What is SEM of concrete?
Scanning Electron Microscopy (SEM) in concrete provides a detailed examination of the microstructure at magnifications up to 100,000x, able to pick out defects 50x smaller than optical microscopes.
SEM facilitates the identification of concrete constituents, including aggregate, binder, and additives, and allows for the assessment of material characteristics such as porosity, water-to-cement ratio, and microcracking at the smallest scales to allow the earliest identification of potential deterioration.
This advanced imaging technique is crucial for evaluating the intricate details of concrete's microstructural behaviour and for the precise characterization of materials such as identification of hydration products or expansive reactions in both standard and sustainable concrete applications.
How does SEM of concrete work?
SEM in concrete operates by scanning a focused electron beam over a carefully prepared specimen surface, producing high-definition images that reveal the intricate microstructure of concrete with submicron precision.
The interaction between the electrons and the concrete's components generates various signals that can be captured to create detailed images. Techniques like backscattered electron (BE) imaging and X-ray imaging (XR) are often used in tandem, allowing for the segmentation of the image into its constituent phases.
This meticulous process requires highly polished specimen surfaces for optimal imaging and enables accurate measurement and analysis of microstructural features.
What is SEM of concrete for?
Deterioration process | Defects | Control of repairs |
- | - |
How do I carry out SEM of concrete?
Based on ASTM C1723-16 (2022), a simplified procedure for conducting SEM examination of concrete structures includes:
Ensure familiarity with SEM equipment, specimen preparation procedures, and complementary techniques as outlined in ASTM C856/C856M.
Choose samples that offer a representative view of the concrete's condition, including both deteriorated and relatively intact sections. This dual perspective helps in understanding the extent and nature of the deterioration or damage.
Cut samples to a manageable size, typically using a diamond saw to prevent alteration of the material properties.
After cutting, the sample should be mounted on a suitable SEM stub or holder. This mounting process often involves using conductive adhesives to ensure a good electrical contact and to prevent charging under the electron beam.
For some analyses, it may be beneficial to polish the sample surface to obtain a smooth area for examination. This is particularly important for techniques that require a flat surface, such as backscattered electron imaging or when performing quantitative EDS analysis.
To enhance conductivity and minimize charging effects, samples are usually coated with a thin layer of a conductive material such as gold, gold/palladium alloy, or carbon. The choice of coating material depends on the analysis requirements and the potential impact on the sample's chemistry during EDS analysis.
Before starting the examination, ensure that the SEM is properly calibrated and all operational checks have been completed. This includes verifying the functionality of the electron beam, detectors, and any software used for image acquisition and analysis.
Adjust the SEM optics, including focusing the electron beam and selecting the appropriate aperture to control the beam's diameter. The beam current and accelerating voltage will significantly affect resolution, depth of field, and the interaction volume within the sample, impacting both imaging and X-ray analysis.
Choose the appropriate detectors for the analysis. Secondary Electron Detectors (SED) for high-resolution surface topography imaging or Backscattered Electron Detectors (BSED) for compositional contrast based on atomic number variations in the sample.
Carefully position the sample on the SEM stage, ensuring it is securely attached and correctly oriented for the area of interest to be examined.
Establish the vacuum conditions in the SEM chamber. For conventional SEM, a high vacuum is necessary, while for variable pressure or environmental SEM (ESEM, LVSEM), the pressure can be adjusted to accommodate moisture in the sample.
Begin with imaging with low magnification to locate areas of interest and gradually increase magnification for detailed examination. Adjust focus, stigmation, and brightness/contrast settings as necessary to obtain clear images.
Produce images for analyses with a focus on important microstructural features like the size and shape of constituents, spatial relationships, and volume fraction of each constituent.
What equipment and expertise are required for SEM of concrete?
Equipment required for carrying out SEM analysis of concrete samples includes:
A Scanning Electron Microscope (SEM) capable of high magnification imaging. It should be equipped with detectors for both secondary electrons (SE) for topographical imaging and backscattered electrons (BSE) for compositional imaging.
Sample preparation equipment including tools for cutting, mounting, and polishing concrete samples, as well as equipment for coating the samples with a conductive layer (such as gold, gold/palladium, or carbon) to minimize charging effects under the electron beam.
A computer equipped with software capable of processing SEM images and allowing for the visualization, measurement, and interpretation of both morphological and compositional information obtained from the SEM/EDS analyses.
Vacuum systems capable of producing the high vacuum level required for traditional SEM.
