Browsing by Subject "Concrete"

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A framework for selecting strategies to impact the success of high volume roadway projects
(Texas A&M University, 2006-08-16) Chabannes, Clayton C.
State Highway Agencies (SHAs) are being forced to focus more on rehabilitation, resurfacing, and reconstruction of existing roadways rather than the construction of new facilities. These activities can create several challenges when they must be conducted on roadways with high traffic volumes. This research identified numerous strategies that can be implemented by SHAs that have the potential to influence the overall success of roadway projects with high traffic volumes. This research also created a framework for when to implement these different strategies. These strategies were identified through an in-depth literature review and through case studies conducted on highway projects that were under construction. Through the case studies the different strategies were documented within the context that warranted their use. Information from the case studies was collected and documented through interviews and site visits. The strategies identified through this research were used to create four matrices that summarize the research findings. A general matrix was created to show the motivating project conditions that warrant the use of each strategy. A public relations matrix was created to display the influence the impacted road user groups have on public relations and information strategies. A traffic management matrix was created to show different types of traffic management strategies and the potential impact they will have on the project. Finally, an interdependency matrix was created to show groups of strategies that are related to each other or require the use of other strategies to be able to influence the success of the project. These matrices could be further developed to create a set of guidelines that could be used by a SHA during the planning phases of a roadway project.
A New Protocol for Evaluating Concrete Curing Effectiveness
(2013-07-22) Sun, Peizhi
Excessive early-age concrete surface moisture evaporation causes many problems of concrete pavements, such as plastic shrinkage cracking and delamination; the use of liquid membrane-forming curing compounds is one of the most prevalent methods to mitigate the issues. However, the present standard test, ASTM C 156-98, ?Standard Test Method of Water Retention by Concrete Curing Materials? has some inherent limitations in assessing the curing effectiveness of concrete. To better apply curing practices and qualify the curing compound, a new evaluation protocol is introduced in this study. The new protocol consists of using measured relative humidity and temperature to calculate an effectiveness index (EI) which serves as an indicator of the effectiveness of curing. Moistures loss and surface abrasion resistance measurements were made on concrete specimen, and were found to have significant correlation with EI, where higher EI were associated with lower moisture loss and higher surface abrasion resistance. EI was also found to be sensitive to ambient wind condition, types of curing compound and the application rate of the curing compound. Dielectric constant (DC) measurements were also made on concrete specimens indicating the free moisture content on the surface concrete. The DC measurements were also found to differentiate the quality of curing under different ambient conditions, with various types of the curing compounds and the w/c of the concrete mixture. The utility of using the new protocol to assess concrete curing compound effectiveness was also evaluated under the field condition. Both EI and DC measurements showed potentials to distinguish the curing quality for concrete pavement construction.
A study of stiffened and unstiffened slab-on ground performance over swelling soil
(Texas Tech University, 1986-05) Tsai, Che-hung
Not available
Advancements in concrete material sustainability : supplementary cementitious material development and pollutant interaction
(2013-05) Taylor Lange, Sarah Clare; Juenger, Maria C. G.; Siegel, Jeffrey A.
Calcined clay and fly ash supplementary cementitious materials (SCMs) used in cement based materials were examined for their chemical and mechanical performance, as well as their pollutant interaction. This dissertation addresses three primary research questions, namely: (i) can zincite additions facilitate the use of calcined clay as SCMs by compensating for reductions in early-age mechanical performance or by compensating for their reduced pozzolanic reactivity, (ii) can cement renders, containing metakaolin calcined clays, be engineered for passive carbon dioxide and ozone removal, and (iii) how do the specific activity and emanation fractions of concrete constituents, including fly ash and metakaolin, as well as assembled concretes impact concrete radon emanation and indoor radon concentrations? The first question relates directly to the development of new, sustainable material options, which can replace a portion of cement in a concrete mixture. Results from the experiments with zincite showed that the treatment method removed the dilution effect that occurs when using less reactive materials to substitute a portion of portland cement, but did not considerably influence mechanical properties. Therefore, zincite additions are not a good means of enhancing the utilization of non-kaolinite clays in concrete. As an integrated system, the latter two questions of this dissertation investigate the interaction between airborne pollutants and the cement based materials containing SCMs. The use of SCMs in render and concrete systems resulted in different pollutant uptake and exhalation behavior, relative to non-SCM control systems. For pollutant uptake, render systems containing metakaolin increased the carbon dioxide ingress while decreasing the ozone uptake. For radon exhalation rates, modeling results demonstrated that concretes without fly ash have a higher probability of containing less total radium and lower radon exhalation rates, when compared to samples with fly ash, assuming an emanation fraction of 5%, as suggested in the literature. Experimental results demonstrated that metakaolin, fly ash and control concretes had emanation fractions of 7%, 9% and 13%, respectively, confirming that (i) an assumed fraction of 5% would underpredict indoor radon concentrations and potential health consequences, and (ii) SCMs can reduce the total concrete emanation fraction. This dissertation demonstrates how the use of sustainable material selections, such as calcined clays and fly ashes, not only influences the microstructure and mechanical performance of the cement based materials, but also alters the interaction of the material with its surrounding environment.
