Concrete--Deterioration

Synthetic fiber reinforced concrete sample sets were exposed to two different environments. One set, of six samples, was exposed to filtered seawater in the lab with wet and dry cycles, while the other set of samples was exposed, on a barge, to the marine environment, in the intracoastal waterways, at SeaTech. The samples were exposed for 8 months, and then removed for experimental and mechanical testing. Upon removal, the barge samples were photographed to observe surface organisms that were attached to each sample. The barge samples, after cleaning, were then exposed to UV light to observe surface bacteria. The barge samples were also taken to Harbor Branch facility for DNA testing, and then sent in for sequencing. This sequencing was used to identify the organisms that were present inside the concrete samples. An Indirect Tensile Strength Test, IDT, was performed on both sets of samples to observe the first crack, max load, and fracture toughness of each sample. The Barge samples had a lower first crack, max load, and fracture toughness, which means that it took less force to break these samples, than the Seawater samples. As the fiber content increased, the Seawater samples grew stronger, while the Barge samples grew weaker. Also, as the fiber content increased, the biodiversity found on the surface of the Barge samples increased as well.

Chloride diffusivity in high performance concrete is influenced by the exposure environment, aside from the concrete mixture properties like, water to cementitious ratio (w/cm) and presence of add-on pozzolans. In this study, a set of concrete specimens (eleven-different concrete mixtures) were cast and exposed to three different environmental conditions (Tidal, Splash and Barge) in which the solution was seawater or brackish water. These exposures simulated environmental field conditions. After the specimens had been wet cured for 32 days (on average), the specimens were exposed to three different field simulation conditions for up to 54 months. The specimens under the field simulated conditions were cored at 6, 10, 18, 30 and 54 months at four elevations and then the chloride profiles were obtained from the cores. The apparent diffusivity values for each profile were calculated based on Fick’s 2nd law. Then, the aging factor “m” was calculated by regression analysis of the diffusivity values vs. time (days) plotted in the log10-log10 scale. This was done for samples exposed to the three different exposure conditions and then the results were compared side-by-side. First, the “m” values were calculated using the exposure duration. Then, to study the effect of including the curing time on “m” value, the curing time was added to the exposure time and a new calculation and “m” value was obtained and compared with the previous results. Moreover, upon inspecting the chloride diffusivity values vs. time plots, it was observed that in some cases, a number of data points showed significantly higher or lower values in comparison with the rest of the data points. It was decided to recalculate the “m” values for these cases, and to only use selected data points instead of all data points (i.e., remove outlier data points). In terms of chloride diffusivity value, it was found that in most cases the specimens with higher water to cementitious (w/cm) ratio showed higher diffusivity, as expected. Further, the presence of pozzolans had a noticeable impact on the chloride diffusivity by decreasing the diffusion rate due to microstructure changes that occurred with time. In terms of “m” values, the result for the field simulated conditions showed a range of “m” values dependent on the specimen’s mixture composition and the elevation at which the specimens were cored. It was observed that the chloride diffusivity declined with time and after a certain amount of time (in this research, almost after 30 months) the diffusivity reduction became small and a transition in the slope of the diffusivity trend appeared in a number of cases. After the transition, the diffusivity trend reached either a plateau zone or continued with a significantly lower slope, depending on the time, composition and exposure. It was found that the specimens under tidal and splash field simulation conditions that had only fly ash in their mixtures showed higher “m” values when compared with samples that contained fly ash and silica fume or fifty percent slag.

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.

Hollow, cylindrical mortar specimens of 0.4 water-cement ratio were prepared without reinforcement and exposed to flowing natural sea water for periods up to one year. Direct currents of 2, 10 and 50 mA were impressed between a mixed metal-oxide titanium substrate electrode positioned within each of these two zones, with a different electrolyte supply and exhaust for the cylinder core and exterior surface. Linear expansion of the specimens was evaluated as a function of exposure duration from the output of embedded strain gages and from dimensional measurement of cylinder length and diameter. It was found that expansion of specimens exposed to direct current exceeded baseline ones (no current). Also, the expansion was anisotropic in that different magnitudes and trends were apparent for the diameter versus length directions. The expansion under free exposure (no current) was determined to be a function of specimen size and of the direction of measurement relative to the cast specimen face.