ALBERT

Description: In this study, the effect of curing temperature on the properties of slag cement concrete after high-temperature exposure was studied, and elevated curing temperature (45 ± 2 °C and 95% relative humidity (RH)) was selected to compare with the standard curing temperature (20 ± 2 °C and 95%RH). Four different concrete mixes with the same mix proportion, except for different slag replacement ratios, were used: 0% (reference), 30% (slag), 50% (slag), and 70% (slag). After high-temperature exposure at 200, 400, 600, and 800 °C, the effect of slag replacement, high temperature, and curing temperature on the compressive strength and mineralogical and microstructural properties of slag cement concrete were studied. Test results indicated that the compressive strength of concrete cured for 7 d at elevated temperatures increased by 28.2, 20.7, 28.8, and 14.7% compared with that cured at the standard curing condition at slag percentages of 0, 70, 50, and 30%, respectively. X-ray diffraction (XRD) and Scanning electron microscope (SEM) results revealed that concrete cured at elevated temperatures exhibited a more condensed phase and contained a higher percentage of hydrates than that cured for 7 d in the standard curing condition. However, after 56 d of curing, concrete in the standard curing condition exhibited a more stable phase and a higher concentration of hydrates.

Description: Oil palm shell (OPS) concrete filled steel tube (CFT) columns are acknowledged to be a new type of sustainable composite column. In this type of column, the conventional coarse aggregate was partially replaced with OPS lightweight aggregate to provide a green composite column. This type of CFT column showed higher energy absorption and flexibility compared to CFT columns with normal weight concrete. This research studied the effect of the strength of OPS concrete on the axial compressive behaviour of CFT columns for two grades of OPS concretes. The behaviour was comparable to that of CFT columns with two grades of normal concrete. The results showed that the CFT columns with OPS concrete achieved a new post-peak behaviour. The experimental results of the axial compressive load were compared with the estimation of two international standards. EC4-1994 and ACI318-14 showed a reliable and conservative estimation of the axial load capacity of CFT columns, respectively.

Description: The shear span-to-effective depth ratio (a/d) is one of the factors governing the shear behavior of reinforced concrete (RC) beams, with or without shear reinforcement. In high-strength concrete (HSC), cracks may propagate between the aggregate particles and result in a brittle failure which is against the philosophy of most design guidelines. The experimental results of six HSC beams, with and without shear reinforcement, tested under four-point bending with a/d ranged from 2.0 to 3.0 are presented and compared with different model equations in design codes. The a/d ratio has higher influence on the shear strength of reinforced HSC beams without shear reinforcement than beams with shear reinforcement. Most of the shear resistance prediction models underestimate the concrete shear strength of the beams but overpredict shear resistance of beams with shear reinforcement. However, the fib Model code 2010 accurately predicted the shear resistance for all the beams within an appropriate level of approximation (LoA).

Description: Prestressed segmentally constructed balanced cantilever bridges are often subjected to larger deflections than those predicted by calculations, especially for long-term effects. In this paper, the case of modular balanced cantilever bridges, which are prestressed segmental bridges obtained through a repetition of the same double cantilever, is investigated. The considered bridges are two typical cases of modular balanced cantilever both subjected to large deformations during their lifetime. In this case, due to the unusual employed static scheme, creep deflections indefinitely evolve over time particularly at the end of the cantilevers and in correspondence with the central joint. These remarkable deflections cause discomfort for vehicular traffic and in certain cases can lead to the bridge collapse. Important extraordinary maintenance interventions were necessary to restore the viability of the bridges and to replace the viaduct design configuration. To this aim, the static schemes of the structures were varied, introducing new constraints, new tendons, and carbon fiber reinforcements. In the present work, time analysis was performed to compare the time-dependent behavior of the bridge according to two different creep models, the CEB-FIP Model Code 2010 and the RILEM Model B3, with the real-time-dependent behavior of the bridge observed during its lifetime. The two different employed models exhibit different behaviors in terms of displacements and bending moments acting on the bridge. Interesting considerations are made on their reliability in simulating the long-term creep effects that evolve indefinitely over time. Moreover, retrofitting techniques have been proposed and modeled to predict their effectiveness in reducing time-dependent deflections.

Description: Calcined hwangtoh is a pozzolanic material that is increasingly being used as a mineral admixture in the concrete industry. This study shows a hydration model for cement–hwangtoh blends and evaluates the various properties of hwangtoh-blended concrete using reaction degrees of binders. First, a kinetic reaction model is proposed for analyzing the pozzolanic reaction of hwangtoh. The reaction of hwangtoh includes three processes: the initial dormant period, boundary reaction process, and diffusion process. The mutual interactions between the binary reactions of cement and hwangtoh are thought to be in line with the items in capillary water and calcium hydroxide. Second, the reaction degrees of cement and hwangtoh are determined based on a blended hydration model. Furthermore, the chemical (chemically combined water and calcium hydroxide contents), mechanical (compressive strength), thermal (hydration heat), and durability aspects (carbonation depth) of hwangtoh-blended concrete are systematically predicted. The results show good agreement with experimental results.

