Cell for investigating triaxial mechanical properties of concrete under internal fluid pressure

Abstract

The present invention relates to the field of civil engineering and materials mechanics and is dedicated to the design and construction of a triaxial stress cell for materials to study the behavior of concrete under multiaxial pressures. In conventional methods, existing triaxial equipment is expensive, complex, and mainly designed for soil materials and is not capable of studying high-strength concrete.\t\nIn this invention, a high-pressure steel chamber containing hydraulic fluid is designed, which simulates confinement conditions by placing a concrete sample inside it and applying lateral pressure by a hand jack. Then, an axial compressive load is applied to the sample with the main hydraulic jack. The fluid pressure is controlled by a manometer and, in case of a sudden increase, it is discharged by a hydraulic pressure relief valve. Also, the presence of a protective piece made of phosphor bronze alloy and a strain gauge mounting hole increases the safety and accuracy of the test.

Description

Description of the invention Title of the invention Cell for investigating triaxial mechanical properties of concrete under internal fluid pressure Technical background of the relevant invention Civil Engineering-Mechanical Engineering-Mechanical Properties of Materials Technical problem and stating the objectives of the invention With the widespread use of concrete and other rock-like materials in engineering structures, understanding their mechanical behavior under different loading conditions has become of particular importance. In many structures such as dams, tunnels, foundations, pressure vessels, underground structures, and massive bridges, materials are subjected to multiaxial stresses that cause changes in the compressive strength, ultimate strain, and failure mode of the materials. In conventional axial compression tests, the effect of lateral confinement and effective lateral stresses on the actual strength of materials is not considered. This leads to a significant difference between the experimental results and the actual behavior of materials in the natural environment. For this reason, investigating the behavior of materials under triaxial stress conditions and applying controlled confining pressure is one of the most essential steps in the study of materials mechanics and the design of structures resistant to multidirectional pressures. Despite technical advances in laboratory equipment, existing triaxial cell devices have several limitations, including: High construction and maintenance costs due to the use of servo control systems and complex equipment; Focusing design on rock or soil specimens, not concrete and high-strength cementitious materials; Lack of safety and protective measures to prevent damage to the cylinder at high pressures; Inability to simultaneously record confining pressure and axial force with high accuracy; Requires constant energy sources to maintain fluid pressure during testing. The present invention aims to overcome these shortcomings by providing a device called a triaxial material stress cell, which is specifically designed for concrete and similar materials. This device allows for the creation of hydraulic confinement pressure within a controlled range and can accurately and stably apply triaxial stresses to the sample. The main objectives of the invention are: 1. Design and manufacture of a device capable of creating lateral confinement pressure by fluid to investigate the behavior of concrete under triaxial stress; 2. Providing the possibility of accurately measuring the compressive strength and strain of concrete in the presence of lateral pressure; 3. Using a manual jack to adjust the initial fluid pressure and the main hydraulic jack to apply axial load, in order to reduce costs and increase test control; 4. Design of a protective piece made of phosphor bronze alloy to prevent damage to the cylinder due to sudden and asymmetric loading; 5. Installing a special strain gauge hole to record the actual strains of the sample during testing; 6. Creating a safe mechanism for relieving excess pressure using a high-pressure hydraulic pressure relief valve; 7. Increasing the accuracy and repeatability of experiments by providing the possibility of simultaneously observing fluid pressure (manometer) and axial compressive force (load cell); 8. Reducing manufacturing costs compared to foreign models and providing the possibility of manufacturing and operating in academic and industrial laboratories. A description of the state of the prior art and the history of developments related to the claimed invention. With the advancement of materials mechanics and the need to simulate real stress conditions in massive concrete and stone structures, devices called triaxial cells have been developed over the past decades. These devices allow the study of the behavior of materials under multidirectional stresses by applying fluid confining pressure around the sample and simultaneously introducing axial load. In early equipment, triaxial cells were designed mostly for soil and rock samples, and the main purpose was to determine the shear strength and plastic strains under drained and undrained conditions. Later, more advanced laboratory specimens with servohydraulic control were introduced, which were capable of independently controlling all three stress components (axial and two lateral components). Although these devices have high accuracy, their application in routine academic and industrial experiments is limited in terms of cost, complexity of construction and maintenance, and the need for advanced peripheral equipment. In recent years, some researchers and laboratory equipment manufacturers have