Research and Development of Intelligent Cleaning and Mud Content Detection Technology for Machine-Made Sand

. In this paper, an improved method of sand and mud content detection for concrete is proposed, and a set of sand and mud content detection device for concrete is designed. In this paper, a concrete gas content meter and a bubble parameter analysis system were used to study the effect of sand clay content on the gas content of commercial concrete, the loss rate of gas content in warp time, the bubble spacing coefficient and the distribution of hardened appearance pores. The pore structure test shows that the increase of mud content will appropriately reduce the proportion of harmless pores in machine-made sand concrete, and play a role in refining the pore structure in machine-made sand concrete. By using PLC controller and automatic detection device of liquid level and turbidity, the problems of complicated operation, inconvenient carrying and poor repeatability of test data are solved. It reduces the influence of human operation error and improves the accuracy of test data.


Introduction
Concrete is composed of cement, sand, stone, water and chemical admixtures, when necessary, formed after cement condensation and hardening, with a certain strength and durability of artificial stone. Sand and stone are one of the main components of concrete, called aggregate, about 70% of the total volume of concrete. Workability refers to the concrete mix is easy to construction operation, can achieve uniform structure, forming dense performance. Workability mainly includes fluidity, cohesiveness and water retention. Mud content is an important index in the quality standard of concrete sand aggregate. High mud content may cause the increase of water demand, affect the workability of concrete mix and the strength of concrete, hinder the full development of cement and aggregate bonding, hinder the normal hydration of cement, or chemical reaction with cement components [1]. There are strict limits on the amount of mud in national and industry standards. Concrete requires that the clay content of natural sand should not be more than 5%, and the clay content of stone should not be more than 1.5%. Some scholars have proposed to use the "washing method in the screen" to detect the mud content of sand. Compared with the standard method, this method has a certain improvement, which can reduce the operation of sand repeatedly pouring back and forth, thus reducing the probability of sand loss. Some scholars have put forward an improved method to solve the problems in the detection method of sand and mud content for concrete. The main method is to screen the sand treated according to the standard method and remove the particles less than 0.075mm that failed to be discharged during the washing process, so as to make the detection result of sand and mud content more accurate. If the above two methods are combined to detect the mud content, its accuracy will be increased, but the possibility of sand loss still exists and the operation is more complicated. This paper designs a set of concrete sand mud content detection device, which can make the sand in the device cleaning, drying and weighing, solve the problem of easy loss of sand, eliminate the influence of human factors. This system is of great significance to control product quality and avoid quality disputes.

System Architecture
In this paper, the concrete sand mud content detection device is designed by mixing system, hair dryer system, 1.18mm square hole screen, 0.075mm square hole screen, water container, handle, base and shell. Among them, the stirring system, hair dryer system, 1.18mm square hole screen, 0.075mm square hole screen are located in the shell, the shell is fixed on the base. Considering the need to be weighed together with the device during the test, light materials are preferred for the manufacturing of the device [2]. The total weight should not exceed 15kg. The profile of the device is shown in Figure 1. The stirring system is composed of a motor, a rotating shaft and a brush stirring leaf. The motor drives the brush stirring leaves placed on the 1.18mm and 0.075mm screens to rotate in the water through the rotating shaft, so as to achieve the effect of sand cleaning. The hair dryer system consists of two high power hair dryers. After the sand is cleaned, start the hair dryer system, which can make the cleaned sand dry quickly under the condition of stirring and blowing hot air, and at the same time discharge the particles less than 0.075mm suspended in water or failed to discharge during the washing process [3]. One side of the shell of 1.18mm square hole screen and 0.075mm square hole screen are provided with small doors to facilitate the feeding and pouring of materials during the test. In this device, the water container is the supporting equipment when the test device is cleaning the sand. Its diameter is slightly larger than the diameter of the test device, and its height is slightly smaller than the height of the upper layer of the device, so as to prevent the water from being immersed into the upper live system when cleaning the sand.

