Validation and Modification of the 4SIGHT Computer Program.
Validation and Modification of the 4SIGHT Computer
Program.
(2459 K)
Snyder, K. A.
NISTIR 6747; 85 p. May 2001.
Available from:
National Technical Information Service
(NTIS), Technology Administration, U.S. Department of
Commerce, Springfield, VA 22161.
Telephone:
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Order number: PB2001-106385
Keywords:
computer programs; validation; concretes; cracking
(fracturing); degradation; Monte Carlo method; drying;
service life; transport
Abstract:
The original version of the 4sight computer program was
written to facilitate the performance assessment of
buried concrete structures based deterministic
calculations of service life, using data supplied by the
user. In an effort to assure reliability in the results,
a means of validation was required. Since the results
from the calculations depend strongly on the diffusive
and advective transport of ionic species, validating the
transport portion of 4sight was paramount. The
validation of the transport portion of 4sight used both
reference and laboratory data. The reference data
include electrolyte densities, activity coefficients,
and binary salt diffusion coefficients. The laboratory
experiments included diffusion experiments performed on
relatively inert ceramic frits, using a range of ionic
species and concentrations. The laboratory experiments
were also used to confirm the fundamental assumption
that multi-species transport in non-reacting systems can
be characterized by two numbers, the porosity and the
formation factor, regardless of the number of diffusing
species. A default analysis for estimating the spacing
and thickness of flexural and drying shrinkage cracks
was added to 4sight. The flexural cracks are calculated
from a rudimentary structural analysis that assumes the
roof of the vault acts as a one-way slab. The drying
shrinkage cracks are estimated, in large part, from the
total water content of the concrete mixture, the
exposure environment, and the geometry of the slab. The
subsequent predicted crack widths, crack spacings, and
location of the neutral axis, are used to predict the
bulk permeability of the concrete structure as a
function of depth; the portion above the neutral axis
being uncracked, the portion below the neutral axis
being cracked. Another addition to 4sight was the means
to perform a Monte Carlo calcuation, based on parameter
uncertainty, of the duration of the useful service life.
The service life calculation is repeated multiple times
using a random number generator to calculate random
deviates for the physical parameters. The results can
then be used to make a probabilistic statement about the
performance of the concrete structure. The resulting
computer program is a state-of-the-art attempt to
characterize the response of a concrete to its
environment. However, there can be no assurances that
the model is exact for every scenario possible,
regardless of parameter uncertainty. Therefore, in cases
that concern public safety, the only way to confirm the
response of the concrete to its environment is through
continuous monitoring and periodic testing. The computer
program, in these circumstances, can also serve as a
useful tool in establishing a testing and monitoring
protocol. The spatial and temporal distribution of the
monitoring and testing protocol can be established using
the computer program as a guide.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899