Description: The purpose of this study is to examine the effects of the seismic wave velocity on vertical displacement of a cable-stayed bridge’s deck under asynchronous excitation. The Quincy Bayview Bridge located in Illinois, USA, and four other generic bridges are selected for the study. Ten records obtained from earthquakes in US, Japan, and Taiwan are used as input for the seismic excitation in the time-history analysis. Two equations are proposed in this study to determine a critical seismic wave velocity that would produce the greatest vertical deck displacement. The critical wave velocity depends on the total length of the bridge, the fundamental period of the bridge, and the C-factor. The C-factor in this study is 0.72, which is based on analyzed results from the five selected bridges. The two equations and the C-factor are verified through application on two 3-span cable-stayed bridges studied previously by Nazmy and Abdel-Ghaffar. The proposed C-factor of 0.72 is recommended for use for typical 3-span cable-stayed bridges with a side-to-main span ratio of about 0.48. The methodology developed in the study, however, can be applied to any specific bridge to examine the excitation of the deck vertical displacement under the longitudinal seismic ground motion.

Description: Structural behavior against the blast load is evaluated for a structure reinforced by high-performance fiber-reinforced cementitious composites (HPFRCC). The Alfred P. Murrah Federal Building, which experienced a terrorist attack in 1995 is taken into consideration for the blast analysis using the finite element method. The continuous surface cap model (CSCM) is used to simulate the behavior of normal concrete and HPFRCC. By reinforcing normal concrete with HPFRCC, damage, and deformation of the structure are significantly reduced. This study presents an efficient reinforcement method by performing an explosion simulation on the structure using HPFRCC and evaluating the behavior according to various reinforcement methods. Specific reinforcements according to the types of members are required to enhance the efficiency of reinforcement. With the optimized reinforcement using high-performance fiber-reinforced cementitious composites (HPFRCC), the resistance to blast load is significantly improved.

Description: Recently, as a new precast concrete (PC) construction method for increasing economy and constructability, the PC double-beam system has been developed for factories or logistic centers, where construction duration is particularly important. In this study, half-scaled PC double beam–column connection was tested under gravity loading and cyclic lateral loading. The major test parameters included the use of the spliced PC column and the addition of reinforcement at the beam–column joint. In the gravity loading test, the flexural behavior of the PC double beam was investigated. The test results showed satisfactory flexural capacity at the PC double-beam section, validating the composite action between the PC and RC members. In the cyclic lateral loading test, the seismic performance of the PC double beam–column connection was investigated. Based on the test results, the failure mode, load-carrying capacity, deformation capacity, energy dissipation capacity, secant stiffness, and shear strength of the PC double-beam system were evaluated and compared with those of a conventional RC double beam–column connection. According to the test results, the structural performance of the PC double beam–column connection was comparable to that of the RC double beam–column connection and satisfied the acceptance criteria of moment frame in the ACI 374.1-05 provision.

Description: The purpose of this study was to investigate the thermal and cyclic behaviors of fire-damaged walls designed with different failure modes, aspect ratios and heated areas. These cyclic behaviors include temperature distribution, maximum lateral load, stiffness, ductility, and energy dissipations, etc. Toward this goal, the concrete wall specimens were exposed to heat following an ISO 834 standard time–temperature curve and the cyclic loading was applied to the fire-damaged walls. The test results showed that exposure to fire significantly reduced the cyclic performance of the RC walls. Especially, it was observed that heated area, designed failure mode, and aspect ratio have influences on maximum lateral loads, stiffness, and ductility of the fire-damaged walls, while almost no effects of the heated area, designed failure mode, and aspect ratio on temperature distribution and energy dissipation were found.

Description: Post-tensioned unbonded tendons are widely used in flat slabs/plates when there is a demand for large span lengths, durable tendons and a reduction in the weight of structure. For post-tensioned flat slab/plates, different tendon layouts have been discussed in the literature. It is vital to compare the structural response (i.e., deflection and stresses) and the clashing of tendons of the proposed tendon layouts in the literature to select an appropriate layout. Hence, this study focuses on the analysis of three different six-panel flat plates (i.e., panel sizes: 6 m × 6 m, 9 m × 6 m and 11 m × 6 m) with five different tendon layouts, using computer programs ADAPT-Floor Pro and FEM-Design 17, based on linear finite element (FE) analysis. Short-term/long-term deflection and stress due to service load obtained from the computer programs has also been compared, to highlight the differences. Ultimate bending moment of resistance was calculated theoretically for different layouts and compared. Results from the analysis show that, when a higher portion of tendons is concentrated instead of distributed, stresses caused by other structural loads are counteracted best. The layout with all tendons concentrated also has the best results in terms of deflections.