Da Chen

Shear Behaviors of Reinforced Ultrahigh Toughness Cementitious Composite Slender Beams with Stirrups

This paper presented the investigation on shear behaviors of reinforced ultrahigh toughness cementitious composite (RUHTCC) slender beams with stirrups under center-point loading, where UHTCC possessed the tensile strain-hardening and multiple-cracking characteristics. Eight RUHTCC beams with four different web reinforcement ratios, as well as two reinforced concrete (RC) counterparts, were tested. The experimental results revealed that the use of slight web reinforcement can transform the brittle shear behavior to ductile flexure-shear behavior for RUHTCC beams with about 3.25% longitudinal reinforcement ratio, whereas the flexural failure was obtained when the stirrup ratio was increased up to 0.55% even for the beam with balance reinforcement ratio. Compared to the RUHTCC beam without a stirrup, the ultimate shear strength of RUHTCC beam with a stirrup was slightly enhanced. The shear contribution of UHTCC itself reduced with the increase in the web reinforcement ratio due to the restriction of stirrups. The multiple diagonal cracking and stable crack propagation behavior were presented in RUHTCC beams with and without stirrups, but the number and total opening of diagonal cracks decreased with the increase in stirrup ratio. An empirical equation for predicting the shear resistance of RUHTCC beams with stirrups was proposed based on the shear contribution from both UHTCC and stirrups.

Comparison of Structural Properties between Monopile and Tripod Offshore Wind-Turbine Support Structures

Offshore wind power provides a new kind of green energy. This paper presents a comparison study on the structural properties of monopile and tripod wind-turbine support structures, which are used extensively in offshore wind farms. Both structures have the same upper tower, but different lower structures, one with a monopile and the other with a tripod. Static, fatigue, and modal analyses indicate that both the tripod and monopile structures are feasible in the field, but that the tripod structure is superior to the monopile structure. Static analysis reveals that the location of maximum stress in the monopile structure is different from that in the tripod structure, and that the tripod structure shows higher stiffness and greater stress-control capacity than the monopile structure. Fatigue analysis indicates that the tripod structure has a longer lifetime than the monopile structure. Modal analysis indicates that the two structures exhibit large differences in their natural frequencies. Unlike the monopile structure, the third and first modes both have a substantial influence on the dynamic response of the tripod structure.

An Elastoplastic Damage Constitutive Model for Cementitious Materials under Wet-Dry Cyclic Sulfate Attack

The mechanical properties of cement mortars subjected to wet-dry cyclic sulfate attack were studied by the compression strength test. The results showed that the ultimate compressive strength increased with number of cycles at the initial stage. However, after a certain time, it started to decrease with further increases in the number of cycles. Moreover, the concentration of the sodium sulfate solution proved to be an important factor affecting the ultimate compressive strength. Based on continuum damage mechanics theory, an elastoplastic damage constitutive model is presented to describe the mechanical behavior of cementitious materials under compressive stress. The results obtained agree well with the experimentally observed elastic, plastic, and damage characteristics of cement mortars under compressive stress.

Effect of Polypropylene and Basalt Fiber on the Behavior of Mortars for Repair Applications

The fresh, mechanical, and durability properties of the polypropylene fiber-reinforced mortar (PP FRM) and the basalt fiber-reinforced mortar (BFRM) with various fiber contents were tested in this paper. The test results show that the presence of polypropylene (PP) fiber and basalt fiber (BF) in the mortar reduces the initial slump flow and increases the slump flow loss rate. The bond strength and flexural strength of fiber-reinforced mortar (FRM) are improved, whereas no obvious improvement on the compressive strength has been observed. Compared with the control mortar, the bond strength of PP FRM and BFRM reinforced with 0.6 kg/m3, 1.6 kg/m3, and 2.6 kg/m3 fiber increases by 16.60%–28.80% and 10.60%–21.40%, respectively. Furthermore, FRM shows lower drying shrinkage, superior abrasion resistance, water impermeability, and freeze-thaw resistance compared with the control mortar. The abrasion resistance strength of PP FRM and BFRM is 77.30% and 38.65% more than the control mortar with 2.6 kg/m3 fiber content. Therefore, PP FRM and BFRM are suitable to be utilized as repair materials, especially in repairing hydraulic structures surfaces with excellent bond strength and abrasion resistance.