Microtomes or ultramicrotomes for sample preparation where thin slicing of materials is required, though this might be more relevant for biological samples than for concrete.
For effective SEM examination of concrete, essential expertise includes operational knowledge of SEM and EDS systems for accurate imaging and analysis, skilled sample preparation, and analytical abilities for interpreting complex data.
Understanding concrete's material science is vital for meaningful analysis, alongside proficiency in software for data analysis. Safety awareness in equipment handling and sample preparation is also critical.
This comprehensive skill set ensures precise, safe examinations yielding insightful conclusions on concrete's condition.
What are the advantages of SEM of concrete?
SEM provides unparalleled resolution and magnification, allowing for detailed observation of features in concrete's microstructure and microcracks as small as 1 nm; essential for identifying minute defects and understanding the material's composition at a granular level.
The technology supports a wide array of applications by generating topographical, morphological, and compositional information. This versatility is invaluable in fields ranging from materials science to forensics.
With advances in technology, SEMs have become user-friendly, requiring minimal training for operation. This accessibility facilitates quick and efficient analysis.
Modern SEMs provide data in digital formats, making the sharing and analysis of findings more streamlined. This efficiency is crucial for both research and industrial applications.
What are the disadvantages of SEM of concrete?
The necessity for samples to be solid, small, and stable under vacuum conditions can be limiting. Materials prone to outgassing at low pressures may not be suitable for SEM analysis.
The significant investment in SEM equipment and the need for a controlled environment free from electric, magnetic, or vibration interference can be a barrier for some labs.
The requirement for conductive coating on non-metallic samples can introduce artifacts, although skilled preparation can minimize these effects.
How accurate is SEM of concrete?
Scanning Electron Microscopy (SEM) is a highly precise tool utilized in petrography for examining the microstructure of concrete, boasting a resolution limit around 1 to 2 nm under optimal conditions.
This exceptional resolution makes SEM particularly effective for detailed analyses of concrete deterioration mechanisms, such as the formation of ettringite or thaumasite from sulphate attack.
These mechanisms typically present in sizes ranging from a few µm to several tens of µm in diameter, well within SEM's capabilities. Alkali-aggregate reaction (AAR) gel, another prevalent deterioration product, forms in thin layers at the aggregate-paste interface, can also be resolved.
The most advanced SEMs, equipped with high-efficiency detectors and utilizing beam deceleration techniques, can achieve imaging at magnifications up to 100,000x or more. This level of detail is crucial for identifying the finest microcracks, often less than 1 µm wide, and other subtle microstructural changes that indicate the early or advanced stages of concrete deterioration.
In the context of concrete analysis, SEM's resolution facilitates the identification and differentiation of various deterioration mechanisms through their unique microstructural signatures.
SEM offers significantly higher resolution than Optical Microscopy (OM); 1 to 2 nm vs 200 nm under ideal circumstances. This substantial difference allows SEM to visualize structures and features on the nanometer scale, which are beyond the reach of OM.
Furthermore, SEM can identify and characterize features in concrete that OM cannot, due to its superior resolution and depth of field. For instance, SEM can reveal the precise morphology of cement hydration products, the texture of aggregates at the nanoscale, and the very early stages of microcrack formation.
This capability is critical for understanding concrete deterioration processes, such as the precise composition of AAR gel or the identification of specific sulphate compounds, which might not be distinguishable with OM.
In summary, while OM provides valuable insights into the microstructural arrangement of concrete components and can detect deterioration mechanisms like ettringite or thaumasite formation and AAR gel, SEM offers a deeper, more detailed examination.
It extends the observable range into the nanometer scale, allowing for the identification of early-stage microstructural changes, detailed morphological features, and chemical composition analysis that OM cannot achieve. This makes SEM an indispensable tool for comprehensive concrete analysis, supplementing the macro- and micro-level perspectives provided by OM.
What are the limitations of SEM of concrete?
The observation and microscopical description are very dependent of the observer experience.
While SEM provides detailed surface information, it may not fully reveal the internal composition or structure of the sample, which could be essential for comprehensive concrete analysis.
The need for conductive coating and dehydration of samples adds complexity to the preparation process, potentially altering the sample's natural state.
Despite advancements, the operation of SEM equipment still requires specialized training, and the cost may limit accessibility for smaller research or testing facilities.
Ancillary information
Maturity of test: > 10 years
Qualification & interpretation : Specialised lab
Service disruption: No
Preliminary works: No
Time consumption Low (< one hour)
Cost High
Access to element 1 face