Accelerated Expansion Test: Switzerland

Chapter 7 Accelerated Expansion Test: Switzerland Andreas Leemann, Christine Merz, and Stéphane Cuchet 7.1 Scope This test method covers the laboratory determination of the expansion rate of concrete cores (designated as “residual expansion potential” in the following context) extracted from structures affected by alkali silica reactions. This test differs considerably from methods where aggregates are extracted from concrete extracted from structures and reused in accelerated mortar tests with added alkali. In the concrete structures the decreasing alkali level in the pore solution seems to be the factor limiting expansion and not the availability of reactive minerals as in the accelerated mortar tests. This method does not encompass an assessment of the expansion taken place before the extraction of the cores. A. Leemann (B) Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Concrete and Construction Chemistry, Überlandstrasse 129, 8600 DÜbendorf, Switzerland e-mail: andreas.leemann@empa.ch C. Merz Merz Ingenieurberatung, Mőriken, Switzerland e-mail: merz@merz-ingenieurberatung.ch S. Cuchet Stéphane Cuchet, Holcim Suisse, Laboratoire des matériaux, Eclépens, Switzerland e-mail: stephane.cuchet@lafargeholcim.com © RILEM 2021 V. E. Saouma (ed.), Diagnosis & Prognosis of AAR Affected Structures, RILEM State-of-the-Art Reports 31, https://doi.org/10.1007/978-3-030-44014-5_7 175 176 A. Leemann et al. 7.2 Principles and Methodology The method described below is based on the method LCPC 44 Fasseu [1]. It has been used for various structures including the ones investigated in a project in Switzerland financed by the Federal road authorities [2]. Some of the results of this project are published in [3]. The residual expansion potential is determined on cores taken from structures. They are stored at 38 ◦ C and 100% RH for several months up to more than one year. The measured parameters are mass change, diametral and longitudinal expansions Fig. 7.1, measurements every 4 weeks at 20 ◦ C). The longitudinal expansion is used to assess the residual expansion potential. The principle of the method is described in the following paragraphs. It is not a manual for conducting the test, but describes its major characteristics. Advantages: 1. 2. 3. 4. no alkalis added. connection between length and mass change. classification of the residual expansion potential in three categories. concrete test. Disdvantages: 1. some alkali leaching, although minimized by protective measures (wrapping) 2. how can the measured expansion be used in models? The method is a test for concrete extracted from structures. It is not an aggregate test and differs considerably from methods where aggregates are extracted from concrete and reused in accelerated mortar tests with added alkali. In the concrete structures the decreasing alkali level in the pore solution seems to be the factor limiting expansion. Fig. 7.1 Core (diameter = 100 mm, length = 200 mm) used for the expansion test. The longitudinal expansion is measured on the length axis using two stainless steel pins fixed with glue in small boreholes. The diametral expansion is measured at two positions perpendicular to the length axis 7 Accelerated Expansion Test: Switzerland 177 7.3 Sampling and Sample Preparation Immediately after being taken from the structure, the cores are sealed in plastic bags. The cores are cut in the lab to appropriate size (typical diameter and length: 100 and 200 mm). Usually, three cores are tested per coring site. Small holes are drilled into the cores fitting the pins. As soon as the drying state of surface permits, the pins are glued (e.g. Araldite 2014-1) into the the small boreholes of the cores and aligned with a jig. Stable glue (e.g. organic two-component glue) has to be used that shows no volume changes at the conditions in the reactor. 7.4 Apparatus The apparatus used for the test (based on AFNOR P18–454 and employed in Swiss standard SIA MB 2042) is shown in Figs. 7.2. Three cores are stored in one container. There is water at the bottom of the containers, but as the cores stand on a grid no 1 Inlet preventing condensed water to drop on samples 2 Cover 3 Sealing 4 Handle with joint 5 Stainless steel grid keeping samples in upright position 6 Stainless steel container 7 3 samples (Ø ~ 100mm, L ~200 mm) 8 Stainless steel grid with opening width of ~ 10 mm 9 Water 38°C ± 2°C Fig. 7.2 Stainless steel container for Empa test 1 2 3 4 5 6 7 8 Cover Stainless steel Thermal isolation Container Water Immersion heater Stainless steel grid Thermocouple for temperature control 178 A. Leemann et al. direct contact with water is possible. The containers themselves are placed into the reactor with a climate of 38 ◦ C and 100% relative humidity. 