The durability of concrete structures is closely linked to the permeability of the thin layer of concrete that covers the reinforcement. This layer, known as the cover concrete, plays a critical role in protecting the reinforcement from corrosion caused by carbonation, ingress of chlorides, chemical effects, frost/thaw or frost/thaw/salt damage, and other aggressive environmental influences. However, the quality and properties of the cover concrete can vary greatly due to factors such as concreting conditions at the site, formulation of micro-cracks, and curing practices, which may differ from those used for producing concrete test specimens for quality control.
Currently, there is no universally accepted method for characterizing the pore structure of concrete and correlating it with durability. However, research has shown that the permeability of concrete to air and water can be a reliable indicator of its resistance to the ingress of aggressive media, both in gaseous and liquid forms, and thus its potential durability. Therefore, a rapid and non-destructive measurement of the permeability of cover concrete can provide valuable information about its quality and durability.
The Permeability Tester is a non-destructive instrument designed for determining the air permeability of cover concrete. It allows for a quick assessment of the durability of the cover concrete, which is critical for the overall durability of the concrete structure. The general arrangement of the permeability tester is illustrated in the figure below. However, there is currently no widely accepted method for determining the permeability of concrete under different environmental conditions and establishing limiting values for permeability. Nevertheless, the Permeability Tester offers a practical solution for evaluating the quality and durability of the cover concrete in a rapid and non-destructive manner.
Fig: General arrangement of Concrete Permeability Tester
Principle of Concrete Permeability Tester:
The permeability of concrete can be determined by measuring the rate at which air can be extracted from the concrete cover. This method provides a way to assess the resistance of concrete to various forms of degradation, such as carbonation and penetration of aggressive ions. Additionally, it can also be used to evaluate the quality of grout in post-tensioned ducts. By measuring the rate of air extraction, valuable information can be obtained about the permeability characteristics of the concrete, which can be indicative of its durability and long-term performance. This method serves as a practical tool for evaluating the effectiveness of concrete cover in protecting against potential damage from environmental factors and ensuring the quality of grout in post-tensioned structures.
Description of Concrete Permeability Tester:
The instrument’s technical details are provided in the following paragraphs:
Paragraph 1: The instrument’s specifications and features are outlined in detail. This includes information on its design, functionality, and performance capabilities.
Paragraph 2: The instrument’s components and materials are described, including details about its construction and build quality. This includes information on the type of materials used, their durability, and how they contribute to the instrument’s overall performance.
Paragraph 3: The instrument’s operating parameters and settings are explained, including information on its power requirements, temperature range, and other environmental considerations. This includes details about how the instrument should be used and maintained to ensure optimal performance and longevity.
Paragraph 4: The instrument’s data output and analysis capabilities are discussed, including information on the type of data it can collect, the accuracy and precision of its measurements, and the software or tools that can be used for data analysis. This includes details on how the instrument can be used for scientific research, quality control, or other applications.
Paragraph 5: The instrument’s safety features and precautions are highlighted, including information on any potential hazards or risks associated with its use, as well as guidelines for safe operation and maintenance. This includes details on how to handle the instrument safely, how to troubleshoot common issues, and what to do in case of emergencies.
(1) Display Unit
The device is equipped with non-volatile memory that can store data for up to 200 measured objects. The results can be displayed on a 128 X 128 graphic LCD screen. The device also features an RS 232 C interface for easy connectivity with other devices. Integrated software allows for printouts of the measured objects and data transmission to a PC. The device can be operated using 6 LR6 1.5V batteries, which provide approximately 60 hours of usage, or it can be powered by a commercial power unit with a 9VDC/0.2A output. The temperature range of the device is from -100°C to +600°C, making it suitable for a wide range of applications. The device comes with a carrying case that has dimensions of 320mm x 285mm x 105mm and a total weight of 2.1 kg, making it convenient to transport and store.
(2) Control Unit and vacuum cell
The volume of the inner chamber and hose, as well as the cross-sectional area of the inner chamber, are crucial terms in the formula used to calculate kT and L. Therefore, it is essential not to alter these values as they directly impact the results of the calculation.
