Measurement Techniques And Investigations On Alkali-Silica Reaction
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Helena Keller |
Alkali-Silica Reaction (ASR) is widely recognized as one of the more prevalent deterioration mechanisms affecting concrete worldwide. The expansion process due to the formation of alkali-silica gel is not uniform process and requires continuous testing of the raw materials used for the construction. The following is a brief overview of the current standards and guidelines for the testing regarding the ASR damage potential. The suitable storage chamber and the newly developed benchtop reactor AKROMAT for continuous online measurement of the length change of the samples at 60 °C are shown later.
Durability And Cracking Tendency Of Infrastructure
Durability is closely connected to the dimension change of the building materials during the whole life time of the infrastructure. The main problem here, is the formation of cracks. A distinction is made between structural and non-structural cracks. Structural cracks are due to the external influences like unstable foundations while the non-structural cracks are local failures due to internally induced stress. Non-structural cracks are the result of interrelated complex processes at early age as concrete properties change rapidly and in the long run due to chemical processes like carbonatization, delayed ettringite formation or alkali-aggregate reaction (AAR). Cracking in cementitious systems appears due to the hydration, remineralisation and drying effects in the matrix and is based on the volume change of the building materials. The volume change of cementitious materials is known phenomenon resulting from structural and environmental factors and is of high interest when focusing on maintaining of durable structures.
The complex dimension change processes take place within the cement matrix resulting in early and long-term shrinkage and expansion. Expansion processes due to Alkali-Aggregate Reaction (AAR) are of a high importance in regards on the cracking formation and thus on durability of infrastructure. Alkali-Aggregate Reaction (AAR) is a reaction in concrete between the alkali hydroxides and certain types of aggregate. Two types of AAR are currently recognized, which are Alkali-Carbonate Reaction (ACR) and Alkali-Silica Reaction (ASR).
Alkali-carbonate reaction (ACR) was first discovered by Swenson in 1957. Up to now, ACR is still restricted to a few locations worldwide and therefore plays a minor role. Compared to ACR, ASR was first described in the USA in the 1930s and reported by Stanton. Now, Alkali-Silica Reaction is widely recognized as one of the more prevalent deterioration mechanisms affecting concrete worldwide.
Alkali-silica reaction and the formation of alkali-silica gel reaction product can lead to abnormal expansion within the concrete matrix and thus to cracking and damages of infrastructures. Once the crack is formed, moisture and alkali transport into the bulk can take place and thus accelerate the alkali-silica reaction. Once concrete has alkali-aggregate reaction, it is nearly impossible to stop the reaction.
Investigations On Alkali-Silica Reaction For Durable Construction
For ensuring durable construction, a number of specifications and practices have been developed in recent years resulting in different test methods for identifying reactive aggregates. The reasons are great variability in the composition of ASR gels and therefore difficulties in regards to basic research, the quantification of mineralogical aspects and in the last time additional problems due to usage of alkali containing de-icers. In addition, the damage occurs at different times after the completion of the infrastructures showing slow and late reacting aggregates.
Generally, test methods can be distinguished between dissolving methods, petrographic and mineralogical investigations mortar bar tests and concrete prism tests. The most common methods for the identification of ASR potential in the first instance are mortar bar and concrete prism tests, where the only one limit for the separation into suitable or unsuitable aggregates is the expansion of the sample. Thus, the detection of the ASR will be carried out by measurement of the length change of the specimen tested after a certain storage period.
According to different standards and guidelines, different storage conditions will be used. This results in different methods where the selecting has to be done between tests that are reliable but have a long test duration (up to one year) or rapid tests that often have a poor correlation with field performance. The search is still on for rapid and reliable test methods. The current objectives of the modern applications are to evaluate project specific concrete mixtures regarding their ASR-damage potential and thus avoid the damages and prolong the lifetime of the infrastructures. These requirements require the development of the new test procedures and new measurement technology.
Storage Chambers
Up till now, depending on the standard or guideline, sample storage is carried out at different temperatures. The most common test of ASR in Europe is the accelerated testing at 60 °C according to RILEM AAR-4 recommendations and the French standard NF P 18454. The ASR testing device Alkali-Silica-Reactor can be used for the sample storage for the investigation of ASR potential (fig. 1).