Aggregates in self-consolidating concrete
(2007) Koehler, Eric Patrick; Fowler, David W.
Self-consolidating concrete (SCC) is an advanced type of concrete that can flow under its own mass without vibration, pass through intricate geometrical configurations, and resist segregation. SCC constituent materials and mixture proportions must be properly selected to achieve these flow properties. The effects of any changes in materials or mixture proportions on hardened concrete performance must be considered in evaluating SCC. A research project was conducted to investigate the role of aggregates in SCC. The objectives of this research were to evaluate the effects of aggregate characteristics and mixture proportions on the workability and hardened properties of SCC, to identify favorable aggregate characteristics for SCC, and to develop guidelines for proportioning SCC with any set of aggregates. The research indicated that although SCC can be proportioned with a wide range of aggregates, the selection of favorable aggregates can significantly enhance the economy and performance of SCC. The effects of aggregate grading; maximum size; shape, angularity, and texture; apparent clay content; and packing density were evaluated. The main effect of aggregates larger than approximately 75 [mu]m was found to be on the minimum required paste volume for achieving SCC workability. It was found that dust-of fracture microfines, defined as mineral material finer than approximately 75 [mu]m produced during the crushing of aggregates, could be an economical choice to comprise part of the paste volume. Based on the results of this research, a mixture proportioning procedure for SCC was developed. The procedure is based on a consistent, rheology-based framework and was designed and written to be accessible and comprehensible for routine use. In the procedure, SCC is represented as a suspension of aggregates in paste. Aggregates are selected on the basis of grading, maximum size, and shape and angularity. The paste volume is set based on the aggregate characteristics in order to achieve workability requirements. The paste composition is established to achieve workability and hardened property requirements.
An analysis of practices for concrete bridge deck durability
(Texas Tech University, 2000-12) Yasmin, Ashfia
With increasing roadway congestion, there is increasing pressure to open newly constructed concrete bridge decks to full traffic early. Generally, highway agencies open bridge decks as soon as possible to traffic without sacrificing long-term durability of the structure. Thus, there is a ''critical loading age" at which time applied design loads have no applicable detrimental effect on the long-term durability of concrete bridge deck. Currently, there are no specific methods to determine this critical loading age for different concrete mixes or for site cast concrete. So, difficulties arise when one attempts to (a) define this critical loading age for a number of concrete mixes and (b) determine when site cast concrete actually reaches this critical loading age. This thesis analyzes the current practice for loading age of concrete. Typically, after a minimum number of days of wet mat curing and a given compressive strength (f'c) are reached, traffic is allowed on a concrete bridge deck. In Texas, the requirement for wet mat curing is 8 days for decks with Type I cement and 10 days for decks with Type II or I/II cement or mixtures containing fly ash. The most recent Special Provision to Item 420, "Concrete Structures," effective 9-98, allows opening to traffic after design strength is achieved and concrete surface treatment has been applied. This allows opening to all traffic after specification requirement of 8-10 days wet mat curing plus a day for the surface to dry and a day to apply concrete surface treatment, for a total of 10 to 12 days. In the past, the minimum number of days before full traffic load has been 30 days is 1982 specification and 21 days, as stated in the 1995 specification. The longer time of loading was preferred because of concerns that the concrete would reach it critical loading age at some point before opening the deck to traffic. The main problem the concrete shows at the early age if it is open to traffic earlier than the critical loading age is the appearance of micro-cracks. Significant efforts have been made to develop concrete mix designs to improve early strength and durability of concrete in recent years. However, the minimum number of wet mat curing days to achieve sufficient concrete durability is not known yet.
