The Marshall Stability Tester is used to evaluate the thermal stability and deformation resistance of asphalt mixtures, and its test results are influenced by various factors. The main influencing factors are comprehensively analyzed from the aspects of environmental conditions, specimen preparation, instrument parameters, operating specifications, and data processing
1、 Environmental conditions
1. Temperature control
-The test temperature should be strictly controlled at 60 ℃ (45 ℃ can be used in cold regions), and a temperature difference exceeding ± 0.5 ℃ may affect the viscosity of asphalt and lead to stability deviation.
-The immersion Marshall test requires the specimen to be immersed in a constant temperature water bath for a certain period of time. If the water temperature fluctuates or the immersion time is insufficient, it may not be possible to accurately simulate the water damage effect.
2. Humidity and power stability
-A humid environment may cause sensors to become ineffective due to moisture. It is recommended to keep the instrument dry and store the sensors in a drying oven.
-Voltage fluctuations in the power grid need to be handled through voltage regulators or purified power sources to avoid data acquisition errors caused by unstable voltage.
2、 Preparation and maintenance of test specimens
1. Quality of test piece forming
-The number of compaction cycles and compaction degree of asphalt mixture directly affect the porosity. If the specimen does not reach the standard density (such as Marshall standard specimen size of Φ 101.6mm × 63.5mm), it will result in lower stability or higher flow value.
-When the surface of the specimen is uneven or has cracks, it needs to be remade, otherwise it may cause stress concentration and early failure.
2. Maintenance time and conditions
-The standard test requires the specimen to maintain a constant temperature at the specified temperature for at least 30 minutes. If the curing time is insufficient, the internal temperature gradient may affect the test results.
-The immersion test must ensure that the specimen is saturated, generally requiring vacuum saturation or soaking for more than 24 hours, otherwise the degree of water damage may be underestimated.
3、 Instrument parameters and equipment status
1. Loading rate and range
-The rate of load increase should be precisely controlled at 50 ± 0.5mm/min. If the rate is too fast, it will increase the impact effect and lead to a virtual increase in stability; If the speed is too slow, it may prolong the test time and introduce creep errors.
-The range of the pressure sensor needs to match the strength of the specimen. Exceeding the range may result in data truncation or instrument damage.
2. Sensor accuracy and calibration
-The accuracy of the flow sensor needs to reach 0.01mm. If the sensor is contaminated or aged, it may cause zero drift and needs to be calibrated regularly.
-Poor grounding or electromagnetic interference may cause signal distortion, so it is necessary to ensure that the instrument is independently grounded and kept away from strong electrical equipment.
4、 Operating standards and human factors
1. Installation and alignment of test specimens
-The specimen needs to be placed vertically at the center of the indenter. If it deviates from the axis, additional bending moments will be generated, resulting in low stability and high flow values.
-Before loading, it is necessary to check the contact surface between the indenter and the specimen, and apply glycerin or anti slip pads to reduce uneven friction.
2. Experimental process control
-During the loading process, continuous monitoring of the force displacement curve is required. If any abnormalities occur (such as sudden drops in pressure or sudden changes in flow values), the machine should be stopped immediately to check the integrity of the specimen.
-After the experiment is completed, the power should be turned off in a timely manner to avoid prolonged power on accelerating instrument aging.
5、 Data processing and result analysis
1. Peak discrimination and correction
-Marshall stability is based on peak load, but if the curve exhibits multiple peaks or oscillations, the effective peak value needs to be determined in conjunction with the flow value. For example, a flow value that is too large (>15mm) may indicate that the specimen has undergone excessive deformation and the data needs to be excluded.
-The stability of immersion test and vacuum saturation test needs to be calculated separately and compared with standard test to evaluate water sensitivity.
2. Calculation of auxiliary indicators
-The Marshall modulus (MS/FL) can reflect the stiffness of asphalt mixtures, and its performance needs to be evaluated together with stability (MS) and flow rate (FL).
-Parameters such as porosity and density need to be measured synchronously with the volume and mass of the specimen to indirectly verify the reliability of the test.