CHLORIDE INGRESS PREDICTION - PART 2: EXPERIMENTALLY BASED DESIGN PARAMETERS

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1 CHLORIDE INGRESS PREDICTION - PART : EXPERIMENTALLY BASED DESIGN PARAMETERS Jens Mejer Frederiksen (1) and Mette Geiker () (1) ALECTIA A/S, Denmark () Department of Civil Engineering, Technical University of Denmark Abstract Prediction of chloride ingress into concrete is an important part of durability design of reinforced concrete structures exposed to chloride containing environment. This paper presents experimentally based design parameters for Portland cement concretes with and without silica fume and fly ash in marine atmospheric and submersed South Scandinavian environment. The design parameters are based on sequential measurements of chloride profiles taken over ten years from 13 different types of concrete. The design parameters provide the input for an analytical model for chloride profiles as function of depth and time, when both the surface chloride concentration and the diffusion coefficient are allowed to vary in time. The model is presented in a companion paper. 1. INTRODUCTION Prediction of chloride ingress into concrete requires both a model and model parameters. As part of the Swedish project Durability of Marine Concrete Structures (BMB) [15] the Träslövsläge Marine Exposure Station was established in 1991 on the South Swedish West Coast 1 km south of the city of Varberg. Laboratory manufactured panels of different types of concrete from the early 199 s are exposed there. Chloride profiles have been measured three to five times during the first ten years of exposure in selected concrete types [15], [17]. Even though much longer exposure times are required for service life modelling of reinforced concrete structures the data may be the best field exposure presently available for calibration of chloride ingress models. This paper demonstrates how experimental data can provide estimates of design parameters needed for chloride ingress modelling. And a means of separating the effects of the concrete composition and the environment and to quantify with as little bias as possible is proposed. Also, the paper presents design parameters for Portland cement concretes with and without silica fume and fly ash in marine atmospheric and submerged South Scandinavian environment based on 1 years field exposure data. An analytical model for chloride profiles as function of depth and time, when both the surface chloride concentration and the diffusion coefficient are allowed to vary in time, is presented in a companion paper [I]. Both a list of symbols and the list of references are given in the companion paper. 3

2 . EMPIRICAL EQUATIONS FOR DESIGN PARAMETERS Empirical equations for generating expectation values of design parameters based on field exposure data from the Träslövsläge exposure station were originally proposed by Frederiksen et al. []. The equations provide estimation of the achieved surface boundary concentration at one year, C 1, and at hundred years of exposure, C 1, plus the achieved diffusion coefficient at one year of exposure, D 1, and the exponent in the equation for the time dependency of the achieved diffusion coefficient (and from also D 1 ). The inspiration for the empirical equations came from experiments carried out and reported by Frederiksen et al. [19] where laboratory tests were performed with different methods for determining diffusivity of concrete without silica fume and fly ash. Figure 1 shows the relation between the transport coefficient respectively the surface concentration and w/c for plain concrete tested according to NT BUILD 3 [1]. It is assumed that the type of correlation observed for plain concrete in the laboratory is valid for naturally exposed concretes with supplementary cementitious materials too; however, efficiency factors are introduced, ref. below. Dpex, mm²/year Dpex Csp D pex =5, exp(- 1/(w /c) ) C sp = 11 w/c R = w /c ratio by mass Figure 1: The relations between the parameters D pex respectively C sp and the w/c ratio are based on data from [19]. The results are from laboratory tested specimens made with a constant cement paste content at 7 % by volume and Danish SRPC (CEM I.5 N (HS/EA/<)). The tests were performed according to NT BULD 3 [1] from an age of days and with an exposure time of 35 days. The data in Figure 1 suggests that the effect of the w/c ratio on the diffusivity and the surface concentration could be functions of the types given below in eq. (1) and (), respectively: C 1 = (11/A) eqv (w/c) b k C1, env [% mass binder] (1) 1 D 1 = B 5 exp kd 1, env eqv ( w [mm ) /year] () c D Csp, % mass binder

