Dominant Role of Cement Paste Content on Bridge Deck Cracking
Presented By: Rouzbeh Khajehdehi
Affiliation: SJCA Inc.
Description: The cracking of concrete bridge decks is a nationwide problem. The results of over 100 crack surveys on 40 monolithic composite concrete bridge deck placements supported by steel girders were used to evaluate the factors contributing to bridge deck cracking. The variables considered were the material properties, such as paste content (volume of cement and water), slump, compressive strength, and air content, and environmental factors, such as air temperature range, high air temperature, and time of placement on the day of construction. A stepwise regression by forward addition was used to determine which of the variables had the greatest, and statistically significant, influence on cracking.
Results showed that, within the ranges studied, paste content has a dominant effect on cracking, with cracking increasing substantially when the volume of paste exceeds a threshold value of 26.4% of concrete volume while being insensitive to changes in paste content below this threshold. Bridge decks placed and finished between midnight and noon exhibited substantially lower cracking than those placed and finished between early morning and late night. Other factors such as slump, compressive strength, and air content affected cracking to a limited degree, with a higher slump, greater compressive strength, and lower air content, resulting in slightly higher cracking, effects that are not statistically significant.
Raman Imaging: A New Technique for Old Problems
Presented By: Nishant Garg
Affiliation: University of Illinois at Urbana-Champaign
Description: Raman spectroscopy has been used in the characterization of cementitious materials for nearly 50 years. However, hyperspectral mapping or Raman imaging is a relatively new technique that produces phase maps or spectral images of sample surfaces by collecting a Raman spectrum on every pixel. Over the last 10 years, the capabilities of Raman imaging have significantly improved, primarily due to hardware and software advancements in the Raman confocal microscopes and spectrometers. In this talk, I will outline recent applications of Raman imaging on cementitious systems and discuss opportunities as well as challenges. Specifically, Raman imaging allows highly accurate and precise mineral mapping of heterogenous ingredients of concrete (ranging from aggregates to cement) with its ability to detect mineral polymorphs at a high spatial, sub-micron resolution. Accurate quantification of mineralogical compositions is a key advance over bulk elemental compositions, and Raman imaging brings us one step closer to such a technological advance.
Hydration Effects of Limestone Replacement in High Volume Fly Ash Concrete
Presented By: Aniruddha Baral
Affiliation: University of Illinois Urbana-Champaign
Description: High volume fly ash concrete (HVFAC) provides lower-cost construction material with better durability, and enhanced sustainability metrics, but favorable early-age properties, such as setting time, air entrainment, and strength gain can be a challenge. Previous research has shown that accelerating the setting time of HVFAC can be achieved by replacing part of the cement with fine limestone powder. Fine limestone powder can accelerate the setting time of HVFAC potentially through two different mechanisms – (1) fine limestone can act as nucleation seeds and thus accelerate C-S-H precipitation, and (2) fine limestone can react with the aluminate phases present in fly ash to form monocarbonate and hemicarbonate phases. In this research, we investigated the setting time acceleration for cement replacement with both micro and nano limestone in HVFAC systems with polycarboxylate and lignosulfonate-based chemical admixtures using isothermal calorimetry. Nano limestone replacement accelerated the setting time more than micro limestone replacement. Moreover, the setting time acceleration by limestone replacement was higher in the class C fly ash mixes containing polycarboxylate-based admixture compared to lignosulfonate-based admixture. Further, the crystalline phases formed in the hydration reaction will be quantified with Rietveld XRD to provide a scientific explanation behind the observed set time acceleration both in terms of different crystalline phase formation and the role of limestone nucleation sites for accelerated C-S-H formation.
ARCTEC (Additive Regulated Concrete for Thermally Extreme Conditions): Development of Multi-Parameter Guidance for Additive Dosage
Presented By: Benjamin Watts
Affiliation: US Army Corps of Engineers
Description: In cold temperatures, fresh concrete can be irreversibly damaged by the formation of ice within the hydrating microstructure before adequate strength has developed. Industry-standard protection measures are frequently laborious, expensive, and time-consuming. The USACE ERDC Cold Regions Research and Engineering Laboratory (CRREL) has developed ARCTEC (Additive Regulated Concrete for Thermally Extreme Conditions) to enable the use of commonly available concrete additives as alternative freeze-protection in cold conditions. ARCTEC builds upon pioneering work performed at CRREL over the last several decades, with the goal of improving the user-friendliness, economy, and utility of the technology.
A core component of ARCTEC is guidance to recommend the required dosage of additive for a successful concrete placement. This recommendation depends upon multiple aspects of a placement, including geometry, mixture proportions, ambient temperature, wind, and time of placement. The number of unique cases implied by these parameters precludes physical testing of every possibility, so a transient finite element thermal model was created to simulate the effects of these parameters on the evolving thermal behavior of concrete placed at a range of additive dosages. Inputs for this model were obtained through laboratory characterization of the thermal and mechanical behavior of concrete at multiple curing temperatures and additive dosages. These inputs, when combined with synthetic daily temperature profiles, variable convective boundary conditions, a range of placement geometries and constructions, and maturity-informed success criteria, result in the ability to define the optimal additive dosage over a broad range of possible placement configurations.