Browsing by Subject "Concrete -- Permeability"

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Chloride ion transport in bridge deck concrete under different curing durations
(Texas Tech University, 2004-08) Ghanem, Hassan Akram
The durability of concrete is usually measured by the number of years of maintenance- free service it provides. If a correct prediction has been made of the environmental conditions to which the concrete will be subjected to, concrete can be made to last for its intended service life. The durability of concrete is closely related to its permeability. The permeability dictates the rate at which aggressive agents can penetrate the concrete and attack the steel reinforcement. During winter, ice accumulates on the top surface of concrete slabs and bridge decks. For the purpose of removing the snow and ice, de-icing agents, such as sodium chloride and calcium chloride, are applied. These salts migrate down to the reinforcing steel through small pores in the concrete. Over time, the chlorides in these salts can react with the reinforcing steel, breaking down the passive layer and causing the steel to corrode. In Texas, the specification requirement for wet-mat curing is 8 days for decks with Type I and III cements and 10 days for decks with Type II and I/II cements and for mix designs with fly ash. Once the concrete is cured sufficiently, the deck is allowed to dry for one day. An additional day is typically allowed for any required surface treatment. The main objective of the research is to explore the possibility of opening the bridge decks earlier than 10 to 12 days by decreasing the number of wet mat curing days. For this reason, concrete mixtures typically used in Texas bridge deck were evaluated using numerous tests such as the Ponding test, the Rapid Chloride permeability Test, and the Abrasion Test. This research focuses on the effect of curing on the chloride ion permeability of different mix design. It is expected that the knowledge gained through this work can be used to optimize the number of curing days and age required before full traffic can be allowed on bridge decks in Texas.
Effect of curing on permeability and freeze-thaw durability of bridge deck concrete
(Texas Tech University, 2002-05) Afroze, Montasheema
Durability of bridge deck concrete may be defined as the ability of concrete to remain fully functional over an extended period under prevailing service conditions for the purpose for which it was designed. The durability of concrete structure is closely related to its permeability. The permeability dictates the rate at which aggressive agents can penetrate to attack the concrete and the steel reinforcement. Also, the most destructive weathering agent is the freezing and thawing of water in the concrete. During cold weather, pavements and bridge decks are subjected to freezing and thawing. For the purpose of removing the snow and ice, de-icing agents are applied, which causes the surface disintegration in the form of scaling and consequently, the chemicals accelerate the corrosion of reinforcement. This thesis is a part of a research project titled "Effects of Wet Mat Curing and Earlier Loading on Long-Term Durability of Bridge Decks" sponsored by Texas Department of Transportation (TxDOT). The project investigates the long-term effects of wet-mat curing duration and "early" loading on the durability of bridge decks. This thesis outlines the effect of curing on the chloride-ion Permeability and freeze-thaw resistance of different mix designs.
Effects of curing on shrinkage cracking in bridge deck concrete
(Texas Tech University, 2003-12) Aamidala, Hari Shankar G
Shrinkage cracking is a critical factor affecting the durability of concrete bridge decks. Since cracks in reinforced concrete decks provide a path for corrosive agents to enter the concrete, thereby accelerating the deterioration of the reinforcing steel. Concrete undergoes volumetric changes due to variation in temperature and moisture. When concrete is prevented by a surrounding structure from undergoing these volumetric changes freely, tensile stresses are developed. Depending on the amount of restraint applied, these tensile stresses potentially lead to cracks in the structures. This type of cracking can be reduced by providing rebars in the form of mesh to take the tensile stresses and/or the use of shrinkage-reducing admixtures (SRAs). Recent studies performed on SRAs show that the use of such admixtures is an effective method to reduce shrinkage cracking. Simulating restraint experienced in highway pavements and bridge decks is a challenge to researchers. This challenge has led to the development of different methods to assess the potential of restrained shrinkage cracking. Currently, there exists no standard test to assess cracking due to restrained shrinkage, though several researchers have simulated the effects of restraint in various ways to better understand the behavior of concrete shrinkage. The objective of this research is the evaluation of the different concrete mix designs used in bridge decks in Texas. Specifically, the influence of curing duration on the cracking potential of concrete bridge decks has been investigated based on the unrestrained (free) linear shrinkage measurements of prismatic specimens. In addition to free shrinkage tests, tests have been performed to study the weight loss, modulus of elasticity and split tensile strength of the concrete mix designs. Using the data obtained from these tests, shrinkage stresses developed due to full-restraint (100%-restraint) have been estimated and compared with the split tensile strength to estimate the age at which a first crack can be observed in the concrete. The approach presented assists in better understanding the effect of shrinkage on the cracking of bridge decks under full-restraint. Using the weight loss measurements and weight of oven-dried specimens, internal moisture content was tracked. This data is used to compare the behavior of the material to the development of shrinkage strains. Comparing the different curing durations did not indicate a significant difference in the development of either modulus of elasticity or split tensile strength. However, a favorable correlation was observed between the tensile strength of concrete and shrinkage stresses developed due to full restraint based on different curing durations. The results did not show a significant delay in the age of first crack with increase in curing duration from 4 days to 7 days. Thus, based on the research to date, one can conclude that 4 day curing suffices for durability of bridge decks from a shrinkage cracking perspective.