Analysis of Causes of Falsework Failures in Concrete Structures

A study of 85 major falsework collapses of bridges and buildings in the past 23 yrs has documented the types of collapsed falseworks and failed permanent structures, the construction stages at the time of collapse, and the causes of failures. Three causes of failure were identified: triggering; enabling; and procedural causes. Most failures occurred because of the interaction of the triggering and enabling events that were, in many cases, produced by inadequacies in the procedural methods. Impact forces resulting from concreting operations have repeatedly triggered falsework failures that were enabled by deficiencies in the falsework bracings, components, connections, foundations, and design. Inadequate review of falsework design and monitoring procedures were frequent problems that facilitated the occurrernce of these events. The findings emphasize the importance of proper delineation of responsibility of each party in the building process in order to reduce falsework failures in the future.     

Coupled Mechanical and Chemical Damage in Calcium Leached Cementitious Structures

Long-term durability of concrete structures must be faced both from the point of view of cracking and physical degradations. In this paper, the relevance and the sensitivity of an existing constitutive relation aimed at modeling mechanical and chemical damage is examined. This constitutive relation is based on a scalar continuum damage model. The chemical degradation mechanism is calcium leaching. It is observed that the model predictions, i.e., the lifetime of cement-based beams subjected to leaching, are very sensitive on the tensile strength and fracture energy of the sound material. The existing model predicts the response of bending beams subjected to various states of leaching prior to any mechanical loading. The simulation of the size effect tests shows that the mechanical internal length and the damage threshold of the material cannot be considered to be constant. The internal length ought to decrease and the damage threshold should increase.     

Corrosion Rate Evaluation and Prediction for Piles Based on Long-Term Field Performance

A study to evaluate corrosion rates was conducted using pile foundations abandoned during the reconstruction of I-15 through Salt Lake Valley, Utah. Corrosion rates were measured for 20 piles extracted from five sites after service lives of 34–38 years. Measurements were made of soil index properties, resistivity, pH, cation/anion concentrations, and water table elevation. The critical zone for corrosion was typically located within the groundwater fluctuation zone; but correlations with soil properties were generally poor. Despite low resistivity, average corrosion rates for pile caps in native soil were typically between 2 and 9?μm/year with a maximum of 19?μm/year and did not pose any structural integrity problems. Nevertheless, for abutment piles where chloride concentration was very high, the average pile corrosion rate increased to 13?μm/year within the embankment and the maximum corrosion rate was 48?μm/year in the underlying native soil. Based on data from this and previous studies, equations were developed to predict maximum corrosion loss for piles in nonaggressive soil as a function of time.     

Note on Chloride-Induced Corrosion of Reinforced Concrete Bridge Decks

This technical note presents numerical results to predict the corrosion initiation time of reinforced concrete bridge decks using measured surface chloride accumulation. Based on actual core measurements, the surface chloride, which is mainly derived from the deicing salts used during winter maintenance operations, is assumed to increase linearly over a period of time and then remains constant afterward. The chloride ions penetrate the concrete by diffusion and corrosion is initiated when the concentration of the ions around the reinforcement steel reaches a critical value needed to break the passive film surrounding the steel. The corrosion initiation time is computed for different values of the diffusion coefficient and the concrete cover. Such results are useful for scheduling bridge deck maintenance and rehabilitation programs.     

Corrosion of High Chromium and Conventional Steels Embedded in Concrete

Corrosion rates of high chromium and conventional steel rebars were measured and compared by conducting two studies. One was on concrete blocks and the other was on bare steel rebars. In the former study, concrete blocks that had been made with two different steel rebars were placed in sodium chloride solutions, and air was blown through the solutions to accelerate corrosion of the embedded steel rebars. These blocks were taken out of the solution periodically, and the corrosion rates of the rebars were measured with a 3LP device. In the latter study, the bare rebars of the two steels were also corroded in sodium chloride solutions through which air was blown, withdrawn periodically, dried, and weighed after the corrosion products were removed. The corrosion rates were measured by the reduction of the weight of the rebars. In the study on concrete blocks, it was found that the corrosion rate increases for both steels as the concentration of sodium chloride in solution increases. It was also found that the corrosion rate of concrete blocks reinforced with conventional steel was about twice as much as that of the concrete blocks reinforced with high chromium steel after 132 days of exposure. From the study on bare steel rebars, it was found that the rate of corrosion of conventional steel was 12 times as much as that of high chromium steel at 0.1% sodium chloride, and the ratio decreased to 2 times as much when the sodium chloride concentration was increased to 3%. It was also found that the corrosion rate of high chromium steel was very sensitive to sodium chloride concentrations whereas that of conventional steel was not sensitive. The corrosion products were analyzed using x-ray diffraction and atomic absorption spectroscopy to identify the minerals present in them. It was found that corrosion products produced on the high chromium steel were predominantly lepidocrocite (γ-FeOOH) and hematite (Fe2O3), whereas that on the surface of conventional steel was predominantly magnetite (Fe3O4). It appears that the former iron oxides form an adherent and nonporous protective layer while the latter iron oxides (magnetite) do not, which can explain the distinct difference in corrosion rates of the two steel rebars.     

