Suarez, Jorge Alejandro

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.

For the purpose of investigating the cause of duct cracking in post-tensioning tendons and improving the durability of this type of structure, a series of experiments was performed. These were divided in two groups, the first to simulate the state of stress inside the tendon ducts and the second to compare the properties of new materials approved according to new regulations on the subject and those used in the construction of the structures. A proposed mechanism of cracking was studied and its validity verified. According to simulations performed on specimens that were fabricated and others developed by the finite element method, new major factors were found to explain the duct failure. Finally, a comparison of mechanical properties indicated a difference between virgin and aged materials.