LEACHING PERFORMANCE TEST ASSESSING DURABILITY OF CONCRETE EXPOSED TO CHEMICAL ATTACK

Transcription

1 LEACHING PERFORMANCE TEST ASSESSING DURABILITY OF CONCRETE EXPOSED TO CHEMICAL ATTACK François Jacquemot (1) and other members of GEF 8 leaching test working group (2) (1) CERIB (Centre d Études et de Recherches de l Industrie du Béton), France (2) See Acknowledgements Abstract This paper presents a methodology for ensuring the durability of concrete exposed to acid attack and demineralised water as defined in the XA exposure classes of standard NF EN The methodology is based on a performance approach, the equivalent concrete performance concept, using a test procedure and relevant durability indicators. The test consists in immersing concrete specimens in an acid solution and keeping the ph constant by adding small amounts of nitric acid. To maintain its leaching ability, the solution is periodically renewed. The indicator of the test is the depth of damaged concrete corresponding to a ph gradient in the concrete pore solution. A previous study compared the degradation of concrete exposed to naturally aggressive water and the same sound concrete tested with the present procedure. Results show that similar mechanisms are involved. The first data on reproducibility from round-robin laboratory work is also presented. 1. INTRODUCTION Increasing the durability of concrete structures is a great challenge for the 21 st century. It would result in better use of natural resources and a decrease in carbon dioxide emissions. The development of new durability approaches is deemed to be of great interest for ensuring long service lives for concrete structures. The performance approach is based on test procedures and relevant durability indicators. Some concrete structures can be exposed to chemical attack. These include drainage channels, floor slats for livestock, water tanks, sewage pipes, radioactive waste containers, etc. To ensure concrete durability, it is essential to define relevant methods taking knowledge of leaching mechanisms into account. The scope of the study is to develop a methodology for ensuring the durability of concrete exposed to acid attack and demineralised water as defined in the XA exposure classes of standard NF EN

2 The principle of the test consists in leaching elements, in particular calcium, from concrete by immersing specimens in a chemically aggressive solution of dilute nitric acid. The ph of the solution, which is kept constant during the 2-month-long test, depends on the level of exposure considered (XA1, XA2 or XA3). A Damage Depth indicator (DD ph ) is determined by measuring the depth of the front of colouration shown by the phenolphthalein ph indicator. This leaching test is being discussed in the GEF 8 working group of the French standardisation concrete committee which has to draft an experimental standard and more generally to help define the content of the performance approach to durability. The names and affiliations of the participants are given in the acknowledgements. The main points argued are the scope of the methodology, the choice of the reference concrete, the relevant test indicators, and the influence of limestone aggregate. Some round-robin laboratory work has been done to assess the reproducibility of the test. 2. METHODOLOGY FOR APPLICATION OF THE TEST 2.1 Equivalent concrete performance concept The equivalent concrete performance concept is described in NF EN [1]. In the recent methodology for application of the equivalent performance concept [2], two concrete mixtures are defined: - A reference concrete: the mix is in accordance with standard durability requirements for a chosen XA exposure class: maximum water/equivalent binder ratio, minimum strength class, minimum equivalent binder content, cement type. Moreover, the reference concrete meets two additional requirements: - The water/equivalent binder ratio is 0.05 points less than the maximum ratio, - The equivalent binder content is 5% higher than the minimum content (table NA.F.1 of NF EN 206-1) or the water absorption coefficient is 0.5 points less than the maximum coefficient (table NA.F.2 of NF EN 206-1). - The concrete to be qualified: the mix is in accordance with standard durability requirements, except for binder specifications. The same aggregates are used for both concretes. The objective of this methodology is to compare the risk associated with both concretes by running the test simultaneously. Validation of the concrete to be qualified is complete when its degradation, measured by an indicator (DD ph for example, see 3.4), is seen to be less than that of the reference concrete: DD ph Concrete to be qualified DD ph Reference concrete 2.2 ph of the leaching solution The ph is kept constant during the test by repeatedly adding small amounts of nitric acid. The ph value depends on the required XA exposure class and corresponds to the lowest value of the ph domain of the exposure class: - ph = 5.5 ± 0.1 for XA1, - ph = 4.5 ± 0.1 for XA2, - ph = 4.0 ± 0.1 for XA3. 373

