Zongjin Li

BEHAVIOR OF CONCRETE COLUMNS CONFINED WITH FIBER REINFORCED POLYMER TUBES

New types of structural columns are being developed for new construction. They are made of concrete-encased fiber reinforced polymer (FRP) tubes. The concrete-filled FRP tubes are cast in place. The tube acts as formwork, protective jacket, confinement, and shear and flexural reinforcement. It can also be used to complement or replace conventional steel reinforcement of the column. This paper presents the results of experimental and analytical studies of the performance of concrete columns conbined with carbon and glass FRP composite tubes. Concrete-filled FRP tubes are instrumented and tested under uniaxial compressive load. Test variables include type of fiber, thickness of tube, and concrete compressive strength. Results show that external confinement of concrete by FRP tubes can significantly enhance the strength, ductility, and energy absorption capacity of concrete. Equations to predict the compressive strength and failure strain, as well as the entire stress-strain curve of concrete-filled FRP tubes were developed. A comparison between the experimental results and those of analytical results indicate that the proposed model provides satisfactory predictions of ultimate compressive strength, failure strain, and stress-strain response. The study shows that the available models generally overestimate the strength of concrete confined by FRP tubes, resulting in unsafe design.

Property improvement of Portland cement by incorporating with metakaolin and slag

The physical and mechanical properties of Portland cement (PC) containing metakaolin (MK) or combination of MK and slag and the compatibility between such materials and superplasticizers were investigated in present study. After MK was incorporated into PC, the compressive strength of the blended cement was enhanced. However, the fluidity of MK blended cement became poorer than that of PC at the same dosage of superplasticizer and the same water/binder ratio. When both MK (10%) and ultra-fine slag (20% or 30%) were incorporated into PC together, not only the compressive strength of the blended cement was increased, but also the fluidity of the blended cement paste was improved comparing to MK blended cement. This indicates that ultra-fine slag can improve the physical and mechanical properties of MK blended cement. The physical and chemical effects of two mineral admixtures were also discussed.

Mussel-Inspired Adhesive and Tough Hydrogel Based on Nanoclay Confined Dopamine Polymerization

Adhesive hydrogels are attractive biomaterials for various applications, such as electronic skin, wound dressing, and wearable devices. However, fabricating a hydrogel with both adequate adhesiveness and excellent mechanical properties remains a challenge. Inspired by the adhesion mechanism of mussels, we used a two-step process to develop an adhesive and tough polydopamine-clay-polyacrylamide (PDA-clay-PAM) hydrogel. Dopamine was intercalated into clay nanosheets and limitedly oxidized between the layers, resulting in PDA-intercalated clay nanosheets containing free catechol groups. Acrylamide monomers were then added and in situ polymerized to form the hydrogel. Unlike previous single-use adhesive hydrogels, our hydrogel showed repeatable and durable adhesiveness. It adhered directly on human skin without causing an inflammatory response and was easily removed without causing damage. The adhesiveness of this hydrogel was attributed to the presence of enough free catechol groups in the hydrogel, which were created by controlling the oxidation process of the PDA in the confined nanolayers of clay. This mimicked the adhesion mechanism of the mussels, which maintain a high concentration of catechol groups in the confined nanospace of their byssal plaque. The hydrogel also displayed superior toughness, which resulted from nanoreinforcement by clay and PDA-induced cooperative interactions with the hydrogel networks. Moreover, the hydrogel favored cell attachment and proliferation, owning to the high cell affinity of PDA. Rat full-thickness skin defect experiments demonstrated that the hydrogel was an excellent dressing. This free-standing, adhesive, tough, and biocompatible hydrogel may be more convenient for surgical applications than adhesives that involve in situ gelation and extra agents.

The relationships between stress and strain for high-performance concrete with metakaolin

Abstract This paper reports the results of a study of stress–strain relationships (tension and compression) and bend strength measurements for concrete incorporating 0%, 5%, 10%, and 15% metakaolin. The test results show that the tensile strength and peak strain increase with increasing metakaolin content whereas the tensile elastic modulus shows only small changes. The descending area of overpeak stress is improved when 5% and 10% of cement is replaced by metakaolin. Also, the bend strength and compressive strength increase with increasing metakaolin content. The compressive elasticity modulus of concrete shows a small increase with increasing metakaolin replacement. The compressive strength increases substantially at early ages, and there is also higher long-term strength. Therefore, the metakaolin is a very efficient strength-enhancing addition. The workability of concrete is little influenced by small metakaolin contents (5% metakaolin). At higher metakaolin contents workability can be controlled effectively by superplasticizer additions.

Mechanism of cement paste reinforced by graphene oxide/carbon nanotubes composites with enhanced mechanical properties

This study presents the enhanced mechanical properties of cement paste reinforced by graphene oxide (GO)/carbon nanotubes (CNTs) composites. The UV-vis spectroscopy and optical microscopy results show that the dispersion of CNTs in the GO solution is much better than in an aqueous solution due to the higher electrostatic repulsion, which allows a completely new approach of dispersing CNTs rather than by incorporating a dispersant. More importantly, the GO/CNTs composite plays an important role in improving the compressive and flexural strength of cement paste by 21.13% and 24.21%, which is much higher than cement paste reinforced by CNTs (6.40% and 10.14%) or GO (11.05% and 16.20%). The improved mechanical properties of cement paste are attributed to better dispersed CNTs and enhanced interactions among CNTs by the GO incorporation. Finally, the space interlocking mechanism of the GO/CNTs/cement paste composite with enhanced mechanical properties is proposed.

