THE INFLUENCE OF FROST ATTACK ON CHLORIDE TRANSPORT IN CONCRETE

Transcription

1 THE INFLUENCE OF FROST ATTACK ON CHLORIDE TRANSPORT IN CONCRETE Miguel Ferreira (1), Hannele Kuosa (1), Markku Leivo (1) and Lasse Makkonen (2) (1) Lifetime Management Centre, VTT - Technical Research Centre of Finland, Finland (2) Efficient Machines & Vehicles Centre, VTT - Technical Research Centre of Finland, Finland Keywords Concrete, durability, degradation, chloride, freeze-thaw, frost attack, service life, migration, diffusion, scaling, internal cracking. Contacts Corresponding author Miguel Ferreira VTT - Technical Research Centre of Finland Kemistintie 3, Espoo P.O. Box 1000 FI VTT, Finland miguel.ferreira@vtt.fi Telephone:

2 THE INFLUENCE OF FROST ATTACK ON CHLORIDE TRANSPORT IN CONCRETE Miguel Ferreira, Hannele Kuosa, Markku Leivo and Lasse Makkonen VTT - Technical Research Centre of Finland, Finland Abstract Research on the durability of reinforced concrete structures is focusing ever more on the combined effect of degradation mechanisms. While most of the research has been directed at combined carbonation and chloride penetration, recently some attention has been given to emulate combined degradation conditions, especially those existing in Nordic countries (frost attack/chloride penetration), with regards to the harsh environmental conditions. Frost attack of concrete affects the chloride penetration by reducing the concrete cover, and more importantly, by changing the characteristics of the surface and internal concrete due to cracking. Recent research has shown there to be a synergetic effect, but no steps have been taken to characterize and comprehend the mechanisms involved, and to develop a procedure to transfer this knowledge to the industry and into practise. To address this need, a research project was set up with the objective of: ascertaining how ice formation in the concrete pore structure influences the transport properties of chloride ions in un-cracked concrete; to establish the characteristics of chloride ions transport in concrete subject to frost attack; and integrate the effect of combined degradation mechanism into a service life model. In this paper the, the preliminary result of this research project are presented. 1. INTRODUCTION In Finland, reinforced concrete structures (RCS) have to perform in rather difficult conditions due to the extremely harsh winters. This results in unique combinations of degradations mechanisms. Traditionally frost attack and carbonation have been considered as the predominant degradation mechanisms and the influence of chlorides as a secondary mechanism. However, in light of recent research in Finland [1, 2] and internationally, more 1

3 attention is now being drawn to other degradations mechanism such as chloride penetration and alkali-aggregate reactions and the relationship of these to frost attack. Concrete infrastructure located in cold climates frequently suffers from the effect of frost action during the winter periods. Frost action can result in the superficial scaling of concrete caused by freezing of a saline solution on the surface of concrete (salt scaling), and in the internal cracking of concrete caused by cyclic freezing/thawing cycles resulting in a reduction of strength and elasticity modulus. Salt scaling and internal cracking are distinct and separate mechanisms that can occur in similar environmental conditions. While scaling occurs mainly in the presence of salts, internal cracking depends mainly on the uptake of water and the critical degree of saturation. In cold climates, salts (mostly NaCl, CaCl 2 ) are regularly used to deice roads. As a result, scaling is one of the major durability issues facing concrete in this climate. While scaling alone will not render a structure useless, it results in accelerated ingress of aggressive species, such as chlorides. It renders the concrete susceptible to corrosion of the reinforcing steel [3-5]. Furthermore, the propensity for a high degree of saturation in the presence of salts can result in strength loss from internal frost action [6-8]. Both of these effects diminish the expected lifetime of concrete [9]. Much research has been done concerning the causes and mechanism of frost action. Due to the initial work of Powers [10-11] in the USA in the 1940s and 1950s, the microstructure of concrete became clearer even though his theories could not explain entirely the phenomena of frost damage. Since then, much research has been done on the topic, with significant Finnish contributions by Kukko [12-15] and Penttala [16], and Fagerlund in Sweden [17-20]. More recently, Jacobsen in Norway [21], Setzer in Germany [22, 23], Marchand in France/Canada [24], and Valenza [25, 26] and Sun [27] in the USA have suggested descriptions for the phenomena of frost damage. Despite all the advances in the research of frost attack, very little attention has been given to its effects on chloride penetration. Frost attack affects this phenomena by reducing the concrete cover, and more importantly, by changing the characteristics of the surface and internal concrete due to cracking [25]. As a result, it has been shown [1] that frost attack affects chloride penetration, but research has noted that this phenomenon is not yet understood. Research by Li [28] into the influence of chloride concentration by the effect of salt concentration, air-entrainment agents and the addition of silica fume in the concrete showed that there is a pessimum concentration of chloride related to the amount of scaling damage of the concrete surface. In addition, the use of air entrainment and silica fume improved the performance of the concrete with regards to chloride penetration (measuring migration coefficients), but does not explain the moisture uptake, chloride transport and binding in the concrete. It also showed that freeze-thaw action accelerated the chloride penetration in concrete because of the significant liquid uptake (pumping effect). Bouteille et al. [29] also studied chloride penetration into concrete during laboratory testing for salt scaling. Based on the probability distribution curves of the chloride concentration fronts and that of the scaled surface they concluded that the phenomena are distinct and not interrelated. While the assumption that they are distinct is correct, they are interrelated as the consequences of frost scaling change the properties of the concrete surface which in turn influences the penetration of chlorides. Leivo et al. and Kuosa et al. [1,2] studied the interaction between different degradation 2