developed triaxial cells for concrete and cementitious materials. However, these models also generally have the following problems: 1. Focusing the design on stone or soil materials and not accurately matching the dimensions and mechanical behavior of high-strength concrete; 2. High manufacturing cost due to the use of multi-axis servo control systems; 3. Lack of a simple and safe system for applying initial fluid pressure without the need for permanent electric pumps; 4. Lack of safety measures to prevent damage to the cylinder due to sudden loading; 5. Inability to directly connect the strain gauge to the sample to record axial and lateral strains inside the cell; 6. Lack of ability to replace the loading series for testing samples of different dimensions (cylindrical and cubic). Internationally recognized examples include fully automated triaxial cells with computer-controlled fluid pressure and axial force (such as the MTS and GCTS systems), which are mostly used in advanced studies or large research projects. In these devices, the focus is on precise and simultaneous control of stresses, but the very high cost and complete dependence on external control equipment have prevented widespread use in educational and industrial centers. In the domestic domain, simpler triaxial cells have been developed mainly for soil materials, which are capable of applying limited pressure (up to about 2 MPa) and are not suitable for high-strength concrete specimens with standard dimensions. Therefore, despite the existence of various triaxial cells, a device that can be designed with low manufacturing cost, high safety, appropriate accuracy, and applicability for high-strength concretes is still not available. Providing a solution to an existing technical problem along with an accurate, sufficient, and integrated description of the invention The present invention is presented to overcome the technical limitations of conventional triaxial testing machines. In conventional triaxial cells, problems such as high cost, high complexity, limitations in applying pressure to high-strength concrete, lack of appropriate safety measures, and inability to install internal strain gauges reduce the accuracy and safety of the tests. To overcome these shortcomings, this invention designs a device with a simple, robust mechanical structure capable of simultaneously controlling lateral pressure and axial load, which enables the investigation of concrete behavior under triaxial stress with high accuracy. This device consists of a main chamber (high-pressure steel cylinder) in which a cylindrical or cubic concrete specimen is placed. The specimen is filled with hydraulic fluid around it to provide the lateral pressure required for confinement. The fluid pressure is adjusted by a hydraulic hand jack and can be viewed and controlled by a manometer. A variable steel loading series is installed on top of the cylinder, which is interchangeable for samples of different dimensions (100 x 200, 150 x 300 mm and 100 mm cube). Axial compressive force is applied to the upper series via the main hydraulic jack to transfer the axial load to the sample. To prevent damage to the cylinder body from sudden impacts or eccentric loading, a piece of phosphor bronze alloy is inserted between the cylinder and the loading head. This piece absorbs the impact energy and prevents stress concentration in the contact area. To control and relieve excess pressure, a high-pressure hydraulic pressure relief valve is installed in the system's hydraulic circuit. It operates automatically in the event of a sudden increase in pressure, preventing danger or damage to the device and sample. To accurately measure the strain of the specimen during testing, a special hole for installing a strain gauge is built into the cylinder body, which allows the wire to pass through and the strain gauge to be installed on the surface of the concrete specimen. In this way, strain changes during axial loading and lateral confinement are continuously recorded. The axial compressive force is measured by a precision load cell or load cell, and the fluid pressure is read by a manometer. The simultaneous combination of these two measurements allows for the accurate determination of the triaxial compressive strength and stress-strain relationships of the concrete. The device has high-pressure hydraulic connections and valves that allow for fluid injection, discharge, and adjustment during testing. Using a manual jack, the initial pressure can be adjusted to the desired value and then axial loading can be performed with the main hydraulic jack, without the need for complex servo control systems or a permanent power source. The cylinder body, caps, steel heads, valves, manometer and pressure relief valve are mounted in a sturdy steel frame that ensures stability, safety and accuracy of the device's operation. All components are designed to be modularly detachable in case of need for repair or replacement. Key features of the technical solution provided 1. Using a manual jack to adjust fluid pressure and eliminate the need for permanent electric pumps; 2. Simple mechanical design that can be manufactured with high precision in domestic workshops; 3. Presence of a pressure relief valve for safety and double pressure control during testing; 4. Ability to install an internal strain gauge to record the actual behavior of the sample; 5. Using a phosphor bronze alloy protective piece to increase cylinder durability; 6. Ability to replace the loading series to test samples of different dimensions; 7. High accuracy in simultaneous reading of lateral pressure and axial force; 8. Reduced manufacturing costs and increased safety compared to similar foreign equipment. Explanation of shapes, maps and diagrams Figure 1) Dimensional