Corrosion Propagation of Reinforcing Steel in High Performance Concrete

The Florida Department of Transportation (FDOT) has been using high performance concrete since the late 1980s early 1990s. Some of the older bridge structures (assuming 1990 as the completion year) are reaching 35 year of age, and depending on the concrete cover thickness (those with thinner covers), and the diffusivity of the concrete, corrosion of the reinforcement might initiate in a few years on some of these structures. The corrosion propagation stage of carbon steel rebar in high performance concrete might last longer than the typically five years usually attributed for carbon steel rebar in concrete with type I/II Portland cement as the only cementitious material. It is well known that reinforcement corrosion typically initiates at a later time on specimens in which the concrete contains supplementary cementitious materials such as fly ash, silica fume, slag or a combination of these materials when compared to reinforced concrete with no cementitious materials of the same w/cm ratio, cementitious content, and concrete cover. There are some reports [1-2] that indicate that the chloride threshold is lower on concrete with supplementary cementitious materials, particularly for concrete with high fly ash content (i.e., 50%); thus, this needs to be considered when modeling service life. There are several reports [3-4] that indicate that the corrosion rate of carbon steel rebar embedded in high performance concrete is lower when compared to rebar corroding on concrete with no supplementary cementitious materials. This observation in part can be attributed to the higher concrete electrical resistivity for concrete with supplementary cementitious materials when comparing concretes with similar moisture content. It is possible that there are reduced macrocell effects. The higher resistivity is in part due to pore refinement and higher tortuosity that develop with time on concrete structures with supplementary cementitious materials. In this investigation chlorides were driven into reinforced concrete specimens via an electro-migration method, as a way to accelerate chloride transport and allow corrosion to initiate after a short period of time. Mature (samples prepared in 2008) and recently prepared (April/2016) reinforced concrete samples were used in this investigation to gain additional insight on the corrosion propagation stage. The cementitious in the concrete of the older samples: 1) ordinary portland cement (OPC), 2) OPC and 20% fly ash, and 3) OPC, 20% fly ash and 8% silica-fume. Recent concrete samples contained 1) OPC and 50% slag or 2)OPC and 20% fly ash composition. All specimens had a w/cm ratio of .41 and 390 kg/m 3 of cementitious material. The older specimens had either a single rebar embedded or four rebars (# 5 rebar/2 inch cover). Recently prepared samples had one rebar (#3 rebar/0.75 inch cover). Solution reservoirs ranged from one inch to 4 inches in length, as a way to vary the anode length. Rebar potential was used to determine if corrosion had initiated. The corrosion propagation was monitored via linear polarization measurements, solution resistance measurements, and rebar potentials. The corrosion propagation monitoring ranged from 300 days to several years. 1. M.D.A. Thomas and J.D. Matthews, “Chloride penetration and reinforcement corrosion in marine-exposed fly ash concretes”. In: Malhotra VM, editor, Third CANMET/ACI International Conference on Concrete in a Marine Environment, ACI SP-164, Detroit: American Concrete Institute; p. 317-38 1996 2. F. Presuel-Moreno, M. Paredes, “16 Years’ Exposure Of Fly Ash And Silica Fume Concretes On Salt Induced Reinforcing Steel Corrosion: Corrosion Potential, Resistivity And Diffusivity”, International Conference in Durability of Concrete, Trondheim, Norway, June 18-21, 2012 (Proceeding published on USB/Electronic form) 3. C Andrade, C Alonso “Test methods for on-site corrosion rate measurement of steel reinforcement in concrete by means of the polarization resistance method” Materials and Structures, v37 p623, 2004 4. W. Morris, A. Vico, M. Vázquez, “Chloride induced corrosion of reinforcing steel evaluated by concrete resistivity measurements” Electrochemica Acta, V 49 pp4447–4453 2004