Xuhui Zhang

Ultra-high performance concrete (UHPC) is widely used for its excellent compactness and durability. However, steel fibers in UHPC may form conductive paths, affecting electrochemical testing of internal rebars. To assess the applicability of current testing standards in UHPC, specimens of UHPC, high-performance concrete (HPC), and normal concrete (NC) with different cover thicknesses are designed. Open circuit potential (OCP), linear polarization resistance (LPR), and Tafel polarization curves are adopted to compare the corrosion behavior during 180 days of chloride immersion. Results show that UHPC has the most negative OCP, followed by NC and HPC. According to ASTM C876, this would indicate the highest corrosion risk for UHPC, contradicting its well-known superior chloride resistance. Hence, ASTM C876 is not applicable to UHPC. Corrosion current density (I_corr) is smallest in UHPC, followed by HPC and NC, consistent with chloride resistance ranking, indicating good applicability of the linear polarization method to UHPC. The anodic Tafel slope is larger than the cathodic one for all specimens, showing anodic control, unaffected by steel fibers. Larger cover thickness leads to higher OCP, higher polarization resistance, and lower I_corr. At 30 mm cover, internal rebars in UHPC are essentially non-corroded.

Structural deflection is a critical factor used for evaluating the effectiveness of reinforcement. This study proposes a method for generating FRP layouts with a local displacement constraint to strengthen damaged structures. A local displacement constraint strategy is developed using the Lagrange multiplier method, integrating the constraint into the objective function and transforming the problem into an unconstrained optimization framework. The design sensitivity formula for strengthening damaged structures is derived based on this displacement-constrained strategy. Additionally, an automatic adjustment strategy of the Lagrange multiplier is given based on the bisection method. Finally, the effectiveness and applicability of the proposed method are illustrated through case studies on damaged RC beams, slabs, and arches. The FRP configurations under various constraints are discussed and compared with the results generated by the BESO method. Results demonstrate that the proposed method can effectively generate FRP configurations for damaged RC structures.

In order to explore the secondary bond anchorage performance between prestressed tendons and concrete after the fracture of steel strands in post-tensioned, prestressed concrete (PPC) beams, a total of seven post-tensioned, prestressed concrete specimens with a size of 3 × 7ϕ15.2 mm were constructed firstly, and the steel strands at the anchorage end were subjected to corrosion fracture. Then, the pull-out test of the specimens was conducted to explore the secondary anchorage bond mechanism of the residual stress of prestressed tendons experiencing local fracture. Moreover, the influences of factors such as the embedded length, release-tensioning speed, concrete strength, and stirrup configuration on anchorage bond performance were analyzed. Finally, the test results were further verified via finite element analysis. The results show that the failure of pull-out specimens under different parameters can be divided into two types: bond anchorage failure induced by the entire pull-out of steel strands and material failure triggered by the rupture of steel strands. The bond anchorage failure mechanism between steel strands and the concrete was revealed by combining the failure characteristics and pull-out load–slippage relation curves. The bond strength between prestressed steel strands and concrete can be enhanced by increasing the embedded length of steel strands, elevating the concrete strength grade, and enlarging the diameter of stirrups so that the specimens are turned from bond anchorage failure into material failure.

To understand the secondary transfer performances of residual prestress after the anchoring failure of end-anchored steel wire strands due to corrosion fracture, six steel wire strand components of post-tensioning prestress were designed and fabricated. One-side fast corrosion was applied to the steel wire strand components using the electrochemical method until anchoring failure was reached. The sphere of influence, stress changes, and the retraction and swelling effect of broken beams after failure were investigated. The influences of factors such as concrete strength, stirrup area, and the length of the component on the secondary transfer length of residual prestress were discussed. Based on the deformation relationship between prestressed steel wire strands and concrete in the stress transfer zone, a stress equation was established and solved through a bond constitutive model. A prediction model of the effective stress transfer length of prestressed steel wire strand after failure was proposed. The results demonstrated that residual prestress can have a secondary transfer after the corrosion fracture of end-anchored steel wire strands, but some effective prestress may be lost. Moreover, the loss of prestress is inversely proportional to concrete compressive strength. When the specimens are relatively short, the prestress loss increases significantly. Concrete strength has significant influences on the length of secondary transfer. The proposed simplified calculation method of the secondary transfer length of residual prestress has a relatively high accuracy, with an average error of 2.9% and a maximum error of 5.2%.

