Hartt, William H.

In previous research, cements with high alkali content (EqA 1.0-1.2 percent) extended the
corrosion initiation time of reinforcing steel in concrete. During this study, laboratory
tests were performed to determine the suitability of high alkalinity cements to improve
concrete durability without modifying physical properties and to control the risk of
alkali-aggregate reaction (AAR).
A mix design for the FOOT-Class V concrete served as base material for this study. On a
cubic meter basis the cementitious material in this concrete included 363 kg of Type l/ll
Portland cement and 83 kg of Class F fly ash. The water-to-cementitious material ratio of
the concrete was 0.40. The fine aggregate used in the experimental concretes was quartz
sand from a Florida source with no history of alkali-silica reactivity (ASR) susceptibility. A number of cement alkali contents were prepared by different additions of sodium
hydroxide to the concrete mix (3.42 - 4.57 kglm\ in some cases, and by using different
cements in others. Thus, effects on concrete susceptibility to ASR, electrical resistivity,
and strength were studied. Pore water alkalinity was measured by ex-situ leaching and
pore water extraction methods. It was concluded that leaching procedures were not
appropriate to determine concrete pore water alkalinity in the presence of fly ash.
Results suggested that it is feasible to use high alkali cement without the risk of ASR or
the loss of strength for two of the seven coarse aggregates studied, given that
supplementary cementitious materials and lithium nitrate admixtures are utilized. Criteria
for qualification of a concrete as being ASR resistant was based on dimensional stability
(less than 0.01% average specimen length change) and the absence of cracking over the
one and two year exposure periods according to ASTM Cl293.
Based on the fundamentals of the electric double layer theory, the incidence of bivalent
cations adjacent to the surface of cement hydrates and reactive silica particles was
proposed to provide an explanation for the effects of alkali addition on the electrical
resistivity of concrete and the source of the expansive nature of the ASR gel.

Effects of reinforcement and coarse aggregate on chloride ingression into
concrete and reinforcement corrosion initiation have been studied with experimental
and modeling (finite element method) analyses. Once specimens were fabricated
and exposed to a chloride solution, various experimental techniques were employed
to determine the effect of reinforcement and coarse aggregate on time-to-corrosion
and chloride ingress and concentration at corrosion locations. Model analyses were
performed to verify and explain the experimental results. Based upon the results, it
was determined that unexpectedly higher chloride concentrations were present on
the top of the rebar trace than that to the side at the same depth and an inverse
concentration gradient (increasing [Cl-] with increasing depth) occurred near the top
of rebars. Also, coarse aggregate volume profile in close proximity to the rebar and
spatial distribution of these aggregates, in conjunction with the physical obstruction
afforded by reinforcement to chloride flow, complicates concrete sampling for Cl- intended to define the critical concentration of this species to initiate corrosion.
Modeling analyses that considered cover thickness, chloride threshold concentration,
reinforcement size and shape, and coarse aggregate type and percolation confirmed
the experimental findings. The results, at least in part, account for the relatively
wide spread in chloride corrosion threshold values reported in the literature and
illustrate that more consistent chloride threshold concentrations can be acquired
from mortar or paste specimens than from concrete ones.

Critical chloride threshold, CT values for initiation of reinforced steel corrosion m
mortar typical of Florida coastal bridge substructures were determined in laboratory tests.
Previous research has reported CT values that vary by more than an order-of-magnitude,
making design life estimation for structures difficult. On this basis, experiments on
piling type specimens focused on [Cl-] contamination in the splash zone and coupling of
this steel to a large surface area submerged anode. The lower portion of simulated piling
was immersed and the region above the waterline periodically sprayed with NaCI
solution. Corrosion potential with respect to height above the waterline was monitored.
A temporary depolarization method for determining as to whether or not corrosion had
initiated is proposed. Chloride distribution at the reinforcement-concrete interface was
determined in piling using energy dispersive x-ray analysis and related to height with
respect to the waterline. Chloride threshold was related to corrosion potential.

The penetration of chloride ions through concrete can compromise the integrity of
a structure. The chloride concentration, [Cl-], at which the corrosion process initiates is
termed the critical chloride concentration or chloride threshold, [Cl-]th. One of the
purposes of this research was to determine the [Cl-]th for various reinforcing alloys.
Furthermore, the time-to-corrosion (TIC) was measured to determine the time at which
bars become active. Both parameters, [Cl-]th and TTC, were found to be distributed;
therefore, statistical analyses were performed to forecast the probability of activity. A
new experimental procedure was introduced to increase the TTC data set by electrically
isolating the top bars as they became active.
The research also compared the [Cl-] for core samples with those values obtained
from along the top rebar trace. In general, this analysis demonstrated that core sample
[Cr] was lower than at the rebar-trace.