3 3. TEST METHOD 3.1 Scope of application This test applies to concretes concerned by the XA1, XA2, or XA3 exposure classes for acid attack associated with the presence of acid or demineralised water. It does not apply to sulphate attack. A working group of the French standardisation concrete committee has been set up to develop the test method, assess its reproducibility, and draft an experimental standard. For now, the test method applies to siliceous-aggregate concretes. Tests on limestoneaggregate concretes are currently in progress within the group and the results should show how the scope of application can be extended to these concretes. 3.2 Preparation of specimens Test specimens are produced for each of the reference concrete and the concrete to be qualified. They are taken from cored cylinders (80 mm dia., 100 mm thick). The cylinders are kept in lime-saturated water until the minimum age of 28 days and the cylindrical surface is then coated with acid-resistant mastic. The test specimens (80 mm dia., 30 mm thick) are sawn from the cylinder immediately before the start of the test. 3.3 Description of method The test consists in leaching calcium from concrete by immersing specimens in a chemically aggressive solution of demineralised water and dilute nitric acid. Calcium and hydroxide ions, mainly from dissolution of portlandite, Ca(OH) 2, are leached out. The ph value required is kept constant by repeatedly adding small amounts of nitric acid at a concentration of 0.25 mol/l. The temperature of the solution is kept constant using a double-wall reactor where water flows at 20 ±1 C (Figure 1). A continuous flow of N 2 purified by a calcium oxide solution is bubbled through the solution to avoid carbonation. F N 2 E H I J Figure 1: Experimental scheme of the test which first version was developed by Adenot [3] A: test specimen; B: test solution; C: demineralised water; D: calcium oxide solution; E: nitric acid at a concentration of 0.25 mol/l; F: refrigerating circulator maintained at 20 ±1 C; G: ph electrode; H: ph regulator system; I: ph value; J: value of nitric acid volume; K: magnetic stirring system. G A K B C F D 374

4 After addition of a total of 30 ml of nitric acid, the aggressive solution is changed and the calcium concentration is measured by atomic absorption spectrometry, inductively coupled plasma or ion chromatography. The volume of nitric acid is calculated to keep a sufficient calcium ion concentration gradient between the solution and concrete so that the solution continuously leaches calcium ions from concrete. 3.4 Indicators Three indicators are studied: - Ca leached (mol/m 2 ) is the amount of leached calcium ion per unit area of concrete exposed to the aggressive solution. The Ca leached indicator is measured throughout the test. For a concrete made of siliceous aggregates, Ca leached should be linear against the square root of time given that the reaction kinetics is controlled by a diffusion mechanism. - DD ph (mm) is a damage depth indicator determined after 2 months of testing by measuring the mean depth of the front of colouration obtained by spraying the phenolphthalein ph indicator. The damage depth is generally small and needs to be measured by numerical analysis of a magnified image of the sawn specimen (Figure 2). 2 mm mm 0.78 mm 0.65 mm 0.99 mm 0.72 mm 0.87 mm 0.77 mm 0.55 mm 0.74 mm 0.77 mm 0.68 mm 0.75 mm 0.78 mm 0.69 mm 0.97 mm 0.96 mm 0.71 mm 0.93 mm 0.96 mm 0.84 mm 0.97 mm Figure 2: Magnified scanned image of a sawn specimen of concrete 375

5 - DD eq (mm) is the equivalent damage depth indicator defined as: DD eq = 1000 x Ca leached / Ca initial Ca initial (mol/m 3 ) is the initial amount of calcium in the binder of the concrete specimen. When the concrete aggregate is limestone, it contributes to the amount of leached calcium. The DD eq indicator as defined is therefore affected. The GEF 8 working group is giving further thought to the indicator. 4. REPRESENTATIVENESS The degradation of a concrete exposed to naturally aggressive water (9-year-old water tank) has been compared with that of the same sound concrete tested with a similar procedure [4] (ph = 5, nitric acid at 0.5 mol/l). SEM analysis (figures 3 and 4) reveals that for natural leaching there is a first zone characterised by a more porous matrix, absence of Ca(OH) 2, and progressive decalcification of other hydrates (C-S-H). Between this first zone and the sound zone, there is an intermediate zone where partial calcium leaching is observed. Results show that similar mechanisms are involved and lead to similar degradations. Figure 3 SEM image of water-tank concrete exposed to naturally aggressive water for 9 years. 1: surface; 2: leached zone; 3 and 4: intermediate zone; 5: sound zone; 6: aggregate Figure 4 SEM image of sound water-tank concrete exposed to the test. 1: surface; 2: leached zone; 3: intermediate zone; 4: sound zone; 5 and 6: aggregate 5. DATA FOR REPRODUCIBILITY This leaching test is being discussed in the French standardisation concrete committee. Two round-robin laboratory test campaigns have been carried out to assess the reproducibility of the test. The following data is from the second campaign of round-robin tests. 376

