Volume 19, Issue 1

The nuclei of unhydrated cement particles left in cementitious materials after maturation, can provide a self-healing capability to micro-cracked matrix under favorable conditions. This study is conducted to characterize self-healing efficiency on cracks in cementitious composites from the viewpoint of a geometrical probability. Based on a practical cracking mode of unhydrated cement nuclei in cementitious composites, the splitting crack mode is presented to deal with the self-healing efficiency on cracks due to rehydration of unhydrated cement nuclei. Recurring to a generalized hydration reaction model of cement particles, the self-healing efficiency model quantitatively considers the influence of the volume fraction, particle size distribution and cracking mode of unhydrated cement nuclei, randomly distributed in cementitious materials. Further, the reliability of these theoretical models is verified via computer simulation technology.

A new numerical approach was developed to study the mechanical properties of cementitious granular materials. At first, an ellipsoid based method was developed to generate aggregate particles of arbitrary shape. These aggregate particles were then mixed in a cement paste using the algorithm-based program HADES and an interfacial transition zone (ITZ) between the aggregate particles and the cement paste was added. All phases including aggregate, cement paste and ITZ were meshed by a special tool Gmsh. The mesh data was transferred into the finite element code, Finite Element Analysis Program (FEAP) to predict the mechanical properties of the material. As a typical cementitious granular material, concrete was simulated using this approach. The method was also validated by way of comparison to existing results available in previous studies of concrete. Additionally, a parameter study was conducted to highlight the influence of aggregate on the mechanical properties. The simulation shows that the method is feasible and effective in predicting the elastic modulus of three-phase cementitious granular materials (cement paste, ITZ and aggregate).

In this study, some mechanical and durability properties of concrete specimens, with a water/cement (w/c) ratio of 0.47 and having a reference slump of 40 mm, produced by addition of steel fibers (SF) and/or polypropylene fibers (PF), were investigated. The additions of SF were 20 kg/m 3 and 40 kg/m 3 and PF additions were 0.3 kg/m 3 and 0.6 kg/m 3 . Workability and unit weight tests were made on fresh concretes. Compressive stress, elastic modulus, and flexural tensile tests were performed on hardened concretes. Also, load-deflection curves and toughness indices were obtained during a three-point flexure tensile test. Freeze-thaw, abrasion and drying shrinkage tests were also carried out to determine the effects of SF and/or PF on some durability properties of concretes. It was observed that fiber inclusion to concrete decreases the workability and that the mechanical properties of concrete can be improved by the addition of fibers. Using hybrid fibers in the concrete both increases and decreases some mechanical properties of concrete. In order to see a greater effect of both SF and PF on the properties of concrete, different hybrid fiber volume fractions for different matrix strengths may be examined, since fiber type and fiber volume fractions are important parameters to be taken into account in the mix design of concrete for the performance

Post earthquake field investigations reveal that frames with inferior detailing are vulnerable to seismic actions. The presence of a large building stock in Turkey, which is vulnerable to an imminent earthquake, entails fast, relatively easy, and feasible alternative strengthening methods. In this paper, the strengthening of brick infilled walls with fiber reinforced polymer (FRP) overlays, and integrating them to the load carrying system of the frame, is considered as an alternative method for building upgrading. The effectiveness of the FRP overlays on frames with different aspect ratios, is an issue to be investigated. For this purpose, three reinforced concrete (RC) frame specimens with one-third scale and infilled with hollow clay tile (HCT) were strengthened with FRP overlays. The frame specimens had a single bay and two stories, and were tested to failure. The strengthening scheme was shifted, depending on the aspect ratio of the specimens. Test results were evaluated in terms of base shear-roof drift hysteretic response, and a macro model using the well-known equivalent diagonal strut concept was proposed to predict the lateral capacity and the response of the strengthened frames. The model predicted secant stiffness and failure load in an acceptable accuracy range. Theoretical and experimental results proved that aspect ratio significantly affects the level of strengthening attained. The study presented here is part of a comprehensive research work on the strengthening of HCT infilled RC frames, with different aspect ratios by means of various carbon fiber reinforced polymer (CFRP) schemes.

The aim of this study was to investigate the effect of montmorillonite as additive on the properties of cement paste and mortar. Montmorillonite was used at 0%, 5%, 10% and 15% additive by weight of cement, and the water/cement ratios of 0.45 and 0.55 are considered for the cement pastes and cement mortars with the sand/cement ratio of 1, respectively. Experimental results indicate that the use of montmorillonite as cement additives increases the compressive strength except the specimens containing 15% montmorillonite at the age of 28 days. The water absorption and initial surface absorption decreased with the increase of montmorillonite additives for the cement pastes. However, the cement mortars had an opposite test result in water absorption and initial surface absorption. The cement pastes and mortars containing montmorillonite have higher adsorption-desorption values than those of the plain cement pastes and mortars. Meanwhile, scanning electron microscopy and mercury intrusion porosimetry tests indicate that the cement paste containing montmorillonite are denser than that of the plain cement paste. X-ray diffraction analysis showed that the main hydration products are portlandite, hatrurite and calcite in cement paste and quartz and portlandite in cement mortar.

