H. C. Wu

Conditions for Pseudo Strain-Hardening in Fiber Reinforced Brittle Matrix Composites

Apart from imparting increased fracture toughness, one of the useful purposes of reinforcing brittle matrices with fibers is to create enhanced composite strain capacity. This paper reviews the conditions underwhich such a composite will exhibit the pseudo strain-hardening phenomenon. The presentation is given in a unified manner for both continuous aligned and discontinuous random fiber composites. It is demonstrated that pseudo strain hardening can be practically designed for both gills of composites by proper tailoring of material structures. 18 refs., 8 figs., 2 tabs.

Application of recycled tyre cord in concrete for shrinkage crack control

Recently, there has been rauch attention focused on the rapidly deteriorating infrastructure. The main cause of the decay of infrastructure is the deterioration of the materials used in the construction and repair of the structures. In many types of infrastructure, such as highway and airport pavement, cracking behaviour is the most important factor for the determination of durability and lifetime of structures. For example, the control of the crack spacing and width is the main design criterion in continuously reinforced concrete pavement [1]. This type of cracking behaviour is related to environmental strain due to either dry shrinkage or thermal change in concrete structures with restrained boundary condition. Such shrinkage-induced cracking is very common in concrete, especially for large surfaceto-volume ratio structures such as highway pavement, slabs for parking garages, and walls [2]. Cracking can cause several problems in a reinforced concrete structure: durability problems related to the corrosion of rebars, spalling of structure surface, and increased permeability through the cracks, etc. Thus, the control of the shrinkage cracking behaviour in cementitious materials is considered an important factor for long-term structural performance. In the past, research was conducted on the shrinkage behaviour of ordinary concrete and reinforced concrete structures. Recently, attention has focused on fibre reinforced cementitious composites [2-5]. It was found that the effect of discrete fibres is to reduce shrinkage cracking rather than reduce free shrinkage, although 10-25% reduction in free shrinkage in cementitious composites having steel fibre reinforcement has been reported [6, 7]. Shrinkage cracking can, as expected, be significantly reduced by increasing the fibre volume fraction in cementitious composites [8]. In the above mentioned studies of shrinkage behaviour of fibre reinforced composites (FRCs), all composites were reinforced with various types of new virgin fibres including polypropylene and steel. Data on the shrinkage properties of concrete reinforced with recycled waste fibres are not available. Many industrial wastes can be recycled and utilized in building and highway construction. At present, most of these wastes are disposed of in landfills, which is of great environmental concern. For example, about 275 million used tyres are disposed of annually, and 3 billion used tyres have accumulated in waste piles across the United States [9]. No efforts have been made to reuse recycled tyre cord (such as nylon, polyester, and Kevlar), however,

Interface Property Tailoring for Pseudo Strain-Hardening Cementitious Composites

It is well known that interfaces in composites play an important role in determining composite properties. In this paper, the role of interface properties on pseudo strain-hardening properties of brittle cement matrix composites is briefly summarized. An example of interface tailoring by plasma treatment on polyethylene fibers and its effect on composite behavior is described.

