Rebar Stress Test Composites Strength and Electrical Properties

First, the material characteristics and production process Epoxy resin and curing agent are produced by Guangzhou Dongfang Chemical Industry Co., Ltd. E-44 epoxy resin and EP-type curing agent. Nanometer carbon black is HG-IP (black powder) manufactured by Shandong Zibo Huaguang Chemical Factory, with a particle size of 33 nm, a nitrogen adsorption specific surface area of ​​1,056 m2/g, and a specific resistance of 0.22o. The process of making cm composite resin: In the first step, the surface of a polished round bar with a length of 200mm is smoothed, cleaned with absolute ethanol, dried, and a thin layer of epoxy resin is applied to the surface (add 50 % Curing Agent Stirs the homogeneous resin about 0.2-0.5mm to insulate the reinforcing steel from the composite resin. And on the surface of the steel foil affixed foil strain gauge, model: BX120-3AA, Zhejiang Huangyan Test Instrument Factory, such as curing one day. The second step, weigh a certain ratio of epoxy resin and curing agent, stir, add thinner and tackifier, stir for a few minutes, and then add a little bit of nano-carbon black, due to the nano-carbon black is light, easy Reunion, not easy to stir, add a few times to add, each determined to have been fully stirred, then add material. In the third step, the composite resin with sufficient stirring is applied within a length of 2.5cm in the middle of the reinforcing bar. The thickness is appropriate, and about 1mm is appropriate. After winding copper wire, four, the inner two are separated by 1cm, and the outer two are separated by 2cm. Finally, a plastic film is wrapped around the outer layer of the composite resin and is cured.

Second, the test method and process The composite resin material was cured after 3 days to measure the resistance polarization curve, and the piezoresistive effect was measured at 7 days. The resistance test method used in the test was a four-electrode method and the resistance measurement instrument was Agilent Co. The HP34401A Digital Multimeter produced by Ltd. The loading equipment is a CMT5105 microcomputer controlled electronic universal testing machine produced by SANS. Before testing Gala Force, first connect the four electrodes of the test piece with the multimeter, and connect the multimeter to the computer with a data cable, and then measure the zero-load power pack. The piezoresistive test is performed when the measured resistance shown is stable. When tension is applied, the specimen is axially stretched at a speed of 0.5 mm/min, and the resistance, tension and strain are collected in real time by a computer.

Third, the test results and analysis As a sensitive material coating, the thickness is a factor that can not be ignored, the same amount of conductive material mixing evenly mixed composite resin, different thickness resistance difference will be very large, this article to take B1 group for research, divided into three Thickness: 0.5mm, 1mm, 1.5mm, three test pieces per thickness, the conductive properties of the composite resin coating at the same mix ratio, there is a greater difference with the thickness, the coating is relatively thin, the resistance Larger, as the thickness of the coating increases, the resistance gradually decreases, which is caused by the change in the conductive path inside the coating as the thickness changes. Epoxy resin is a thermosetting material that is not electrically conductive. Under the action of the curing agent, a space polymer structure is formed, and the addition of nano-carbon black increases the conductivity of the polymer due to the conductivity of the nano-materials, and the same ratio of the composite resin, the inner conductive channel with the thickness The increase increases, decreases and decreases, which is the reason that the resistance increases with the decrease of the thickness. Considering the problems in the process, if the resin coating is too thick, it is not easy to apply evenly, and the defects caused by using the structure are large and the stress concentration is serious. Take a thickness of about 1mm.

The properties of epoxy resin are largely related to the curing agent. The amount of the curing agent has a great influence on the resistance value of the composite resin, and it also affects the seepage curve of the composite resin. The curing agent accounts for the epoxy. The percentage of resin is: 25%, 50%, 75%. With the decrease of the content of the curing agent, the overall resistance drops significantly. Especially when the nanometer content is relatively low, the resistance value is significantly reduced by an order of magnitude. With the increase of the nanocarbon black content, the difference becomes smaller and smaller. With the increase of nano-materials, the density of the internal conductive vias increases, and the increase or decrease of the curing agent only changes the relative number of internal circuits, and the influence on the conductive paths is reduced. The piezoresistive effect test of the composite resin coating was carried out in two groups with curing agent contents of 25% and 50%, respectively. The composite resin with nanometer content of 15% has poor piezoresistive effect, taking 20%, 22.5%, 25%, and 27.5% to study its sensitivity. With cyclic loading, the maximum loading values ​​are 30KN and 40KN, respectively. Five cycles are loaded for each level. The steel bar is HPB235 steel with a diameter of 14mm, and its yield strength fy=32.31KN can be calculated. Loaded across two stages of elasticity and plasticity. R0 is the initial relative stable resistance value. The piezoresistive curves of the two groups of composite resins at seven days are obtained.

In the first group, the ratio of curing agent to epoxy resin was 0.25:1. From the smart characteristic curves of four different nanometer percentage composite resin coatings, we can see that, under the action of the circulation of constant amplitude loads, There is a good mapping relationship between the piezoresistive curve and the stress-strain curve of the resin. As the stress of the steel bar increases, the rate of change of the resistance changes approximately linearly. In the four different percentages of nano-doped composite resin, the nanoparticle content is 20 The two-line curves at % and 22.5% were more linear than those at 25% and 27.5%. As the content of nanocarbon black increases, the linear change in the rate of change in resistance is slightly worse.

In the second group, the ratio of the curing agent to the epoxy resin is 0.5:1. From the graphs of the piezoresistive test results of four different nano-components, we can see that the composition of nanometer particles in this group is 20%. The sensitivity of the resin becomes unstable. During the cyclic loading process, the resistance change rate curve begins to appear more nonlinear, and the 22.5% group still shows a good linear mapping relationship. The 25% and 27.5% groups follow the cycle addition. The performance of the unloaded resistivity curve is also not very stable and the linear mapping relationship deteriorates.

IV. Conclusions 1. With the adjustment of the proportion of epoxy and curing agent, the regularity of the resistance change, the greater the content of the curing agent, the greater the composite resin resistance of the same nano-carbon black content, and the resistance of the composite resin with curing. When the amount of the agent is decreased, when the ratio of the epoxy resin and the curing agent is fixed, the change in the resistance decreases as the amount of the nanocarbon black increases, and even if the same specification is used due to the limitation of the manufacturing process and the characteristics of the material properties. The composite resin has a certain degree of dispersion in the resistance value.

2. The performance of the alertness also changes with the percentage of curing agent and epoxy resin and the regularity of the amount of nano-carbon black. Under the action of equal amplitude cyclic loading at two stress levels, the composite resin shows force and resistance. The good mapping relationship of the rate of change shows that when the rebar enters the plastic stage from the elastic stage, the stress curve and the resistance change rate curve and the strain curve show a good correspondence. With the increase of the cyclic load action time, when the residual strain occurs in the steel bar, the resistance change rate also irreversibly changes.

3. Nano-carbon black composite epoxy coating is possible as a sensing material for stress and strain of steel bars.