top plate of the bridge in the negative bending moment zone is susceptible to cracking under the load of vehicles, as well as due to the shrinkage and creep of concrete. This poses a risk to the safety, applicability, and durability of the bridge structure and may lead to a reduction in these aspects. Ultra-high performance concrete (UHPC) has excellent tensile performance and stronger crack control capability, which can effectively improve the anti-cracking and durability of the wet joints of the PSBG. In the present study, the research focused on the negative bending moment zone of a three-span continuous small–box girder bridges. This zone was selected as the research object and served as the basis for designing a stepped wet joint girder, where UHPC is partially cast on the top plate. By conducting three-point bending tests, we compared the bending mechanical properties of the joint girders when poured with normal concrete (NC) and UHPC in the joint top area. Further, the key factors affecting the anti-cracking performance and bearing capacity, including the length and thickness of the UHPC layer and the width of the wet joint, were analyzed by finite element simulation. The results show that the contribution of the top plate UHPC to tensile stress cannot be ignored. Compared with the NC joint girder, the UHPC joint girder has better toughness and plasticity, and the cracking load and bearing capacity increased by 28.6% and 9.7%, respectively. According to the simulation results, as the increase of the length and thickness of the UHPC layer and the width of wet joint, the anti-cracking and bearing capacities of the PSBG with the wet joint were enhanced. The thickness and length of the UHPC layer had a significant effect on improving the anti-cracking performance of the wet joint, and the joint width only needed to meet the transfer length of rebar bond stress.

Flexural performance of a prefabricated steel–UHPC composite deck under transverse hogging moment

This paper aims to investigate the flexural performance of a prefabricated steel–ultra-high-performance concrete (UHPC) composite deck (PSUCD) consisting of an orthotropic steel deck (OSD) and a reinforced UHPC layer under transverse hogging moment. Ten full-scale steel–UHPC composite slab specimens were subjected to four-point flexural tests, which varied shear connectors, steel reinforcement, and wet joint interfaces. The results showed that all specimens exhibited typical flexural failure with excessive opening of critical cracks in the UHPC layer. The specimens utilizing notched perfobond strip connectors (NPBLs) with perforating rebars had higher nominal cracking strength (maximum crack width of 0.05 mm) and ultimate capacity by 15% and 27%, respectively, compared to those using studs. The perforating rebars in NPBLs improved post-cracking performance, increasing nominal cracking strength and ultimate capacity by 14% and 16%, respectively. The lateral interfaces of rectangular-tooth-shaped wet joints resulted in a local weak link in PSUCDs, leading to a 17% decrease in nominal cracking strength, and as much as 26% if interfacial roughening was omitted. Increasing the diameter of surficial reinforcement from 10 mm to 12 mm improved nominal cracking strength and ultimate capacity by 20% and 17%, respectively. Considering the influence of wet joint interfaces, prediction models for maximum crack width, nominal cracking strength, and ultimate capacity were developed and validated for the performance design of PSUCDs under transverse hogging moment.

Experimental study on protection of prestressed concrete box girder under vehicle explosion of bridge deck

AbstractThis paper presents a study on the explosion protection of prestressed concrete box girder, which is a commonly used in concrete bridges. The explosion protection tests of four prestressed box beams were designed with 1:5 scale. In order to evaluate the explosion performance of different protective layers, steel plate, ultra‐high‐performance concrete (UHPC) layer, and composite protective layer were used to protect the box girder. The experimental results revealed that under the effect of deck explosion, local penetrating failure occurred in the top plate of the box girder, which exhibited characteristics of punching failure. The web and bottom plate of the box girder as a whole exhibited bending characteristic. Compared with the unprotected box girder, the damage of the top plate of the protected box girder was less severe, whereas the damage of the bottom plate was greater. In working conditions 2–4, the UHPC protective box girder suffered less damage to the top plate, whereas the other protective box girders suffered more. The explosion dynamic responses of the box girder were compared, and the influences of different protective layers on the explosion dynamic response of box girder were studied. Based on the box girder damage, the protective efficiency of each protective layer under near‐explosion was studied. Based on the explosion tests, three‐dimensional numerical analysis model of box girder explosion was established. The accuracy of the numerical simulations of the box girder explosion were verified by comparing with the experimental failure data. Through the numerical results, the explosive energy dissipation capacity of different protective materials was compared and analyzed. Finally, based on the dynamic response of the box girder under different working conditions, the sensitivity of the protective layer to the dynamic responses of the box girder were studied. The analysis showed that the UHPC layer was sensitive to the change of prestress and explosion damage area of the box girder but had little influence on the rebars stress and the overall response of the box girder.