Economic Evaluation of Reinforced Concrete Structures with Columns Reinforced with Prefabricated Cage System

A new reinforcement system termed the prefabricated cage system (PCS) that can be used as an alternative to the rebar reinforcement cage is economically evaluated. PCS is a prefabricated reinforcement that enables easier, faster, and more reliable construction. Use of PCS shortens the construction schedule time and lowers total construction cost. This is important to both owners and construction contractors. The engineering economics methods presented in this paper would also be of interest to researchers. Reinforced concrete structures with PCS reinforced columns have been considered in this research, as it is one of the major applications of PCS. Various parameters affecting the economics of PCS are reviewed and a case study structure is analyzed comparing the costs of the structure with rebar reinforced columns to costs of the structure with PCS reinforced columns. The investigation shows that using PCS results in a 33.3% time savings and a 7.1% cost savings over rebar for each column. This results in an average of 3.6% savings on total project cost; an average of 22.2% savings on total column costs; 20.4% savings on total project time period, and 33.3% savings on columns construction time period. The cost savings are estimated based on production of small quantities of PCS reinforcement. Mass production of PCS reinforcement would result in even higher cost savings.     

High-Cycle Fatigue of Diagonally Cracked Reinforced Concrete Bridge Girders: Field Tests

Large numbers of conventionally reinforced concrete deck–girder (RCDG) bridges remain in-service in the national highway system. Diagonal cracks have been identified in many of these bridges, which are exposed to millions of load cycles during service life. The anticipated life of these bridges in the cracked condition under repeated service loads is uncertain. RCDG bridges with diagonal cracks were inspected and instrumented. Strain and crack displacement data were collected under ambient traffic conditions and controlled test trucks. Results indicated relatively small stirrup stresses and diagonal cracks exhibited opening and closing under truck loading.     

Analysis of Life-Cycle Maintenance Strategies for Concrete Bridge Decks

This paper presents the development of a project-level decision support tool for ranking maintenance scenarios for concrete bridge decks deteriorated as a result of chloride-induced corrosion. The approach is based on a mechanistic deterioration model and a probabilistic life-cycle cost analysis. The analysis includes agency and user costs of alternative maintenance scenarios and considers uncertainties in the agency cost and the corrosion rate in the deterioration model. The tool presented in this paper can be used to find the optimal condition index of a given bridge deck that minimizes life-cycle cost. Based on the results obtained on three existing bridge decks, it is shown that the total life-cycle cost (user cost plus agency cost) is a nonlinear function of the maximum tolerable condition of the deck, Sm, and that for a practical range of Sm, the relationship between total life-cycle cost and Sm is convex.     

Stiffness Degradation and Time to Cracking of Cover Concrete in Reinforced Concrete Structures Subject to Corrosion

Corrosion-induced cracks in reinforced concrete (RC) structures degrade the stiffness of the cover concrete. The stiffness degradation is mainly caused by the softening in the stress-strain relation in the cracked concrete. Limited efforts have been made to model the cracking and the corresponding effects on the cover concrete, despite of its importance in assessing and modeling the behavior of RC structures. This paper proposes a stiffness degradation factor to model the stiffness degradation of the cover concrete subject to cracking. The proposed factor is computed in terms of the cracking strain corresponding to the maximum opening of the concrete cracks based on an energy principle applied to a fractured RC structure. The time to cracking of the cover concrete is then determined as the time from the corrosion initiation needed by the crack front to reach the outer surface of the cover concrete. The proposed stiffness degradation factor and the method to compute the time to cracking are illustrated through two numerical examples. The times to cracking of the cover concrete that are predicted using the proposed method are in agreement with the measured values from laboratory experiments.     

Analytical and Field Investigation of Roma Suspension Bridge

The Roma–Ciudad Miguel Aleman International Suspension Bridge is an historic unstiffened suspension bridge with a 192 m (630 ft) main suspended span, originally constructed in 1928. In 1997 the bridge was inspected and a full-scale nondestructive load test was conducted. The resulting experimental data are evaluated and compared to the results of analyses by finite-element method modeling. The history of the bridge is reviewed, with an emphasis on modifications and retrofits to the structure. The unique behavior and attributes of unstiffened suspension bridges are discussed in the specific context of this particular bridge.     

