Advanced Cement Based Materials最新文献

Air void spacing equations have been proposed in the literature by a number of authors: Powers; Philleo; Attiogbe; and Pleau and Pigeon. Each proposed spacing equation attempts to characterize the true “spacing” of entrained air voids in concrete. While efforts have been made to correlate these spacing equation calculations to freeze-thaw performance, no test has been performed to assess the geometrical accuracy of these spacing equations. Herein is a computerized accuracy test of these proposed spacing equations. A computer model of air void systems is used, and various “spacings” are measured in the model system. The results of these measurements are then compared to the appropriate spacing equation prediction, along with equations developed by Lu and Torquato.

An experimental and numerical study of the long-term interaction between silica fume and Portland cement in concrete subjected to air, water, or sealed curing is outlined. About 250 kg of eight qualities of each concrete were studied at four different ages each over a period of 7 years between 1989 and 1996. Parallel studies of strength, hydration, and internal relative humidity were performed. Half of the concretes contained silica fume. New and original results and analyses of the interaction between Portland cement and silica fume related to compressive strength, split tensile strength, hydration, and internal relative humidity are presented. The specimens are available for future measurements.

Accurate measurements of the surface area of cement based materials using small-angle neutron scattering (SANS) require determination of the neutron scattering contrast between the C-S-H gel and H₂O. Because the C-S-H gel has a poorly understood structure and a variable composition, calculated values of the C-S-H:H₂O neutron scattering contrast based on an assumed C-S-H chemical composition and density are subject to error. The C-S-H:H₂O neutron scattering contrast was determined experimentally by measuring the change in apparent surface area as the H₂O in a hydrated cement specimen was replaced with D₂O, resulting in a new value of 6.78 × 10²⁸ m⁻⁴. This new contrast value increases some previously reported SANS surface area values by 21%. The results also confirm that C-S-H interlayer water should be included in the solid phase whereas gel pore water should be excluded.

Portland cement pastes 1 day to 1 month old were each subjected to a single cycle of 1250 V/m electric field (15 s forward, 15 s off, 15 s reverse) and analyzed for microstructural/transport changes by impedance spectroscopy. All samples experienced an irreversible increase in resistance, as much as 20–25%, which decreased with increasing age of the paste. Subsequent applications of field produced no additional changes. The resistance increases were shown to be attributable to decreases in pore network connectivity rather than to changes in overall degree of hydration, capillary porosity, or pore fluid conductivity. It is proposed that electro-osmotic swelling of product near “bottleneck” pores results in the decreased connectivity. Ramifications for electrocuring, electromigration, electrochemical chloride treatment, and permeability studies of cement based products and structures are also discussed.

This study presents a method for estimating the strength of lightweight aggregate. Cylindrical specimens with various aggregate volume ratios (volume of coarse aggregate/total aggregate volume) were cast and tested. Micromechanics method was applied by considering a perfect bond between mortar and aggregate. The approximate aggregate strengths determined from the concrete strength, component properties, and the volume ratio of aggregate are between 15 and 30 MPa. Both matrix strength and composite strength are much higher than the lightweight aggregate strength.

Metakaolin (MK) additions to cement-quartz pastes autoclaved at 180°C affect the binding material and physical properties. At MK additions of up to 12%, the amount of tobermorite formed decreases when cement is partially replaced, whereas the opposite effect is observed in the case of partial replacement of quartz. An overall decrease in both compressive strength and drying shrinkage occurs with MK additions. In autoclaved systems, MK provides a source of silica that is more reactive than ground quartz.

The limitations of the traditional material design approach in driving properties to extreme values and handling multiple design criteria and variables can be overcome by applying the optimization approach. Fiber-reinforced cement based composites based on strong aggregates and exhibiting approximately bilinear fiber pullout behavior are optimized in the present study for a given compressive strength f^′_c with a view to maximizing their uniaxial tensile strength f^′_t and fracture energy G_F. Relations for the bridging stresses prior to and during fiber pullout are established using fracture mechanics. The mix design leads to nonlinear, single, or multicriterion maximization problems for the objective functions f^′_t and l_ch = EG_F/f^′_t² (characteristic length), subject to an equality constraint on f^′_c. In this way, optimal values for the microstructural parameters (fracture toughness of paste, length of fiber, and volume fractions and diameters of aggregate and fiber) are obtained.