Experimental Research on Fatigue Behavior of Reinforced UHPC-NC Composite Beams under Cyclic Loading.

Ultra-high-performance concrete (UHPC), a new cement-based material that offers high mechanical strength and good durability, has been widely applied in construction and rehabilitation projects in recent years. An optimum bending system is achieved by positioning the UHPC layer at the bottom tensile zone of the composite beam and placing the normal-strength concrete (NC) layer at the upper compression zone, which is described as the UHPC-NC composite beam. The fatigue behavior of reinforced UHPC-NC composite beams was described in this study, with an emphasis on the effects of UHPC layer thickness and fatigue load level on the fatigue life of the beam, deformation of the interface between UHPC and NC layers, as well as the bending stiffness of the beam. A total of 9 reinforced UHPC-NC composite beams were tested under cyclic loading. The test variables include UHPC layer thicknesses (zero, 200, and 360 mm), reinforcement ratios (1.184% and 1.786%), and the upper load levels (0.39~0.65). The results showed that good bonding had been achieved without delamination between UHPC and NC layers prior to the final fatigue failure of the beam, and the bending stiffness of the composite beam experienced a three-stage reduction under cyclic loading. Furthermore, an equation was proposed to predict the stiffness reduction coefficient of UHPC-NC composite beams under cyclic loading.

Experimental and theoretical study on seismic behavior of a new UHPFRC sandwich wall and wall-wall joint

A new type of precast ultra-high performance concrete (UHPFRC) sandwich wall is proposed in this study. The proposed UHPFRC sandwich wall is composed of the UHPFRC layers, extruded polystyrene foam (XPS) board layer, and short UHPFRC columns connecting the UHPFRC layers. Compared with the traditional reinforced concrete wall, the UHPFRC sandwich wall possesses lower self-weight, less reinforcement, and higher strength. In addition, two types of wall-wall vertical joint connection methods, flexible cable connection and embedded steel plate welding connection, are proposed. The pseudo-static tests on five UHPFRC sandwich wall specimens were conducted to study the effects of UHPFRC layer thickness, reinforcement ratio, and width of edge solid area on the seismic behavior of the wall. Then, the seismic performance of different connection methods was studied by pseudo-static tests on two T-shaped UHPFRC sandwich wall-wall joint specimens. Test results demonstrate that the unreinforced UHPFRC sandwich wall, with 20–30 mm UHPFRC thickness and 150 mm edge solid area, showed excellent cracking resistance and seismic behavior. The cracking loads are more than 50% of the peak loads, and the bottom shear capacity to self-weight ratios are generally more than 35. The seismic behavior of the wall-wall joints connected by the flexible cable and embedded steel plate welding are both reliable. Finally, formulas for cracking and peak load of the UHPFRC sandwich wall are proposed and the fiber model is developed for analysis of the wall-wall joint, which is verified by the test results to be accurate and reasonable.

