To investigate the effects of long-term prestress loss on concrete box girders strengthened with external prestressing, a large-span box girder, in service for over 20 years and strengthened with external prestressing, was monitored for four months. Prestress loss in the longitudinal, vertical, and transverse directions of the box girder was calculated according to Chinese code requirements. Magnetic flux rope force transducers were used to monitor the prestress loss in the external prestressing cables. Fiber Bragg Grating (FBG) sensors were used to monitor deflection changes at the mid-span of the bridge. Finally, the effect of prestress loss in the longitudinal, vertical, and transverse tendons on mid-span deflection was investigated through simulations using ABAQUS software. The results show that instantaneous prestress loss accounts for most of the total loss compared to long-term loss, and that longitudinal prestress loss has the most significant effect on mid-span deflection. The impact of longitudinal prestress loss on deflection before and after strengthening was also compared. The downward deflection and up-ward arch caused by longitudinal tendon prestress loss were reduced after strengthening, con-firming the effectiveness of the external prestressing method.

The feasibility of composite-activated coal gangue (CACG) as the primary cementitious material for concrete was experimentally studied in this paper. The effects of concrete strength grade on slump and slump flow, compressive strength, splitting tensile strength, axial compressive strength, elastic modulus, and drying shrinkage of alkali-activated coal gangue concrete (AACGC) were experimentally investigated. Experimental results indicated that the slump and slump flow of the AACGC were smaller than that of ordinary Portland cement concrete (OPCC). The mechanical properties of the AACGC were superior to those of the OPCC. The compressive strength, splitting tensile strength, axial compressive strength, and elastic modulus of the AACGC were 1.17 times, 1.04 times, 1.47 times, and 1.04 times those of the OPCC, respectively. With the increase in concrete strength grade, the mechanical properties of the AACGC have gradually increased. The difference in failure modes of axial compressive strength between the AACGC and OPCC was analyzed. Moreover, the empirical formulas of the elastic modulus and compressive strength for the OPCC in various regions codes were summarized, and found that the empirical formula in GB 50010-2002 code and EN 1922 Eurocode 2 was also applicable to the AACGC. Finally, the mass-loss rate and drying shrinkage for the AACGC at different concrete strength grades were systematically analyzed, and a hyperbolic prediction model was proposed to reflect the drying shrinkage behavior of the AACGC.

The feasibility of preparing coal gangue-based geopolymer mortar (CGBGM) with composite-activated coal gangue was experimentally explored in this paper. The effects of water-to-binder ratio (W/B), alkali content (A), and slag content (S) on the fluidity and performances of the CGBGM were experimentally investigated. The ANOVA method was employed to evaluate the significance level of influenced factors. Moreover, the microstructure, element compositions, and qualitative microanalysis of the CGBGM at various curing ages were verified by ESEM-EDS. Test results denoted that the mechanical properties of the CGBGM are superior to that of ordinary Portland cement mortar (OPCM). The flexural and compressive strength of the CGBGM rapidly increases before 7 d and then tends to slow afterward. According to the impact degree on the flexural and compressive strength of the CGBGM, the S is ranked first, followed by the A, and finally the W/B. The flexural and compressive strength of specimens with 40% slag increased by 45.97% and 90.75%, respectively, compared to the control group. However, the A and W/B have little effect on flexural strength. In addition, the hydration productions filled in the crevice and healed the entrapped microcracks in the hardened paste with the increase in curing ages, forming a dense microstructure. The Ca/Si decreased from 0.5 at 3 d to 0.06 at 28 d, and the Ca/Al decreased from 0.25 at 3 d to 0.05 at 28 d. Finally, the difference in drying shrinkage behavior between the CGBGM and OPCM was systematically analyzed.

The coal gangue after composite activation treatment is considered a potential low-carbon and green cementitious material, so the feasibility of employing composite-activated gangue to partially or entirely replace cement for building materials is systematically studied in this paper. The effects of alkali content, slag content, and water-to-binder ratio on the mechanical properties and frost resistance of alkali-activated coal gangue mortar (AACGM) were experimentally investigated. An ESEM was employed to observe the microstructure of the AACGM. Moreover, the microstructure damage to the AACGM was analyzed by a pixel-based image processing algorithm. The research was carried out in accordance with standards JGJ/T70-2009 and GB/T 50082-2009. Experimental results indicated that the mechanical properties and frost resistance of the AACGM were superior to those of ordinary Portland cement mortar (OPCM). Compared with the OPC group, the compressive and flexural strength of the W0.50 group increased by 16.01% and 14.19%. Moreover, the loss rate of mass, flexural strength, compressive strength, and microstructure damage of the AACGM were less than those of the OPCM. Between freeze–thaw cycles 25 and 100, the cracks and pores of specimens gradually grew, and the maximum crack width increased by 277.78%. In addition, the slag was beneficial in improving the flexural strength, compressive strength, and frost resistance of the AACGM. Finally, the freeze–thaw damage mechanism of the CGBG mortar was systematically analyzed.

External prestressing technology has been widely applied to mitigate the cracking and deflection of variable cross-section continuous box-girder bridges. However, it remains a challenging task to establish a reliable safety monitoring and evaluation method in engineering applications. This paper takes a practical project as the research background and installs a sensing monitoring system on a bridge before strengthening. The evolutions of the strain at the characteristic sections, crack width, and deflection of bridges before and after strengthening were compared and analyzed. The construction process’s safety and strengthening measures’ effectiveness were monitored and evaluated. According to the construction process monitoring, the prestress tensioning process did not bring torsion to the bridge, and the strain at each monitoring point was within safe boundaries. After strengthening, the bridge appeared to camber with a value of $3–6\mathrm{mm}$ . The typical cracks were mitigated to different extents, and the closure degree of the crack width was $(2.5–4)\times {10}^{-2}\mathrm{mm}$ . The strain distribution of the bridge was more stable after strengthening, and the frequency of large strain was significantly reduced, which reflected the improvement of section stiffness. The study can provide meaningful references for the optimization design and strengthening strategies of similar projects.

This paper proposes a procedure of synthetic detection for the location of a change point and outliers in bilinear time series models with a change after an unknown time point. Based on Bayesian framework, we first derive the conditional posterior distribution of the change point and from that distribution estimate the position of the change point. Then we use these results to detect the outliers in the time series before and after that change point via Gibbs sampler algorithm. Our simulation studies show that the proposed procedure is effective.