Analysis and response mechanisms of blast-loaded reinforced concrete columns
(2009-05) Williams, George Daniel; Williamson, Eric B., 1968-; Williamson, Eric B., 1968-
Terrorism has been an international threat to high occupancy civilian structures, government buildings, and military installations for many years. Statistical data from past terrorist attacks show that transportation infrastructure has been widely targeted, and a bombing of an ordinary highway bridge is a realistic scenario. Recent threats to bridges in the U.S. confirm this concern and have caught the attention of the bridge engineering community. Given that many ordinary highway bridges in the United States support critical emergency evacuation routes, military transportation plans, and vital economic corridors, the loss of a key bridge could result in severe national security, economic, and socioeconomic consequences. Therefore, in this research, a simplified procedure is developed to predict blast loads on bridge columns, and an understanding of the mechanisms that cause damage and ultimately failure of blast-loaded reinforced concrete bridge columns is advanced. To that end, computational fluid dynamics models are constructed and validated using experimental data. These numerical models are used to characterize the structural loads experienced by square and circular bridge columns subjected to blast loads, which is followed by the formulation of a simplified load prediction procedure. Additionally, nonlinear, three-dimensional, dynamic finite element models of blast-loaded reinforced concrete bridge columns are developed and validated using qualitative and quantitative data from recent experimental tests. The results of these analyses illustrate the fact that circular columns cannot be assumed to experience less base shear demand than a square column simply because they experience less net resultant impulse. Furthermore, the column response models developed in this research are used to identify and explain the mechanisms that lead to the spalling of side cover concrete off blast-loaded reinforced concrete members observed in recent experimental tests. Therefore, the results of this research advance the understanding of the structural loads on and the resulting response of reinforced concrete bridge columns subjected to blast loads, and as such these contributions to the structural engineering community enhance the security of the U.S. transportation infrastructure.
Assessment of long-term corrosion resistance of recently developed post-tensioning components
(2012-08) Moyer, Kevin Lee; Breen, J. E. (John Edward), 1932-; Wood, Sharon L.; Wheat, Harovel G.
The forensic analysis of fourteen post-tensioned beam specimens after six years of aggressive exposure testing is the focus of this thesis. Funding for this research came from TxDOT and FHWA. Current post-tensioning materials and construction practices have been deemed inadequate due to fairly recent corrosion failures. Recently developed post-tensioning components and systems were assessed to determine their suitability to prevent durability concerns that had been found in older structures. Testing was conducted on the following variables: Strand Type, Duct Type, Duct Coupler Type, Anchorage Type, Electrically Isolated Tendons. Non-destructive and destructive testing methods were used to study the specimens and were evaluated on their effectiveness in predicting corrosion. Service life analysis was done on a structure using the strands and ducts study in the project. Galvanized duct showed substantial pitting and area loss. The majority of the plastic ducts had no observed damage. However, tendon grout chloride concentrations in most cases were extremely elevated with both galvanized and plastic ducts. This indicated that moisture had entered the duct, through either the couplers and/or grout vents. Except for strands from one specimen, the strands had minor corrosion with occasional mild pitting. The exception had heavy mild pitting confined to a small portion of the strand due to a hole in the duct. Backfill quality was good but it did not bond well with the base concrete. Therefore, moisture and chlorides entered the anchorage region. The electrically isolated tendon did not perform as well as expected. The grout chloride concentrations and level of corrosion damage were comparable to the concentrations and corrosion damage from the more conventionally protected specimens.