Study on early shrinkage and cracking of slab in UHPC light-weight composite deck based on ambient temperature and humidity effects

This study investigates the early performance and potential issues of ultra-high-performance concrete (UHPC) light-weight composite decks (LCBDs) used in municipal highway bridge design. The major objective of this research was to understand the complex evolution of temperature and humidity within immature UHPC thin layers, which can lead to deformation and secondary stress, potentially causing cracks and threatening the long-term applicability and safety of steel-concrete composite bridges. Understanding of the mechanism of this process will be the basis for subsequent work. The early performance of UHPC with ordinary concrete is compared. Besides, discussions are conducted on the basic thermal conductivity theory of three-dimensional porous media and the hydration mechanism of cement-based materials that are appropriate for early-stage UHPC. Subsequently, a refined thermo-mechanical coupling analysis method for steel-UHPC LCBD was proposed, implemented using ABAQUS, and validated through a UHPC composite beam test. This method was applied to calculate the early shrinkage of the UHPC layer in a steel-UHPC LCBD and evaluate the risk of cracking in an actual bridge expansion project, providing a rationality evaluation of a bridge design and its construction procedure.

Experimental and numerical simulation research on the flexural performance of beams reinforced with GFRP bars and three-sides UHPC layer

This paper presents the results of the experimental investigation and numerical investigation conducted to study the flexural response of reinforced concrete beams reinforced with glass fiber reinforced polymer (GFRP) bars using three-sides ultrahigh performance concrete (UHPC) layer. The interface treatment of bubble wrap was applied in the internal surface of UHPC layer to ensure the enough interfacial bond behavior between UHPC layer and original concrete. Three tested beams, including two beams reinforced with three-sides UHPC and a reference beam, were studied through four-point bending tests and the effectiveness of flexural strengthening scheme associated with three-sides UHPC layer was evaluated. The results showed that the cracking load and ultimate load of beams reinforced with UHPC increased by 36.7%, 55.3%, and 30.0%, 42.7%, respectively, relative to the reference beam. The employment of UHPC layer could effectively control the development of surface crack width in beams. In addition, parametric studies were conducted to investigate the effects of the height of the UHPC, the thickness of the UHPC on both sides, and the longitudinal length of the three-sided UHPC layer using finite-element (FE) analysis. The results of numerical simulation were good agreement with the experimental results, indicating that the proposed finite element model could accurately predict the flexural responses of beams.

Data-driven moment-carrying capacity prediction of hybrid beams consisting of UHPC-NSC using machine learning-based models

This paper presents, for the first time in the literature, a study on the development of data-driven machine learning (ML) models to predict the moment-carrying capacity of ultra-high performance concrete (UHPC)-normal strength concrete (NSC) hybrid beams. A database of 56 specimens of rectangular-section UHPC-NSC hybrid beams subjected to flexural loading is adopted to train the models. In this context, ten ML algorithms that are most preferred in structural engineering applications are selected to develop prediction-based models: linear regression (LR), lasso regression (LASSO), ridge regression (RR), support vector regression (SVR), multilayer perception (MLP), random forest (RF), extremely randomized trees (ERT), extreme gradient boosting (XGBoost), K-nearest neighbors regression (KNN), and adaptive boosting regression (AdaBoost). Moreover, the Shapley additive explanation (SHAP) method is used to assess the impact of the input features on the prediction results. Lastly, user-friendly a graphical user interface (GUI) has been developed to ensure the interpretability of the prediction models and to overcome the "black box" problem of ML methods. The GUI, which is designed based on the model with the most effective prediction ability obtained from this work, allows design engineers to analyze their own data and customize the parameters of the model for the prediction of the moment-carrying capacity of UHPC-NSC hybrid beams. The results indicated that ML models can be an effective tool for predicting the moment-carrying capacity of UHPC-NSC hybrid beams. In this regard, notably, the XGBoost model exhibited superior performance in terms of prediction accuracy and generalization ability (R2 = 0.996 and 0.945 in the training and test datasets, respectively). On the other hand, according to the SHAP analysis results, the three most important input parameters influencing the moment-carrying capacity of UHPC-NSC hybrid beams are the effective depth (d), UHPC thickness at the bottom of the beam (UHPCbottom layer) and compressive strength of UHPC (fc,UHPC), respectively. Moreover, it has been found that the placement of the UHPC layer at the bottom of the beam rather than at the upper part of the beam is more effective in enhancing the moment-carrying capacity of UHPC-NSC hybrid beams.