In-Situ Evaluation of Two Concrete Slab Systems. II: Evaluation Criteria and Outcomes

The primary objective of in-situ load testing is to evaluate the safety and serviceability of an existing structural system with respect to a particular load condition and effect. In light of technological advances in construction methods, analytical tools and monitoring instrumentation, new different evaluation criteria are being proposed in addition to different in-situ load test methods. Some criteria may be more appropriate than others based on the expected damage and failure mechanisms of the structure being considered. The companion paper describes the rationale and application of both a consolidated and an alternative approach to the determination of load level, loading procedure and instrumentation requirements for two case studies. This paper discusses in detail the evaluation criteria and outcomes of these two field projects consisting of a posttensioned concrete slab with structural deficiencies due to tendon and mild reinforcement misplacement and a floor bay of a two-way reinforced concrete slab showing cracking at the positive and negative moment regions. After discussing the relative merits of the evaluation methodologies and the significance of their respective acceptance thresholds, concepts for the development of a new global criterion are discussed.     

In-Situ Evaluation of Two Concrete Slab Systems. I: Load Determination and Loading Procedure

The primary objective of in-situ load testing is to assess the safety and serviceability of an existing structural system with respect to a particular load effect. At this time, the most appropriate loading level and procedure, as well as the associated evaluation criteria are being reconsidered in light of technological advances in construction methods, analytical tools, and monitoring instrumentation. The in-situ load test method for reinforced concrete systems described in the ACI Building Code Requirements for Structural Concrete, namely the 24–h load test method and its evaluation criteria, has been in use for several decades, but may no longer serve the needs of contemporary construction and engineering practices. As a result, other load test methodologies and associated evaluation criteria are under development. This paper and a companion paper describe the rationale and application of an alternative approach to the determination of load level, loading procedure, instrumentation requirements, evaluation criteria and outcomes for two field projects. The first case study is relative to a posttensioned concrete slab where many areas were characterized by tendon and reinforcement misplacement, resulting in inadequate flexural strength and inadequate shear/flexure transfer at column/slab intersections. The second case study is the structural evaluation of a typical floor bay of a two-way reinforced concrete slab system, presenting distributed cracking at the positive and negative moment regions. Finite-element-method models were created for both structures to aid the load test design. The numerical models validated the field observations.     

Field Evaluation of a Glass-Fiber Soil Reinforcement System

A soil reinforcement system using the combination of glass-fiber-reinforced polymer (GFRP) pipe and pressure grouting has been developed by a Korean company. The system is being adopted as slope stabilization measures in Hong Kong. Individual GFRP pipes were designed and installed as soil nails with the exception that two-stage pressure grouting was applied instead of gravity grouting. A full-scale field evaluation on constructability and in situ performance of the system was performed in Hong Kong. The performance of the constructed nails was evaluated by pullout tests. The construction procedure, site adaptations of the Korean product made in Hong Kong, field test procedure, and performance of the constructed soil nails are reported in this paper.     

Effect of Fiber-Reinforced Polymer Wraps on Corrosion Activity and Concrete Cracking in Chloride-Contaminated Concrete Cylinders

This paper presents the results of an experimental study designed to investigate the effect of fiber-reinforced polymer (FRP) wraps on corrosion activity and concrete cracking in chloride-contaminated concrete cylinders. Thirty-five concrete cylinders, each having 102?mm diameter and 204?mm height, concentrically reinforced with one steel reinforcing bar, were subjected to accelerated corrosion exposure for 80?days. Test parameters included level of applied potential, presence of FRP wraps, and bar diameter. The corresponding current and concrete expansion were continuously monitored throughout the corrosion exposure. At the end of the test, the steel bars were extracted, cleaned of rust, and weighed to determine the actual steel mass loss. The results showed that, for the same applied fixed potential, FRP wraps effectively reduced the corresponding current, the concrete expansion, and the steel mass loss. For the same applied potential, the current density increased as the bar diameter decreased. For the same corrosion depth, the circumferential expansion of the cylinder caused by corrosion decreased as the concrete cover-to-bar diameter ratio (c/d) increased.     

Service Life Prediction of RC Bridge Structures Exposed to Chloride Environments

For a long-span coastal bridge structure, the corrosion initiation time is controlled by the speed of chloride ions transfer and the depassivation process within the structure. These processes are significantly influenced by the actual variation of the environmental conditions on the concrete surface throughout its service life. From the regional climate characteristics through local climate conditions, the microclimate variation on the concrete surface is studied in this research. A set of realistic environmental condition profiles is proposed, based on the exposure conditions and the material properties of the components. Moreover, a 2D integrated corrosion performance assessment model is constructed to capture the change in environmental conditions and simulate the coupled diffusion process and the corrosion performance in the time domain. Two typical locations (Hong Kong and Michigan) are chosen as numerical examples for implementing the proposed corrosion performance assessment model, and control of the environmental factors of the various chloride exposures is highlighted. These factors are used to construct an integral empirical equation together with the general critical material and geometrical parameters.     

