More than a dozen years ago, the research team of Hanoi University of Science and Technology successfully built the prestressed concrete core core that previously only a number of developed countries could produce.
In the construction structure, especially large bridges or high -rise buildings, concrete beams with good compression strength but less durable before traction. To overcome, engineers often use the pre -stress method – placing steel cables inside the beam, stretching them before concrete freezing. When concrete clinging to the cable, steel strands will tend to shrink, create a residual compressive force to help the beams bear the load better and minimize the risk of surface cracking.
But how to keep the cables always tight and stable throughout the life of the building? That is the task of the NEO system – in which the anchor core is the most important part. The anchor core is in direct contact with the cable, transmitting force when stretching and holding tightly so that the cable does not slip. It must be hard enough to wear abrasion, durable enough to not crack, and flexible enough to not break crispy under large loads.
Since 1974, when building Thang Long Bridge – Vietnamese engineers have tested the production of anchor core at Ngo Gia Tu Mechanical Factory and the No. 1 Tool Cutting Joint Stock Company. With appropriate material and heat treatment process, the test products are not meeting mechanical requirements, not being used.
In 2007, under the guidance of the late Professor Le Thi Chieu, the research team at Hanoi University of Technology embarked on the manufacture of the prestressed concrete core that replaced the imported anchor core.
From zero, the group started surveying international documents, looking for used anchor core for analysis. However, foreign documents are only generally described, absolutely does not disclose the detailed production process. “It is a core technology, which no country is easy to share,” said Assoc.Prof. Dr. Nguyen Duong Nam, Deputy Director of the Institute of Mechanical Engineering, Vietnam Maritime University, a member of the research team recalled.
The research team has analyzed and discovered that the imported anchor core made of alloy steel with low carbon content. After performing steel thermal thermalization with surface hardness on 62-63 HRC, it will ensure the resistance to abrasion for details; Besides, the core of the material reaches hardness from 38-40 HRC to create compression resistance and ensure good toughness. So the group has studied heat treatment in this direction.
After two years of tinkering, the research team of GS Chieu made hundreds of SCM420 steel anchors for experiments. To increase surface hardness while retaining the flexibility, the toughness of the material – the key factor helps the anchor core to withstand a large traction without breaking, the group uses the method of carbon -nitrogen absorbent in the gas, with the source Gas supply is completely produced in the country.
Neo core. Image: NVCC
Carbon-acid-absorbent technology has a low temperature and shorter processing time than carbon absorbent technology, which helps to limit the form and growth of the grain of steel after absorbent. The presence of nitrogen creates nitrite particles combined with carbides formed during permeability, increased abrasion resistance, special features of this heating process will create toughness for steel. “The simultaneous combination of carbon and nitrogen in the process of heating helps enhance both abrasion and tough resistance, especially suitable for prestressed cable anchor – the product must be under the abrasion. And requires high toughness, “said PGS Nam.
Test results at the laboratory of Hanoi University of Technology are very positive: the anchor core of the research team could withstand 200 cable pulls, while Chinese anchor core only reached 60 times.
In order for the product to be put into practice, Prof. Le Thi Chieu and his colleagues bring the anchor core to the Institute of Transport Science and Technology, the Ministry of Transport, to test. The criteria of traction, the falling cable are all international quality. The group’s anchor core became the first product manufactured by Vietnam to be granted a certificate.
In early 2009, the core product was first appointed by Tuan Thinh Construction Joint Stock Company at Kien Hung Bac Bridge through Nhue River and then some prefabricated concrete beams at Xuan Mai concrete company.
Anchor trial at Kien Hung Bridge. Image: NVCC
According to Mr. Nguyen Do Ha, Institute of Research, Development and Application of new technologies (Irdat), the maximum value of the anchor core is to meet technology requirements. This type of surface technology has a hardness of 65 HRC but the core has a hardness lower than 38-40 HRC. This allows holding the cables in high toughened, not slipped but also does not break the anchor core, which can be applied in the pre -stress concrete industry as well as many other industries that require both hard and tough as gears. “The fact that Vietnam is proactive in anchor production technology also demonstrates the high -tech position in the world that only a number of developed countries such as Germany, Italy, Japan, China can do it,” said Mr. Ha. speak.
The anchor core is the “heart” of the bridges and the building has a 40 m long aperture without column. However, Mr. Ha said that to commercialize requires very high -scale and licenses evidence. At present, the domestic production core has just stopped at a small production scale for special needs to be imported, not commercialized to universal.