3 Based on a preliminary study of the results from the field exposure the long time development of the parameters was suggested by Frederiksen et al. [] to be of the types given in eq. (3) and (): C 1 = C 1 k C1, env [% mass binder] (3) α = (U eqv (w/c) D + V) k α, env () Based on () and () D 1 is derived from: 1 D 1 = D 1 (5) 1 Frederiksen et al. [] introduced environmental correction parameters and efficiency factors for puzzolanas in order to generalize eq. (1) to (). The efficiency factor related to a property of concrete is defined as the mass of cement that can be replaced by one unit mass of puzzolana while maintaining the property. When taking the efficiency factors in to account the equivalent w/c ratio is obtained. 3. FIELD EXPOSURE DATA The data used for the establishment of design parameters were chloride profiles measured repeatedly on specimens at a Swedish marine exposure station. Sandberg et al. [15] describe the marine exposure station in detail, and the most recent set of data are given in [17]. Selected information on the concrete compositions is given in Table 1. All specimens were manufactured in the laboratory of the Swedish Testing and Research Institute, SP, in Borås and placed in the marine environment at an age of approx. 1 days. The panels were cured sealed at 1 ± ºC until exposure at an age of approx. weeks. A panel (1 7 1 mm) of each composition is mounted in an upright position on a pontoon in such a way that two local environments in principle are obtained: marine atmospheric zone and marine submerged zone. The pontoons are placed in Träslövsläge Harbour at the south west coast of Sweden, 1 km south of Varberg [15]. The chloride content and the temperature of the seawater in Träslövsläge Harbour (1 ± g/l and 11 ± 9 º C) represent an average marine environment from a Danish point of view, as the marine environment lies between the environment of the North Sea and the Baltic Sea. The panels have a thickness of 1 cm with the form and finished sides through which the chloride ingress is studied. Therefore, the chloride profiles in this study were all related to the amount of binder. For most of the chloride profiles the relation to the amount of binder was based on the calcium content of each ground sample from the cores, but some of the early chloride profiles were simply recalculated with respect to the average binder content in the concrete. 3.1 Evaluation of input data At the planning of the exposure station it was the intension to measure the chloride profile in the three different environmental zones: SUBmerged, SPLash and ATMosphere. The original thoughts were in fact followed [], [1], but due to the floating pontoons the splash environment became a very narrow band on the specimens and at the ten years inspection the measurements in this area were omitted. Therefore, in this study the splash zone were not considered. α 5

4 Table 1: Composition of panels at the Träslövsläge Marine Exposure Station []. Binder constituents: Cement A: Swedish sulphate resistant Portland cement (SRPC), cement DK: Danish SRPC, Norwegian silica fume (added as slurry), Danish fly ash. ID Number w/b ratio Cement Silica fume Fly ash Water Paste Cementitious materials Calculated density A % A % 15 Ö.3. A % A % A % 1 H A % DK % A % 5 H A % H A % H.3 5. A % 39 H. 93. A % 399 H A % 51 Unit By kg/m³ kg/m³ kg/m³ kg/m³ % volume % mass kg/m³ mass concrete concrete concrete concrete concrete concrete concrete The number of chloride measurements in the two local marine environments is 33 nos. in the ATMosphere zone and 5 nos. in the SUBmerged zone in total 797 chloride versus depth measurements. The measurements cover an exposure time from. to 1 years and they formed the basis for optimising the 1 parameters in the system of models. The first point of all chloride profiles and other dubious points were omitted when fitting the parameters to the chloride profiles. Examples of the chloride profiles from different exposure times obtained on one panel in the two marine environments: SUBmerged and ATMosphere are shown in Figure 1 in [I]. In Figure profiles degree of saturation in the two local environments are shown for six of the panels covered by this investigation [] ATM, 1.3 y Deg. of cap. Sat., S cap Depth below exp. surface, mm.7..5 SUB, 1.3 y Deg. of cap. Sat., S cap 1 Depth below exp. surface, mm Figure : Profiles for the degree of saturation at 1.3 years of exposure only for concrete nos.: 1-;1-35; H1; H3; H5; H.

5 . ESTIMATED DESIGN PARAMETERS By linking all the spreadsheets dealing with the Mejlbro-Poulsen Model, e.g. [I], the estimation equations (1) to (5) with information on concrete (Table 1) it was possible to perform one multiple regression analysis for optimising the 1 design parameters by a least sum of square fit. The derived design parameters are given in Table. The equations for some of the parameters cover both the atmosphere zone and the submerged zone. Therefore, the optimisation of the system of equations was performed using all data simultaneously. Thus the design parameters are influenced by data from both exposure zones. The correlation between the predicted (y mod ) and measured (y measured ) chloride content is shown in Figure 3. Table : Derived design parameters. Coefficient Value Coefficient Value B 7 k C1, env ATM. Parameters A 3 Surface k C1, env SUB 1.9 U.1 concentration k C1, env ATM V.9 k C1, env SUB 3.5 k D1, env ATM. Efficiency factors, k FA Diffusion k D1, env SUB.7 transport k MS 1 coefficients k α, env ATM.7 Efficiency factors, k FA.1 k α, env SUB. binding k MS. 5 Points estimated by the model, % mass binder 3 1 R ² SUB =.1 R ² ATM =.5 x=y ATM (33 points) SUB (5 points) Submerged Atmosphere Measured points up to 1.3 years of exposure, % mass binder Figure 3: Illustration of the quality of the fit. The figure comprises 797 nos. of corresponding chloride and depth measurements in naturally exposed concrete panels. 7

