Hartt, William H.

Failure of prestressed steel tendon in concrete due to corrosion or environmental cracking (or both) has become a major technological concern because of increased utilization of this form of construction in corrosive exposure such as coastal bridge deck structures. At the same time cathodic protection has evolved as the most practical means for arresting corrosion of reinforcing steel in existing structures. This technology also applies to pretensioned tendons. However, concerns have been raised that cathodic protection might cause embrittlement, since yield stress of these materials is relatively high. For the purpose of analyzing this embrittlement tendency a series of constant load and slow strain rate experiments were performed and the data evaluated. Influence of potential, chloride concentration, pH and precharging on the cracking susceptibility have been evaluated. The cracking process has been explained based on test results and SEM analysis.

The objective of this study was to determine the influences of chlorides, pH and surface films on the corrosion of reinforcing steel in alkaline solutions. Anodic and linear polarization experiments were conducted on 1018 steel specimens exposed to these environments. The results indicate that there is a threshold amount of [Cl-] beyond which passivity is compromised. The i-passive in the electrolyte of pH 12.24 was approximately 0.01 of that in the electrolyte of pH 10.13. At least a one day period was necessary for the effect of the excess crystals of Ca(OH)2 to be felt. Corrosion rates of specimen with a Ca(OH)2 surface film were lower than without this film. [Cl-] /[OH-] in the range 2.5 to 3.5 appeared to be the threshold for breakdown of passivity.

Knowledge of the reversible potential for corrosion reaction under condition of deep sea exposure and how this is influenced by parameters such as temperature and pressure is of importance to the understanding and design of deep sea cathodic protection systems, since this potential is used as the cathodic protection criterion. Also, if the polarized potential is more negative than the reversible potential of the hydrogen reaction, then hydrogen embrittlement may occur. Thermodynamic analysis methods are used to calculate Gibbs free energy change and reversible potential of reactions involved in cathodic protection of steel for temperatures and pressures typical of deep sea. Based on the assumptions employed it is found that the reversible potential for the iron reaction becomes less negative with depth for the first 1000m and remains approximately constant beyond this.

Study of selected candidate steels for offshore application was undertaken to observe the effects of cathodic protection and cyclic frequency on corrosion fatigue life. Keyhole Compact Tension Fatigue experiments under constant amplitude sinusoidal loading and stress ratio of 0.5 were performed on 25.4 mm thick specimens in natural sea water and also in air upon three different steels (Y.S. 500-563 MPa). These steels represented different strengthening techniques, namely precipitation hardening, direct quenching--a thermomechanical control process (TMCP), and controlled rolling. Cathodic polarization was in the range between freely corroding and -1.10 Volts (SCE). The tests were performed at a frequency of 0.3 and 1.0 Hz. The results are presented in the S-N and potential versus cycles to initiation format. No effect of frequency (1.0-0.3 Hz) was observed at cathodic protection of -1.10 V (SCE). The steels showed an increase in fatigue life to an optimum potential, and excessive potentials were detrimental. The fatigue life in dry air was greater than in laboratory air (~50% RH).

In order to investigate the environmental cracking tendency of prestressing steel tendons under cathodically polarized conditions, a series of slow strain rate tests were conducted using smooth and notched specimens in simulated concrete porewater environments. Applied potential was varied from -0.500 V to -1.500 V(SCE) at intervals of 0.200 V, and the pH range was 8.0 to 12.5. Based on the test results presented in terms of time-to-failure, reduction in crosssectional area and load at failure, potential regimes for susceptibility to hydrogen embrittlement in saturated Ca(OH)2 solution were identified. SEM analyses performed for both notched and smooth specimens tested at -0.700 V and -1.300 V in saturated Ca(OH)2 solution showed that different fracture mechanisms operate at these two potentials. The cracking processes have been explained based on the test results and SEM analyses. Based upon the data obtained, the steel showed higher resistance to environmental cracking at pH 9.8 and 8.0 than for 12.5. The applicability of the results to cathodic protection of prestressed steel in concrete is discussed.

