Reinforced concrete

Traditional techniques of observing cracking within reinforced structures can be invasive, leading to an increased risk of added corrosion to structures already undergoing corrosive processes. The research presented in this document improves upon a nondestructive method for detecting early crack formation in reinforced concrete. This method includes using acoustic signaling to add a layer of salt water between the sensor and analyzed sample. Following the collection of surface and rebar echo responses, an adapted version of the novel Biot-Stoll method is used to model sound propagation for poro-elastic mediums. Testing of model parameters and variables has improved the root mean square error (RMSE) by up to 63.7% when studying the full signal, and up to 62.6% for the rebar echo locations. These improvements signify better curve fitting between simulated and measured responses, which lead to increased accuracy in the model parameter outputs.

Reinforced concrete (RC) is the building block of modern architecture and industry. The failure of which is costly and dangerous. Typically made with carbon steel rebars, corrosion resistant alloys provide an alternative method of delaying failure. Stainless steels, while more expensive than carbon steels, provide excellent corrosion resistance, but less is known about the long term monitoring of corrosion activity for stainless steel than for carbon steel. This study looks at samples prepared between 2005 and 2009 using 304SS, 316SS, and 2304SS rebars, as well as SMI and Stelax stainless steel clad carbon steel reinforcements embedded in three different concrete mixes. These selected samples are split into two exposure environments, inside humidity chambers within the laboratory and outdoor exposure. Measurements reported here were made monthly over the course of 250 plus days using the Galvanostatic Pulse method, Electrochemical Impedance Spectroscopy, and a Gecor 8 device. These methods were used to determine corrosion current, isolated corrosion current density, and solution resistance.
Corrosion current density values calculated from measurements by the Galvanostatic Pulse and Electrochemical Impedance Spectroscopy method are too small to indicate corrosion, based on value ranges provided by Andrade. However, Gecor 8 corrosion current density values indicate low levels or moderate levels of corrosion for all samples compared to the Andrade’s value ranges. The area used by the Gecor is unknown, so it’s possible this is driving up the measured values.

The detection of rebar corrosion in reinforced concrete is important due to the high costs of corrosion related damages to infrastructure. One such method of rebar corrosion lies in the use of non-destructive ultrasonic testing. To date, acoustic methods require either the training of an artificial neural network or a theory of acoustic wave propagation. Using a more complete acoustic model such as the Biot-Stoll model avoids algorithm training requirements by directly modeling the acoustic environment. A problem with this method lies in the complexity of the model and the selection of free parameters. The problem of parameter selection is addressed by a series of targeted measurements using ultrasonic transducers on a set of existing reinforced concrete samples placed in a saltwater solution. This data can then be analyzed by a non-linear least squares solver to produce a better fit for the acoustic signal.

In order to study the mechanical performance of dry-cast synthetic fiber reinforced concrete (SynFRC), samples of varying geometry, fiber content, and environmental exposure were developed and tested using the modified indirect tensile test. The samples created consisted of three different thicknesses (with two different geometries), and six different fiber contents that differed in either type, or quantity, of fibers. Throughout the duration of this research, procedures for inflicting detrimental materials into the concrete samples were employed at a number of different environments by implementing accelerated rates of deterioration using geometric adjustments, increased temperature exposure, wetting/drying cycles, and preparation techniques. The SynFRC samples studied were immersed in a wide range of environments including: the exposure of samples to high humidity and calcium hydroxide environments, which served at the control group, while the sea water, low pH, and barge conditioning environments were used to depict the real world environments similar to what would be experienced in the
Florida ecosystem. As a result of this conditioning regime, the concrete was able to imitate the real-world effects that the environments would have inflicted if exposed for long durations after an exposure period of only 20-24 months. Having adequately conditioned the samples in their respective environments, they were then tested (and forensically investigated) using the modified indirect tensile testing method to gather data regarding each sample’s toughness and load handling capability. By analyzing the results from each sample, the toughness was calculated by taking the area under the force displacement curve. From these toughness readings it was found that possible degradation occurred between the fiber-matrix interface of some of the concrete samples conditioned in the Barge environment. From these specimens that were immersed in the barge environment, a handful of them exhibited multiple episodes of strain softening characteristics within their force displacement curves. In regard to the fibers used within the samples, the PVA fibers tended to pull off more while the Tuff Strand SF fibers had the highest tendency to break (despite some of the fibers showing similar pull off and breaking failure characteristics). When it comes to the overall thickness of the sample, there was clear correlation between the increase in size and the increase in sample toughness, however the degree to which it correlates varies from sample to sample.