6 The distribution of the relative deviation ((y measured y mod )/y mod ) for each set of data in Figure 3 is ( ; s) = (.;.5) and ( ; s) = (.;.) for the atmosphere and the submerged zone, respectively. However, for single values a considerable deviation between the predicted and measured data can be observed. To demonstrate the application of the derived design parameters and the mathematical model presented in [I], chloride profiles for varying exposure time and paste composition (CEM I with and without fly ash and silica fume, w/b from.35 to.5) are shown in Figures and 5. Paste composition: w/b =.5; CEM I 1 %; fly ash %; silica fume % Parameters: (D aex, α, S p, p)= (35,.,.5, 1.3) Parameters: (D aex, α, S p, p)= (175,.57, 1.35,.) 1 1 ATM 1 y ATM 1 y SUB 1 y SUB 1 y Paste composition: w/b =.; CEM I 1 %; fly ash %; silica fume % Parameters: (D aex, α, S p, p)= (151,.,., 1.9) Parameters: (D aex, α, S p, p)= (19,.5, 1.,.1) 1 1 ATM 1 y ATM 1 y SUB 1 y SUB 1 y Paste composition: w/b =.35; CEM I 1 %; fly ash %; silica fume % Parameters: (D aex, α, S p, p)= (15,.,.19, 1.) Parameters: (D aex, α, S p, p)= (91,.5, 1.5,.1) 1 1 ATM 1 y ATM 1 y SUB 1 y SUB 1 y Figure : Estimated chloride profiles for 5, 1, 5, 5 and 1 years in the marine atomsphere and the submerged zone. The paste composition is stated above graphs. The units for the parameters given above the graphs are [D aex, α, S p, p] = [mm²/year; 1; %mass; 1].

7 Binder composition: w/b =.; CEM I 95 %; fly ash %; silica fume 5 % Parameters: (D aex, α, S p, p)= (3,.5,., 1.7) Parameters: (D aex, α, S p, p)= (19,.5, 1.3,.1) 1 1 ATM 1 y ATM 1 y SUB 1 y SUB 1 y Binder composition: w/b =.; CEM I %; fly ash 15 %; silica fume 5 % Parameters: (D aex, α, S p, p)= (1,.,.9, 1.) Parameters: (D aex, α, S p, p)= (5,.55, 1.5,.) 1 1 ATM 1 y ATM 1 y SUB 1 y SUB 1 y Binder composition: w/b =.; CEM I 5 %; fly ash 15 %; silica fume % Parameters: (D aex, α, S p, p)= (1,.,., 1.) Parameters: (D aex, α, S p, p)= (157,.55, 1.53,.) 1 1 ATM 1 y ATM 1 y SUB 1 y SUB 1 y Figure 5: Estimated chloride profiles for 5, 1, 5, 5 and 1 years. The paste composition is varied according to the information given above each of the graphs. The units for the parameters given above the graphs are [D aex, α, S p, p] = [mm²/year; 1; %mass; 1] 5. DISCUSSION The estimated chloride profiles in Figures and 5 reflect findings of a faster chloride ingress in the submerged zone compared to the atmosphere zone, cf. e.g. Figure 1 in [I]. Also, the estimated profiles reflect expectations of a reduced rate of chloride ingress with decreasing w/b, cf. e.g. Figure 1 (this paper). The derived design parameters cause contrary to expectations no effect of cement replacement by silica fume, whereas fly ash is observed to reduce the rate of chloride ingress. Compared to the effect of silica fume and fly ash 9

8 observed in investigations covering shorter exposure periods, e.g. [], [1] the transport related efficiency factors obtained based on the present data set is lower for silica fume (1, cf. Table ), but higher for fly ash (, cf. Table ). It should be noted that a separate efficiency factor is used for the effect on binding. Also contrary to expectations, the ageing effect is not significantly different between concrete with and without puzzolana. Further investigations should be made to verify the design parameters. Also, the design parameters should be used together and single values not be exchanged with single values from other studies. It shall be noted that the time dependency of the diffusion coefficient as well as the surface concentration is based on 1 years of field exposure data, only. By applying the mathematical model presented in [I] it is implicitly assumed that the observed trend will continue without a time limit. Thus, further investigations should be made to verify the effect of prolonged exposure time on the design parameters. Also the moisture state of the concrete should be considered. As shown in Figure six of the panels are not near saturation after 1 years; only some of the panels have a high degree of saturation and only in the outer surface of the submerged part of the panels. Possible increases in degree of saturation will increase the rate of chloride diffusion. From a durability point of view, prediction of chloride ingress should be combined with information on threshold values for corrosion onset due to chloride ingress. Also, or design purposes a safety concept should be applied. However, both issues are outside the scope of the present paper. 5. CONCLUSION Based on chloride profiles obtained over ten years from test panels of 13 different types of concrete naturally exposed at Träslövsläge Marine Exposure Station, Sweden, empirical design parameters for an analytical chloride ingress model have been derived. The applied analytical model describes the result of chloride ingress into concrete, when both the surface chloride concentration and the diffusion coefficient are allowed to vary in time without any time limit. Further investigations should be made to verify the empirical design parameters. The design parameters should be used together and single values not be exchanged with single values from other studies. ACKNOWLEDGEMENTS Thanks are due to B.Sc. students Nicolai H. Hoffmann, Tobias Høiagaard, Ronnie R. H. Frederiksen, Techn. Univ. of Denmark, for the initial preparing of the spread sheets with data. NOTATION Notations are explained in the companion paper [I]. REFERENCES All references are given in the companion paper: [I] Frederiksen, J.M. & Geiker, M.R.: Chloride ingress prediction Part 1: Analytical model for time dependent diffusion coefficient and surface concentration. ibid 9