STRESS–STRAIN MODEL STUDY OF FERROUS TAILING CONCRETE SHORT COLUMN RESTRAINT BY HOOPING

: This study analyzed the stress–strain curve of stirrup restraint ferrous tailing concrete by test and theoretical analysis. Twenty short column in strength grade 5 are used in the test with WC30, WC35, WC40, WC45 and WC50. The stress–strain data is achieved after the axial compression behavior test. It is found from the stress–strain curve that the stress–strain relationship is similar with natural sand concrete. Moreover, the whole stress–strain curve equation of stirrup restraint ferrous tailing concrete is put forward. The calculation results show that the data calculating by the equation are in good agreement with the experimental data.


INTRODUCTION
The natural sand is mainly used as fine aggregate of concrete at present. Natural sand is a kind of non-renewable resource, and it has caused poor resources and even serious consequences of soil erosion because of excessive exploitation of natural sand. At the same time, the number of abandoned tailings in China is also increasing year by year. Land resource was occupied by abandoned tailings, and the environment was destroyed. Ferrous tailing sand concrete was prepared by taking tailing sand as the fine aggregate. Material properties and working performance of ferrous tailing concrete is similar to ordinary concrete prepared by nature sand. In recent years the research of _________ material properties of tailings sand concrete have been achieved (Hu 2014;Kang et al. 2015;. But there are few studies on the properties of ferrous tailing sand short columns applied axial compression, especially the column which is made of ferrous tailing sand instead of nature sand. So it is necessary to study the axial compressive behavior of concrete short columns with iron tailings, and it will promote the application of tailings concrete in engineering. Based on this, the test was developed to the axial compressive behavior of concrete short columns with iron tailings, and the stress strain was analysed in this paper.

TEST CONDITION
The raw materials used in this test: local iron tailings from Tangshan and Dunshi cement produced by Jidong Cement Company, China, were used. Experiments of tailings sand concrete restrained by stirrups applying axial compression were carried out. In order to obtain the stress and strain data of tailings sand concrete restrained by stirrups, specimens of WC30, WC35, WC40, WC45 and WC50 were selected in the test. The concrete were used to fabricate short column sample. HRB400 steel bar was used for short column reinforcement. Two kinds of longitudinal forced reinforcement and stirrup were configured in the column. 1216 longitudinal reinforcements with 2.79% reinforcement percentage were used in each column. The diameter of stirrup was 10 mm. For each strength grade concrete, 4 short columns with different characteristic value of stirrup were produced. The specimen size was 300 × 300 × 1200 mm. The thickness of concrete cover of the sample was 15 mm. In order to prevent concrete crushing at both ends, a ring of steel plate hoops with a width of 100 mm and a thickness of 3 mm was wrapped at each end of the column. The details of specimen preparation are shown in Table 1.
The test was carried out in the structural laboratory of the Civil Engineering College of Tangshan University. The long column testing machine with the pressure of 10 000 kN which was produced by Jinan Sanyue Testing Instrument Co. Ltd. was used for loading. In order to obtain the data of the descending stage of stress-strain curve conveniently, belleville springs rigid element was set at the four corners of the specimen in the experiment. The test loading was carried out in accordance with Standard for Test Method of Mechanical Properties on Ordinary Concrete GB50081-2002GB50081- (2002.
In this test, there were two failure modes of reinforced concrete short columns under axial pressure. The first failure mode was located between 1/2 and 2/3 of the column height. Several vertical cracks at the length of 5-10 cm appeared on the surface of the short column before the destruction, then the cracks penetrated transversely, the concrete protective layer gradually peeled off, and the longitudinal reinforcement protruded outward in the shape of "lantern". The concrete in the core area was crushed, and the short column was damaged. The second kind of failure was located between 1/3~2/3 of the column height. Before the failure of the short column, there were several small vertical cracks at 1/3 and 2/3 of the high surface of the column. Then the cracks ran through obliquely. The concrete protective layer at the oblique cracks spalled, the steel bar buckled, and the upper and lower parts of the oblique cracks slightly dislocated. The angle between the inclined crack and horizontal plane was about 70 degrees. Longitudinal bars usually failed to yield, and the dislocation of longitudinal bars led to the dislocation of stirrups in the plane. Finally, concrete in the core area was crushed and specimens were damaged.
Processing the test data, the stress-strain curve can be drawn. The curve of specimen WC40 is shown in Fig. 1. It was found that the stress-strain curve of stirrup confined tailings concrete was similar to that of the ordinary concrete prepared by nature sand.

STRESS-STRAIN MODEL OF CONCRETE WITH COMPRESSION RESTRAINT BY STIRRUP
Stress-strain curve is the basis to study the bearing capacity and deformation of concrete members. For a long time, many scholars have devoted themselves to the study of the complete stress-strain curve of concrete. They have put forward many kinds of complete curve equations for core concrete confined by stirrup. Stages of ascending and descending were contained in these equations. , ( 1 ) The difference lies in the parameter values in the equation: / cc The parameters of ascending curve can be determined by the following formula: where f cc stands for the peak value of confined concrete in the core zone and  cc stands for the strain corresponding to peak stress.

EQUATION TO STIRRUP CONSTRAINT CONCRETE
Professor Qian Jiaru put forward the curve equation of the descending curve containing the characteristic value of stirrup to introduce the effect of stirrups on concrete in core zone: ( 2 ) The two parameters in Eq. (2)  The comparison between the calculated results and the test curves of WC40 concrete confined by stirrups which used different stress-strain curve models is shown in Fig. 1. It was seen from Fig. 1 that the calculation result of the curve equation was more consistent with the test value. It also can be seen that the rising part of the three curve models were more similar, but the falling part were different. The stirrup restraint curve model showed the best ductility, while the curve model proposed by Standard for Test Method of Mechanical Properties on Ordinary Concrete showed the worst. Moreover the proposed curve was the closest to the test value. It was because that the stirrup restraint curve model constrained the parameter of characteristic value of stirrup and the advantageous restraint effect of stirrup was considered.

CONCLUSION
In this paper, the axial compression performance test of reinforced concrete short columns prepared by iron tailings concrete was carried out. According to the experimental data, the stress-strain curve of ferrous tailing concrete were obtained. Combining experimental data with theoretical analysis, the conclusion can be summarized as follows: The stress-strain characteristics of stirrup confined tailings concrete column is similar to natural sand concrete; Because of stirrup restraining, the stress-strain curves become fuller, which shows the better ductility of tailings sand concrete restraint by stirrup.
The calculation of the stress-strain equation is in good agreement with the experimental data to the tailings sand concrete column of WC40 used in the test.