An experimental test was performed to study the bond behavior between steel rebar and concrete reinforced by polyvinyl alcohol (PVA) fibers. Twenty specimens were prepared and subjected to the pull-out test. Four different fiber volume contents (0%, 0.2%, 0.4%, and 0.6%) were considered and the concrete with the strength grade of C35 was designed in the present study. The effects of PVA fibers, rebar diameter, and cover depth on bond behavior were clarified. The effects of PVA fibers on the mechanical property of concrete were also studied. The results show that PVA fibers decreased the compressive strength of concrete, but increased splitting tensile, flexural, and direct tensile strength. PVA fibers negatively affected bonding in the ascending branch both for the pull-out and the splitting failure cases, but improved the bonding in the descending branch after peak stress for the splitting failure case. In the present test, the maximum decrement of bond strength was about 16.2% for specimens with less than 0.6% PVA fibers. PVA fibers restricted both the macro-cracking and micro-cracking for the splitting cases, the former were much more significant than the latter. The effects of rebar diameter and cover depth on bonding became slight and significant with the increasing content of PVA fibers, respectively.

Corrosion-effected bond behavior between polyvinyl-alcohol-fiber-reinforced concrete and steel rebar under a chloride environment is the experimental subject studied in the present work. Twenty-four pull-out specimens are designed and subjected firstly to an accelerated corrosion test. The effects of polyvinyl alcohol fibers on the cracking behavior, chloride penetration of concrete members and the corrosion loss of steel rebars during the corrosion test are discussed. After this, these corroded specimens are subjected to a pull-out test. The failure mode, the bond-slip curves and the typical bond-stress values are measured during the test. The effects of polyvinyl alcohol fibers and corrosion loss on bond behavior between polyvinyl-alcohol-fiber-reinforced concrete and steel rebar are clarified. Results show that the polyvinyl-alcohol-fiber-reinforced concrete exhibits worse resistance to corrosion damage than plain concrete. The cracking width, chloride penetration depth in concrete and the corrosion loss of steel rebar are more serious for the specimens with more polyvinyl alcohol fibers. The polyvinyl alcohol fibers also negatively affect bonding in ascending branches for both the specimens, but improve the bonding in descending branches after peak stress in the case of splitting. In the present test, the bond strength of corrosive specimens is increased slightly and then decreases gradually with the deepening of corrosion loss. The failures of specimens change from pull-out to splitting-pull-out as the corrosion time exceeds 30 days. Compared with uncorroded specimens, the maximum degradation of bond strength is about 50.1% when the corrosion is increased from 0% to 15%.

In order to study the bond-slip constitutive model between prestressed tendons and concrete, the pull-out tests of single-strand and multi-strands specimens are carried out. The effect of the number of prestressed tendons on the failure mode, slip characteristics and concrete strain of the pull-out specimens are analyzed, and the constitutive models of bond-slip between for single- strand and multi-strand tendons are also proposed. The results show that the bond mechanism between steel strand and concrete is basically similar to that of deformed steel bar, but the slip process of steel strand along the axis direction is accompanied by rotation phenomenon because it is twisted. Moreover, compared with the single-strand prestressed tendon, the average ultimate bond stress of each tendon of the three-strand prestressed tendons decreases by 13.2%. In addition, the calculated result of the pull-out limit load for the single-strand prestressed tendon is only 2% higher than the experimental value, while the corresponding value of three-strand prestressed tendon is only 3.74% lower than the experimental value. This means that the proposed bond-slip models for single- strand and multi-strand tendons have high reliability.

The fundamental notion of ‘smart’ in building materials discourse is responsiveness—the ability of materials to react to environmental stimuli by manifesting a noticeable physical change when there is a difference in the conditions of their immediate surroundings. This notion, however, is also interchanged with ‘intelligence’, which involves an array of control protocols. Notwithstanding, both notions are used synonymously and as occupant comfort and energy efficiency strategies in buildings. The current study aimed to underscore the fundamental issues in the conceptualization of both notions in building materials colloquy by systematic review of published literature following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 checklist. The review revealed that while smart responses are direct, predictable and reversible, requiring no external control system, computer systems and networks which require a constant supply of energy are essential for intelligence. In fact, the relationship between intelligent systems, energy efficiency and occupant comfort depends on external computer control and machine components of learning, resulting in complex systems with longer payback times, whereas smart materials and systems respond directly and immediately without additional energy or occupant control. The discussions present an attempt towards promoting zero additional energy demand for buildings using smart materials.

The filling of strand corrosion products during concrete crack propagation is investigated experimentally in the present paper. The effects of stirrups on the filling of corrosion products and concrete cracking are clarified. A prediction model of crack width is developed incorporating the filling proportion of corrosion products and the twisting shape of the strand. Experimental data on cracking angle, crack width, and corrosion loss obtained from accelerated corrosion tests of concrete beams are presented. The proposed model is verified by experimental data. Results show that the filling extent of corrosion products varies with crack propagation. The rust filling extent increases with the propagating crack until a critical width. Beyond the critical width, the rust-filling extent remains stable. Using stirrups can decrease the critical crack width. Stirrups can restrict crack propagation and reduce the rust filling. The tangent of the cracking angle increases with increasing corrosion loss. The prediction of corrosion-induced crack is sensitive to the rust-filling extent.