Advanced Cement Based Materials最新文献

Microstructural change of hardened cement paste and mortar under various stresses was studied to obtain the basic data for judging the safety of concrete structure repeatedly receiving the stress. The crack revealed generally at 60% of the fracture stress, and it grew rapidly, exceeding at 80% of the fracture stress. The growth of the cracks in the hardened body was more conspicuous in paste than in mortar, in fly ash cement than in normal portland cement and blastfurnace slag cement, at high W/C than at low W/C, and in repeated loading than in single loading. Results were discussed in connection with the changes of pore structure and backscattered electron images of hardened bodies after loading the stress.

The moisture permeability of hardened cement paste is normally not a constant, because it depends on both the porosity and the relative humidity (RH) in the pores of the cement paste when RH exceeds approximately 60 to 70%. The measured results show a strong relationship between the porosity of the cement paste and the moisture permeability.

The present article deals with the influence of microwave treatment on the properties of glass-fiber reinforced composites. The cement used in the matrices of the composites was either plain ordinary portland cement or metakaolin-blended cement. Two levels of microwave power were investigated: 40 W and 80 W for 1 hour 30 minutes. The behavior of such composites was compared to that of specimens cured at ambient conditions (20°C) and at 60°C and 90°C. The mechanical behavior was assessed by means of three-point flexure tests. The microstructure was investigated using differential thermal analysis, infrared spectrometry, and scanning electron microscopy. The results obtained show that microwave treatment enhanced the pozzolanic reaction between ordinary portland cement and metakaolin and the bond between the fibers and the matrix. Interfaces between fibers and matrix were also modified. The mechanical performances of the microwave-processed composites were very interesting at early ages, but microwave heating degraded some long-term properties, such as work of fracture and toughness.

A filament winding system was developed for manufacturing cross-ply cement based composites using continuous fibers. The computer-controlled system is used for manufacturing composite laminates, pipes, and pultruded sections. The electrical and mechanical components of the system are discussed in detail. Composite laminates are manufactured using continuous glass and polypropylene fibers. Their mechanical response is measured using closed loop uniaxial tensile and flexural tests. Results indicate that tensile strength of composites can exceed 50 MPa using 5% alkali-resistant glass fibers. By varying the stacking sequences, the ultimate strain capacity can be increased to more than 2% for glass fiber composites and 8% using polypropylene composites. The software/hardware system may be easily adapted for design and manufacturing of full-scale cement based composite laminate systems.

This article deals with the effect of both initial temperature and subsequent high curing temperature on the degree of hydration of normal strength concrete at the age of 28 days. Variations in compressive strength were observed and presented previously. Here, their causes are investigated through a difference in the hydration state in relation with the initial mixing and curing conditions. We present two ways of measuring the degree of hydration and compare the results, first by means of an original technique of image analysis on flat polished sections observed by scanning electron microscope, and second by classical measurement of the chemically bound water. Two kinds of specimens are studied. Cylinders of 11 cm × 22 cm were prepared with two different temperature formulae of constituents: 20°C (water)—20°C (aggregate)—20°C (cement); and 20°C (water)—70°C (aggregates)—70°C (cement), so as to obtain an initial temperature of the mixes between 20°C and 50°C, respectively. Specimens were sealed and cured under either controlled laboratory conditions or simulated conditions of hot weather. In each case, all the specimens were stored in water up to 28 days.

This article presents a computationally efficient method for analyzing the performance of short-fiber reinforcement in cementitious composites. Each fiber is modeled as a discrete entity. Realistic, nonuniform fiber distributions can be specified as program input. Discrete element systems are used to represent the matrix material. Fiber response is constrained to the kinematics of the discrete elements; the number of system degrees of freedom is therefore independent of the number of fibers. Pre-cracking contributions of the fibers are modeled using an elastic shear lag theory. Post-cracking contributions depend on pullout relations based on the micromechanics of the fiber-matrix interface. In either case, there is a direct link between fiber-local actions and composite response. Numerical results for both aligned and randomly oriented fiber composites are compared with theoretical predictions based on simple mixture rules.

In this work, a new analytical model for predicting the elastic modulus of discontinuous fiber reinforced cement composites in uniaxial tension is presented. The proposed elastic modulus model depends on the macroscopic and microstructural parameters (interfacial bond modulus, matrix porosity, and so forth) of the composite. The microstructural scale is the fiber. The composite is considered to be a set of many fiber-matrix cells. The shear-lag concept is used to determine the forces and stress fields in the unit cell. The linkage between the microstructural and the macroscopic properties (homogenization) is made through an averaging process based on continuum mechanics, probabilistic considerations, and tensorial transformations. The model is applicable to all fiber orientations, probabilistic or deterministic. The particularities are that the model is triphasic and integrates the probabilistic density function of fiber orientation as a variable. Good agreement is observed between the model predictions and the limited experimental data available. The model is applicable to other discontinuous fiber reinforced quasibrittle matrix composites.