Electronic control device
The electronic control system of the measuring device is composed of liquid level sensing device, turbidity detection device, A/D analog-to-digital conversion module, PLC control interface, inlet solenoid relay valve, inlet solenoid relay valve, water spraying device and power module, etc. ( Figure 2). PLC transmits the signal of liquid level sensing device and turbidity detection device to the upper computer through the analog-todigital conversion module [4]. The related configuration software queries the data in the library several times to determine whether the liquid level in and the turbidity in the discharge reach the set upper limit. Then, PLC controller according to the test program written in advance. By controlling the inlet solenoid valve and outlet solenoid valve switch, the mud content test is completed. The controller of the control system itself is a complete set of independent control unit, but also a basic data acquisition node, which can realize the automatic analysis of data and command execution.

Operation Process
First, open the lid, load 500-1000g sampling sand into the screen sand cylinder, manually start the hot air-drying device of the lid, and begin to dry the sand. After 3h, manually stop the hot air device of the barrel cover and open the balance weighing device of the barrel cover. The balance displays the weight G of sand after drying. PLC controller is energized, open the water inlet pipe valve, 15cm away from the upper part of the screen sand cylinder, cleaning cylinder wall is provided with a level induction device, when the level reaches the set level, induction signal transmission to the PLC controller, PLC controller through the electromagnetic relay valve control closed water [5]. After 120min, the PLC controller controls the outlet electromagnetic relay valve and inlet electromagnetic relay valve to open simultaneously. At this point. Manually start the switch of the precision augmenting electric mixer on the barrel cover, and the sand cleaning process begins. The outlet is connected with an online turbidity detection device. When the effluent turbidity index is lower than the set value, the detection signal is transmitted to the PLC controller, and the PLC controller controls the closing of the inlet electromagnetic relay valve. At the same time, the spray washing device on the inner wall of the fuselage is controlled to start, and the remaining sand on the sand cylinder wall of the screen is washed. At this time, manually turn off the precision augmenting electric mixer. After lmin, the spray washing device will automatically stop working, and all the water in the cleaning cylinder will flow out. The sand cleaning process is over. Manually start the hot air device of the cylinder cover again for sand washing and drying.

Raw Materials
The raw materials are 42.5R ordinary Portland cement, sand, stone, water and mud. The mix ratio was C30 concrete, and the same mix ratio was used in the test. Natural sand with different mud content was mixed to carry out the sand mud content test [6]. Sand with mud content of 1.0%, 2.5%, 3.5% and 5% were selected. In the test, all other materials were the same except for the different clay content of sand, and the test mix ratio was shown in Table 1. The sand clay content test is based on "Standard of Sand and Stone Quality and Inspection Method for Ordinary Concrete", and the concrete performance test is based on "Test Method for the Performance of Ordinary Concrete Mix".