6 5.1 Concretes tested Two concretes with siliceous aggregates were made for the round-robin work. Concrete C1 is formulated with CEM I cement, C2 with CEM V/A (S-V) cement. The compositions and characteristics of both concretes are shown in table 1. Table 1: Mix designs and characteristics of concretes for round-robin testing Concrete C1 Concrete C2 Mix Designs Siliceous sand 0/0.315 mm (kg/m 3 ) Siliceous sand 0.315/1 mm (kg/m 3 ) Siliceous sand 0/4 mm (kg/m 3 ) Siliceous gravel 4/8 mm (kg/m 3 ) Siliceous gravel 8/12.5 mm (kg/m 3 ) CEM I 52.5 N CP2 (kg/m 3 ) 382 CEM V 42.5 N PM ES CP1 (kg/m 3 ) 383 Water (kg/m 3 ) Characteristics Fresh density (kg/m 3 ) Air content (%) Slump (mm) f c, cube, 28 days (MPa) f c, cube, 90 days (MPa) Concretes C1 and C2 were chosen because of their reputations for distinctly different performance relative to acid attack. Concrete C1 does not fit requirements prescribed by standard NF EN for the XA exposure class because of the nature of the cement. Concrete C2 fits all the requirements prescribed by standard NF EN for the XA2 exposure class. Tests are made with ph = Results A cylinder of each concrete (80 mm dia., 100 mm thick) made by a single laboratory and kept in lime-saturated water until the age of 90 days was sent to the four participating laboratories. Round-robin testing applied to the method described. Figure 5 shows that Ca leached is a linear function of the square root of time due to the fact that leaching works through a diffusion mechanism. Because of the properties of cementitious matrices, and especially the nature of cements, it can be seen that the Ca leached indicator for concrete C2 is less than that of C1. 377

7 4,0 Ca leached 3,5 Concentration (mol/m²) 3,0 2,5 2,0 1,5 1,0 0,5 0, Time (day 1/2 ) Lab 1 / C1 Lab 2 / C1 Lab 3 / C1 Lab 4 / C1 Lab 1 / C2 Lab 2 / C2 Lab 3 / C2 Lab 4 / C2 Figure 5: Round-robin results for Ca leached indicator Table 2 shows results for both indicators (DD ph and Ca leached ) after 56 days of testing. The variation coefficients for the Ca leached indicator for both concretes are considered to be satisfactory enough. Even if every laboratory verifies that DD ph C2 DD ph C1, the variation coefficient on this indicator is still high for C1. A round-robin campaign for measuring DD ph is now in progress. This work tends to show that the time interval between spraying phenolphthalein and measuring DD ph affects the value of the indicator. For future leaching tests, the time interval will be set at 24 hours as the colouration front has not been found to move after this time. Table 2: Data for reproducibility C1 C2 Lab 1 Lab 2 Lab 3 Lab 4 Mean value Standard deviation DD ph (mm) Ca leached (mol/m²) Var. (%) DD ph (mm) Ca leached (mol/m²) Another campaign of tests is planned for the near future: As the present procedure has proven to be satisfactory for siliceous-aggregate concretes, limestone-aggregate concretes are 378

8 to be tested for different XA exposure classes. The results should show if and how it is possible to extend the scope of application to these concretes. During this next campaign, the influence of time of curing should be evaluated on concretes formulated with CEM V cement containing pouzzolanic and latent hydraulic materials. 6. CONCLUSIONS The use of the equivalent concrete performance concept contributes to sustainable development through better use of natural resources and a decrease in carbon dioxide emissions. The method described, the choice of durability indicators, and the scope of application are the result of the work of a panel of experts from the French standardisation concrete committee. This leaching performance test has been proven to be representative of the in situ degradation of a 9-year-old water tank exposed to chemical attack, and its reproducibility has been assessed by round-robin laboratory test campaigns. The study shows that the main difficulty in developing a test in the field of a performance approach to durability is not necessarily the test procedure itself but could be the choice of the indicator for interpreting the results. It requires good understanding of the physico-chemical mechanisms involved and analysis to determine the limits of all indicators. ACKNOWLEDGEMENTS Members of GEF 8 leaching test working group, co-authors of the paper (in alphabetical order): A. Ammouche (LERM), C. Badoz (CERIB), A. Bertron (LMDC), J. Bosmet (Vicat), P. Brédy-Tuffé (Vicat), P. Cassat (CTG-Calcia), G. Escadeillas (LMDC), C. Foussat-Prévot (EDF), P. Francisco (CERIB), X. Guillot (ATILH), F. Jacquemot (convenor, CERIB), A. Jeanpierre (EDF), P. Le Bescop (CEA), A. Loukili (GeM-ECN), N. Meddahi-Trepy (CEA), A. Pavoine (LCPC), P. Rougeau (CERIB), E. Rozière (GeM-ECN), and G. Zammout (EDF). REFERENCES [1] NF EN 206-1, 2004: Concrete - Part 1: Specification, performance, production and conformity. [2] Cussigh F., 2009, Méthodologie d application du concept de performance équivalente des bétons, FNTP. [3] Adenot F., 1992, Durabilité du béton : caractérisation et modélisation des processus physiques et chimiques de dégradation du ciment, Thèse de doctorat de l Université d'orléans, France, 238 pages. [4] Francisco P., Badoz C., Rougeau P., 2007, A performance approach for durability of concrete exposed to acid attacks and aggressive water, Proceedings of the 5 th International Conference on Concrete Under Severe Conditions Environment and Loading, LCPC. 379