Behavior of the shear studs in composite beams at elevated temperatures
(2015-12) Dara, Sepehr; Engelhardt, Michael D.; Helwig, Todd A; Williamson, Eric B; Ghannoum, Wassim M; Ezekoye, Ofodike A
In order to improve the fire safety and at the same time to provide more economical design of composite floors in fire, it is important to understand the behavior of these systems under fire exposure. An important step needed to reach this goal is to better understand the behavior of shear studs in composite beams at elevated temperatures, which was the focus of this research study. Typically, corrugated metal decks are used in construction of composite beams. These decks act as formwork and provide reinforcement for the concrete. For this study, however, the corrugated deck was not included. Rather, this study focused on cases where there is a solid concrete slab over the steel beam. The purpose of this limitation was to first gain a thorough understanding of shear stud behavior under fire exposure for this simpler configuration. This study on shear stud behavior at elevated temperature in solid slabs included both experiments and numerical simulations. The objective of the experimental test was to develop additional data on the load-slip behavior of shear studs in solid concrete slabs at elevated temperatures, and to compare the measured shear stud strength values with the limited test data and code provisions available in the literature. Two different specimen heating scenarios were introduced. One was meant to result in a temperature gradient in the specimen to simulate a fire condition. The other scenario was meant to result in a uniform temperature throughout the specimen for comparison purposes with the other scenario. One of the conclusions was that the shear stud strength and initial stiffness in the shear stud load-slip behavior have strong correlations with bottom of stud temperature, regardless of the heating scenario. Therefore, choosing the bottom of stud temperature as a reference temperature in predicting the shear stud ultimate strength and initial stiffness is reasonable. The objective of the numerical simulations was to develop a finite element (FE) model which can predict the thermal and mechanical behavior of shear studs in solid concrete slabs at elevated temperatures, and to validate the model against the experimental data. Different aspects of modeling the specimen using the general purpose finite element software, Abaqus, were discussed. Results of the analyses were compared with the experimental results of this study. Temperatures resulting from the heat-transfer analysis were found to be in a good agreement with experimental results at some locations in the specimen. However, at some other locations the difference between the experimental and FE results were more than 100 ºC. The existing level of uncertainty in the input data highly contributes to the errors in the temperature results, and emphasizes the difficulty that exists in heat transfer modeling. The load-slip curves found from FE analysis were presented for all the tests. The ultimate strength and the initial stiffness of the specimens were predicted well by the FE analyses. However, the slip capacity did not match between the experiments and FE analyses. Several parametric studies using the finite element model were conducted to investigate the sensitivity of the analysis results to various model parameters, both for heat transfer analysis and structural response analysis. The studied parameters included thermal conductivity of concrete, convective heat transfer coefficient, resultant emissivity, thermal joint conductance coefficient, Concrete Damaged Plasticity model parameters, steel stress-strain curves recommended by two different code provisions, and concrete tensile strength. The current gaps in our knowledge about these parameters were discussed.
Characterization of fly ash for evaluating the alkali-silica reaction resistance of concrete
(2012-12) Jasso, Andres Jose; Folliard, Kevin J.; Ferron, Raissa D
Fly ash has been used extensively to control deleterious alkali-silica reaction in concrete. The majority of fly ashes can be used to control ASR induced expansion. Fly ashes with high CaO contents are less effective at reducing expansion and fly ashes with high alkali contents can be counter active. Class C fly ashes are less effective at reducing the pH of the pore solution because they are less pozzolanic. The pozzolanic reaction in Class F fly ashes enhances the ability for the hydration products to bind alkalis. This prevents the availability of these alkalis for ASR. This project aims to characterize fly ash in a way that best predicts how it will perform in concrete with an emphasis on ASR. Fly ashes with a variety of chemical compositions were evaluated using a range of analytical and characterization techniques. Research data from several universities were used to correlate their long term data with this project’s accelerated tests. This research aimed at evaluating the mineralogical, chemical, and physical characteristics that most affect the ability of a given fly ash to prevent ASR-induced expansion and cracking.
