What is Water soluble alkali testing?
Water-soluble alkali testing in concrete assesses the active alkalis remaining within the material, providing critical insight into potential for Alkali Aggregate Reaction (AAR) and deterioration of concrete structures.
How does Water soluble alkali testing work?
The water-soluble alkali test has been standardized for the analysis of cement and powdered concrete in ASTM C114 and BS EN 196-2 and involves extraction of the alkalis using hot-water from a powder concrete sample. The total soluble alkalis content is determined in solution by Fundamental Parameters (FP), Atomic absorption spectroscopy (AAS) or Inductively coupled plasma (ICP) spectroscopy.
What is Water soluble alkalis testing used for?
Deterioration process | Defects | Control of repairs |
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How do I carry out Water soluble alkalis testing?
A simple test procedure based on BS EN 196-2 includes the following steps:
Select a coring schedule and extract concrete samples from the structure, ensuring a representative cross-section for analysis.
Crush and then finely grind the concrete samples to a homogeneous powder.
Accurately weigh a precise amount of the powdered sample to determine the concentration of alkalis based on the solvent volume used for dissolution.
Use distilled or deionized hot water as the solvent to dissolve the alkalis, maintaining purity and preventing external contamination.
Select a ratio of solvent to powdered sample based on the expected soluble alkalis content as per BS EN 196-2 to ensure adequate dissolution without excessive dilution.
Stir the mixture of powdered concrete and solvent for a predetermined period at an elevated temperature to allow complete dissolution of alkalis.
Filter the solution post-dissolution to remove undissolved particles, ensuring the clarity and purity of the solution for analysis.
If necessary, adjust the pH of the solution to optimize alkali detection during analysis, depending on the sensitivity of the chosen analytical method.
Analyse the clear, filtered solution using Atomic absorption spectroscopy (AAS) or Inductively coupled plasma (ICP) spectroscopy to quantify the alkali content.
Interpret the results in the context of the concrete's durability and potential for alkali-silica reaction (ASR), considering the type of aggregates and environmental exposure.
What equipment and expertise are required for Water soluble alkalis testing?
Equipment required includes:
Coring equipment and a concrete saw for sample collection/preparation.
Mortar and pestle or ball mill to powder cores for chemical analysis.
- A precise electronic scale capable of measuring to at least 0.001 gram for weighing the samples.
An atomic Atomic absorption spectroscopy (AAS) or Inductively coupled plasma (ICP) spectroscopy test device.
Filter paper.
Hot, distilled or deionised water.
What are the advantages of Water soluble alkalis testing?
Distinguishes between internal and external alkali sources, critical for assessing alkali-silica reaction (ASR) risks and formulating mitigation strategies.
Supports the development of concrete durability strategies by providing crucial alkali content data, guiding material selection and additive use to enhance structural lifespan.
Can be used as a quality control and compliance test for concrete providers ensuring concrete mix and materials meet specific standards regarding alkali content, essential for quality assurance and regulatory compliance.
Can help to identify potential environmental sources of alkalis, aiding in environmental impact assessments and protection measures for construction projects.
Acts as a diagnostic tool for evaluating existing structures, identifying ASR risks, and informing remedial actions.
Relatively quick and low-cost test.
What are the disadvantages of Water soluble alkalis testing?
Involves labor-intensive preparation steps, including crushing and grinding concrete, which can be time-consuming and requires careful handling to ensure representativeness and avoid contamination.
Figure 5. Atomic absorption spectroscopy (AAS) and Inductively coupled plasma (ICP) spectroscopy test device
Necessitates access to precise scales and analytical instruments, such as AAS or ICP, and a high level of technical expertise for accurate execution and result interpretation, limiting accessibility for some organizations.
The testing may not extract all soluble alkalis, potentially underestimating the true alkali content if dissolution conditions are not fully optimized, affecting the accuracy of the analysis.
Only assesses alkalis soluble in water, potentially overlooking other alkali sources bound within the concrete matrix, thus not providing a complete picture of total alkali content.
The use of chemicals and solvents in sample preparation and analysis may pose environmental and safety risks, requiring stringent handling and disposal procedures.
How accurate is Water soluble alkalis testing?
The accuracy of water soluble alkalis testing for predicting alkali-silica reaction (ASR) in real structures is determined by the precision of test procedures, the skill of the operator, and the quality of the equipment used.
While this testing method is valuable for identifying potential ASR risks by quantifying water-soluble alkalis, its ability to fully predict ASR occurrence is contingent on integrating it with a broader assessment framework. This includes considering aggregate types, environmental conditions, and the overall composition of the concrete.
Advanced research suggests that a comprehensive evaluation combining direct testing, observational studies, and predictive modelling is essential for an accurate assessment of ASR mechanisms and potential in concrete structures. The effectiveness of water soluble alkalis testing, therefore, relies heavily on meticulous execution and contextual analysis within the broader scope of concrete durability studies.
What are the limitations of Water soluble alkalis testing?
Water soluble alkalis testing, while invaluable for assessing the risk of alkali-silica reaction (ASR) and ensuring the durability of concrete structures, comes with inherent limitations that necessitate cautious interpretation of results.
The method exclusively measures the alkalis that are soluble in water and not bound alkalis that could contribute to ASR. This could possibly result in an underestimation of AAR risk.
Additionally, the effectiveness of water soluble alkalis testing can be influenced by the age and condition of the concrete. These factors introduce variability that can complicate the interpretation of test results, necessitating a nuanced approach that considers the broader context of the concrete's service environment and history.
Operational conditions during testing, such as the precision of weighing scales, the purity of water used for dissolution, and the consistency of sample preparation techniques, can affect the reliability of the measured alkali concentrations. Ensuring strict adherence to standardized procedures and conditions is crucial for minimizing these sources of variability.
Despite these challenges, water soluble alkalis testing remains a critical tool in the assessment of concrete durability, with its limitations highlighting the need for comprehensive testing protocols that incorporate a range of analytical methods and consider the concrete's service life condition.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Specialised lab
Service disruption: No
Preliminary works: No
Time consumption Medium (one day)
Cost Low
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