Fatigue response of selected high strength steels in seawater was investigated
under conditions relevant to tension leg platform tendon applications. This involved both
freely corroding and cathodically protected, displacement controlled experiments upon
constant taper, bending specimens which were either notched or welded, ground and post
weld heat treated. High stress range freely corroding results indicated enhanced life
compared to structural steel, whereas at low stress range the opposite was true.
Corrections made to the high strength steel and structural steel data to correspond to
similar test conditions revealed no benefit of enhanced material strength on corrosion
fatigue life. Fatigue life for all cathodically protected specimens exceeded that for the
freely corroding ones, and with one exception testing was terminated prior to failure.
The single cathodically protected specimen failure involved a stress range approximately
one-third the value for other specimens which were run-outs. This suggests that an
environmental cracking process may operate precipitously at relatively low stress
amplitudes (alternately, high R values). Fatigue life for freely corroding notched
specimens was less than for grounded PWHT specimens, the difference increasing with
decreasing amplitude.

This thesis deals with the representation of discrete signals using triangular basis functions. Signals are usually represented by Fourier series expansions where the basis functions are cosine and sine functions which are all mutually orthogonal. The triangular basis functions used here are called TRIC (triangular cosine) and TRIS (triangular sine) functions. The TRIC and TRIS functions are like their cosine and sine function counterparts except that they are linear. The TRIC and TRIS functions are not all mutually orthogonal, though most of them are. A matrix method of representing discrete signals using TRIC and TRIS functions is presented. A discrete triangular transform matrix is developed and a method of deriving this matrix is presented. A Fortran program is written to derive the discrete triangular transform matrix and to prove the reconstruction of several basic functions like impulse, step, pulse and sinusoidal waveforms.

The objective of this study was to determine whether the presence of undissolved calcium hydroxide at the steel interface helped maintain or delay the breakdown of passivity under adverse conditions, such as the presence of chlorides and carbonation. Saturated calcium hydroxide solution was used as an electrolyte in the test cell, and steel specimens were exposed to a range of chloride ion concentration and carbonation. The results indicate that undissolved calcium hydroxide has an important role in the passivation of reinforcing steel. Also, the critical chloride to hydroxide ratio may be more relevant in characterizing the breakdown of passivity than the threshold value of total or soluble chloride present in the electrolyte. It was noted that in the presence of undissolved calcium hydroxide, steel remained passive for as high a chloride ion concentration as 0.54% (by weight of electrolyte).

Experiments have been performed where cylindrical 1018 steel specimens were polarized to -0.900 V.(SCE) in natural sea water at 24 and 3C and with rotation speeds of 0 and 0.83 Hz. The nature of the calcareous deposits which formed was characterized, first, by monitoring current density during the experiments and, second, by post-test SEM investigation of deposit morphology and thickness and by EEDS analysis of deposit composition. The dependence of cathodic current density and film composition upon temperature and exposure time has been established, and the observed trends have been interpreted in terms of (1) the temperature dependence of the solubility limit and relative nucleation and growth rates for Ca and Mg rich phases and (2) possible electrical conductive behavior on the part of Mg rich deposits.

The fatigue response of selected high strength low
alloy steels, that are under consideration for future
offshore structural applications, was investigated. Freely
corroding and cathodically polarized experiments were
conducted under conditions relevant .to these applications.
Cathodic polarization experiments involved testing of A537
Direct Quenched (A537 DQ) steel at different potentials in
the -0.80 V to -1.10 V (SCE) range. The damage to freely
corroding high strength steel was more than on structural
steel for calculations based on the Wirsching wide band
spectrum. Polarization to -0.80 V (SCE) improved the
fatigue life of A537 DQ steel over the freely corroding
life. A definite conclusion with regard to the influence of
different levels of cathodic polarization on the fatigue
life of the steel investigated can not not be drawn based on
the results from this study due to data variability.