Comparison between epoxy-coated steel and glass fiber reinforced polymer bars in a concrete highway bridge deck
(Texas Tech University, 2002-12) Bice, Jacob
The use of fiber reinforced polymer (FRP) bars as reinforcement in concrete highway bridges potentially provides a means that can extend the useful life of the bridges in selected cases. Resistance of glass fiber reinforced polymer (GFRP) rebars to corrosion associated with the use of de-icing salts on bridges makes these bars particularly attractive to the transportation industry. However, questions concerning the bars' relatively low modulus of elasticity, bond slip properties, and in situ corrosion resistance must be answered prior to widespread implementation in highway bridges. The Sierrita de la Cruz Creek Bridge near Amarillo, Texas provides a means of making a direct comparison between the performance of an epoxy coated steel (ECS) reinforced bridge deck and a fiber reinforced polymer (FRP) reinforced bridge deck. Short-term comparisons are presented in this thesis, and the long-term monitoring potential of the two bridge sections is also discussed. With the exception of a longitudinal crack that formed in the cast-in-place bay in the GFRP-reinforced span, this thesis found that the GFRP-reinforced sections perform almost identically to the steel-reinforced sections. The research performed for this thesis also determined that the FRP may be exposed to internal temperatures which may cause long-term deterioration of the FRP bars. The data collected in the live load tests set a benchmark for any future live load testing performed on the bridge. Long-term monitoring of the bridge will be required to assess the durability of the GFRP bars in the concrete bridge deck.
Concrete: Hill Country home
(Texas Tech University, 1990-05) Spiva, Scott
Nature has thousands of elements that are essential in the success of its biological phenomenon, and human life plays just a small role. Each biological niche has many elements that work together, and each element has its own order of elements. Living off the sun's light and energy; breathing the earth's air; consuming natural resources of animal, vegetable, and mineral, is a biological phenomenon and provides a structure of our place in the phenomenon. All of the elements of architecture must work, as nature does, in a smooth flowing manner to create a workable used architecture that is an excellent example of design. Just as most buildings reflect nature, they must also respond to our phenomenon of human life and user needs. Architecture needs to be environmentally responsive to its local site, climate, and economy, and this responds through a reflection of nature. Natural lighting in a majority of interior spaces becomes important in design as a psychologically comforting element. Outdoor spaces should be an integral part of overall design and help bring together a design by making the outside bring about an intrigue to the interior. An observation of nature shows that the basis of life in each organism is its structuring elements, and thus, structure becomes a basis of design. Architecture is a participant in the human environment. When architecture is natural in its origin, it is environmentally responsive, for without a concern for the environment, the design's natural forms become hypocritical. Natural lighting, fresh air circulation, and natural materials are also of importance. Nature, in design, ties together the parts in a whole.
Determination of aggregate shape properties using X-ray tomographic methods and the effect of shape on concrete rheology
(2005) Erdoğan, Sinan Turhan; Fowler, David W.; Garboczi, Edward J.
The shape of aggregate particles can significantly influence certain properties of concrete, both in its fresh and hardened states. Therefore, there is a need to be able to completely characterize the shape of aggregate particles, in three-dimensions, in order to develop computational models which accurately predict properties. In the past, numerous methods have been suggested for this task; however these methods are often only applicable to two-dimensional images of particles, they output a single or a few values, and fail to characterize the true shape of the particle. X-ray tomographic techniques allow the capturing of the true shape of particles and have been applied to concrete aggregates. Computed tomography has been used to characterize coarse and fine aggregate particles, while X-ray microtomography has been used to characterize particles passing the 75µm sieve. Sample preparation methods and scanning parameters applicable to concrete aggregates have been developed. The spherical harmonic method was used to efficiently store shape information, and to calculate useful parameters for individual particles, such as volume and surface area. Comparisons of the results to properties determined using other techniques were made and it was determined that the results of indirect or two-dimensional shape and size characterization methods can be misleading. The shapes of aggregate particles particularly influence the rheological properties of concrete mixtures. However, it is not clear to what degree different-scale shape properties (the overall shape, angularity and texture) influence flow separately. Artificial aggregates were prepared in the laboratory and simplified test cases were chosen to independently investigate the effect of overall shape and surface texture on the yield stress and plastic viscosity of mixtures and to obtain a set of results which could be used to calibrate computational models. These tests revealed that the overall shape of coarse aggregate particles significantly influences the plastic viscosity of a mixture, but does not affect the yield stress visibly. Particle surface texture does not seem to noticeably influence either viscosity or yield stress, for the cases tested. The results were also used to verify the “Dissipative Particle Dynamics” model and showed good correlation with the predictions.
Development of a Reaction Signature for Combined Concrete Materials
(2010-07-14) Ghanem, Hassan A.