Fig. 1: ASR Chamber with Sample Container and Measurement Gauge for Testing Length Change of the Specimen
The temperature inside the chamber can be adjusted in the range of room temperature to 60 °C ± 2°C. The samples will be placed in the sample container and stored in the chamber. In addition, the chamber can be used as a fog chamber due to the fog generation system, which is recommended for the storage conditions at lower temperatures such as 38 °C or 40 °C. As example, results of the dimension change for the sample A and sample B are shown in Fig. 2 for the storage in ASR chamber at 60 °C and in Fig. 3 for a storage in a fog chamber at 40 °C. From the German national regulations, the maximum strain of 0.6 mm/m for the 40 °C fog test and 0.3 mm/m for the accelerated test method at 60 °C are allowed, to be specified as inactive aggregates.
Using such classical storage chambers, the samples needs to be taken out for the length change measurement. This takes place only few times providing limited number of measurement points. Due to the handling of the samples and the manual readout the variation of the measurement results is still high. In addition, taking out of the sample results in a temperature gradient due to the cooling down to the room temperature and later on, after the measurement is done, the heating up in the storage chamber. This leads in the build-up of additional stresses within the cement matrix showing additional length change and higher cracking potential.
Using such classical storage chambers, the samples needs to be taken out for the length change measurement. This takes place only few times providing limited number of measurement points. Due to the handling of the samples and the manual readout the variation of the measurement results is still high. In addition, taking out of the sample results in a temperature gradient due to the cooling down to the room temperature and later on, after the measurement is done, the heating up in the storage chamber. This leads in the build-up of additional stresses within the cement matrix showing additional length change and higher cracking potential.
Online Monitoring Of The Length Change During The Storage
The combination of the storage chamber and the continuous dimension change measurement is not trivial. The adjustment of the length change measurement techniques to the storage conditions for a reliable measurement faces major challenges. The high temperature and the high humidity as well as the high alkalinity offer very rough test conditions for sensors. The current available sensors are very sensitive especially to the high humidity and alkaline environment.
 Fig. 4: The Benchtop Reactor AKROMAT |
The new testing device AKROMAT was specially developed for an automated monitoring of the expansion of concrete prisms during the storage (fig. 4). Using inductive displacement transducer, it is possible to get continuous and reliable measurement results of the length change during the storage at 60 °C and high humidity. The resolution of the length change measurement is better than 0.5 mm and the accuracy is better than ± 1.5 mm. The testing device AKROMAT was designed as a benchtop reactor for the storage up to 6 samples. Due to the online measurement,the storage time can be shorten as soon the set length change is reached. The storage and the measurement procedure of the next sample can be started immediately afterwards independently from the other samples. |
Compared to the currently used testing methods, effort and time consumption are notably reduced. No manual readout is necessary due to the continuous recording of the length change during the storage. Hence, the interruptions of storage due to the take out of the samples, which is necessary for manual measurements, are avoided. Therefore, the study of ASR-mechanisms in more detail is possible. As example, a comparison of the measured results for sample A and sample B is shown in fig. 5.
In dependence on the storage conditions, the results show comparable tendency but different expansion values. For the non-continuously storage the expansion values are higher. Due to Wallau et al. and Krütt et al., the reason could be an additional dimension change due to the temperature change during the manual measurements. The influence of such effects is not well studied yet. Nowadays, the availability of suitable measurement techniques allows to focus the investigations and carry out the tests.
Fig. 5: Comparison of the Measurement Results of the Sample A
Conclusion
The most used building materials are cement based. During the whole lifetime the cementitious building material running highly interrelated processes and mechanisms depending on the structural factors and environmental conditions. The most critical process on durability is the formation of cracks and thus the shortening of the lifetime of the infrastructure.
The formation of cracks due to the build up of internals stresses is closely connected to the dimension change of the cementitious matrix. Cementitious materials are a multiphase system, where the hydration and crystallisation processes running in parallel. The understanding of these processes requires appropriate testing, which is very complex and not trivial. Depending on the task, design of the equipment as well as the test setup have to be selected and adjusted.
Nowadays, the ability of a modern testing equipment allows research and quality control on dimension change of the building materials from the fluid state right after water addition to the long-term investigations. In particular, the possibility of continuous measurement of the shrinkage and expansion behaviours at the special storage conditions opens up a multitude of possibilities to investigate hydration processes more in details. The durability of the infrastructures can only be achieved by understanding of mechanisms and the application of the knowledge for the building construction.
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