STEEL CORROSION IN CONCRETE: A COMPREHENSIVE EXPERIMENTAL PROGRAM AND PRELIMINARY RESULTS Steel corrosion in concrete

Transcription

1 STEEL CORROSION IN CONCRETE: A COMPREHENSIVE EXPERIMENTAL PROGRAM AND PRELIMINARY RESULTS Steel corrosion in concrete C.Q. LI, M. CLEVEN and F. ISAAC Department of Civil Engineering, Monash University Victoria, 3145, Australia Durability of Building Materials and Components 8. (1999) Edited by M.A. Lacasse and D.J. Vanier. Institute for Research in Construction, Ottawa ON, K1A 0R6, Canada, pp National Research Council Canada 1999 Abstract The emphasis of current research in steel corrosion is on the corrosion mechanisms and on the prevention and mitigation of corrosion from a material perspective. Little is known about how the material induced degradation affects the residual strength and serviceability of a concrete structure in a quantitative way. The intention of this paper is to discuss issues related to steel corrosion in concrete structures. A comprehensive experimental program aiming to investigate strength and serviceability deterioration of concrete structures is presented. The thrust of the program is that full size specimens are tested under service loading and subjected to simulated environment in an environmental chamber. The program will provide vital data on corrosion initiation, propagation and its effect on residual strength and serviceability of concrete buildings which can be used as a rational basis for rehabilitation and replacement of deteriorated concrete structures. Some preliminary results are also presented. Keywords: Reinforcing steel, corrosion, concrete structures, residual strength 1 Introduction Reinforced concrete structures are one of the most widely used engineered structures and are frequently subjected to extreme natural hazards, such as wind, earthquake etc. These structures also suffer material and structural deterioration to certain extent. Of three major mechanisms that cause structural concrete deterioration (Warner, et al 1989), steel corrosion in concrete is the most severe form of deterioration and is most difficult to deal with. Considerable research on

2 reinforcing steel corrosion has been undertaken. But the emphasis of current research is on the corrosion mechanisms and on the prevention and mitigation of corrosion from a material perspective. Little is known about how the material induced degradation affects the residual strength and serviceability of in-service concrete structures in a quantitative way. The corrosion induced structural deterioration is attributed to a combination of factors including area reduction and bond loss. This combined effect of steel corrosion on structural performance has not been fully investigated and an understanding of this effect can only be gained through experiment. Prediction of the remaining safe life of corrosion-affected structures must be based on models of structural deterioration (strength and serviceability). Such models can only be derived from realistic and accurate data. Field data is usually poor in accuracy and has a high degree of variation (uncertainty). Laboratory data so far is obtained from tests without a service load and hence they are not fully applicable to structures in-service (i.e. under loading). Research into the corrosion induced bond loss and its effect on structural strength and serviceability under operating loads is virtually non-existent as shown in literature, due mainly to a lack of experimental facilities capable of simulating a desired aggressive environment and maintaining load during the corrosion process. The intention of this paper is to discuss issues related to steel corrosion in concrete structures. Based on the understanding of the problem, a comprehensive experimental program aiming to investigate strength and serviceability deterioration of concrete structures, caused by corrosion of reinforcing steel in concrete, is presented. The thrust of the program is that service loading is to be applied to test specimens which at the same time are subjected to simulated marine environment in a large corrosive environmental chamber with salt spray facility. The proposed research will provide vital data on physical characteristics of corrosion initiation, propagation and its effect on residual strength and serviceability of concrete structures which can be used as a rational basis for rehabilitation and replacement of deteriorated concrete structures. Some preliminary results are also presented. 2 Current issues related to steel corrosion It is believed that research into structural durability should be focused on how to assess the performance over the time during the service life of the structures. Only in this way can the research of structural durability be of any practical significance and the benefit to the society and community be observed. Since the factors that affect structural durability are highly uncertain and timevariant, only a time-dependent reliability approach is justifiable in dealing with the problem. The theoretical framework for structural assessment based on timedependent reliability theory has been layed but substantial work is needed, particularly in experimental research, to make the methodology practically useable. Based on this understanding it can be envisaged that the followings are demanding issues in the research of concrete structural durability.