Silica composites (2.5D quartz fibers-reinforced) were prepared by the silicasol-infiltration-sintering method. The fiber volume fraction was 50.5%. The composites were sintered in a temperature range of 400∼450°C. Mechanical properties of the composites were determined by pc-controlled electronic universal testing machines. The microstructure of the composite was observed by scanning electron microscopy (SEM). The density of the composite was 1.77 g/cm 3 . The flexural strength and the shear strength of the composites were 50.35 MPa and 22.41 MPa, respectively. The stress-deflection curves exhibited mostly nonlinear behavior; the failure was not catastrophic. Some fibers pull-out indicated that the composite had part toughness. However, abundant fibers fractured in the same plane revealed that the composite presented serious brittleness. The fracture mechanism of the composite was a mixture of ductile and brittle modes.

An elastoplastic shear-lag analysis was developed to analyze the stress distribution around a fiber break in a composite to consider the plastic deformation in the matrix. A one-dimensional shear-lag analysis, which considers the fiber, the matrix, and the interphase shear layer, was conducted. It is assumed that the fiber consists of an elastic material, and the matrix and the interphase shear layer consist of elasto­plastic materials. The method of successive elastic solutions was used to determine the plastic strain distribution in the elastoplastic materials.

The mechanical properties of centrifuged Al-Si-B composites containing aluminum diboride and aluminum dodecaboride particles were studied using optical and scanning electron microscopy (SEM), hardness tests, and nanoindentation. SEM was used to further analyze the Rockwell indentations, while nanoindentation experiments were completed on the matrix and the reinforcing particles. The outcomes of this study were compared with the results obtained in superficial Rockwell hardness and Vickers microhardness tests to determine which of the boride reinforcements was more beneficial for the mechanical properties of the Al-Si matrix composite. Aluminum diboride was found to improve bulk mechanical properties more than did aluminum dodecaboride particles. On the other hand, aluminum dodecaboride showed to be a better reinforcement at the micron and submicron scales, as revealed by nanoindentation experiments. A discussion of these findings helped understand the underlying mechanisms behind this apparent discrepancy.

Zinc-aluminum coatings were prepared; parts of them were sealed with an organic coating; and the behavior and mechanism of corrosion were studied by means of in situ plates test, salt spray test, and electrochemical impedance spectroscopy. A scanning electron microscope was used to assess the surface of the zinc-aluminum coatings, which showed roughness and some defects that would reduce the protective effect. Organic coatings improve the protective effect by sealing the defects of the metal coating. A multilayer structure is not only expensive but would also destroy the continuity of the coating if the zinc corrodes.

This study aims to investigate the relation of hazard rate in terms of the corresponding reliability for composite laminates under constant-amplitude cyclic stresses when the static strength is described by the Weibull distribution and the S-N curve is given. On the basis of the strength-life equal rank assumption, Yang’s residual strength degradation model is considered to derive the reliability by retrieving the residual strengths to the initials. Thus, a reliability-dependent hazard rate function is proposed as h ( R )= e g +μ (-ln R ) ξ , where e g denotes the intrinsic weakness of composites during fabrication, μ the hazard scaling parameter, and ξ the curve trend parameter. Parameter e g can be assumed as a very small positive value. Both ξ and μ are connected to the shape parameter of the Weibull static strength distribution and the parameter of fatigue life distribution, and μ is correlated to a power function of the applied maximum cyclic stress. The proposed model can be reduced to either the Gompertz model as ξ =1 or the Weibull model when e g =0 and ξ <1. It has been verified that the proposed model agrees well with the fatigue data of composites under cyclic stresses as well as with various failure data of mechanical components.

The blends were successfully prepared by the copolymerization of 2,2-bis [4-(3, 4-dicyanophenoxy) phenyl] propane (BAPh) and epoxy resins E-44 (EP) in the presence of 4,4′-diaminodiphenyl sulfone (DDS) as curing additive. With the loading of BAPh in blends increasing from 10% to 50%, the excellent flame retardancy properties of the copolymers with the weight ratio of 20:80 of BAPh/EP achieved over 30 according to the limiting oxygen index test (LOI) values and V-0 grade of UL-94 measurement compared with the pure EP. The temperatures at weight loss 5% ( T 5% ) of the pure EP were improved 16.4°C in nitrogen and 64.6°C in air when the ratio was 20:80. In addition, the gelation time was shortened to 3 min when the prepolymerization temperature was 190°C.

No abstract available