Civil Engineering ETDs

Publication Date

2-8-2011

Abstract

Moisture damage is one of the major problems of asphalt pavements in United States. The moisture damage problem in asphalt has been studied for decades; still it remains an unsolved problem. Traditional macro-scale tests and methods failed to describe how and what factors affect moisture damage because moisture damage in asphalt is related to asphalt chemistry and adhesion characteristics, which are below micron scale phenomena. To this end, asphalt chemistry and adhesion values are studied at nano-scale to understand moisture damage in this study. Nano-scale measurements are conducted using an Atomic Force Microscope (AFM) in the laboratory. In an AFM test, adhesion forces of dry and wet asphalt samples are measured by probing the sample surface with AFM tips. Nano-scale pull-off force or adhesion between sample molecules and tip molecules are measured. To facilitate the study of asphalt chemistry, AFM tips are modified using chemical functional groups such as carboxyl (-COOH), hydroxyl (-OH), ammin (-NH3) and methyl (-CH3), representing the chemistry of asphalt binder. Thus functionalized tips facilitate the measurement of adhesion within the asphalt binder. In addition, silicon nitride (Si3N4) tips are used. Silicon nitride resembles aggregate molecules (e.g. silica aggregate) that are used to produce asphalt concrete or pavements. Thus adhesion value measured using a silicon nitride tip can be considered as the adhesion value of asphalt-aggregate interface. It is shown in this study that the adhesion within the asphalt binder varies depending on the chemistry of asphalt. AFM testing on asphalt is non-trivial and very challenging as AFM tips stick to the asphalt surface due to the viscous and soft nature of asphalt binder. AFM testing also requires smooth surface of a test sample. This study has developed a methodology for asphalt sample preparation for AFM testing. Simply, pouring asphalt binder on a glass substrate and melting to free flow and then cooling it generates an AFM asphalt sample with a root mean square surface roughness below 10 nm. Such sample surface is smooth enough for AFM testing. Through trial and error, this study has calibrated a set of AFM testing parameters that are suitable for successful adhesion measurement in asphalt binder. In all cases, a set of AFM samples are tested under dry condition, and a set of identical samples are tested after wet conditioning. Polymer is almost an essential component of asphalt binders now-a-days. However it is not known whether polymer modification helps reduce moisture damage potential of asphalt. Therefore, both base binder and polymer modified asphalt binder are characterized herein using AFM. Two common polymers Styrene-Butadyne (SB) and Styrene-Butadyne-Styrene (SBS) are used to modify the base asphalt. The goal is to examine whether polymer modification helps reduce moisture damage at nano-scale. In addition to polymer, a chemical modifier known as Elvaloy is included in this study to examine whether Elvaloy is more effective than polymer in regards to moisture damage. It is shown that both base and modified asphalt binders are vulnerable to moisture damage to some degree. However, base binder is the most susceptible to moisture damage among all the binders. It is evident that the SB polymer modification of asphalt is good for interface adhesion, whereas the SBS polymer modification is good for achieving higher adhesion within the asphalt binder. Antistripping agents are commonly used to reduce moisture damage potential of an asphalt binder. A number of antistripping agents are available in the market. However, it is not know which antistripping works better than others. To examine, five common antistripping agents such as lime, klingbeta, wetfix, morlife and unichem are considered for AFM testing in this study. It is evident from this study that moisture damage occurs in asphalt binder having an antistripping agent. Hydrated lime provides higher moisture damage resistance to asphalt binders that the liquid antistripping agents such as morlife, unichem, klingbeta, and wetfix. Statistical analysis of the adhesion test results is performed. Based on Pearsons p-value (significance test), it is concluded that the adhesion value measure by an AFM varies with the type and amount of antistripping agent present in an asphalt sample. Finally, an attempt is made to correlate nano-scale adhesion value of an asphalt binder to macro-scale strength value representing moisture damage. Only polymer modified binders are considered for examining such correlations. Macro-scale indirect tension tests are conducted on wet and dry asphalt concrete samples. A good correlation exists between the macro-scale indirect tensile strength ratio, and nano-scale adhesion ratio of wet and dry samples.'

Keywords

Asphalt--Moisture--Testing, Polymer asphalt--Moisture--Testing, Asphalt--Absorption and adsorption--Testing, Polymer asphalt--Absorption and adsorption--Testing.

Sponsors

National Science Foundation (NSF)

Document Type

Dissertation

Language

English

Degree Name

Civil Engineering

Level of Degree

Doctoral

Department Name

Civil Engineering

First Committee Member (Chair)

Maji, Arup

Second Committee Member

Ng, Percy

Third Committee Member

Huerta, Gabriel

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