3 2.1 Initiation of steel corrosion under service loads A theoretical prediction of initiation time of steel corrosion in concrete is through Fick s law to estimate the diffusion of Cl - to the surface of the steel. Unfortunately, the assumptions built in this law in application to initiation prediction are not always met in practice. In particular, the service load induced cracks would invalidate the assumption that concrete be homogeneous material, even though it may be assumed to be homogeneous in the absence of cracks. Also the diffusion process assumed in Fick s law applies to semi-infinite solids with constant concentration of chloride on the surface, which limits its application to more common situations, in particular, the chloride concentration on the surface varies. Also half cell potential measurement for the initiation of steel corrosion seems to be very random. It is therefore desirable that a model be developed to predict the initiation of steel corrosion in a situation that is more realistic, i.e., a concrete structure in service under loading and aggressive environment. 2.2 Residual strength As has been noted steel corrosion is essentially a two-stage electrochemical oxidation process. The current research should be focused on the second stage, i.e., determining the strength deterioration which depends on corrosion rate and, in turn, various factors that are related to material and structural properties. However, very limited research has been undertaken using advanced stochastic approaches. Although in some countries, e.g., in Europe, considerable research has been undertaken (Andrade, et al 1996), care should be taken in considering the fact that substantial differences exist (in different countries) in environments, material constituents, construction practices and design loads. Theoretical models based on area reduction of the cross-section of rebars are more or less simplistic. The model is of the form as follows (Li and Melchers 1993) As (0) As ( t) Ar ( t) = (1) A (0) s where A s (t) is the area of reinforcing steel. It is evident that the residual strength of a concrete structure is not only related to area of rebar (although it may be dominant). It depends on a combination of factors, such as the area reduction, the bond loss at the interface of steel and concrete, expansion of cracks (deepening) and so on. These factors may also interact, resulting adverse affect on the strength of the structural section. All this means that any theoretical model needs to be verified by experiment. 2.3 Acceptable probability of deterioration failure In performance assessment, the probability of deterioration failure is checked against an acceptable probability of structural failure. How to determine the acceptable probability of structural failure is of great significance in decisionmaking in infrastructure (asset) management. The acceptable limit for the probability of structural failure can be determined in the framework of reliability-

4 based economic optimisation of structures. The basic idea of economic optimisation is to find an optimal balance between the total cost of a structure (including initial cost, maintenance cost, rehabilitation cost etc.) and the safety level that is required for the structure, which should be based on a socio-economic criterion (Li 1995). Although the concept of reliability-based optimisation as illustrated in Fig. 1 is not new, the topic has not been fully explored and a general solution to obtain the optimised safety index (acceptable failure probability) and to incorporate it in engineering structures is highly desirable. Fig. 1: Optimised safety level The total cost, C T, of a concrete structure (including initial cost, C I, maintenance and/or repair cost, C M,R, the cost for structural failure, C F, etc.), can be expressed as C T = C I + C M,R p f C F (2) Theoretically, a target acceptable limit for the failure probability can be determined by minimising the total cost of equation (2). 2.4 Durability design Traditionally the durability design of concrete structures is based on implicit rules for materials, material compositions, working conditions, structural dimensions, etc (SAA 1994). These deemed-to-comply rules are also related to the type of environmental exposure. Modern structural design codes should increasingly be based on the performance of structures. It must be ensured that the required performance exists throughout the service life of the structures. With deemed-to-comply rules it is not possible to give an explicit relationship between the performance and service life. For concrete structures these relationships are not yet available as design tools. They need to be developed. 3 A comprehensive research program on steel corrosion A comprehensive experimental program aiming to investigate strength and serviceability deterioration of concrete structures caused by corrosion of