Numerical and experimental study of a segmentally prefabricated orthotropic steel–ultra-high-performance concrete composite deck for long-span cable-stayed bridges

Utilizing the high strengths, toughness, and durability of ultra-high-performance concrete (UHPC), a segmentally prefabricated orthotropic steel–UHPC composite deck (POSUCD) system has been developed for long-span cable-stayed bridges. The deck system combines an orthotropic steel deck (OSD) with a reinforced UHPC layer using notched perfobond strips (NPBLs). The UHPC layer is prefabricated onto the OSD of the girder segment in the factory to form the POSUCD. After the precast UHPC layer is cured, the girder segments are transported to the bridge site and assembled. The precast UHPC layers are then connected via transverse wet joints before tensioning stay cables. Against the background of the Danjiangkou Reservoir Bridge with a main span of 760 m, finite element (FE) analysis was conducted to examine the stress state and flexural tests were carried out on local full-scale models to verify the feasibility of POSUCD. The results show that the prefabricated UHPC layer reduces the maximum static compressive stress of the steel deck in the girder system by 21%, and the stress ranges at fatigue-prone details of the steel deck are decreased by 28%–90% compared to traditional OSDs with asphalt overlay. Since the prefabricated UHPC layer helps withstand dead axial load in the main girder, the high compressive strengths of UHPC can be utilized more effectively than in deck systems with UHPC only used as a cast-in-situ overlay to resist vehicular wheel loads. The transverse wet joint interfaces were identified as the weakest aspect of the POSUCD, whether under tension or compression; however, their nominal flexural strengths along the longitudinal direction meet the required design standards. The shear resistance of NPBLs was also well confirmed by the minimal interfacial slip on the specimen when it reached the ultimate capacity under positive bending moment.

Shear behavior of pre-damaged RC beams strengthened with steel plate and UHPC

To address the deficiencies in the durability of the bonded connection, an innovative strengthening method combining steel plates with UHPC (ultra-high-performance concrete) is proposed. In this method, the UHPC layer is located between the steel plate and the original damaged RC (reinforced concrete) structures, mainly acting as a binder to enhance the bonding effect. To verify the effectiveness of the proposed method, five RC beams are manufactured and carried out to investigate the shear characteristics and capacity. In this experiment, some factors that may affect the strengthening effect are considered, such as the initial damage degree (represented by different crack widths), steel plate thickness, and strengthening method (two-side or one-side). The experiment results show that the proposed strengthening method can significantly improve the shear capacity (increased by at least 56%) of pre-damaged RC beams without significantly enlarging the cross-section and self-weight. In addition, during the load test, there is no peeling phenomenon for the steel plate-UHPC interface and the UHPC-RC interface. This demonstrates that it is feasible for the UHPC layer to replace epoxy resin glue as a bonding agent to enhance the bonding effect between steel plate and RC. The proposed strengthening method shows promising applications in strengthening old/defective bridges. Moreover, a simple formula for predicting the shear-bearing capacity of strengthened beams is derived. The shear-bearing capacity predicted by the proposed formula is in good agreement with the experimental results.

