Response of Reinforced Concrete Building Subjected to Northridge Earthquake

The study of instrumented buildings helps to improve the tmderstanding of how structures respond to earthquakes and to decrease losses due to damage in future earthquakes. Traditional methods for modeling reinforced concrete elements may be used to provide estimates of the building response, and these methods can then be evaluated based on the measured response. This study focuses on the modeling and response modification of a reinforced concrete building designed in 1964 and subjected to the 1994 Northridge earthquake. The significance of the study is the investigation of the response of a reinforced concrete building with poor detailing and subjected to moderate earthquake demands. The structure is a reinforced concrete frame building located in Sherman Oaks, California with 13 stories and 2 sublevels. The building was instrumented with 15 sensors distributed on 5 floors. The maximum drift response of the structure was determined to be 0.7% of the total height of the structure with an associated maximum recorded ground acceleration of 0.23g. The study discusses the challenges of simulating the response of a structure having poorly detailed reinforced concrete columns that is subjected to moderate earthquake demands as well as how to apply a strength reduction factor to study the modification of the structural response

Correlating Nonlinear Static and Dynamic Analysis of Reinforced Concrete Frames

Nonlinear static and dynamic analyses for the design of reinforced concrete frames for strong ground motion are explored in the study. The objectives of the study are to determine 1) the correlation between results from nonlinear static and dynamic analyses, 2) the optimallateralloading distnbution for static analysis, 3) the simplest lateral load distribution that provides adequate results, and 4) the parameters that are reasonably calculated using static analysis for use in design. Parameters included in the study were four number of stories, three frame configurations, four lateral loading distributions for use in static analysis, and ten strong ground motion records for use in dynamic analysis. The key design items were base shear, location of member yielding, column ductility, controlling mechanism, distorted shape of the frame, story drift ratio, and shear and rotation in the members. Results indicated that static analysis provided fair estimates of base shear, general member yielding, distorted shape, and story drift, but gave insufficient estimates of member shear and rotation and the exact location of the controlling mechanism in the frames. The uniform loading distribution best estimated base shear and member shear and rotation, whereas the loading distribution based on provisions in FEMA-356 best estimated the distorted shape, story drift, and column ductility. Overall, precise results from static analysis can not be expected because the results from dynamic analysis vary widely

Analysis of Method for Improving the Performance of Reinforced Concrete Frame Buildings During Earthquakes

This thesis describes a method for improving the performance of reinforced concrete frame buildings during seismic events. The criteria used to assess performance were the location of plastic hinges and the controlling mechanisms, the displacement characteristics of the frames including the inters tory distortion, and the deformation of the structural elements. The goal for improved performance was to reduce hinging in columns and force the yielding into the girders, resulting in the formation of the structural mechanism. A limit analysis was used to develop the method for a strength based relationship to ensure that the structural mechanism would be the controlling mechanism for any set of frame parameters. The relationship to improve performance reduces the flexural strength of upper floor level girders in the frames a prescribed amount. Non-linear static and dynamic analyses were used to test the method on several regular reinforced concrete frames. The results indicate that the performance of the frames improved when using the method discussed in this thesis

Reduction of Column Yielding During Earthquakes for Reinforced Concrete Frames

An investigation to reduce the number of columns that are vulnerable to yielding in reinforced concrete frames subjected to earthquakes is described. Simple limit analysis is used to demonstrate that a reasonable minimum column-girder strength ratio cannot be defined to eliminate yielding in columns of regular frames. A method is developed for reducing the number of columns vulnerable to yielding by applying a strength reduction factor to the girders in the upper floor levels of the frames. Nonlinear static and dynamic analyses of 16 reinforced concrete frames demonstrate that drift can be reduced by using the suggested girder strength reduction factor. Application is limited by the initial stiffness of the elements because of the increased drift demands in the top portion of the frame and by the allowable reduction in girder strength that will satisfy gravity-load demands

Evaluation of Multiple Corrosion Protection Systems and Corrosion Inhibitors for Reinforced Concrete Bridge Decks

