VALIDATION OF THE METHOD TO EVALUATE THE CORROSION PROPAGATION STAGE BY HYGROTHERMAL SIMULATION Simo ILOMETSa, Targo KALAMEESa, Jukka LAHDENSIVUb a
Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia,
[email protected] Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia,
[email protected] b Tampere University of Technology, P.O. Box 600 FI-33101 Tampere, Finland,
[email protected] a
Abstract Evaluating the propagation period for reinforcement corrosion in concrete facades is an important but complex task which contains a high level of uncertainty. Corrosion current intensity during the propagation period have been measured in a large number of studies and there is a general consensus in regard to factors affecting carbonation induced corrosion. Hence, a proper evaluation of hygrothermal conditions in concrete facade becomes crucial. In this study a method to calculate the corrosion propagation period was validated based upon a field survey of prefabricated concrete facades in large-panel apartment buildings. The method combines existing corrosion propagation models and the Delphin dynamic hygrothermal simulation tool, and takes into consideration material properties, carbonation depth, concrete cover depth, indoor and outdoor climate loads. With the proposed method, propagation consists of a time that is required for a concrete cover to begin cracking and a further expansion of the crack to open to 0.3 mm in width. As a result, the method is validated via the correlation between measured and calculated propagation periods across a range of twenty years. The sensitivity of the results are also studied. The method allows for an evaluation to be carried out on degradation, residual service life, and the need for the renovation of reinforced concrete facades. Keywords: corrosion model, corrosion propagation, concrete damage, service life, hygrothermal simulation
1
Introduction
The durability of concrete facades that were put up in the 1960-s and 1970-s has become a very relevant field of research, since degradation mechanisms - carbonation induced corrosion and frost damage - have become very evident. Detecting a correlation between actual deterioration and climate loads is a highly complicated task thanks to a large number of uncertainties such as, for example, material properties (depending on moisture conditions), local climate loads on the building site, and the complexity of deterioration phenomena. Hygrothermal simulation tools have a wide field of application when it comes to modelling heat, air and moisture transfer. Since the hygric conditions in concrete are mainly
responsible for corrosion intensity via resistivity, applying dynamic simulation tools becomes highly useful. The proposed method is attempting to fill in a gap that is related to the evaluation of carbonation induced corrosion. The aim of the paper is to validate the method by comparing the calculated corrosion propagation rates against the results of a field survey. An illustration of a research problem and a description of a wall that has been studied, with indoor and outdoor climate loads affecting its performance, is shown in Fig. 1.
Fig. 1 Illustration of a research problem (left and middle) and cross-section of a studied wall (right). Reinforcement corrosion takes place in exterior concrete if the carbonation depth dcarb>cover depth dc.
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Corrosion model 2.1
Background and theory
Corrosion of concrete reinforcement can be divided into two main stages: initiation and propagation, Otieno (2011). During the initiation stage, carbonation (i.e. the penetration of CO2 from ambient air into the concrete by diffusion) in concrete reaches the depth of the embedded steel and breaks down the passive film that surrounds the reinforcement (the onslaught of a grey tinge in Fig. 1, right, leading to the neutralisation of the concrete’s pH). The propagation stage covers the actual deterioration of the steel and the concrete cover. Corrosion current during propagation is an electrochemical process that comprises steel as a connector of anode and cathode, and pore water as an electrolyte. Electrical resistivity is related primarily to relative humidity (RH) in concrete, since resistivity/conductivity depends on the volume of water molecules in the pores. There is a thicker layer of water molecules on the pore walls at higher RH, causing higher conductivity i.e. lower resistivity. Corrosion current during the propagation stage depends on the following: the electrical resistivity of concrete (depending on concrete RH), temperature, oxygen availability, and the pH of the pore solution. A correlation between corrosion rate and resistivity has been found for carbonated concrete by Bouteiller et al. (2012), but this is controlled by resistivity up to certain critical RH ~90-95%, Yu et al. (2014). Above this figure, corrosion of steel is under the control of the cathodic reaction (oxygen diffusion) and the corrosion rate decreases due to a shortage of oxygen - see Fig. 2, left. Higher temperatures accelerate corrosion since there is more intensive electrochemical reaction that is caused by electrons that are moving faster. Broomfield (1997) has estimated the change in corrosion current at between five and ten times with a 10°C temperature drop.