In a cement hydration product-electrolyte system the hydration products are micron-sized charged particles. The principles of colloid electrochemistry predict that in such a system each solid particle is surrounded by a layer of concentrated solution of its counter-ion. The actual concentration of this layer depends on the surface charge and the concentration of the solution away from the particle, i.e., bulk solution. Divalent counter-ions preferentially concentrate around the solid particles. The principles also predict that the intrinsic diffusivity of an ion is proportional to the square root of its concentration in a solution. The literature on the expressed pore solution, ionic diffusivity through cement based materials, the formation factor, alkali-silica reaction, etc., has been examined from the point of view of colloid electrochemistry. Many of the reported but unexplained phenomena could be explained from the principles of colloid electrochemistry.

The role of fibers in fiber reinforced cement based composites was studied by means of finite element method. The study was conducted in two steps. The first step simulated the fiber pullout from a cementitious matrix and resulted in pullout force vs. slip displacement response. The pullout response was used in the second step as the bridging pressure applied over the crack length in composite specimen. The contribution of fiber’s closing pressure was quantitatively measured through calculation of the J-integral and the effective stress intensity factor. The interfacial zone was characterized as a third phase with a lower stiffness and strength as compared to matrix and fiber. The debonding criterion was based on a yield surface defined by normal and shear strength of the interface. After debonding, Coulomb friction was introduced in the debonded zone. Effects of interfacial adhesional strength, clamping pressure, and fiber length on the fiber pullout response were studied. In the composite response simulations, the fibers across a prescribed crack length were modeled as nonlinear spring elements. The pullout force vs. slip displacement was used for the stiffness of the spring elements. J-integral was evaluated for the two cases of with and without fibers, and the difference between the two was used as the toughening contribution of fibers. The fiber toughening effect was studied for different fiber lengths and interface parameters. Results were compared with analytical simulations of crack growth using R-curves and a simplified approach based on linear crack opening–closing pressure relationship.

This article presents expansion and microstructural data for a series of concrete mixes containing reactive flint aggregate, with a range of fly ash levels, exposed to various alkaline salt solutions. This study was undertaken to determine whether fly ash has any influence on alkali-aggregate reaction beyond changes in pore solution chemistry; in these tests the external source of alkalis should neutralize pore solution effects. Fly ash was found to be effective in reducing expansion even after extended periods (44 months) of exposure in 1N NaOH at 80°C, notwithstanding the presence of abundant reactive silica and an inexhaustible supply of alkali hydroxides. Higher levels of ash (40%) prevent damaging expansion and cracking in this environment despite considerable evidence of reaction. In some cases, flint grains had been completely removed by dissolution. The addition of Ca(OH)₂ at the mixing stage was found to increase the expansion of all the concretes; the effect on concrete with 40% ash was most marked, the expansion increasing by nearly 20 times. The most noticeable difference between deteriorated control specimens (no ash) and concrete with 40% ash was the formation of a calcium-alkali-silica rim on certain flint grains in concrete without ash. Such particles were invariably sites of expansive reaction with cracks emanating from them. The absence of such a feature in concrete with 40% ash is probably linked to the reduction in Ca(OH)₂ at the cement-aggregate interface. It is possible that the formation of this reaction rim produces expansive forces itself or acts as a semi-permeable membrane preventing diffusion of alkali silicate solution from the reaction site, thereby leading to osmotic pressure generation. Regardless of the actual mechanism, the presence of Ca(OH)₂ appears to be critical for the development of expansion due to alkali-silica reaction. It was observed that the alkalis of the reaction product were distributed in bands. In the Portland cement concrete specimens, the distribution of the gel consisted of a high calcium reaction rim at the aggregate-cement interface with a sodium-rich silica gel adjacent to it, followed by a potassium-rich silica gel. The potassium-rich silica gel appears to have a crystalline, needle-like structure, whereas the sodium-rich silica gel is amorphous. In fly ash concrete specimens in which the formation of calcium-rich reaction rim was prevented, it was observed that the sodium-rich gel had diffused into the surrounding cement matrix, and the potassium-rich gel had remained within the original aggregate boundary.