7.5 Experimental Procedures The expansion of the cores can be divided into three different phases, Fig. 7.3: Phase 1: The initial measurements of dimension and mass are performed before sample conditioning. Sample conditioning is done at 20 ◦ C in containers with a thin layer of water at its bottom permitting capillary suction of the cores. The mass of the cores has to be measured daily. Conditioning ends when constant mass is reached. This is usually achieved after one week. Immediately after reaching constant mass, the cores have to be moved into the reactor. The water saturation is accompanied with a first swelling of the cores (hygroscopic swelling). Remark: Conditioning is mandatory, because otherwise the expansion related to moisture uptake cannot be distinguished from expansion related to ASR, after the samples have been placed into the reactor. This is the only way to ensure identical conditions as a starting point for cores of different structures making possible comparisons and classifications for the residual expansion. Phases 2 & 3: The cores are stored in the reactor at 38 ◦ C and at 100 % RH. After the cores are wrapped in a plastic film (minimizes mass loss and alkali leaching), they are moved into the containers. The containers are placed into the reactor. For the length measurements the containers are removed from the reactor and stored at 20 ◦ C for 24 h before the cores are measured. Afterwards the containers are put back into the reactor. The same apparatus as for the French performance-test AFNOR P18–454 is used. At the beginning of the storage at 38 ◦ C and 100% RH a strong but quickly levelling off expansion during 30–60 days is usually observed (phase 2). In the following phase 3 the expansion rate is constant. However, it can also level off in certain cases. Remark: Mass control is very important, but not required in all published methods [e.g. 1]. An preparation of the length change is difficult without having the data for the mass change available. The mass change is used as a control parameter in phases 2 and 3. It allows detecting unintentional changes of RH in the reactor or containers and assessing the reasons for irregularities in the expansion behavior of cores. Phase 4: The cores are removed from the reactor after 160 days. Afterwards, they are dried at 60–70% RH until they reach the identical mass as before conditioning started (phase 1). The difference in length change is defined as “irreversible” expansion and enables to distinguish between hygroscopic swelling due to the formation of newly formed ASR products. This step requires 2–8 weeks. So far, the longitudinal expansions during phases 2 and 3 are used to assess the residual expansion potential of the cores. However, a first assessment is usually 7 Accelerated Expansion Test: Switzerland 179 Fig. 7.3 Different phases of expansion during the residual expansion measurements in longitudinal and diametral direction of the cores already possible at the end of the non-linear expansion (phase 2). The determination of the irreversible expansion in phase 4 is used as a control parameter. It gives a rough indication about the magnitude of hygroscopic swelling and ASR-induced expansion. 7.6 Preparation Calculate the difference between the initial length of the specimen and the length at the end of each period. The mean of the longitudinal length change at the beginning of phase 2 (non linear expansion rate) and the mean of following linear length change (phase 3, linear expansion rate) are used to assess the residual expansion potential Fig. 7.4. The diametral length changes are used as a control parameter. They can indicate an inhomogeneity of expansion over the length of the cores. Constant relationship between longitudinal and diametral expansion has been observed during expansions test by several authors as Smaoui, Bérubé, Fournier, and Bissonnette [4], Larive [5] Multon, Leclainche, Bourdarot, and Toutlemonde [6] and ourselves. For instance we use the diametral expansion as control parameter and also to detect an eventual warping of the cores. The method has been used to assess the residual expansion potential of numerous structures in Switzerland, Fig. 7.4. The cores can be classified into three different levels. 180 A. Leemann et al. Fig. 7.4 Results of residual expansion measurement of cores extracted from concrete structures in Switzerland The differently colored domains are defined empirically, based on the experience from Swiss concrete structures. Blue domain: Green domain: Red domain: concrete with a residual expansion potential zero or very low. concrete with a moderate residual expansion potential. concrete with a high residual expansion potential. The domains shown in Fig. 7.4 were defined based on the experience gained from Swiss concrete structures They take into account the degree of damage of the structure at the time of core extraction, age and exposition of the structure and concrete composition (alkali content, cement type, aggregates). As these additional characteristics are considered and influence the assessment of the structure, the domains overlap. The measured values give an indication about range of expansion rates to be expected in a structure. These values are influenced by the expansion that already took place and by the exposition of the studied structure or component. As an example, cores taken from a part of the structure with little ASR-induced damage may show a higher expansion compared to cores taken from a strongly damaged part. This emphasizes that the degree of ASR can vary widely within a structure. It is advantageous if structures are monitored and the measured expansions can be compared with the one determined in the laboratory test. This facilitates the assessment of the structure. 