Experiments have been performed which determined the fatigue crack growth rate (FCGR) of short cracks (a > 0.1mm) for five high strength steels (yield stress 370-570 MPa) in air and in natural seawater with and without cathodic protection. Attention was focused upon Regions I and Il of the classical FCGR-stress intensity range(Delta K) curve with particular consideration of the near-threshold behavior for short cracks. Single edge notch (SEN) three-point bend specimens and a direct current potential drop (DCPD) crack monitoring system were employed, and test parameters simulated offshore structure conditions. The results indicated enhanced FCGR for short cracks compared to macrocracks by 3-20 times in air and 2-6 in seawater free-corroding(FC). Also, the Delta Kth for short cracks was apparently lower than for long ones in both environments. The transition from short to long crack behavior occurred at constant $\Delta$K in each environment (15.6 MPa m in air and 10.0 MPa m in seawater(FC)) irrespective of initial Delta K (Delta K(0)). The transition crack length ranged from 0.25 to 1.6 mm and was inversely proportional to $\Delta$K(0). Scanning electron microscope fractography showed that the mechanism of enhanced crack growth rate was associated with secondary crack (SC) formation in air and SC or inter-granular cracking (or both) in seawater (FC). The enhanced FCGR for short cracks was minimized by polarization to -950 mV(SCE). Through an elastic-plastic fracture mechanics analysis using the J-integral parameter it was found that the influence of plastic deformation at the crack tip was approximately independent of crack length (short versus long), and the linear-elastic fracture mechanics analysis gave a realistic representation for fatigue behavior.

The present research focused on the behavior of arc sprayed zinc and zinc-aluminum coatings on concrete specimens exposed to specific relative humidity environments (100, 85, 60 and 25% RH) and with specific chloride contents (0.0, 3.0, 5.9 and 11.8 kg/m^3). All specimens experienced a decline in current output with time. Anode wastage and formation of oxidation products were mainly responsible for this lack of protection in 100 and 85% RH, whereas for 60 and 25% RH, drying of the concrete and long-term polarization of the anode were the key factors. Validity of the DC measurements was verified with slope parameter analysis on selected specimens. In addition, Electrochemical Impedance Spectroscopy was performed on the galvanic anode and an equivalent circuit model was obtained for the zinc-concrete interface.

This report describes the results obtained from reinforced concrete slabs having different fly ash and silica fume content. The specimens are submitted to periodic seawater ponding. PH measurements as well as free and total chloride analyses were achieved at 1213 days in order to study the alkalinity, resistance to chloride ingress and binding properties afforded by each mix design. Water absorption experiments were also conducted at different relative humidities and in water. Pore water pH was found to decrease with increasing admixture content and increasing relative humidity. The permeability to chloride ions was significantly reduced in the fly ash blends whereas most of the silica fume blends exhibited effective diffusion coefficients and chloride concentrations at the depth of steel marginally better than the controls. Both the fly ash and silica fume blends showed similar binding capacity at a given replacement ratio, the percentage of bound chloride increasing with increasing admixture content.

An analytical model for cathodic protection (CP) attenuation along a one-dimensional structure such as a pipeline is developed in terms of non-dimensional polarization and, alternately, cathode potential. The derivation is based upon the fundamental principle that (1) potential is, in fact, the potential difference across the metal/electrolyte interface, (2) an assumption of a linear potential and current density relationship, and (3) the treatment of Galvanic anodes as spherical current sources. The resulting expression characterizes the distribution of protection in terms of physical design variables and electrochemical properties of the cathode. Polarization is presented graphically in terms of two parameters which are comprised of the aforementioned variables and make for a novel method of analysis. Two sample pipeline CP systems are analyzed by this method, one of which is an existing design and the other a hypothetical design used to illustrate the effect of the varying parameters. Some sample anode life calculations based upon the analytical model are performed, and a method for correcting the model for the existence of a non-spherical bracelet anode is proposed. It is concluded that electrolyte resistance (or alternately anode resistance) is of greater importance in all practical Galvanic CP attenuation calculations than is the metallic (Ohmic) resistance.

A study was performed for the purpose of developing a non-destructive method to evaluate corrosion damaged pre-stressing steel in jacketed concrete piles by using a cover meter. The procedure first involved taking cover meter readings with the pile jacket in place. Subsequently, a portion of the jacket was removed and tendon diameter measurements were taken. A relation for normalized tendon diameter as a function of normalized cover was determined, where normalized cover increased with a decrease in normalized tendon diameter. Finally, it is projected that the cover meter is a useful non-destructive tool to estimate the remaining tendon diameter in the inspection of jacketed, prestressed concrete piles and to this end to characterize the residual structural integrity of piles.

Pretensioned concrete members require a certain bond strength between the steel and concrete to maintain prestress force. Previous studies have indicated that when reinforcing steel in concrete is cathodically polarized, a chemical change of the cement near the steel-concrete interface occurs; and this can reduce the bond strength. In the present research experiments were conducted with concrete specimens that contained either a non-stressed seven wire steel tendon or a single strand through the longitudinal direction. The specimens were cathodically polarized with current densities ranging between 50 and 2500 mA/m^2 of steel. Upon achieving a pre-determined charge density transfer, the steel was pulled relative to the concrete until the bond was broken. Results indicate that a total charge density transfer of up to 14000 A*h/m^2 of steel, may introduce an average 16 percent decrease in ultimate bond strength. This and other data were evaluated in order to assess if cathodic protection, as utilized for corrosion control, is likely to compromise structural integrity of pretensioned concrete members and structures.