4 mechanisms and showed that the combined effect of degradation mechanisms can have a synergetic effect on concrete deterioration. The results focused on the comparison of concrete performance/durability indicators in an attempt to ascertain the influence of the combined degradation mechanisms. It was found that internal freeze-thaw degradation, as defined by the relative dynamic modulus (RDM), tended to increase the chloride migration coefficient as determined after the freeze-thaw. Obviously the reason for this was the increased amount of internal microcracking caused by freeze-thaw. 2. OBJECTIVES The scientific aim of the research project described in this paper is to investigate the influence of frost attack of concrete (freeze-thaw with salt solution on the surface) on the transport characteristics of chloride ion in concrete. An attempt is made to minimise the influence of scaling and internal cracking. The aims will be addressed through three specific objectives: ascertain how ice formation in the concrete pore structure influences the transport properties of chloride ions; define the characteristics of chloride ions transport in concrete subject to surface scaling due to frost attack; develop more reliable service life design models that take into account the interacted effect of frost attack with (de-icing) salt on chloride transport into concrete. 3. METHODOLOGY The project is clearly inter-disciplinary as a result of the various expertise from different areas that are integrated: concrete materials science (degradation mechanisms), cement chemistry, ice physics and mechanics, probabilistic analysis and statistical quantification, material and structural modelling. Inevitably, many of these areas are intrinsically connected to one another. 3.1 Task structure The research project is divided into three main parts: i) Acquisition of knowledge; ii) Laboratory testing; and iii) Performance modelling. Each of these parts depends on the outcome of the previous part, and is divided into several tasks: Part 1 Acquisition of knowledge covers the preparation of two literature review reports on Freeze-thaw testing [30] and the Performance & durability of concrete in extreme cold environment [31], and international networking; Part 2 Laboratory testing looks at test setup modifications to simulate more realistic environmental loading. Aspects considered are the simulation of chloride movement in concrete pores due to ice formation, and characterization of chloride movement due to freeze-thaw loading (scaling with salt and possibly some internal cracking); Part 3 Performance modelling cover the development of design models (numerical simulation vs. testing result) with in situ performance calibration and validation, resulting in the specification of performance based design recommendations. 3

5 4. PRELIMINARY RESULTS At the time of writing, the concrete samples were undergoing different freeze-thaw cyclic loadings. Therefore, this paper presents only a description of the reference concrete and its performance, as well as a description of the testing regime and some preliminary results. 4.1 Concrete composition and basic characteristics Two concrete mixes were prepared with a 0.42 and 0.55 w/b ratio (B42 and B55). A CEM I 42,5 N-SR3 was used to minimise the possibility of chloride binding. A plasticizer (VB- Parmix) was used in the C42 mix, and an air entrainment agent (Ilma-Parmix) for both mixes. Details of the concrete mixtures and their workability are presented in Table 1. The characteristics of the air entrainment measured on hardened concrete is presented in Table 2. The concrete batches were produce in a vertical axis mixer with a 200 litre capacity. Cubic specimens of 100 mm and 150 mm in dimension and cylinders of 100 mm in diameter and 200 mm in height were cast in moulds and compacted using a vibrating table. 24 hours after casting the specimens were removed from the moulds and permanently stored in a climate room at RH 95% at 20 ± 2 ºC until testing. Table 1: Concrete compositions and workability Water Binder Aggregate Total (kg/m 3 ) & fractions (%) Slump Series w/b (l/m 3 ) (kg/m 3 ) (mm) Total <0.125 <0.250 <4.0 <10.0 B B Table 2: Characteristics of the air pores (AP) of hardened concrete. Series Protective AP mm (%) Compaction AP > mm (%) Total AP (%) Specific surface for protective AP (mm 2 /mm 3 ) B B Air (%) Spacing factor for protective AP (mm) From Table 2, it can be concluded that very good air entrainment was achieved in both concretes, as can be seen by the low spacing factor and the specific surface. 4.2 Initial testing durability performance The performance of the concretes was characterised by initial testing with the following tests: compressive strength, capillary absorption and porosity parameters, chloride migration, chloride ponding with profile determination, reference freeze-thaw scaling with salt solution including relative dynamic modulus, moisture uptake based on weight change and chloride profiles. In Figures 1 6 the results of these tests are presented. 4.3 Further testing arrangements Four different testing cycles were defined to quantify the possible influence of freeze-thaw cycles on the transport of chlorides in concrete. All test specimens were prepared in 4