The critical depth for steel corrosion (a cross-section loss) which causes cracks, chipping, and spalling of the concrete covering is 15–40 μm, Broomfield (1997), 15–50 μm, Alonso (1998), 54 μm on average (Köliö et al. 2015), and up to 100 μm. Critical depth depends on cover thickness and a lower value of 15 μm represents a small cover depth while the upper part at 100 μm is rather typical of a large cover depth. Empirical, analytical or numerical models to calculate the corrosion and service life of a structure have been developed, most of them reviewed by Ahmad (2003), El Maaddawy (2007), and Jamali (2013). Since the corrosion mechanism depends on many factors, calculation results are never accurate and tend to be valid only for the conditions in which they were developed. Rouchier et al. (2013) found that cracking affects the hygrothermal performance of the façade. There is a porous zone at the reinforcement/concrete intersection, but expansion of corrosion products creates internal stress that itself causes cracks in the concrete cover, Šavija et al. (2013). After the formation of the initial crack (w=0.05 mm, see Fig. 2, middle), propagation accelerates locally due to increased penetration of wind-driven rain (WDR). For the crack width to become visible, a range of 0.1–0.3 mm is needed, but this depends on a large number of factors - surface characteristics, surface covering, the distance of the observer, lighting conditions, etc. Limit criterion proposed by Khan et al. (2014), for a crack width wmax=0.3 mm was chosen in this study in order to evaluate the service life.
Fig. 2 On the left, corrosion current depending on the RH of concrete (valid for a temperature of +5°C), based on Tuutti (1982). In the middle, the ratio of the reinforcement’s cover depth/diameter causing the first crack (w=0.05 mm) and further crack evolution (w=0.3 mm, right) Alonso (1998).
2.2
Corrosion calculation
The process that covered the calculated propagation period tp,c was carried out by observing the following steps: Hourly values for temperature and RH were calculated inside the external concrete at the depth of the reinforcement dc (Fig. 1, right) by using a dynamic hygrothermal simulation tool (a description of the buildings and a simulation tool follows in a later chapter); Hourly values for corrosion current Icor were calculated from the RH according to Tuutti (1982) in Fig. 2, left (MS Excel post-processing); Hourly values for corrosion current Icor were corrected with the temperature (by 7.5 times with a 10 °C temperature change by reference to a 5 °C baseline); A cross-section loss of the reinforcement was calculated according to Faraday’s Law (Eq. 1), from El Maaddawy (2007): M loss
M I corr t zF
(1)
where: Mloss is the mass of steel dissolved at the anode during the overall time t, kg/m2; M is the molecular weight of corroding steel (M=55.8 g/mol); Icorr is the corrosion current, A/m2 ; t is the corrosion duration, s; z is the valence of corroding metal, i.e.
the number of electrons involved in the electrochemical reaction (z=2 for steel); F is Faraday’s constant, F=96487 A∙s/mol. A cross-section loss x0 that relates to the first visible crack (w=0.05 mm) depends on cover depth c, mm and diameter of the reinforcement d, mm, being calculated (see Fig. 2, middle): x0=7.53+9.32·c/d. The uniform corrosion of a cylinder-shaped reinforcement and the density of the steel ρ=7.85 g/cm3 is assumed. Further attack penetration for the crack’s opening, from 0.05 mm up to 0.3 mm (Δ0.25 mm crack width corresponds to Δ80 μm attack penetration) is independent of cover/ diameter ratio from Fig. 2, right. Alonso (1998) models were chosen since those showed the best performance in a solid comparison carried out by Jamali et al. (2013).
Finnish prefabricated multi-storey concrete-element dwellings from a low-rise district of Helsinki called Siltamäki were used in the study. The district was built up between 1967–1971 and a field survey was carried out in 2007. By this point, the age of the buildings covered by the field survey was between 36–40 years. The buildings have an external wall that comprises three layers - 150 mm of internal concrete, 80 mm of mineral wool, and 60 mm of external concrete - see Fig. 1, right, and Lahdensivu (2012). The profound dynamic hygrothermal simulation tool, Delphin 5.8.1, was used for the hygrothermal calculations. The software has been validated by Scheffler (2008) and Sontag et al. (2013) - see more information in Nicolai (2008) and in the user manual. Delphin has also been validated in a study in which the calculated results for hygrothermal performance were compared to the results from the field measurements in Ilomets and Kalamees (2013). Façades facing in various directions - south, west, etc - were selected according to the real-life situation of the building, and temperature and RH values during the propagation period were calculated in the outer layer of the concrete. WDR loads for the vertical façade, depending on wind speed, wind direction, and rain intensity, were calculated by the user according to the standard rain model contained within the Delphin software. This approach lead to an average catch ratio of η~0.2 which was representative of the lower part of the façade, i.e. the first floor for a building that had the dimensions of 100·10·20 m3 in Blocken et al. (2013). The WDR load was doubled (η~0.4) for the second floor and quadrupled (η~0.8) for the third floor. Delphin`s default material data and functions were mostly used in a study, although some of the hygrothermal properties of the concrete specimen were measured in the laboratory. Thermal conductivity was at λ=1.35 W/(m·K), specific heat capacity at ce=900 J/(kg·K) and water uptake coefficient at Aw=0.016 kg/(m2√s) - see also Tab. 1. The hourly outdoor climate readings (temperature, RH, wind direction and velocity, rain intensity, and diffuse and direct solar radiation) as measured by the Finnish Meteorological Institute during the propagation period tp from Helsinki was used. An indoor air temperature of +22 °C (with an outdoor temperature of