The corrosion performance of different corrosion protection systems is evaluated using the mortar-wrapped rapid macrocell test, bench-scale tests (the Southern Exposure, cracked beam, and ASTM G109 tests), and field tests. The systems include conventional steel with three different corrosion inhibitors (DCI-S, Hycrete, and Rheocrete), epoxy-coated reinforcement with three different corrosion inhibitors and ECR with a primer coating containing microencapsulated calcium nitrite, multiple-coated reinforcement with a zinc layer underlying an epoxy coating, ECR with zinc chromate pretreatment before application of the epoxy coating to improve adhesion between the epoxy and the underlying steel, ECR with improved adhesion epoxy coatings, and pickled 2205 duplex stainless steel. Conventional steel in concretes with two different water-cement ratios (0.45 and 0.35) is also tested. Of these systems, specimens containing conventional steel or conventional epoxy-coated steel serve as controls. The critical chloride thresholds of conventional steel in concrete with different corrosion inhibitors and zinc-coated reinforcement are determined. The results of the tests are used in an economic analysis of bridge decks containing different corrosion protection systems over a design life of 75 years. The results indicate that a reduced water-cement ratio improves the corrosion resistance of conventional steel in uncracked concrete compared to the same steel in concrete with a higher water-cement ratio. The use of a corrosion inhibitor improves the corrosion resistance of conventional steel in both cracked and uncracked concrete and delays the onset of corrosion in uncracked concrete, but provides only a very limited improvement in the corrosion resistance of epoxy-coated reinforcement due to the high corrosion resistance provided by the epoxy coating itself. Based on results in the field tests, the epoxy-coated bars with a primer containing microencapsulated calcium nitrite show no improvement in the corrosion resistance compared to conventional epoxy-coated reinforcement. Increased adhesion between the epoxy coating and reinforcing steel provides no improvement in the corrosion resistance of epoxy-coated reinforcement. The corrosion losses for multiple-coated reinforcement are comparable with those of conventional epoxy-coated reinforcement in the field tests in uncracked and cracked concrete. Corrosion potential measurements show that the zinc is corroded preferentially, providing protection for the underlying steel. Pickled 2205 stainless steel demonstrates excellent corrosion resistance, and no corrosion activity is observed for the pickled 2205 stainless steel in bridge decks, or in the SE, CB, or field test specimens after four years. ECR, ECR with increased adhesion, and pickled 2205 stainless steel are the most cost-effective corrosion protection systems based on the economic analyses of a 216-mm (8.5-in.) thick bridge deck over a 75-year design life

Use of Innovative Concrete Mixes for Improved Constructability and Sustainability of Bridge Decks 2010-2011

Bridge deck crack surveys were performed on twelve bridges on US-59 to determine the effects of mixture proportions, deck type, and girder types on the crack density of reinforced concrete bridge decks. Of the twelve, eight have prestressed concrete girders and four have steel girders. Four of the decks with prestressed girders have partial-depth precast deck panels, two are monolithic, and two have overlays. Of the four decks with steel girders, two have overlays and two are monolithic. The surveys were performed, crack maps were analyzed, and cracking trends were observed. The results for the US-59 bridge decks were compared with crack densities obtained in a study of low-cracking high-performance concrete (LC-HPC) bridge decks in Kansas. The monolithic concrete bridge decks supported by prestressed concrete girders within this study exhibit less cracking than decks supported by steel girders in the first three years. At an age of approximately three years, the US-59 monolithic decks on prestressed girders with deck panels are not displaying significant cracking at the joints of the panels. The US-59 decks supported by prestressed girders without overlays exhibit significantly less cracking than the decks on prestressed girders with overlays. No benefits of using fibers in either the overlay or in the deck have been observed in this study

Evaluating Free Shrinkage of Concrete for Control of Cracking in Bridge Decks

The effects of paste volume, water-cement ratio, aggregate type, cement type, curing period, and the use of mineral admixtures and superplasticizers on the free shrinkage of concrete are evaluated with the goal of establishing guidelines to reduce cracking in reinforced concrete bridge decks. Three concrete prisms were cast and tested in accordance with ASTM C 157 for each mixture up to an age of 365 days under controlled conditions of 23 ± 2°C (73 ± 3°F) and 50 ± 4 percent relative humidity. The work was organized in five test programs. The first program included mixes with water-cement ratios of 0.40, 0.45, and 0.50, and aggregate contents of 60, 70, and 80 percent, with Type I/II cement and Type II coarse-ground cement. The second program included the mixes with one of three coarse aggregate types, granite, limestone, and quartzite. The third program evaluated the effects of Class C fly ash, ground granulated blast-furnace slag, and silica fume as partial volume replacements for portland cement. The fourth and fifth programs were used, respectively, to evaluate the effect of curing period (3, 7, 14, or 28 days) and the use of different superplasticizer types and dosages. The results indicate that concrete shrinkage decreases with an increase in the aggregate content (and a decrease in the paste content) of the mix. For a given aggregate content, no clear effect of water-cement ratio on the shrinkage is observed. In general, granite coarse aggregates result in lower shrinkage than limestone coarse aggregates. A similar conclusion cannot be made with quartzite coarse aggregate, although in some cases shrinkage of concrete containing quartzite coarse aggregate was lower than that of concrete containing limestone. The use of partial volume replacement of portland cement by Class C fly ash without changing the water or aggregate content generally leads to increased shrinkage. The use of partial volume replacement of portland cement by blast furnace slag without changing the water or aggregate content can lead to increased early-age shrinkage, although the ultimate shrinkage is not significantly affected. An increase in the curing period helps to reduce shrinkage. The use of Type II coarse ground cement results in significantly less shrinkage compared to Type I/II cement. The use of superplasticizers in concrete appears to increase in shrinkage to a certain degree. The results, however, do not present a clear picture of the effect of superplasticizer dosage on shrinkage