Buckling of bridging fibres in composites

In brittle materials such as concrete and ceramics, fibre reinforcement has been widely accepted as an effective way of improving their strength and toughness. In addition, a notable pseudo strain-hardening phenomenon can contribute to a significantly enhanced ductility of the composite when an adequately designed fibre system is used. This condition was first proposed by Aveston et al. [1], and later extended by Marshall et al. [2] for continuous aligned fibre reinforced brittle matrix composites. More recently, further extensions to randomly oriented discontinuous fibre reinforced composites have been presented [3, 4]. Upon satisfying the conditions described in the above mentioned micromechanical models, the ultimate tensile strains of the composites are usually improved by two orders of magnitude [5-7]. Total fracture energy reaching 35 kJ/m 2 was also reported for a 2% polyethylene fibre reinforced cement paste [8]. This kind of ductile fracture resembles metal instead of brittle materials. The pseudo strain-hardening behaviour of fibre reinforced brittle matrix composites is associated with multiple cracking, and results from adequate stress transfer capability of bridging fibres. Studies are typically conducted under monotonic tensile loading only. In reality, composites are usually subject to cyclic loads. As a preliminary report of ongoing research on the cyclic behaviour of pseudo strain-hardening cementitious composites, we present initial findings on buckling of bridging nylon fibres across fracture planes in a cement composite after complete unloading in tension. An analytic model is also proposed to account for this buckling phenomenon. Type I Portland cement, silica fume and superplasticizer were used to form the cement paste with water/cementitious ratio of 0.27. Discontinuous nylon fibres (Lr = 21 mm, dr = 25/+m, and Ef = 5.2 GPa) were used to reinforce the paste at a volume fraction of 2%. Tensile coupon specimens of size 304.8 × 76.2 x 12.7 mm were prepared and tested under direct tension in a servo hydraulic tester. Detailed mix proportions and testing procedures can be found elsewhere [9]. Tensile stressstrain curves were recorded. An optical microscope with 50 times magnification was used to examine the bridging fibres after the specimen was completely unloaded. The stress-strain curve is shown in Fig. 1, where four peaks are identified, corresponding to four multiple cracks which occurred within a length of

Control of Cs leachability in cementitious binders

Nuclear waste, including the production of watersoluble radioactive metal atoms, is inevitable from the production of nuclear energy, and presents a great threat to human health and to the environment [1]. Immobilization of radionuclides is usually accomplished, first, by binding the nuclear waste in suitable solid waste forms. Subsequently, these waste forms are stored at a repository site having multiple physical and chemical barriers. During the long period of containment, any contaminant release must be limited to permissible rates. This can be ensured by stabilizing radionuclides in suitable binder phases, and restricting groundwater flow which may transport soluble radioactive species outside of the containment zone. Although cementitious materials have been actively used as binder and diffusion barriers because

Thermodynamic calculation of partial phase diagram of Al-Si alloy at high pressure

Generally phase diagrams represent the relationship between temperature and composition under atmospheric pressure. However, the effect of pressure on the phase diagram is less well understood. In particular, metastable phases may occur at high pressures. Most often these phases are detected by experimental examination. Theoretical prediction of metastable phases appearing under conditions of high pressure is, at best, difficult. Efforts towards determination of the phase diagram of A1-Si binary alloy at normal pressure are abundant and fruitful [1]; this alloy is of interest due to its ample applications in industry. In addition, rapid quenching processes were applied to achieve supersaturation in A1-Si alloys. The silicon contents of these metastable solid solutions were increased significantly from a maximum of 1.59 at % at equilibrium conditions to 11 at % [2, 3]. Subsequently, followed by an ageing treatment, very fine silicon precipitations possibly contribute to a dispersionstrengthened alloy [2]. Furthermore, a pressure effect on the solubility of Si in A1 was studied experimentally [4]. They found that the solubility limit was extended beyond 15 at % at 5.4 MPa in comparison with 1.59 at % at atmospheric pressure. A thermodynamic analysis of high-pressure phase equilibria was also conducted [5]. Similar conclusions were drawn about the significant increases of Si solubility due to pressure. Their approach was based on the approximation of regular solutions and the independence of volume, energy and entropy on temperature, pressure and composition. The latter, which neglects thermal expansion and compressibility, may be inadequate. Also, research by the author on the high-pressure effect in solidifying an AI-Si alloy revealed the potential importance of the rapid pressure solidification process over the more conventional quenching method [6]. The pressure effects on microstructures of the test alloy were found to be significant, whereas mechanical properties such as the yield strength and hardness showed only minor differences in samples solidified under atmospheric and pressurized conditions. This may be due to the competing effects of the dendritic and eutectic phases in the resultant structure [6]. In this study a partial phase diagram of AI-Si alloy at 0.69 MPa (105 lb in -2) was predicted on the basis of the Clapeyron equation and method of resolution into simple diagrams. In general, the condition for the coexistence of