As part of a research project being funded by the Federal Highway Administration (FHWA), the National Institute of Standards and Technology (NIST) is developing a computer program to predict the service life of concrete pavements and bridge decks exposed to sulfate attack and/or freeze-thaw conditions [1]. For these models, the major mechanism of transport of water and sulfate ions being considered is sorption by the partially saturated concrete. The uptake of water and the sulfate ions contained within it is dependent on many variables including the time-of-wetness of the concrete surface, the saturation of the concrete, and its surface temperature. The objective of this report is to describe the details of a computer model for predicting these three variables as a function of the environmental exposure for hardened concrete. Models for the thermal properties and temperature distribution of fresh hardening concrete have been presented in the literature previously [2,3].
The developed computer model (CONCTEMP) is based on a one-dimensional finite difference scheme [4] and includes heat transfer due to conduction, convection, and radiation. The top surface boundary conditions (and the lower surface boundary conditions for bridge decks) are established based on the typical meteorological years weather data (TMY2DATA) provided by the National Renewable Energy Laboratory (NREL) [5]. Material thermal properties and convection coefficients are taken from the available literature. Outputs from the model include the variation of concrete surface temperature during the course of a year, a file detailing all time-of-wetness events (start time, duration, concrete surface temperature, and exterior relative humidity prior to event), and a file detailing time-of-freezing events (start time, duration, and minimum surface temperature achieved during freezing). These files have been created for twelve representative geographical locations throughout the United States. A complete listing of the computer code used to estimate the concrete surface temperature for pavement structures can be found in Appendix A. The program has been written in the C programming language. It has been written in a modular fashion with separate subroutines for reading in the input material properties and for updating the temperatures of all nodes within the 1-D structure. The log files for wetting and freezing events are created within the main program routine, which is also responsible for accumulating time throughout the year and reading in the hourly weather data values.
This software was developed at the National Institute of Standards and Technology by employees of the Federal Government in the course of their official duties. Pursuant to title 17 Section 105 of the United States Code this software is not subject to copyright protection and is in the public domain. CONCTEMP is an experimental system. NIST assumes no responsibility whatsoever for its use by other parties, and makes no guarantees, expressed or implied, about its quality, reliability, or any other characteristic. We would appreciate acknowledgement if the software is used.
The U.S. Department of Commerce makes no warranty, expressed or implied, to users of CONCTEMP and associated computer programs, and accepts no responsibility for its use. Users of CONCTEMP assume sole responsibility under Federal law for determining the appropriateness of its use in any particular application; for any conclusions drawn from the results of its use; and for any actions taken or not taken as a result of analyses performed using these tools.
Users are warned that CONCTEMP is intended for use only by those competent in the field of cement-based materials and is intended only to supplement the informed judgment of the qualified user. The software package is a computer model which may or may not have predictive value when applied to a specific set of factual circumstances. Lack of accurate predictions by the model could lead to erroneous conclusions with regard to materials selection and design. All results should be evaluated by an informed user.
The algorithms, procedures, and computer programs described in this report constitute a methodology for predicting the surface temperature and time-of-wetness of concrete under typical weathering conditions. They have been compiled from the best knowledge and understanding currently available, but have important limitations that must be understood and considered by the user. The program is intended for use by persons competent in the field of cement-based materials and with some familiarity with computers. It is intended as an aid in the materials selection, optimization, and design process.