ESTIMATING EARLY-AGE THERMALLY-INDUCED STRESSES IN CONCRETE BRIDGE DECKS

ESTIMATING EARLY-AGE THERMALLY-INDUCED STRESSES IN CONCRETE BRIDGE DECKS

Abstract

Early-age cracking has been identified as a significant contributor to increased life cycle cost and decreased service life of concrete bridge decks. Newly placed concrete is susceptible to cracking due its low tensile strength at an early age, commonly considered to be within a few months post concrete placement. Temperature variations within the deck caused by environmental conditions as well as heat of hydration have been identified as potential contributors to early-age cracking. Adjusting the placement time to avoid the adverse effects of different environmental factors could be a strategy to further mitigate early-age cracking. However, there is inconclusive guidance regarding the time of deck placement and corresponding environmental conditions that would effectively limit tensile stresses within the deck at an early-age. In this study, an investigation of the effect of environmental conditions, such as ambient air temperature and solar radiation, and early-age concrete properties, such as heat of hydration and elastic modulus development, on the principle tensile stresses within a concrete deck during the first 29 hours after concrete placement (including an assumed 5 hour set time from placement) is summarized. The investigation was conducted using thermo-mechanical finite element modeling to analyze the stresses in a composite concrete deck-steel girder bridge system for different environmental and design scenarios. The results of this study showed that reducing the concrete heat of hydration within the deck was most effective at limiting the maximum principle tensile stresses within the deck during the first 29 hours

following                    placement                    of                    the                    concrete                    deck.

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Table of Contents

List of Figures…………………………………………………………………………………………………………………… v

List of Tables…………………………………………………………………………………………………………………… vi

 

Chapter 1: Introduction ………………………………………………………………………………………………………………… 1

Chapter 2: Background ………………………………………………………………………………………………………………… 3

Chapter 3: Methodology ………………………………………………………………………………………………………………. 6

Finite-element model ……………………………………………………………………………………………………………….. 6

Analysis Steps ……………………………………………………………………………………………………………………….. 15

Bridge Deck Placement Scenarios ……………………………………………………………………………………………. 16

Sensitivity Study ……………………………………………………………………………………………………………………. 20

Chapter 4: Results ……………………………………………………………………………………………………………………… 23

Sensitivity Study ……………………………………………………………………………………………………………………. 23

Bridge Deck Placement Scenarios ……………………………………………………………………………………………. 24

Chapter 5: Summary and Conclusions ………………………………………………………………………………………….. 31

References ………………………………………………………………………………………………………………………………… 33

Chapter 1 Introduction

Newly placed concrete bridge decks are susceptible to early-age cracking because the tensile stress development, due to restraint and other factors, can exceed the tensile strength at which point a crack forms. Among the causes of early-age cracking in concrete decks, changes in temperature and moisture within the concrete have been identified as potential contributors (Miller et al. 2006, TRB 2006, MnDOT 2011). Spatial temporal temperature variations within the deck cause thermal deformations and shrinkage that, when combined with restraint, induce stresses within the deck. Temperature variations within the deck occur due changes in the ambient air temperature, solar radiation, as well as the heat of hydration within the newly placed bridge deck. Changes in temperature and moisture cause volumetric movement in the form of thermal deformations and shrinkage in the concrete. When thermal deformation in the concrete deck is restrained, for example by composite action with the supporting girders in the case of bridge decks, tensile stresses can develop within the concrete deck, potentially contributing to cracking if the tensile stress exceeds the tensile strength of the concrete prior to the concrete reaching maturity.

Several experimental studies have been performed in which bridge decks were instrumented and monitored for temperature and stress (Miller et al. 2006, Subramaniam et al. 2010 Choi et al. 2011a, Choi et al. 2011b, Hossain et al. 2014). Additionally, numerical methods have been used to investigate temperature and stress within bridge decks. The numerical studies have focused on the behavior of fullymatured concrete, i.e. concrete that has reached its ultimate strength (Elbadry and Ghali 1983, Digler et al. 1983, Fu et al. 1990), or using experimental data to validate a numerical model (Choi et al. 2011a, Choi et al. 2011b, Hossain et al. 2014). Among numerical methods, the Finite Element (FE) method has shown the capability to estimate temperature and subsequently thermally-induced stresses within concrete bridge decks (Choi et al. 2011b, Lin and Chen 2012), allowing a significant number of analyses to be performed in lieu of analytical and experimental techniques. Although previous studies have shown environmental conditions can significantly affect tensile stresses in bridge decks the effect of the thermal loading due to heat of hydration was not considered. In this study, the FE method is employed to estimate

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thermally induced stresses and to study the influence of the various environmental design factors. The complexity of the various factors, that is ambient temperature, solar radiation, heat of hydration, and scale preclude a parametric experimental study.

The objective of this study is to identify the factors that have the greatest influence on the maximum tensile stresses in concrete bridge decks at an early age. Factors that will be considered include ambient air temperature, solar radiation, heat of hydration and elastic modulus development. Stresses in the composite concrete deck will be estimated using the FE method. An array of environmental conditions, concrete material properties, and bridge deck placement times are organized into deck placement scenarios for the parametric study. The maximum principal tensile stress is the primary metric for assessing the effect of the various environmental and design factors. A sensitivity study was carried out for the sensitivity of the maximum principle stress in the concrete deck to variations in model parameters, more specifically heat transfer coefficients, to guide the refinement of the finite element model prior to the scenario analysis.

The results of the parametric study were analyzed to identify deck placement times for different environmental conditions that were best at limiting the maximum principal tensile stresses in the concrete deck. It is assumed that, identifying scenarios that reduce the principal tensile stresses within a deck will reduce the potential for early-age cracking thereby increasing service life for bridge decks and, in turn, reduce life cycle costs. Based on the principal tensile stress results from FE analysis of 128 deck placement scenarios, reducing the concrete heat of hydration by using concrete mix designs with reduced Type I Portland cement contents was found to be the most influential factor to limit maximum principal tensile stress development within the deck during the 29 hours after concrete placement.

ESTIMATING EARLY-AGE THERMALLY-INDUCED STRESSES IN CONCRETE BRIDGE DECKS

 

 

 

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