5 reinforcing steel in concrete has been developed at Monash University since The first step was to apply for funding to build large corrosive chamber with salt spray facility. The overall objective of the program is to develop a reliabilitybased methodology to quantitatively evaluate structural deterioration of concrete structures and to predict the remaining service life. The experiments proposed examine those factors from materials, service loads and exposed environment that contribute to steel corrosion, including corrosion rate and debonding, which leads to loss of structural integrity and serviceability. 3.1 Large corrosive environmental chamber Funded by Australian Research Council, a large corrosive environmental chamber has been built in the Department of Civil Engineering, Monash University, Australia. It is believed to be a world first and largest environmental chamber with salt spray facility. The chamber has the following functions: Operating temperature range of 10 to 60 o C Operating relative humidity (RH) range of 50% to 100% Content of carbon dioxide of up to 4% Controlled salt spray facility (different content of sodium chloride, NaCl) The features of chamber are as follows: Plan dimension of 4 by 8 meters World largest with salt spray facility Fully computerised Full size structural members can be tested Service loads can be applied Tests can be controlled and accelerated Different environments can be simulated The tests that can be done include: Effect of material degradation on structures Effect of material constituents on material degradation Structural residual strength and deterioration rate under service loads Structural failure mechanisms Effect of material degradation on fatigue Diagnose and remedial strategies 3.2 Initiation of steel corrosion under service loads This is the first project of the comprehensive program, the aim of which is to determine the initiation time of steel corrosion in concrete from a probabilistic perspective. Specifically the project examines three factors that are deemed to contribute to the initiation of steel corrosion under service conditions. These factors are service loading, chloride content in the environment and concrete

6 constituents. The objective of the project is to develop a stochastic model to predict initiation of steel corrosion. The outcomes of the project will be: A quantitative understanding of the effect of service loading, chloride content and concrete constituents on initiation of steel corrosion in concrete; Stochastic models to predict the time of initiation of steel corrosion Details of specimens There are two types of test specimens: full size beams and model beams. The cross-section of the specimens is as shown in Fig. 2. There are 60 model beams: 54 to be tested under accelerated conditions, including: 3 cement type; 2 water cement ratios; 3 time points to obtain a function over time; and 3 replicates for statistical study. Concrete ingredients of model beams are as shown in Table 1. There are six full size beams to be tested under accelerated conditions, consisting of different mixes of concrete with 3 cement types: ordinary Portland cement; fly ash blended cement (3:7); and blast furnace slag blended cement (3:7); and 2 water cement ratios: 0.45 and 0.6. Y12 Y12 R (a) Full size Fig. 2: Cross-section of test specimens 62 (b) Model 62 Table 1: Concrete mixes used in model beams w/c = 0.45 Ingredient Normal Portland Fly ash Furnace slag Sand 62.5 kg 62.5 kg 62.5 kg Cement 31.4 kg 21.9 kg cement 9.6 kg fly ash Coarse Aggregate 94 kg 94 kg 94 kg Water 14.2 kg 14.2 kg 14.2 kg 21.9 kg cement 9.6 kg slag Test method The test rig for model beams is as shown in Fig.3. Loads up to 60% of the ultimate flexure strength are applied to model beams by tying rods. This load is

7 kept constant during the test duration. The acceleration is achieved by manually spraying the salt solution on the beams for wetting, and then being left in oven at 50 0 C for drying. There are two wet-dry cycles in a week. The test rig for full size beams is as shown in Fig.4, where the loads are applied using lead ingots, which are up to 60% of ultimate strength of the beam. For the full size beams the cycles will be controlled in the environmental chamber. Extra model beams and full size beams are subjected to natural cycles of wet and dry to obtain the actual cycle period of wet and dry under natural conditions Parameters to measure Three parameters are measured in the tests: (1) half cell potential; (2) depth of chloride penetration and; (3) chloride concentration. All these parameters are measured over time. The initiation of corrosion is determined when the chloride concentration on the surface of steel bars exceeds a threshold of 0.06% of concrete by weight and this initiation is confirmed with visual inspection by breaking open the specimens after cleaning. Half cell potential is also used as an indication of corrosion. tying nut A plate A tying washer to monitor the static load Section A-A beam support tying rod Fig. 3:Test rig for model beam specimens P P Fig. 4: Sketch of test rig for full size beams (symmetric) 3.3 Determination of residual strength The aim of this project is to investigate strength deterioration of concrete structures. In particular, it is to examine the strength deterioration due to a combination of area reduction of steel bars and bond loss. The factors to be examined are the same as in Section 3.2. The objective of the project is to develop