Although concrete is widely considered a very durable material, if conditions are such, it can be vulnerable to deterioration and early distress development. Alkali-Silica Reaction (ASR) is a major durability problem in concrete structures. It is a chemical reaction between the reactive silica existent in some types of rocks and alkali hydroxides in the concrete pore water. The product of this reaction is a gel that is hygroscopic in nature. When the gel absorbs moisture, it swells leading to tensile stresses in concrete. When those stresses exceed the tensile strength of concrete, cracks occur. The main objective of this study was to address a method of testing concrete materials as a combination to assist engineers to effectively mitigate ASR in concrete. The research approach involved capturing the combined effects of concrete materials (water cement ratio, porosity, supplementary cementitious materials, etc.) through a method of testing to allow the formulation of mixture combinations resistant to ASR leading to an increase in the life span of concrete structures. To achieve this objective, a comprehensive study on different types of aggregates of different reactivity was conducted to formulate a robust approach that takes into account the factors affecting ASR; such as, temperature, moisture, calcium concentration and alkalinity. A kinetic model was proposed to determine aggregate ASR characteristics which were calculated using the System Identification Method. Analysis of the results validates that ASR is a thermally activated process and therefore, the reactivity of an aggregate can be characterized in terms of its activation energy (Ea) using the Arrhenius equation. Statistical analysis was conducted to determine that the test protocol is highly repeatable and reliable. To relate the effect of material combinations to field performance, concrete samples with different w/cm?s and fly ash contents using selective aggregates were tested at different alkalinities. To combine aggregate and concrete characteristics, two models were proposed and combined. The first model predicts the Ea of the aggregate at levels of alkalinity similar to field conditions. The second model, generated using the Juarez- Badillo transform, connects the ultimate expansion of the concrete and aggregate, the water cement ratio, and the fly ash content to the Ea of the rock. The proposed models were validated through laboratory tests. To develop concrete mixtures highly resistant to ASR, a sequence of steps to determine threshold total alkali in concrete were presented with examples. It is expected that the knowledge gained through this work will assist government agencies, contractors, and material engineers, to select the optimum mixture combinations that fits best their needs or type of applications, and predict their effects on the concrete performance in the field.
Development of rapid, cement-based repair materials for transportation structures
(2013-05) Zuniga, Jose Ricardo; Folliard, Kevin J.
The deterioration of today's infrastructure particularly roadways and bridge decks has continued to increase over the years due to the larger axle loads, higher traffic volumes of densely populated cities. These highly congested areas have required the need to repair and rehabilitate the affected pavements in a timely manner with minimal traffic interruptions. Different rapid hardening binders were tested in this project to evaluate and characterize their performance when subjected to concrete distresses such as alkali-silica reaction, delayed ettringite formation, corrosion, freezing and thawing, salt scaling, sulfate attack, material incompatibility and volume changes. Among the cements tested were calcium aluminate cement, calcium sulfoaluminate cement, accelerated portland cement, alkali-activated fly ash, and three other proprietary blends available to the public. This thesis will summarize the preliminary findings of a comprehensive laboratory study focusing on rapid repair materials -- the final results of this study will be included in future publication (theses and final project report).
Durability testing of rapid, cement-based repair materials for transportation structures
(2014-05) Garcia, Anthony Michael; Folliard, Kevin J.; Drimalas, Thanos, 1980-
For repairing concrete transportation infrastructure, such as pavements and bridges, much importance is placed on early-age strength gain as this has a major impact on scheduling and opening to traffic. However, the long-term performance and durability of such repair materials are often not satisfactory, thus resulting in future repairs. This research project focuses on the evaluation of the durability of various rapid-setting cementitious materials. The binders studied in this project include calcium aluminate cement (CAC), calcium sulfoaluminate cement (CSA), Type III portland cement, alkali-activated fly ash (AAFA) , and various prepackaged concrete materials. In addition, selected CAC and CSA mixtures were further modified with the use of a styrene-butadiene latex. The durability aspects studied include freezing-and-thawing damage and the implications of air entrainment in these systems, alkali-silica reaction, sulfate attack, and permeability of the concrete matrix and potential corrosion.
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.