7 Accelerated Expansion Test: Switzerland 181 7.7 Examination at End of Test After the final length measurements, the condition of the wrapping, the stability of the pins and the characteristics of each core are examined. This includes: 1. Presence, location, and pattern of cracking, 2. Appearance of surfaces, surface mottling, and 3. Surficial deposits or exudations, their nature, thickness and distribution. 7.8 Report The report has to contain the following information: 1. Distribution and severity of damages in the structure, preferably including crackindices. 2. Location of the coring sites. 3. Microscopic investigation including petrography of aggregates, type of cement, mineral additions, microcracks with crack-indices on the microscopic scale, alterated aggregates, presence of ASR products and information about other processes like leaching, ettringite formation, carbonation etc. 4. Alkali content of the concrete as mass percent of potassium oxide (K2 O), sodium oxide (Na2 O), and calculated sodium oxide (Na2 O) equivalent. Refer to used method for extracting alkalis (water soluble and/or acid soluble alkalis). 5. If possible: source of aggregate/concrete. 6. If possible: Type and source of Portland cement/mineral additions with their respective alkali contents. 7. Any relevant information concerning the preparation of concrete core. 8. Development of length change of each set of three cores during phases 1–4. 9. Any significant features revealed by examination of the specimens during and after the test. 7.9 Limitations and Applicability to Analysis The method characterizes the kinetics of expansion at the conditions in the reactor (temperature of 38 ◦ C and RH of 100%). As such, it does not provide a value for residual expansion of the tested concrete. The expansion measured in this test shows a strong positive correlation to the crack-index determined in strongly damaged parts of structures with an age in the range of 40 years [3]. This correlation seems to justify using a parameter determined in this test for finite element analysis. Based on the experience with the method up to 182 A. Leemann et al. now, the nonlinear expansion in phase 2 is influenced by the degree of damage of the concrete at the time of coring. The linear expansion in phase 3 does not show such an influence and consequently better represents solely the kinetics of expansion. Therefore, it is recommended to use the approximately constant expansion rate in phase 3 for finite element analysis. It is likely that the expansion rate in the structure is lower due to the lower temperatures. However, a correction for the temperature at the location of the structure seems inappropriate, because the decrease of expansion by decreasing the temperature from 38 to 20 ◦ C is concrete- and aggregate-specific, as shown in Merz and Leemann [3]. As such, the use of the expansion rate in phase 3 enables the modelling of a worst case scenario and should allow identifying the areas of the structure for which expansion is the most critical. 7.10 Precision and Bias The dominating factor for the uncertainty of the expansion is the variation of the concrete cores. The magnitude of error of the measurement itself is negligible (±0.002 mm). Depending on the homogeneity of the concrete at the coring site, significant variations in the expansion of single cores within a set of three cores are possible. Therefore, all cores have to be considered, even if there is an outlier. References 1. Fasseu, P.: Alcali-Reaction Du Beton-Essai D’Expansion Residuelle Sur Beton DurciProjet De Methode D’Essai. Technical report No. 44. Laboratoire Central des Ponts et Chaussées (LPC) (1997). http://www.ifsttar.fr/fileadmin/user_upload/editions/lcpc/ MethodeDEssai/MethodeDEssai-LCPC-ME44.pdf 2. Merz, C., Leemann, A.: Validierung der AAR-Prüfungen für Neubau und Instandsetzung. Technical report 648. Bundesamt Fuer Strassenbau (ASTRA)/Office Federal Des Routes (OFROU) (2011) 3. Merz, C., Leemann, A.: Assessment of the residual expansion potential of concrete from structures damaged by AAR. Cem. Concr. Res. 52, 182–189 (2013) 4. Smaoui, N., Bérubé, M., Fournier, B., Bissonnette, B.: Influence of specimen geometry, orientation of casting plane, and mode of concrete consolidation on expansion due to ASR. Cem., Concr. Aggreg. 26(2), 1–13 (2004) 5. Larive, C.: Apports Combinés de l’Experimentation et de la Modélisation à la Comprehension del’Alcali-Réaction et de ses Effets Mécaniques. Ph.D. thesis. Paris: Laboratoire Central des Ponts et Chaussées (1998). https://hal.inria.fr/docs/00/52/06/76/PDF/1997TH_LARIVE_ C_NS20683.pdf 6. Multon, S., Leclainche, G., Bourdarot, E., Toutlemonde, F.: Alkali-Silica reaction in specimens under multi-axial mechanical stresses. In: Proceedings of CONSEC 4 (Concrete Under Severe Conditions), pp. 2004–2011. Seoul (2004)