Corrosion Resistance of Duplex Stainless Steels and MMFX Microcomposite Steel for Reinforced Concrete Bridge Decks

Chloride-induced corrosion of reinforcing steel in concrete is one of the major durability concerns in reinforced concrete structures. In Northern America, the cost of maintenance and replacement for highway bridges due to corrosion damage is measured in billions of dollars. Of corrosion protection systems, reinforcing steels with inherently good corrosion resistance have received increased attention. In this study, the corrosion performance of duplex stainless steels, including 2101 and 2205 duplex steels in both “as-rolled” and pickled conditions, and MMFX microcomposite steel were compared with the corrosion performance of conventional and epoxy-coated steel using laboratory tests. These tests include rapid macrocell tests, corrosion potential tests, bench-scale tests (the Southern Exposure and cracked beam tests), and two modified versions of the Southern Exposure test to determine the critical chloride threshold. The rapid macrocell tests were modified by replacing the simulated concrete pore solutions at the anode and cathode every five weeks to limit the effects of changes in the pH of the solutions. The corrosion resistance of the steels was evaluated based on the corrosion rates, corrosion potentials, mat-to-mat resistances, and critical chloride thresholds measured in these tests. Based on laboratory results, along with data from bridge deck surveys and field experience, the service lives of the steels for bridges decks were estimated and the cost effectiveness was compared based on a life-cycle cost analysis. Results show that, in all rapid macrocell tests, replacing the test solution helps maintain the pH and reduces the corrosion rate and loss of steel. It is recommended that the test solution be replaced every five weeks. Statistically, effective chloride thresholds for reinforcing steel can be determined based on chloride samples from modified Southern Exposure and beam specimens. Results show that conventional steel has the lowest corrosion resistance, with chloride thresholds ranging from 0.91 to 1.22 kg/m3 (1.53 to 2.05 lb/yd3) on a water-soluble basis. Epoxy-coated steel [with four 3.2-mm (0.125-in.) diameter holes in the coating in each test bar to simulate defects of 0.2 to 1% of the bar area] has good corrosion resistance, with corrosion losses ranging from 0.4 to 6% of the values for conventional steel. MMFX microcomposite steel exhibits higher corrosion resistance than conventional steel, with corrosion losses between 16% and 66% and chloride thresholds, 3.70 to 4.07 kg/m3 (4.72 to 6.86 lb/yd3), equal to three to four times the value of conventional steel. Bridge decks containing MMFX steel will be less cost effective than decks containing epoxy-coated steel. Pickled 2101 steel and nonpickled and pickled 2205 steel exhibit significantly better corrosion resistance than conventional steel, with corrosion losses, respectively, ranging from 0.4% to 2%, 0.4% to 5%, and 0.2% to 0.5% of the value of conventional steel. Conservatively, the chloride thresholds of the steels are more than 10 times the value of conventional steel. Overall, 2205 steel has better corrosion resistance than 2101 steel, and pickled bars are more corrosion resistant than nonpickled bars. Pickled 2205 steel exhibits the best corrosion resistance of all the steels tested, while nonpickled 2101 steel has similar corrosion resistance to MMFX steel. The life cycle cost analyses show that in most cases bridge decks containing duplex stainless steels provide lower total life-cycle costs than bridge decks containing conventional, epoxy-coated, or MMFX steel. Pickled 2101 steel represents the lowest cost option

Lightweight Aggregates as an Internal Curing Agent for Low-Cracking High-Performance Concrete