Early-age shrinkage characteristic of UHPC in strengthening stone masonry arch: Influence of restraint from arch structure

Ultra-high performance concrete (UHPC) is frequently employed in strengthening existing bridges, due to its extremely high mechanical qualities and exceptional durability. Nevertheless, the early shrinkage of UHPC layer restrained by the arch structure may result in the risk of cracking, when used for strengthening masonry arch bridges. This paper aimed to investigate the shrinkage and cracking potential of ultra-high-performance concrete (UHPC) at early age under restraint from masonry arch structure. For that, different types of masonry model arches were respectively strengthened by using UHPC and then the shrinkage behaviors of UHPC layer were tested. The effects of four different strengthening modes, i.e., F-hoop (wrapping for four sides), T-hoop (wrapping for three sides), Intrados and Extrados, three different rise-span ratios, i.e., 1/5, 1/4, 1/2 and two different arch ring widths, i.e., 0.42 and 0.15 m, were considered. Results indicate that the shrinkage strain time-varying curve of UHPC layer was not only related to the material characteristics, but also negatively related to the change in external environment temperature. For different strengthening methods, the final shrinkage strain of UHPC layer follows: F-hoop < T-hoop < Intrados < Extrados. The shrinkage strain was unaffected by different strengthening modes along the thickness and arch axis directions. The shrinkage effect of UHPC generates the maximum tensile stress value (1.0–2.0 MPa) at the arch top of the strengthening layer, which was significantly less than the tensile strength of UHPC during the same risk period. The arch foot is the most complex area under stress and should be taken seriously.

Experimental study on thermal behaviors of two-layered functionally graded concrete slabs subjected to fire

In order to improve structural thermal performances, this paper designed a two-layered functionally graded concrete (FGC) slab composed of an ultra-high performance concrete (UHPC) layer and a lightweight aggregate concrete (LWAC) layer. UHPC layers are used as structural layers to bear loads while LWAC layers are used for thermal insulation. Fire tests were conducted to investigate the fire resistance behaviors of the FGC slabs with different LWAC thicknesses under the ISO834 heating regime while homogeneous UHPC and normal concrete (NC) slabs were prepared as a contrast. The experimental results revealed that the LWAC could significantly reduce the temperature transferred to UHPC layers, where the interfacial temperatures of FGC slabs with 40 mm and 30 mm thick LWAC layers were 197 °C and 325 °C respectively. Meanwhile, the maximal mid-span deflections ranged from 26.1 mm to 31.5 mm respectively, approximately 55%-65% lower than those of homogeneous material specimens. Besides, FGC slabs presented good explosive spalling resistance, no delamination or debonding occurred at interfacial regions, and LWAC layers adhered to UHPC layers in the form of “multi-point attachment”. The studies provided a solution to the fire resistance design of reinforced concrete structures under high temperature based on the structural level, and the thickness of LWAC layers was optimized by finite element analyses.

Flexural behavior of the hollow slab girders strengthened with ultra-high performance concrete: Field full-scale experiment and analysis

Due to high strength and well durability, ultra-high performance concrete (UHPC) can be designed to strengthen damage hollow slab girder. The main objective of this study is to evaluate reinforcement feasibility of compression zone of hollow slab girder via field full-scale experiments and finite element (FE) analysis. To this end, UHPC layer was firstly proposed to reinforce top plate of damaged hollow slab girder. Then, four-point bending tests were conducted to investigate flexural behavior of hollow slab girder with/without UHPC layer. FE models were developed to simulate bending failure process of hollow slab girders. The deflection, crack propagation and stress distribution of hollow slab girders were analyzed as well as failure modes. The results show that UHPC layers can significantly improve flexural stiffness of hollow slab girder, and it results in an increase of 11.5% in cracking load and 23.7% in ultimate load, respectively. Moreover, reinforced girder exhibits well ductility and relatively high flexural bearing capacity. Furthermore, the parameter analyses were performed to investigate the effects of the thickness of UHPC layer and prestress on flexural behavior of reinforced girder. The results show that the prestress significantly affects the ductility and cracking load. Whereas, it seems to be independence of ultimate load. Finally, theoretical analysis was performed to obtain calculation formulas, which determine cracking and ultimate loads of reinforced hollow slab girder. Overall, this study has verified effectiveness of proposed reinforcement method in improving stiffness and flexural bearing capacity of hollow slab girder.