6 accordance to the CEN/TS :2006 [32], except with regards to the age of the specimens. Three of the freeze-thaw testing cycles have one day duration, and follow the Figure 1: Compressive strength. Figure 2: Total porosity as function of depth. (a) (b) Figure 3: Pore volume type as a function of depth from surface (a) B42, and (b) B55. Figure 4: Chloride migration results. Figure 5: Chloride profiles from ponding test and reference freeze-thaw test. reference test procedure curve for freeze-thaw scaling, varying only in the minimum temperature reached: -5, -10 and -20 C. The rate of freezing was kept as close as possible to the reference test [33]. The fourth freeze-thaw testing cycle also reached -20 C, but with a cycle length of three and a half days in which the length of the extreme freezing temperature exceeded two days of duration. The total number of freeze-thaw cycles is 104 for the 1 day test cycle and 30 for the 3.5 5

7 day test cycle, respectively. The choice of different lower (negative) limit temperature in the cycles, and also different lengths of cycle was to promote different freezing-thawing behaviour of the brine solution in the pore structure of concrete. As the temperature decreases, or the longer a certain temperature is maintained, a greater volume of the pore structure is frozen. In addition, two ponding tests are carried out, i.e. at +20 C and +5 C. (a) (b) Figure 6: Surface scaling of concrete with salt solution (a) B42, and (b) B55. Figure 7: Temperature measurements of the +20/-20 C test cycle for concrete B42 and B55 at 5mm, 10mm and 15mm depths from the surface. During testing, scaled material, weight variation and fundamental frequency are measured. In addition, at certain intervals specimens are removed and the chloride profiles determined. To understand the temperature gradient across the concrete samples, a specimen for each concrete type was instrumented with thermocouples at 5, 10 and 15 mm depth from the 6

8 surface. Figure 7 presents the results of temperature measurements for the +20/-20 C test cycle. From the results it can be seen that the denser B42 concrete (lower air entrainment 4%) reacts faster to temperature change than the B55 concrete (higher air entrainment 6%). The temperature gradient in depth from the concrete surface can also been seen, however the change is minimal as expected. For the test cycles +20/-20 C setup, it can be seen that the concrete samples are subject to approximately hours of negative temperatures within the concrete, of which approximately 4 hours are lower than -20 C. 5. CONCLUSIONS This paper presents an overview of the research project where the main objective was to investigate the influence of frost attack of concrete on the transport characteristics of chloride ion in concrete. The results presented are limited to the performance characterization of the concretes used in testing and a description of the test setup adopted. The characterization results confirm that good air entrained concrete was produced for both water/binder ratio chosen (0.42 and 0.55), aiding the minimization of the effect of internal damage. ACKNOWLEDGEMENTS This research was supported by a TEKES FiDiPro Program grant for the project CSLA - Concrete Service Life Assessment: modelling frost attack degradation in the presence of chlorides (TEKES-40083/12). REFERENCES [1] Leivo, M., Sistonen, E., Al-Neshawy, F., Piironen, J., Kuosa, H., Holt, E., Koskinen, R., Nordqvist, C., Effect of interacted deterioration parameters on service life of concrete structures in cold environments. Laboratory results VTT Research Report. VTT-R , p. [2] Kuosa, H., Leivo, M., Holt, E., Ferreira, R.M., Effect of varying surface ageing on frost-salt scaling. Proc. 6th Int. Conf. Bridge Maintenance, Safety & Management. Italy, Stresa. July [3] Bertolini, L., Elsener, B., Pedeferri, P. and Polder, R. Corrosion of steel in concrete: prevention, diagnosis, repair. Wiley-VCH Verlag GmbH p. [4] Reddy, B., Glass, G.K., Lim, P.J., Buenfeld, N.R., On the corrosion risk presented by chloride bound in concrete. Cem. & Concr. Comp., 24(1) (2002) 1-5. [5] Alonso,C., Andrade, C., Castellote, M., Castro, P., Chloride threshold values to depassivate reinforcing bars embedded in a standardized OPC mortar. Cem. Concr. Res. 30 (2000) [6] Fagerlund, G., The international cooperative test of the critical degree of saturation method of assessing the freeze/thaw resistance of concrete, Mater. Constr. 10 (58) (1977) [7] Scherer, G.W., Crystallization in pores, Cem. Concr. Res. 29 (1999) [8] Scherer, G.W., Valenza, J.J., Mechanisms of frost damage, in: J. Skalny, F. Young (Eds.), Materials Science of Concrete, vol. VII, American Ceramic Society,( 2005) [9] Vesikari, E., Ferreria, R.M., Frost Deterioration Process and Interaction with Carbonation and Chloride Penetration Analysis and Modelling of Test Results. VTT Research Report. VTT-R , 40. 7

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