Browsing by Author "Meyerson, Richard M."

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    Development of concrete shrinkage performance specifications
    Mokarem, David W.; Meyerson, Richard M.; Weyers, Richard E. (Virginia Center for Transportation Innovation and Research, 2003-08-01)
    During its service life, concrete undergoes volume changes. One of the types of deformation is shrinkage. The four main types of shrinkage associated with concrete are plastic, autogenous, carbonation, and drying shrinkage. The volume changes in concrete due to shrinkage can lead to the cracking of the concrete. In the case of reinforced concrete, the cracking may produce a direct path for chloride ions to reach the reinforcing steel. Once chloride ions reach the steel surface, the steel will corrode, which itself can cause cracking, spalling, and delamination of the concrete. The unrestrained drying shrinkage and restrained cracking tendency of concrete mixtures typically used by the Virginia Department of Transportation (VDOT) were assessed to establish an appropriate limit on drying shrinkage for use in a performance specification. Five existing shrinkage prediction models were assessed to determine the accuracy and precision of each model as it pertains to the VDOT mixtures used in this study. The five models assessed were the ACI 209 Code Model, Bazant B3 Model, CEB 90 Code Model, Gardner/Lockman Model, and Sakata Model. The CEB 90 model performed best for the portland cement concrete mixtures, while the Gardner/Lockman Model performed best for the supplemental cementitious material mixtures. Based on a comparison of the unrestrained drying shrinkage and restrained cracking tendency, it was determined that the potential for cracking could be minimized by limiting the unrestrained shrinkage of the concrete mixtures. Based on the results of this study, the recommended percentage length change specification limits are 0.0300 at 28 days and 0.0400 at 90 days for the portland cement concrete mixtures. For the supplemental cementitious material mixtures, the percentage length change specification limits were 0.0400 at 28 days and 0.0500 at 90 days.
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    Evaluation of models for predicting (total) creep of prestressed concrete mixtures
    Meyerson, Richard M.; Weyers, Richard E.; Mokarem, David W.; Lane, D. Stephen (Virginia Center for Transportation Innovation and Research, 2002-09-01)
    Concrete experiences volume changes throughout its service life. When loaded, concrete experiences an instantaneous recoverable elastic deformation and a slow inelastic deformation called creep. Creep of concrete is composed of two components, basic creep, or deformation under load without moisture loss and drying creep, or deformation under drying conditions only. Deformation of concrete in the absence of applied load is often called shrinkage. The deformation due to creep is attributed to the movement of water between the different phases of the concrete. When an external load is applied, it changes the attraction forces between the cement gel particles. This change in the forces causes an imbalance in the attractive and disjoining forces. However, the imbalance is gradually eliminated by the transfer of moisture into the pores in cases of compression, and away from the pores in cases of tension. Designs typically use one of the two code models to estimate creep and shrinkage strain in concrete, ACI 209 model recommended by the American Concrete Institute or the CEB 90 Eurocode 2 model recommended by the Euro-International Committee. The AASHTO LRFD is based on the ACI 209 model. Three other models are the B3 model, developed by Bazant; the GZ model, developed by Gardner; and the SAK model developed by Sakata. The objectives of this research was the development of performance limits for compressive creep of concrete mixtures used by the Virginia Department of Transportation, specifically concrete mixtures used for prestressed members (A-5 Concrete) and the determination the accuracy and precision of the creep models presented in the literature. The CEB 90 Eurocode 2 model for creep and shrinkage is the most precise and accurate predictor. The total creep strain for the VDOT portland cement concrete mixtures discussed in this study were found to be between 1200 +/- 110 microstrain at 28 days, and 1600 +/- 110 microstrain at 97 days, at a five percent significant level. It is recommended that the CEB 90 model be used in the AASHTO LRFD rather than the ACI 209 model to improve the prediction of prestress loss.