Graduation Year

2021

Document Type

Thesis

Degree

M.S.C.E.

Degree Name

MS in Civil Engineering (M.S.C.E.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Abla Zayed, Ph.D.

Committee Member

Gray Mullins, Ph.D.

Committee Member

Natallia Shanahan, Ph.D.

Committee Member

Dhanushika Mapa, Ph.D.

Keywords

Delayed Ettringite Formation, High Alumina Slag, Internal Sulfate Attack

Abstract

Delayed ettringite formation (DEF) is a durability issue that typically occurs in concrete exposed to high temperatures at an early age such as precast elements and massive concrete structural elements. There are many advantages to precast concrete such as its quality control, better safety control, and reusable forms, which make them cheaper than casting on site [1]. Heat curing of precast concrete is conducted to obtain high early-strength. However, in doing so, durability issues can arise in the form of internal sulfate attack also known as DEF, if the internal temperature of concrete exceeds 70°C. Past this threshold, ettringite decomposes and after subsequent years in service and in the presence of sufficient moisture, the ettringite will reform within the concrete resulting in expansion and eventually failure. Ground granulated blast furnace slag (GGBFS, slag) is often used in blended concrete mixtures for both economic and environmental purposes but it also possesses durability benefits. ACI 201.2R-16 (Guide to Durable Concrete) [2] recommends a minimum of slag replacement level of 35%, for concrete elements which can potentially exceed 70°C, is sufficient to reduce the potential of DEF occurrence. However, this replacement threshold does not account for variable cement and slag chemistries, mineralogy, and physical characteristics. Depending on the chemistry of the cement or slag and the level of replacement, slag can mitigate the effects of DEF or prevent expansion entirely from occurring. Therefore, this study aims to identify the characteristics of cementitious systems that are of significance to DEF phenomenon.

Six different cements were used in this study; namely, Type I, Type I high alkali, Type IL, Type II moderate heat cement (MH) of low alkali content, and Type II (MH) of moderate alkali content. These were blended with 6 different slags of variable chemical and physical characteristics at two replacement levels of 60% and 35% by weight. The heat treatment cycle used had a maximum temperature of 95 °C and a residence time of 36 hours. The specimens were subsequently stored in saturated lime water and measured for expansion regularly over the course of the experiment. X-Ray diffraction scans were also performed to confirm the presence and evolution of ettringite among other phases in each mix.

The results show that 35% high alumina slag replacement does not necessarily prevent expansion when blended with Type I high alkali cement. The degree of expansion was found to be related to the slag alumina content with high alumina slags showing earlier and higher expansion. Increasing slag replacement levels from 35% to 60% suppressed DEF for all blended systems studied. The results of this study indicate the need to further examine ACI recommendations for the minimum replacement level for heat-cured blended cementitious materials that should be used to suppress DEF especially in the light of the variability of modern cements and slag chemistries.

The work reported in this thesis was partially funded by the Florida Department of Transportation (FDOT) under contract number BDV25 977-63 (Correlation of Slag Cement Composition with Durability of Portland Cement-Slag Concrete). Most of the collected data (charts and tables) are shared between the Final Report of the named project and this thesis.

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