8 a reliability-based methodology to quantitatively determine residual strength of concrete structures and to predict the remaining service life. The outcomes of the project will be: A quantitative understanding of the effect of the build-up of corrosion products on residual strength and failure mechanisms induced by corrosion; A stochastic model of strength deterioration of concrete structures under corrosion attack and service loadings; A reliability-based method to predict the remaining service life of deteriorated concrete structures Details of specimens There are 28 beam specimens to be tested in the project, broken down as follows. 2 cement types: normal Portland cement and fly ash blended cement (3:7); 2 water/cement ratios: 0.45 and 0.6; 2 duplicates for statistical study; and 3 time periods: 3, 6 and 9 months duration respectively. An additional 4 beams (2 cement types, 2 water/cement ratios) will be used for long term testing to verify the data acquired under accelerated conditions. Typical cross-section of the beam is as shown in Fig. 5. Fig.5: Details of cross-section of fuss size beams Test method The test rig is as shown in Fig. 4. The specimens are exposed in the chamber for acceleration. A typical cycle of wet and dry is as follows: spayed with salt water for 2 hours until saturated. The salt (NaCl) content is 3.5% by weight to comply with Australian Sea Water; dry purge at 50 C for 24 hours. Generally there are 2 cycles a week. Service loads of 60% ultimate load are applied by lead ingots and kept constant during the test duration Measurement The following parameters will be investigated in the experiment: (1) Bond loss, which is determined by slips at the interface of concrete and steel and recorded with DeMag gauge; (2) Corrosion rate, which is determined by the cross

9 section reduction of steel bars; (3) Ultimate strength, which is determined by external loads at cantilevered ends; (4) Deflection, which is measured at the cantilever ends; and (5) Cracks. Crack width, depth and patterns will be measured and monitored 4 Preliminary results Preliminary results can be summarised as follows: Corrosion occurs where cracks are (see Fig. 6). So Fick s law is not accurate to predict the initiation time of steel corrosion when service loads may induce considerable cracks. In the tests cracks width varied from 0.2 to 0.5 mm. Half cell potential measurement is very unreliable (Fig. 7). At the time of corrosion, the range of half cell potential varied from 350mV to 600 mv. Chloride content seems to be more accurate than other measurements (Fig. 8). The bond failure in a pioneer test to determined the bond strength is in Fig. 9. Fig. 6: Corrosion initiation at cracks N FA BFS N FA BFS HCP (mv) Time (weeks) Fig. 7: Half cell potential varies with time

10 0.12 Chloride content (%) normal cement FA blended BFS blended Depth from surface (mm) Fig. 8: Chloride content (by weight of concrete) varying with the depth from surface The relationship between slip and ultimate load will be presented in the conference due to length limit of the paper. Fig. 9: Slag Blended Cement 200mm Transmission Length (Beam 3) 5 Conclusion Current issues related to steel corrosion in concrete structures have been discussed. A comprehensive experimental program aiming to investigate strength and serviceability deterioration of concrete structures has been presented. Preliminary results from this program have also presented. It can be concluded that corrosion initiation and propagation in concrete structures without loading are different to those under loading. The focus of research on concrete durability should be on how to assess the structural performance and hence to determine the maintenance time and service life of the structures.

11 6 References Andrade, C, et al (1996), Advances in the On-Site Electrochemical Measurement of Reinforcement Corrosion and Their use for Predicting Residual Life, Proc.13th Int Conf. on Corrosion, Keynote Paper 3. Guirguis, S., (1990), Durable Concrete Structures, TN57, Cement & Concrete Association of Australia. Li, C.Q., (1995), A Case Study on Reliability Analysis of Deteriorating Concrete Structures, Struct. & Bldg., I.C.E., 110, (3), Li, C.Q. and Melchers, R.E., (1993), Simulation of Corrosion of Reinforcing Steel in Concrete Structures, Proc. 13 th Aust. Conf. on Mech. of Struct. and Mater., 5 7 July, Wollongong, Sagoe-Crentsil K.K, et al, (1989), Steel in Concrete: A Review, Magazine of Concrete Research., 41, (149), Sarja, A and Vesikari, E., (1996), Durability Design of Concrete Structures, RILEM Report 14, E & FN Spon, pp165. Standards Association of Australia, (1994), Concrete Structures, AS3600, Sydney. Warner, R.F., Rangan, B.V. and Hall, A.S., (1989), Reinforced Concrete, Longman Cheshire, (3rd Edition), Melbourne.