High-Early-Strength Magnesium Phosphate Cement with Fly Ash

To understand the mechanism of fly ash in a novel magnesia phosphate cement (MPC), the microstructure and properties were investigated in the present study. Reference specimens without the incorporation of fly ash were also investigated for comparison purposes. The effect of fly ash content on the properties of MPC was studied with two dead burnt magnesia materials with different MgO contents and fineness. The results demonstrated that fly ash does improve the bonding and compressive strength of MPC, even at very early ages. Fly ash content of 30 to 50% has the best improving effect on MPC, despite the two types of magnesia. Due to the difference of MgO content and particle fineness of two dead burnt magnesia materials, MPC mortars with finer magnesia revealed higher compressive strength. The incorporation of fly ash does not retard the setting reaction of MPC, but it does reduce its total heat evolution. The hydrates and microstructure of MPC paste were examined by an x-ray diffractometer, scanning electron microscopy-energy dispersive x-ray analysis (SEM-EDX), and Fourier transform infrared (FTIR) spectroscopy. These techniques revealed that the products formed in MPC paste were crystal magnesium potassium phosphate hexahydrate and amorphous species. The particles of fly ash fill the voids of MPC paste and strongly bond together with hydrates of MPC. The microanalysis showed that the strengthening of fly ash to the cement might come from physical and chemical effects.

FLEXURAL BEHAVIOR OF BAMBOO - FIBER-REINFORCED MORTAR LAMINATES

Abstract This paper reports the flexural behavior of bamboo–fiber-reinforced mortar laminates. The laminate considered in this study is a sandwich plate combined with reformed bamboo plate and extruded fiber-reinforced mortar sheet. Due to its high strength to weight ratio, the reformed bamboo can remarkably strengthen the mortar and reduce the total weight of the laminate. Test results show that, for the laminates with reformed bamboo plate on the bottom as tensile layer and fiber-reinforced mortar sheet on the top as compressive layer, the flexural strength values can be improved to greater than 90 MPa.

2-2 Piezoelectric cement matrix composite: Part II. Actuator effect

In this paper, experimental results of the actuator effect of a 2-2 piezoelectric cement matrix composite are presented. A desirable actuator effect was observed in the composite. In the first part of this study, the actuator effect of the composite under free conditions was studied. It was found that the amplitude of the response increases, but the phase angle of the response decreases, both almost linearly with the frequency of the actuator effect. The second part of the study focused on the behavior of the actuator in a structure. Distinct differences were observed in the behaviors of the composite under free conditions and in a structural system (simulated by precompression in the frame of an MTS machine). Clearly, the behavior of the actuator in a structure is influenced by factors including the properties of other components of the structure and the interaction between the actuator and other components, which was represented here by the precompression level in the experiment.

Uniaxial Tensile Behavior of Concrete Reinforced with Randomly Distributed Short Fibers

A study was made of the behavior of concrete reinforced with randomly distributed short fibers under uniaxial tension. Five kinds of fibers were used to prepare the fiber reinforced concrete specimens for the uniaxial tension test. These included two kinds of steel fibers, two kinds of polyvinyl alcohol fibers, and one kind of polypropylene fiber. Fiber volume fraction varied from 2% to 6% in most cases. The matrices were normal strength concrete and/or admixture modified concrete. Plain concrete specimens were also prepared as a reference for the purpose of comparison. The adaptive control test method was used in the experimental study to obtain complete stress-deformation responses for both plain concrete and fiber reinforced concrete. Both strain softening and strain hardening behavior was observed in the experiment, and it seemed that a transition point did exist for the composites. It was concluded that strain hardening and multiple cracking responses can be achieved for concrete incorporating short fibers when the parameters are carefully selected.

Investigation of Chloride Diffusion for High-Performance Concrete Containing Fly Ash, Microsilica, and Chemical Admixtures

This study evaluated the influence of addition of fly ash, microsilica (MS), and calcium nitrite solution (CN) on chloride diffusion of high-performance concrete. The concentrations of chloride ions penetrating specimens through diffusion were measured periodically, up to one year, by using an electronic ion/pH meter. When the diffusion flux reached a steady state, the diffusion coefficients of all mixes were calculated based on Ficks first law. The test results demonstrated that the 25% addition of fly ash by weight of cement improved the mixs resistance to chloride diffusion. The improvement was especially evident when 25% of fly ash and 10% of MS by weight of cement were added into the mix together. However, the addition of CN into the mix resulted in deterioration in improvement of resistance to chloride diffusion enhanced by the incorporation of fly ash and MS. Through the mercury intrusion porosimetry test, the influence of CN on chloride diffusion is studied further. The results showed that the addition of CN leads to the increase of micropore diameter. Therefore, CN should be adopted cautiously in the practical engineering in which higher resistance to chloride diffusion is required.