Durability Based Performance Evaluation of Bridge Deck High-Performance Concrete (HPC) Mixes through Innovation in Pore Solution and Transport Properties

Abstract

Texas Department of Transportation (TxDOT) employs High Performance Concrete (HPC) predominantly for bridge construction in Texas. In the past, achieving high strength was the basis of the main design criteria to formulate high-performance concrete. However, TxDOT's current Class S HPC mix design options (Option 1-5 & 8) (TxDOT, 2014) consists of binary and/or ternary combinations of supplementary cementitious materials (SCMs) such as Class F, Class C fly ashes & silica fume, wherein the replacement levels were primarily designed to address alkali silica reaction (ASR) mitigation. As a result, current specifications are predominantly prescriptive and use a deemed-to-satisfy approach to address other durability aspects such as shrinkage, permeability reduction to resist chloride ion ingress, freeze-thaw durability, and others through minimal performance control based on requirements for min strength, max w/cm ratio, and permeability (ASTM C 1202). While SCMs densify concrete microstructure and reduce permeability, they also modify concrete mixes' concrete pore solution chemistry.

Therefore, the current research focuses on evaluating two major chemical deterioration mechanisms- (a) alkali silica reaction and (b) chloride-induced rebar corrosion that influence durability performance of concrete structures in service through innovative approaches and modeling tools addressing pore solution chemistry and transport properties of HPC mixes.

In current research, a comprehensive pore solution model is developed to estimate : (a) pore solution alkalinity (PSA) of concrete mixes based on combined effect of soluble alkali contribution from cement and water soluble alkali from fly ashes into concrete pore solution (critical for ASR evaluation), and (b) long-term pore solution chemistry (PSC) of concrete mixes based on a balanced consideration of total soluble alkali contribution from all ingredients and alkali binding by pozzolanic hydration products (critical for formation factor (FF) determination and FF based transport properties evaluation). ASR mitigation is addressed through the development of a screening tool that predicts optimum fly ash dosage (i.e., total SCM replacement level) based on concrete PSA vs. aggregate threshold alkalinity (THA) relationship, i.e., PSA ≤ THA to mitigate ASR. Furthermore, State DOTs and highway agencies are currently using concrete resistivity tests towards mixture qualification and QA/QC. The comprehensive long-term PSC prediction model determines formation factors of HPC mixes based on laboratory or field resistivity measurements. A primary benefit of using the formation factor in specifications is that an easy resistivity measurement in a field or lab can be used to assess durability performance through service life models based on formation factor – transport properties relationships.

Addressing the durability design of HPC mixes requires incorporating methods/model approaches that predict concrete performance characteristics effectively. The findings/individual models developed from current research are used to develop a comprehensive durability-based performance evaluation of HPC mixes that quantifies and connects (a) ingredient composition, (b) mix design, (c) pore solution chemistry, (d) concrete microstructure, and (e) environmental conditions.