2 research outputs found
On the global hydration kinetics of tricalcium silicate cement
We reconsider a number of measurements for the overall hydration kinetics of
tricalcium silicate pastes having an initial water to cement weight ratio close
to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica
mole numbers can be well characterized by two power-laws in time, . For early times we find an `accelerated' hydration
() and for later times a `deaccelerated' behavior (). The crossover time is estimated as . We
interpret these results in terms of a global second order rate equation
indicating that (a) hydrates catalyse the hydration process for , (b)
they inhibit further hydration for and (c) the value of the
associated second order rate constant is of magnitude 6x10^{-7} - 7x10^{-6}
liter mol^{-1} s^{-1}. We argue, by considering the hydration process actually
being furnished as a diffusion limited precipitation that the exponents and directly indicate a preferentially `plate' like hydrate
microstructure. This is essentially in agreement with experimental observations
of cellular hydrate microstructures for this class of materials.Comment: RevTeX macros, 6 pages, 4 postscript figure
Evaluation of bond using FRP rods with axisymmetric deformations
This paper presents experimental results obtained with the direct pull-out test using machined and wrapped glass/vinylester, carbon/vinylester, and carbon/epoxy FRP rods with axisymmetric lugs. The typical results are given as nominal shear stress vs. free- and loaded-end slip. Experimental results obtained from strain probes used during the pull-out test are presented as shear stress vs. strain. Machined glass/vinylester FRP rods with embedded lengths including five and 10 lugs, and different lug widths and heights were studied. The failure mode consisted of the shearing off of the lugs without concrete damage. Four concrete mixtures with strengths ranging from 32 to 66.1 MPa were examined. Provided that enough confinement is used, it was found that the concrete strength has no noticeable effect on the shear strength and failure mode of FRP rods. Results showed that the FRP-concrete bond is controlled by the lug dimension and shear strength of the resin. The shear strength of the wrapped lugs is less than that of machined ones due to fiber orientation and weaker interfacial bond between the wrapped strands and rod surface. (C) 1999 Elsevier Science Ltd. All rights reserved