Hybrid Bonding of FRP to Reinforced Concrete Structures

The adhesive attachment of fiber-reinforced polymers (FRP) laminate to the external face of reinforced concrete structures is currently one of the most popular and effective methods for retrofitting and strengthening concrete structures. With this method, the additional strength of the attached reinforcement is transmitted into the concrete members through adhesion. However, the relatively weak adhesive interface fundamentally limits the efficacy of the method. Much effort has been made in the research community to improve the bond strength and develop bond models, but a satisfactory solution has yet to be found. Mechanical fastening is another more traditional technology that is used to bond one material to another. This paper introduces a new hybrid bonding technique that combines adhesive bonding and a new type of mechanical fastening. The new mechanical fastening technique does not rely on bearing to transmit the interfacial shear, but instead increases the interfacial bond by resisting the separation of the FRP laminate from the concrete substrate. Experimental tests demonstrated that the bond strength with this new hybrid bonding technology was 7.5 times that of conventional adhesive bonding. Furthermore, the new bonding technique is applicable to all types of commercially available FRP laminate (fabric, sheet, plate, and strip), and in principle is also applicable to materials other than FRP.     

Nondestructive Assessment of Reinforced Concrete Structures Based on Fractal Damage Characteristic Factors

Nondestructive damage assessment of civil engineering structures has become a focus of increasing interest for recent decades. Its core is to extract effective damage characteristic information capable of reflecting structural damage status. In this study, fractal theory is adopted to extract the fractal damage characteristic factors of a reinforced concrete structure by characterizing its surface-crack distributions. The concentrated and even load spaces are generalized as applicable spaces for employing fractal-to-structural damage assessment. As demonstrated in the damage assessment of reinforced concrete beams under four-point bending and aged crossbeams of an operation bridge in a sluice, the surface-crack distributions of reinforced concrete structures exhibit monofractal character in the concentrated load space, and multifractal character in the even load space. The physical damage interpretations of the extracted monofractal and multifractal damage characteristic factors in the respective load spaces are then established by analyzing the correlations between the monofractal dimension and the natural frequency, and between the multifractal singular spectrum and the average carbonized depth and residual material intensity, respectively. The closely linear fitting relationships between the fractal quantities and traditional damage characteristic factors indicate that the fractal (i.e., monofractal and multifractal) quantities can serve as viable and novel damage characteristic factors in the online damage assessment of concrete structures. It is significant that the proposed fractal damage characteristic factors overcome some disadvantages of traditional damage characteristic factors in practical applications, and they extend the technique of fractal into the meaningful damage assessment of reinforced concrete materials.     

Fatigue Behavior and Nondestructive Evaluation of Full-Scale FRP Honeycomb Bridge Specimen

An experimental investigation was performed to assess the projected fatigue performance of a fiber-reinforced polymer honeycomb bridge that has recently been completed in Troupsburg, N.Y. The laboratory specimen was representative of a 305-mm-wide strip of the completed bridge. The specimen was first subjected to fatigue loading. Load, displacement, and strain were measured every 25,000 cycles. The data indicated minimum signs of degradation after 2 million cycles of fatigue loading, as reflected in slightly increased values of vertical deflection and strain at midspan. After completion of the fatigue loading, the specimen was evaluated with acoustic emission. Load was statically applied and increased incrementally until failure occurred at a load level exceeding 16 times the fatigue level loading. The results of the static testing also indicated that only minor damage occurred due to fatigue. Field load testing of the actual bridge has been completed by the New York State Department of Transportation, and the results are discussed as they pertain to the fatigue and static load testing programs described.     

Field Experiment Study of Transient Stratified Flow in an Estuary

An experimental study of stratified flow was conducted in this study. An electromagnetic measurement instrument, the S4 current meter, was used in field data collections of salinity and currents in the lower Apalachicola River estuary, Florida. The S4 current meter has an advantage in field deployments for long periods of time due to its preprogrammable capability for automatic data sampling and recording of fluid flow. Time series of surface and bottom salinity and currents obtained from field experiments were used to characterize the stratified flow at the measurement location in the Apalachicola River. Analysis of field data indicated that the river was strongly stratified. The stratification was affected by the upstream river flow and the downstream tidal variations. Stratification was stronger at high tide than at low tide. By removing the tidal signal using low-passing filtering, subtidal salinity, and currents were obtained to investigate the salinity stratification and currents responses to the changes of fresh water input. Subtidal vertical salinity and velocity profiles were presented at different flow conditions. At high flow conditions, both surface and bottom subtidal currents were in the seaward direction. At low flow conditions, the bottom subtidal currents were in the upstream direction due to the strong effects of density gradients. Empirical regression equations were obtained to quantify the effects of river flow on the subtidal salinity and the bottom currents. Regression analysis indicated good linear relationship between subtidal salinity stratification and the bottom currents.