The vacuum connection is a small flange with a 16 KF size, which is a standard measurement for vacuum systems. This flange is designed to connect the inner chamber and hose securely, ensuring a proper vacuum seal.
The carrying case provided for this equipment has dimensions of 520 mm in length, 370 mm in width, and 125 mm in height. It has been specifically designed to safely transport the equipment, providing protection during storage or transportation. The total weight of the carrying case, including the equipment inside, is 6.3 kg, making it manageable for transportation purposes. The dimensions and weight of the carrying case are standardized to ensure convenience and ease of use.
(3) Resistance probe WENNER –PROCEQ
The spacing between the electrodes is set at 50 millimeters. This distance refers to the measurement between the two electrodes, which are used in various applications such as electrical circuits, sensors, and electrochemical processes. The 50-millimeter electrode spacing is an important parameter that affects the performance and efficiency of these applications. It determines the distance over which electrical signals or reactions occur, and plays a crucial role in the overall functioning of the system. The specific electrode spacing of 50 millimeters is carefully chosen based on the requirements and specifications of the particular application, taking into consideration factors such as electrical conductivity, sensitivity, and desired outcomes. Proper electrode spacing is essential for ensuring optimal performance and reliability in a wide range of electrical and electrochemical systems.
(4)Vacuum pump:
The instrument in question is designed to be used in conjunction with a commercially available vacuum pump. The technical specifications of the instrument adhere to the DIN 28400 standard. The suction capacity of the instrument is rated at 1.5 cubic meters per hour. The final total pressure that the instrument can achieve is approximately 10 bar. The suction side connection of the instrument is equipped with small flanges, either 10 KF or 16 KF in size, depending on the specific configuration. One notable feature of the instrument is its high tolerance to water vapor, making it suitable for applications where water vapor may be present in the vacuum environment.
Methodology of Concrete Permeability Tester:
The method of measurement involves using a two-chamber vacuum cell and a pressure regulator to ensure air flow at right angles to the surface and into the inner chamber. The measurement is carried out on a dry surface without cracks, and care is taken to ensure that the inner chambers are not located above the reinforcement bar. The pressure loss is calibrated periodically, especially after significant changes in temperature and pressure. The electrical resistance of the concrete is measured 3 to 6 times, and the mean value is used to calculate the coefficient of permeability (kT) based on a theoretical model. When testing dry concrete, the results obtained using this method are in good agreement with laboratory methods such as oxygen permeability, capillary suction, and chloride penetration. The quality class of the cover concrete is determined from the calculated kT value using a table as shown below.
Table – Quality class of cover concrete
The permeability of concrete is influenced by various factors, with humidity being one of the main ones. To compensate for the effect of humidity, an additional measurement of the electrical resistance of the concrete is taken. The combination of the electrical resistance (denoted as ?) and the temperature (denoted as kT) is used to determine the quality class of the concrete. A monogram, as shown in the figure below, is used for this purpose.
Limitations of Concrete Permeability Test
When determining kT and p, it is important to avoid measuring on wet surfaces as moisture could potentially damage the pressure regulator’s membrane. The most accurate results are obtained when measuring dry concrete, and measuring p may not be necessary. To accurately assess the quality of the cover concrete for a structure or finished component, multiple measurements should always be conducted.
The classification of cover concrete quality based on table and monogram data is applicable to young concrete, typically aged around 1-3 months. However, measurements on concrete that is several years old may require additional considerations as the classification from the table and monogram may not directly apply.
Moisture content of concrete significantly affects gas permeability, and correction for this effect through electrical resistance measurement usually provides satisfactory results for young concrete. However, for old concrete, further investigations may be necessary.
The investigations were conducted using a vacuum pump with a suction capacity of 1.5 m3/h and a motor power of 0.13 kW, capable of achieving a vacuum of a few mbar. It is recommended to use pumps with similar power to ensure consistent results, as pumps with lower power may not reach the desired vacuum range of 10-50 mbar.
There could be three potential reasons for not achieving the desired vacuum range:
- The concrete cover may be too permeable, which is within the normal function of the unit.
- The concrete surface may be too uneven, as the rubber seals can only compensate for a certain degree of unevenness, indicating abnormal function.
- The unit may have a leak, also indicating abnormal function.