The use of lightweight aggregates to supply a source of internal curing for Low Cracking, High Performance Concrete (LC-HPC) is evaluated. Prior research is used as a basis to estimate the amount of with lightweight aggregate replacement needed to optimize the amount of moisture available in the mix for internal curing. An aggregate optimization program (KU Mix) is revised to include modifications for the addition of aggregate with different specific gravities, such as lightweight aggregate, for the purposes of internal curing. Fourteen concrete mixes are designed to evaluate the free shrinkage and strength properties of LC-HPC mixes with lightweight aggregate for the purposes of internal curing. Six mixes in Program I are used to evaluate different replacement levels of lightweight aggregate. Eight mixes in Program II are used to evaluate the use of lightweight aggregate with Grade 100 slag. All mixes have a water/cement ratio of 0.44, 24.7% paste content (equivalent to a cement content of 540 lb/yd3) and an air content of 8%. Both 7-day and 14-day curing periods are evaluated for the free shrinkage specimens. Cylinders are cast for every batch and tested for the 28-day strength. The effect of adding lightweight aggregate does not significantly decrease the strength of any one mix. The addition of the lightweight aggregate increases the amount of internal curing water available and reduces shrinkage. The recommended mixes to reduce free shrinkage from Programs I and II were the 14-day cured lightweight aggregate mix with the highest level of replacement and the 14-day cured lightweight aggregate mix with a 30% cement replacement of slag, respectively

Development and Construction of Low-Cracking High-Performance Concrete (LC-HPC) Bridge Decks: Free Shrinkage Tests, Restrained Ring Tests, Construction Experience, and Crack Survey Results

The development, construction, and evaluation of low-cracking highperformance concrete (LC-HPC) bridge decks are described based on laboratory test results and experiences gained during the construction of 13 LC-HPC bridge decks in Kansas, along with another deck bid under the LC-HPC specifications but for which the owner did not enforce the specification. This study is divided into four parts covering (1) an evaluation of the free shrinkage properties of LC-HPC candidate mixtures, (2) an investigation of the relationship between the evaporable water content in the cement paste and the free shrinkage of concrete, (3) a study of the restrained shrinkage performance of concrete using restrained ring tests, and (4) a description of the construction and preliminary evaluation of LC-HPC and control bridge decks constructed in Kansas. The first portion of the study involves evaluating the effects of the duration of curing, fly ash, and a shrinkage reducing admixture (SRA) on the free-shrinkage characteristics of concrete mixtures. The results indicate that an increase of curing period reduces free shrinkage. With 7 days of curing, concretes containing fly ash as a partial replacement for cement exhibit higher free shrinkage than concretes with 100% portland cement. When the curing period is increased to 14, 28, and 56 days, the adverse effect of adding fly ash on free shrinkage is minimized and finally reversed. The addition of an SRA significantly reduces free shrinkage for both the 100% portland cement mixture and the mixture containing fly ash. The second portion of the study investigates the relationship between the evaporable water content in the cement paste and the free shrinkage of concrete. A linear relationship between free shrinkage and evaporable water content in the cement paste is observed. For a given mixture, specimens cured for a longer period contain less evaporable water and exhibit lower free shrinkage and less weight loss in the free shrinkage specimens than those cured for a shorter period. The third portion of the study evaluates the cracking tendency of concrete mixtures using the restrained ring tests. Different concrete ring thicknesses and drying conditions have been tested. The results indicate that specimens with thinner concrete rings crack earlier than those with thicker concrete rings. Exposing specimens to severe drying conditions results in the earlier formation of cracks, although it does not result in increased crack width. Mixtures with a lower watercement (w/c) ratio crack earlier than mixtures with a higher w/c ratio. Concretes with a higher paste content crack earlier than concretes with a lower paste content. The final portion of the study details the development, construction, and preliminary performance (with most bridges at three years of age) of LC-HPC and control bridge decks in Kansas. The results indicate that the techniques embodied in the LC-HPC bridge deck specifications are easy to learn. Contractor personnel can be trained in a relatively short time. The techniques used for LC-HPC bridge decks are effective in reducing bridge deck cracking. The crack surveys indicate that LC-HPC bridge decks are performing much better than the control decks, with average crack densities reduced by about seventy five percent at three years of age. The factors that may affect bridge deck cracking are analyzed. The analyses indicate that an increase in paste content, slump, compressive strength, maximum daily air temperature, and daily air temperature range causes increased crack densities. Contractor techniques influence cracking