Corrosion Activity on CFRP-Strengthened RC Piles of High-Pile Wharf in a Simulated Marine Environment

We report test results from an experimental study to investigate the effectiveness of carbon fiber-reinforced polymer (CFRP) against reinforcing steel bar corrosion. Twelve reinforced-concrete pile specimens of 180 mm square by 1,600 mm long were cast. Three pile specimens were corroded to 5% steel mass loss and then strengthened with CFRP sheets; four specimens were strengthened by using CFRP sheets, whereas the remaining five specimens were not strengthened. The specimens were placed in a simulated marine environment, and corrosion was induced by an impressed current technique. At different theoretical corrosion degrees, nondestructive tests were performed to investigate the corrosion activity of the pile specimens, and destructive tests were performed to determine reinforcing steel bar mass loss. Based on the findings, the effectiveness of the CFRP-strengthened RC piles under aggressive marine environmental conditions was established.

Experimental Study on the Performance and Microstructure of Cementitious Materials Made with Dune Sand

This paper presents the results of an investigation on the utilization of dune sand from waterway regulation engineering as the main raw materials to produce cementitious materials. The mechanical and durability properties of the cementitious materials were studied. Furthermore, a scanning electron microscope (SEM) and mercury intrusion porosimeter (MIP) were used to identify the microstructure of the specimens. The results show that the compressive and splitting tensile strength of cementitious materials can be improved due to the addition of ground granulated blast-furnace slag (GGBS) which mainly attributes to a better grain size distribution and pozzolanic effect compared to the specimen added cement alone. The specimen with the addition of suitable cement, GGBS, and gypsum shows low dry shrinkage and excellent abrasion resistance. Correspondingly the specimens present a lower porosity and total volume of pores at different curing ages. The SEM observation indicates that there are quite a lot of hydrate products such as calcium silicate hydrate gel in the matrix which verifies the formation of cementitious compounds. The results obtained suggest that there is potential in manufacturing cementitious material with dune sand in substitution of ordinary concrete to use in hydraulic engineering.

The mechanical properties of coastal soil treated with cement

The influences of cement type, cement content, and curing time on the unconfined compression strength (UCS) of soil-cement were investigated. The influence of groundwater on UCS of soil-cement was also studied. The experimental results indicate that the soil treated with high grade cement presents a higher UCS. Additionally, the UCS of soil-cement presents linearly increased with the cement content. A logarithm correlation between UCS and curing time presents to forecast the strength development. Compared with the UCS of samples immersed in distilled water, those immersed in groundwater present a higher value.

Degradation of mechanical properties of cementitious materials exposed to wet-dry cycles of sulphate solution

Abstract In order to study sulphate attack effects on the mechanical behaviour of cementitious materials, the compressive behaviour of such materials subjected to wet–dry cycles of sulphate solution was investigated. Then, their variation of mass was recorded. According to the experimental results, it was observed that the mortars subjected to wet–dry cycles of sulphate attack were more severely damaged than ones immersed in a sulphate solution. In addition, these experimental results showed that the ultimate strength, elastic moduli and mass of samples initially increased with the number of wet–dry cycles and then declined after a certain number of wet–dry cycles. However, the peak strain corresponding to the compressive strength presented a decrease in the initial stage and then increased with the number of wet–dry cycles. Finally, the relationships between the mechanical behaviours and mass increment of such materials were analysed and qualitatively established.

The influence of plastic deformation on the structure of passive films on carbon steel in simulated pore solution

The process of passivation of carbon steel when experiencing plastic deformation in simulated pore solution has been studied using electrochemical tests and atomic force microscopy (AFM). The polarization results show that the activity of the carbon steel increased with increasing degree of deformation. Before the passive films were ruptured, the heavily deformed samples presented a high open circuit potential (OCP). On the other hand, the pitting incubation time decreased with increasing plastic deformation. The Mott-Schottky results suggested that the high deformation caused the passive films to be heavily doped. In addition, the space charge layers of passive films were thinned when the plastic deformation increased. The AFM observations indicated that the passive films become more inhomogeneous as the deformation increased. These results demonstrate that passive films on the deformed carbon steel become unstable when the plastic deformation increases.

Experimental Study of Aged and Seriously Damaged RC Beams Strengthened Using CFRP Composites

This paper presents results from experiments on aged and seriously damaged reinforced concrete (RC) beams strengthened with different arrangements of external carbon fiber-reinforced polymer (CFRP) laminates and end anchorages. Seven RC beams from an old bridge, measuring 250 × 200 × 2300 mm, were tested. All specimens were loaded to yield load to evaluate initial mechanical properties. Then, these seriously damaged specimens were repaired using different CFRP-reinforcing schemes and reloaded to failure. The yield load growth due to CFRP reinforcement ranged from 5% to 36%. Different parameters including CFRP dimension and position, bonding length, and end anchorage were investigated and facilitated conclusions on beam ductility, load-midspan deflection response, and failure mode. This research contributes to knowledge about the CFRP repair of aged and seriously damaged beams to ensure better performance in overloaded conditions.