The load-carrying capacity of aging bridge members may at times be found insufficient due to deterioration and a historical trend towards increased truck axle loads beyond their design capacity. Structurally deficient bridges are problematic for bridge owners and users because they restrict traffic usage and require bridges to be posted (operate at less than their ideal capacity). Structural deficiency is the primary motivation for bridge owners to retrofit bridges to meet a specified operating demand. It may be required to replace or retrofit a portion or all of a deficient bridge. The replacement of an entire bridge or even a part of the bridge is generally less desirable than a retrofit solution because retrofits are generally a cheaper alternative to the entire replacement of a structure and usually do not require the bridge's closure. Standard strengthening solutions for corroded members include bolting or welding steel cover plates, replacing sections of the girder, or adding external prestressed tendons. However, these methods also have several challenges, including required lane closures, high installation costs, increased dead weight, and continuing corrosion issues. One alternative to conventional retrofits is the use of carbon fiber-reinforced polymer (CFRP) laminates, which can be adhered to increase both strength and stiffness. CFRPs are a highly tailorable material with an extremely high strength-to-weight ratio, ease of installation and can potentially mitigate further corrosion concerns. Fiber Reinforced Polymers (FRPs) have already been widely accepted as a means of retrofitting reinforced concrete structures (AASHTO 2012, 2018a; ACI 2002, 2017; National Academies of Sciences, Engineering 2010, 2019) but have not yet been widely adopted in the steel industry due to the retrofit's material limitations (lower elastic modulus [less than 29,000 ksi], unanswered questions related to debonding, and no unified design or installation guides). However, newly developed materials and manufacturing processes have allowed for the economic development of stiffer CFRP materials suitable for steel structures, such as the high modulus (HM) CFRP strand sheet. This research analytically and experimentally investigates how newly developed HM strand sheets perform in small scale tensile testing and large scale flexural testing (laboratory and in situ testing). During the laboratory testing, these HM strand sheets were compared against normal modulus (NM) CFRP plates to draw conclusions on these different retrofitting materials (strength, stiffness, bond behavior, and applicability of the retrofit). Another central point in examing these different retrofit materials is how CFRPs perform when attached to structural steel with significant corrosion damage. Corrosion damage typically results in a variable surface profile, which may affect a CFRP retrofit's bond behavior. While limited laboratory testing has been conducted on CFRP attached to steel structures with simulated deterioration, the surface profile does not represent realistic conditions. The effects of a variable surface profile on the NM plate material and HM strand sheet were investigated using small scale tensile testing and large scale flexural testing. All the variable surface profiles tested for bond strength were fabricated based on "representative" simulated corrosion samples or on specimens with significant corrosion. Once all the variables pertaining to the new materials and the effect of a variable surface profile on CFRP retrofits had been examined in a laboratory setting, these retrofitting techniques were implemented on deteriorated in-service steel bridge structures. This research was the first to retrofit deteriorated in-service bridge structures with HM CFRP strand sheets in the United States. This in situ testing was used to compare the laboratory test data of an individually retrofitted girder to the behavior of a single girder that had been retrofitted in a bridge structure. This information was used to verify results on the behaviors of strengthening, stiffening, effects on live load distributions, and modeling assumptions of retrofitted bridge structures. The results from the laboratory testing and in situ testing of CFRP retrofits on corroded steel structures were synthesized to provide information on performance and design guidance for future retrofits. This dissertation provides additional information on CFRP retrofits applied to variable surface profiles and provides data on new CFRP materials (HM strand sheets). With this information, Departments of Transportation (DOT) can be confident as to where and when different types of CFRPs are a suitable retrofit material for corroded or uncorroded steel structures. / Doctor of Philosophy / The capacity of aging bridges may at times be found insufficient due to deterioration and a trend towards increased loading. Structurally deficient bridges are problematic for bridge owners and users because they restrict traffic usage and require bridges to operate at less than their intended capacity. Inadequate capacity are the primary motivation for bridge owners to repair (retrofit) bridges to meet specified traffic demands. Repairs usually do not require the bridge's closure to traffic. Standard repairs for corroded steel members include bolting or welding steel cover plates, replacing sections of the girder, or adding external prestressed tendons. However, these methods also have several challenges, including required bridge closures, high installation costs, increased weight, and continuing corrosion issues. One alternative to conventional repairs is the use of carbon fiber-reinforced polymer (CFRP) laminates, which can be adhered to the deteriorated members to increase strength and stiffness. CFRPs are an extremely versatile material with high strength, high stiffness, ease of installation and can potentially mitigate concerns about further corrosion. Fiber Reinforced Polymers (FRPs) have already been widely accepted as a means of retrofitting reinforced concrete structures(AASHTO 2012, 2018a; ACI 2002, 2017; National Academies of Sciences, Engineering 2010, 2019) but have not yet been widely adopted in the steel industry due to the lack of literature and economical implementation of the CFRPs on steel. However, over the past 20 years, research has been completed on the application of CFRPs on steel, and newly developed materials were created for the economic implementation of CFRP materials suitable for steel structures. In particular, this material is a high modulus (HM) CFRP strand sheet, which has a higher stiffness than a conventional CFRP. This research investigated how newly developed HM strand sheets perform in small-scale laboratory testing and large-scale laboratory testing. Where material strengths, bondability, and the efficacy of different repairs were examined against conventional means on steel structures with and without corrosion deterioration. Once all the variables pertaining to the new materials and the effects corrosion had on CFRP retrofits had been examined in a laboratory setting, these retrofitting techniques were implemented on a deteriorated in-service steel bridge structure (field study) that required repair. This research was the first to repair deteriorated in-use bridge structures with HM CFRP strand sheets in the United States. This information was used to verify results on the material's behavior. The laboratory testing and field testing of CFRP retrofits on corroded steel structures were summarized to provide information on performance and design guidance for future retrofits. This dissertation provides additional information on CFRP repairs applied to corroded steel and provides data on new CFRP materials (HM strand sheets). With this information, Departments of Transportation (DOT) can be confident as to where and when different types of CFRPs are a suitable retrofit material for corroded or uncorroded steel structures.