plan of the loading series from the top view Figure 2) Plan view of the cell without hydraulic equipment (dimensions are in centimeters) Figure 3) View of the cell with hydraulic equipment Numbering of parts in Figure 3: 1-Hydraulic drain valve 2-Hydraulic manometer 3-Double outlet hydraulic pressure breaker 4-High strength hydraulic connection 5-Loading series with the ability to separate the lower section to change dimensions 6-Phosphor-bronze pieces to protect the device against unbalanced loading 7- Detachable head of the device for connection with specific hydraulic systems (connection by screws) 8-Packings to seal the cell during loading 9-Sample holder with adjustable dimensions for samples of different sizes 10-Strain gauge input to check the strain of the sample during loading Figure 4 - Perspective view of the cell A clear and precise statement of the advantages of the claimed invention over prior inventions. Considering the review of technical records and existing devices in the field of triaxial testing, the present invention has significant advantages over previous domestic and foreign equipment in terms of design, safety, efficiency, and performance accuracy. These advantages are expressed as follows: 1. Simplicity of construction and low cost: The proposed device does not require complex servo control systems and expensive external equipment and is designed in a simple mechanical and hydraulic way. This feature significantly reduces construction and maintenance costs. 2. Precise control of lateral and axial pressure: Using a manual jack to adjust fluid pressure and a main hydraulic jack to apply axial force enables precise control of both stress components at low cost. 3. High safety: The installation of a high-pressure hydraulic pressure relief valve prevents sudden increases in fluid pressure and ensures the safety of the user and the device during testing. 4. Increased cylinder durability: Using a protective piece made of phosphor bronze alloy between the head and the cylinder body prevents mechanical damage due to uncertain loading and increases the useful life of the device. 5. Ability to install an internal strain gauge: This feature, which is not available in many common triaxial cells, allows for accurate recording of the actual strain of the sample and significantly increases the accuracy of the results. 6. Ability to test a variety of samples: The design of the variable loading series allows for testing cylindrical (100×200 and 150×300 mm) and cubic (100 mm) samples. 7. High accuracy in data recording: Both fluid pressure and axial force can be measured and monitored simultaneously by precise mechanical instruments (manometer and load cell). 8. Ability to manufacture domestically and easy to repair: All components of the device, including cylinders, valves, fittings, and protective parts, can be manufactured and replaced domestically and do not require special foreign equipment. 9. Easy to use and suitable for education and research: The ergonomic, safe, and simple design of the device makes it a suitable tool for educational and research laboratories in universities and industrial centers. Description of at least one implementation method for implementing the invention To perform a test using a triaxial stress cell for concrete materials, the following steps are performed: 1. First, a standard concrete sample (cylindrical or cubic) is placed inside the high-pressure cylinder. 2. The empty space around the sample is filled with hydraulic fluid until complete contact of the fluid with the sample surface is established. 3. Using a hydraulic hand jack, the initial fluid pressure is applied to the desired value (for example, 2 to 10 MPa) and controlled by a manometer. 4. After stabilizing the lateral pressure, the axial compressive load is applied to the sample from the top of the chamber using the main hydraulic jack. 5. During loading, the axial force is recorded instantaneously by the load cell and the lateral pressure by the manometer. 6. If strain measurement is required, the strain gauge is connected to the sample through a special hole in the cylinder body and axial and lateral strain changes are recorded. 7. At the end of the test, by opening the drain valve and hydraulic pressure relief valve, the fluid pressure is safely released and the sample is removed from the cylinder. This implementation method allows for examining the behavior of concrete under triaxial pressures and determining the effective compressive strength of materials with high accuracy. Explicit mention of the industrial application of the invention The present invention has wide industrial application in the field of civil engineering and materials mechanics and can be used in the following fields: 1. Concrete and building materials research laboratories to investigate the behavior of concrete under multiaxial stresses and determine ultimate strength and strain parameters; 2. Academic research and postgraduate theses in the field of concrete mechanics, behavioral modeling, and triaxial creep; 3. Design and evaluation of special structures such as dams, tunnels, underground structures and foundations that are subject to significant lateral pressures; 4. Construction industries and consulting companies to control the quality of materials and check the strength of special high-strength or fiber-reinforced concretes; 5. Application in domestic educational and industrial centers as a cost-effective alternative to expensive imported triaxial cells. Overall, this device, with its domestic manufacturing capability, high safety, appropriate accuracy, and low production cost, can be effectively used in the construction industry, materials research, and civil engineering education, and help improve the country's laboratory technology.