Experimental Results
At present, with the popularization of engineering construction mechanization, the fluidity and slump loss of concrete is more and more high requirements, poor liquidity, slump loss is larger, will make the quality of concrete and construction performance will be seriously affected. As an important index affecting concrete performance, sand clay content should be paid enough attention to. The ratio in Table 2 was used in the test, and the single factor changed the mud content of river sand to 0%, 2.0%, 4.0% and 6.0% respectively. The influence of different mud content of river sand on concrete slump and expansion was investigated, and the test results were shown in Figure 3. It can be seen from the experimental results that the expansion degree of each grade concrete decreases with the increase of mud content, and the slump of C40 and C60 concrete also shows the same change trend, but the slump of C25 concrete increases with the increase of mud content within a certain range. Because the amount of cementitious material of C25 concrete is small, the cohesivity and the amount of coating is not good [7]. When the mud content is small, the slump is easy to appear like "straw hat", resulting in small slump and large expansion. When the clay content increases, the cohesiveness of C25 concrete gradually becomes better. This is because mud, as a powdery material, has a certain complement to the cementitious material of concrete, plays the filling role of admixture, and can improve the workability of new mixed concrete. When the mud content increased to a certain amount, the adsorption of polycarboxylic acid admixture increased obviously, and played a leading role, which showed that the concrete fluidity became worse and the loss became faster. FIG. 4 Experimental results show that, with the increase of mud content, the admixture content of each grade concrete increases continuously when it reaches the corresponding slump, the mud content increases from 2% to 6%, and the admixture content that meets the requirements of fluidity basically doubles about. This is due to the adsorption of clay particles on the admixture, resulting in the reduction of the water reduction effect of the admixture. Under the same conditions, the admixture amount is reduced, so that the concrete cannot reach the expected effect. In addition, with the increase of mud content in the test process, although the concrete can reach the target slump by increasing the admixture content, the expansion degree of concrete mix still decreases. This is because the increase of mud content improves the cohesiveness and water retention of concrete [8].
Although the slump of the mixture can reach, the fluidity becomes worse. The comparison samples without mud showed poor cohesiveness, bleeding and bottom grabbing, and the above phenomena were improved with the increase of mud content, which also indicated that clay had strong adsorption capacity to water and admixtures. The distribution of pores in concrete hardened appearance is an important component to characterize the appearance quality of concrete, but the causes of the distribution of hardened pores are complex, including mix ratio, admixture and construction technology, etc., will have a great impact on the distribution of pores. From the perspective of concrete performance, there are mainly two influencing factors: The first is the bubble structure inside the concrete. After the plasticity of the concrete is lost, the bubbles close to the formwork wall form pores. The other is the rheological properties of the slurry, the yield stress and plastic viscosity of the slurry directly affect the interface properties between the slurry and the template (figure 5). It can be seen from FIG. 5 that with the increase of mud content in machine-made sand, the average size of concrete appearance pores gradually increases, indicating that the increase of mud content has an adverse effect on the appearance bubble structure. However, the increase of plastic viscosity caused by mud is beneficial to the stable adhesion of bubbles to the formwork interface, and the increase of yield stress enhances the stability of the thin wall of the bubble slurry and weakens the agglomeration effect of the stirred bubbles. In general, bubbles with continuous size distribution are formed inside the concrete to adhere to the interface between the formwork and the slurry, thus increasing the average size of bubbles with hardened appearance. The increase of plasticviscosity of slurry caused by the increase of mud content not only increases the size of pores in appearance, but also increases the number of pores, which is mainly due to the influence of the increase of total gas content. The increase of mud content in machine-made sand not only has a certain ability of auxiliary air entraining for concrete, but also results in the rapid increase of the number of bubbles of all sizes in concrete, and the increase of mud content in an appropriate range can improve the bubble structure of concrete. However, when the mud content increases to more than 4%, the number of pores in concrete hardened appearance gradually decreases with the increase of mud content. In this case, it is not only affected by the decreasing effect of mud on air entraining, but also affected by the continuous increase of the average size of bubbles, that is, the bubble structure inside concrete presents a state of "integration" under certain gas content conditions. The increase in the proportion of large bubbles in all bubbles ultimately leads to a decrease in the number of pores per unit area in the hardened concrete appearance.

Conclusion
Whether the clay content of sand used in common concrete mix is accurate or not will have a profound influence on the quality of the final concrete pouring building. The sand and mud content i Bedding device for concrete has solved the disadvantages of complicated operation in conventional test, inconvenient equipment to carry, poor repeatability of test data and too long test operation time. The arrangement of PLC controller, automatic detection device of liquid level and turbidity not only reduces the influence of human operation error. It also improves the accuracy of the test data: the limited space in the barrel is adopted to optimize the spatial structure to the maximum extent, and the portability and flexibility of the concrete sand and mud measurement device is ensured on the premise of ensuring no influence on the test results. Under the same condition of controlling the initial slump of concrete, the gas content of concrete increases first and then decreases with the increase of the mud content of machine-made sand. When the mud content is less than 6%, the gas content increases with the increase of the mud content. When the mud content is more than 6%, the gas content decreases with the increase of mud content.