Effect of Portland cement concrete characteristics and constituents on thermal expansion
(2014-08) Siddiqui, Md Sarwar; Fowler, David W.; Juenger, Maria W; Bhasin, Amit; Won, Moon; Wheat, Harovel G
The coefficient of thermal expansion (CTE) is one of the major factors responsible for distresses in concrete pavements and structures. Continuously reinforced concrete pavements (CRCPs) in particular are highly susceptible to distresses caused by high CTE in concrete. CRCP is a popular choice across the U.S. and around the world for its long service life and minimal maintenance requirements. CRCP has been built in more than 35 states in the U.S., including Texas. In order to prevent CRCP distresses, the Texas Department of Transportation (TxDOT) has limited the CTE of CRCP concrete to a maximum of 5.5 x10-6 strain/oF (9.9 x10-6 strain/oC). Coarse aggregate sources that produce concrete with CTE higher than the allowable limit are no longer accepted in the TxDOT CRCP projects. Moreover, CTE is an important input in the Mechanistic-Empirical Pavement Design Guide (MEPDG). Small deviations in input CTE can affect the pavement thickness significantly in MEPDG designs. Therefore, accurate determination of concrete CTE is important, as it allows for enhanced concrete structure and pavement design as well as accurate screening of CRCP coarse aggregates. Moreover, optimizing the CTE of concrete according to a structure’s needs can reduce that structure’s cracking potential. This will result in significant savings in repair and rehabilitation costs and will improve the durability and longevity of concrete structures. This study found that the CTEs determined from saturated concrete samples were affected by the internal water pressure. As a result, the TxDOT method yielded higher values than did the American Association of State Highway and Transportation Officials (AASHTO) method. To further investigate the effect of internal water pressure, an analytical model was developed based on the poroelastic phenomenon of concrete. According to the model, porosity, permeability, and the rate of temperature change are the major factors that influence the internal water pressure development. Increasing the permeability of concrete can reduce the internal water pressure development and can thus improve the consistency of measured CTE values. Preconditioning concrete samples by subjecting them to several heating and cooling cycles prior to CTE testing and reducing the rate of temperature change improved the consistency of the CTE test results. Concrete CTE can be reduced by blending low-CTE aggregates with high-CTE aggregates and reducing the cement paste volume. Based on these findings, a concrete CTE optimization technique was developed that provides guidelines for the selection of concrete constituents to achieve target concrete CTE. A concrete proportioning technique was also developed to meet the need for CTE optimization. This concrete proportioning technique can use aggregate from any sources, irrespective of gradation, shape, and texture. The proposed technique has the potential to reduce the cement requirement without sacrificing performance and provides guidelines for multiple coarse and fine aggregate blends.
Effects from Alkali-Silica Reacton and Delayed Ettringite Formation on Reinforced Concrete Column Lap Splices
(2012-07-16) Eck, Mary
Reinforced concrete bridge columns can deteriorate prematurely due to the alkali-silica reaction (ASR) and/or delayed ettringite formation (DEF), causing internal expansion and cracking on the surface of the concrete. The performance of the longitudinal reinforcement lap splice in deteriorated concrete columns is the focus in this research. This thesis presents the results from the deterioration of large-scale specimens constructed and placed in an environment susceptible to ASR/DEF deterioration, the experimental results from four-point and three-point structural load tests, and an analytical model based on bending theory characterizing the specimen behavior during the structural load tests. Fourteen large-scale specimens were constructed, placed in an environment to accelerate the ASR/DEF deterioration mechanisms, and instrumented both internally and externally to measure the internal concrete expansions, and surface expansions and crack widths. In addition, two control specimens were constructed and kept in a laboratory, preventing ASR/DEF deterioration. Post-tensioning was used to simulate axial load on a bridge column. Structural load tests were performed on eight specimens with no ASR/DEF damage to late stage ASR and minimal DEF damage. Comparing the specimen behaviors during the loading testing, it was found that the yield strength increased about 5-15%, and post-cracking stiffness up to first yielding of the deteriorated specimens was about 25-35% stiffer than the control specimens. The increased specimen strength and stiffness likely occurred from volumetric expansion due to ASR/DEF damage which engaged the reinforcement, further confining the concrete and causing a beneficial increase in the axial post-tensioning load. The analytical model matched the control specimens well and matched the non-control specimens when the axial load was increased.
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.