Claims

Claim What is claimed: Claim 1) What is claimed is a laboratory device consisting of a chamber containing high-pressure fluid (triaxial cell) that, by placing concrete samples inside this chamber and placing the chamber head, pressure is applied to this chamber by a hydraulic loading jack, allowing the simulation of lateral confinement for the sample and the calculation of compressive strength under triaxial stress. The components of this frame include: - A cylinder for placing the fluid and sample, - A series with the ability to change the size of the sample loading, - A piece of phosphor bronze alloy on the cylinder to prevent damage to the cylinder during loading, - High-pressure hydraulic discharge valves for feeding and discharging fluid to the system, - Manometer for observing the fluid pressure inside the cylinder, - Pressure relief valve for adjusting the fluid pressure inside the cylinder, - Possibility of placing a strain gauge to measure the strain of concrete samples - Hydraulic jack for applying force to the cylinder series for horizontal axis loading - Manual jack for initial adjustment of the fluid pressure inside the cylinder - Axial force measurement system applied by the hydraulic jack Claim 2) According to claim number 1, a manual jack is used to apply the initial fluid pressure after the sample is placed inside the cylinder, and after reaching the desired confining pressure, the load is applied via a hydraulic jack. Claim 3) According to claim number 1, the fluid pressure inside the cylinder is observed by a manometer, and this value is equal to the pressure inside the chamber, according to the principle of fluid pressure. Claim 4) According to claims 1 and 2, it is possible to control the fluid pressure inside the cylinder using a manual hydraulic jack and adjust it by observing this pressure value with a manometer. Claim 5) According to claim number 1, after the axial loading of the sample by the hydraulic jack begins, the high-pressure hydraulic pressure relief valve enables the double pressure relief for the samples and the control of the fluid pressure during the test. Claim 6) According to claim 1, a piece of phosphor bronze alloy is attached to the cylinder to protect the cylinder body from impact and uncertain loadings to prevent damage to the cylinder by absorbing the damage. Claim 7) According to claim number 1, to calculate the concrete strain inside the pressure chamber, a hole was made in the cylinder that allows the strain gauge to enter and connect to the concrete sample. Claim 8) According to claim number 1, it is possible to examine cylindrical samples of 100 x 200 mm and 150 x 300 mm and cubic samples of 100 mm in size by the variable series.

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