This research was conducted to better understand the corrosion propagation stage on dry-cast reinforced concrete pipes (DCRCPs) while exposed to high moisture conditions and chlorides. Corrosion initiation and propagation were studied in instrumented specimens obtained from segments of dry-cast reinforced concrete pipes. All specimens were subjected to accelerated chloride transport by the application of an electric field. Corrosion of the steel wire mesh initiated after a few days to a few months rather than several years. The specimens were then transferred to high moisture environments (immersed in water, high humidity and/or covered with wet sand) during the corrosion propagation stage. Reinforcement potentials, linear polarization resistance (LPR) and Electrochemical Impedance Spectroscopy (EIS) measurements were carried out periodically. During the propagation stage in different exposures, reinforcement eventually reached negative potentials values (< –-0.55 Vsce), which suggest mass transfer limitations. These specimens showed no visual signs of corrosion such as cracks or corrosion products except the ones exposed to high humidity and laboratory environments; where some corrosion products have reached the concrete surface. Moreover, the apparent corrosion rate values obtained suggest high corrosion rates. No crack appearance on specimens exposed to other conditions could be explained by the porosity of the specimens; the corrosion products moved into saturated pores. It is speculated that although there might be mass transfer limitations present, the current demanded by the anode is being balanced by a larger cathode area due to macrocell effects since the high moisture conditions likely reduced the concrete resistivity and increased the throwing power. The corrected polarization resistance (Rc) was calculated by subtracting the solution resistance from the apparent polarization resistance measured. The Rc values measured over time were used to obtain the calculated mass loss (using Faraday’s Law). Most specimens were forensically analyzed and the measured mass loss compared to the calculated mass loss. The forensic examination includes the measurement of the actual corroding areas. The measured corroding areas were used to obtain corrosion current density (icorr) values. A comparison was made of the calculated corrosion current densities obtained using the linear polarization resistance method (LPR) and the extrapolation method from cyclic polarization tests. It was evident that most of the specimens’ corrosion rates were significantly high. The corrosion products filled the wet-pores inside the concrete and provide an explanation for no cracks or corrosion bleed outs being visually observed on the specimens.

An experiment was conducted to evaluate the durability, toughness, and strength
of Synthetic Fiber Reinforced Concrete after being immersed in five separate
environments for one year at FAU SeaTech. The specimens were molded and reinforced
with two-inch Polypropylene/Polyethylene Fibers in a concrete aggregate matrix and
were cut into identical sizes. Some of these environments had accelerated parameters
meant to increase degradation to simulate longevity and imitate harsh environments or
seawater conditions. The environments consisted of: a high humidity locker (ideal
conditions), submerged in the Intracoastal Waterway (FAU barge), seawater immersion,
a wet and dry seawater immersion simulating a splash/tidal zone, and another in low pH
seawater. The latter three were in an elevated temperature room (87-95°F) which
produced more degradative properties. The specimens were monitored and the
environments were controlled. The specimens were then evaluated using the IDT test
method using force to initiate first-cracking and post-cracking behaviors.

Carbon Fiber Reinforced Plastics has recently has been recognized as an alternative to conventional steel reinforcement in concrete due to its excellent resistance to corrosion. Four rectangular concrete beams and four concrete columns reinforced with CFRP bars were cast for the study of the long term behavior under uniform sustained loading. The beams were simply supported and subjected to uniform sustained loading. The columns were arranged in a steel reaction framework. The beams and columns were instrumented and monitored to observe the change in the behavior due to the creep and shrinkage of concrete. An analytical method is developed to predict the long term behavior of CFRP reinforced concrete members. The calculated deformations compare reasonably with the experimental values. A modified equation for the calculation of the long term deflection is proposed for CFRP reinforced concrete beams. A simplified equation for the calculation of the creep coefficient is also proposed.

In a highly corrosive environment, corrosion is the main factor leading to deterioration and eventual failure of conventional reinforced or prestressed concrete structures. Carbon Fiber Reinforced Plastics (CFRP) are considered as an alternative to steel reinforcement due to its excellent corrosion resistance. This investigation was conducted to establish the feasibility of using CFRP cables as reinforcing elements in reinforced concrete columns. Besides investigating durability of CFRP cables in adverse environments (alkali and sea water) experimental and theoretical studies were carried out to study the behavior of CFRP reinforced concrete slender columns under combined axial load and bending moment. Exposure to air, sea water and alkali environments with alternating wet/dry cycles had no adverse effect on the strength of the CFRP cables. The CFRP reinforced concrete columns subjected to eccentric loads exhibited excellent ultimate load capacity. Feasibility of using CFRP cables in the reinforced concrete columns is assessed based on deflections, strains, curvatures, crack distributions, first crack loads and ultimate loads.

Initiation of corrosion of reinforcing steel in concrete is
often caused by chlorides. Using a pressurized method for
the purpose of accelerating penetration of sea water into
concrete, the threshold chloride ion concentration for
corrosion of reinforcing steel in Type I portland cement
concrete has been studied in detail. The variables that
have been investigated include water-cement ratio and steel
surface preparation. When corrosion was detected by
electrochemical potential measurement, the test was
terminated; and chloride ion concentration was evaluated. No
correlation between threshold concentration and water-cement
ratio was found. With regard to surface condition, the pre-rusted
steel specimen showed a tendency to corrode at a
lesser chloride ion concentration than for the other preparation
techniques, which were sand blasting and
pretreatment in a saturated calcium hydroxide solution.
The results are presented and discussed within a perspective
of established concrete and corrosion technologies.

This thesis presents an experimental investigation for evaluating effects of fire exposure on properties of structural elements retrofitted by carbon fiber reinforced polymers. Mechanical properties of CFRP-strengthened reinforced concrete members, protected with secondary insulation, were investigated, before and after (residual) direct fire exposure. Direct fire contact resulted in a reduction in capacity of 9-20% for CFRP-strengthened RC beams, and 15-34% for CFRP-strengthened RC columns. Furthermore, a dimensional analysis was developed for a heat transfer relationship between full and small-scale specimens, allowing a ¼ exposure time reduction for the latter. Results from experimental investigations demonstrate benefits of employing secondary fire protection to CFRP-strengthened structures, in spite of the glass transition temperature being exceeded in the early stages of the elevated-temperature exposure. Therefore, it is suggested, that fire protection is necessary for a CFRP-strengthened structural member to retain integrity throughout the duration of the fire exposure, and upon return to ambient temperature.