Technical introduce

The experimental investigation of the structural feasibility of adopting CFRP cloth material as pre-stress slab



The experimental investigation of the structural feasibility of adopting CFRP cloth material as pre-stress slab Li Fangyuan, Li Guoping, Chen Airong

Bridgework Engineering Department of Tongji University
Abstract

  CFRP cloth material's production techniques are quite mature. It is widely used in the concrete structure's reinforcement.
  Utilize the CFRP cloth material to exert pre-stress can give fuller play to its high tension and corrosion prevention.
This thesis gives a clear analysis and description of the feasibility of pre-stress exerted by CFRP cloth material and introduces the test instance of adopting CFRP cloth material as pre-stress slab
Keyword: CFRP cloth anchor, pre-stress slab, pre-stress stretch
1 Summary
   Engineering practice shows that CFRP composite can adapt to modern times engineering structure development trends towards big span, soar, and heavy load, high intensity, light weight and it can meet the demands of adverse condition.
It conforms with the industrialization requirements of modern times engineering technique and is increasingly applied to many structures such as bridge, different types of civilian construction, ocean, off sea O as well as subterranean structures. There are two application methods: one is, directly apply it to newly constructed structures; the other is, use it for the maintenance and enforcement of old structures to achieve good economic efficiency.
Our country started the formal research on the application of CFRP to civil construction in 1996-1997 and launched the actual engineering practice in 1998. Up till now, there have been nearly ten scientific research institutes, over ten institutions of higher learning in our country. They do research on the technical performance of CFRP and make substantive scientific research payoffs. However, domestic research achievement and object show that our country's research basically gives priority to structure reinforcement and restoration
  Obviously, it lags behind the overseas countries which have already adopted CFRP to substitute the reinforcing steel bar and pre-stress bar or cable.
  Although the high tensile strength and the low elastic modulus make CFRP bar bundles suitable to exert pre-stress, it has the drawbacks of high heterogeneity and friability. This heterogeneity makes the CFRP bar bundles easy to destroy at the pressured anchor time, whereas its friability makes it more hazardous than steel products at stressing time. So far, there is no anchorage system which can guarantee the best conveyance of pre-stress among the CFRP force bars. This thesis is also one of the architectural researches on CFRP pressured anchor.
  At present, the research on CFRP as pre-stress stuff mainly focuses on exploitation correlation pre-stressed anchor tool, at the same time, develop the high performance of CFRP stuff.
Whereas the CFRP sheet has been applied to the constructional engineering earlier, its application and exploitation are rather mature, and the application, quite prevalence.
The application of high performance concrete, in particular, that of self-leveling concrete, by dint of the incorporation of mature CFRP cloth material and pre-stress technology, will tender new application mode and clue to CFRP stuff and pre-stress technology.
  This thesis mainly introduces the CFRP stuff the writers are developing and the writers' research on the performance and construction technique of slab structure. The relevant report about research on adopting CFRP cloth as pre-stress stuff is unavailable for the moment.
2、 Merit and meaning of adopting CFRP cloth material to exert pre-stress
Carbon fiber reinforcement plastics have the merits of high strength, fine corrosion proof and durability, etc. People both at home and abroad all have started the applied research of substituting it for steel products, but many of the researches are limited to the use of CFRP as extrinsic reinforcement material.
The maturity of the development technique of CFPR stuff in recent years and the rapid fall of its price give people good opportunity to proceed the structural application of CFRP. The conception of adopting CFRP as pre-stress bar has existed for a long time, but its friability poses great difficulty in its anchorage. Therefore, the writers' research is on exerting pre-stress by CFRP cloth material, which enjoys mature technology, instead of by CFRP steel bar, which requires pre-stressed anchor tool.
Exerting pre-stress by CFRP with the form of cloth pattern has merits as follows:
(1) need no anchorage, or develop correlative plumb stage equipment to substitute the anchorage required by bar material, thus saving the anchorage expense.
(2)exert pressure by flaky material itself, thus avoiding rupture, which is the result of adopting conventional anchor tool.
(3) Cloth pattern of CFRP has bigger surface and can have sufficient contract area with concrete, thus guaranteeing the complete and plenary felt between CFRP and concrete.
(4) The cloth pattern of the weft's interlace and stretching can conveniently exert the pre-stress at the portrait and transverse direction.
(5)Flexible arrangement and relatively skim room occupation of CFRP in cloth pattern can avoid the construction inconvenience resulted from the superabundance use of steel bar.
  The report of research on adopting CFRP cloth material as pre-stress stuff is unavailable now both at home and abroad. This thesis uses concrete slab as research object, and gives an introduction about slab stress and section performance when adopting cloth materials to exert pre-stress. Besides, this thesis supplies theory gist for adopting CFRP cloth material to exert pre-stress in wider scope. In the slab structure of constructional engineering and bridgework, including prefabrication and cash irrigate method, adopting pretension method can achieve a more perfect effect as far as CFRP cloth material pre-stress technology is concerned. This method can be used for reference when adopting CFRP cloth for reinforcement of pre-stress tensile structure.
3 the effects of exerting pre-stress by CFRP cloth materials.
This test adopts Tonggu CFRP cloth, CFC2-2 type, manufactured by Shanghai Yichang carbon fiber material Co.Ltd. Material parameters are as follows:
Product thickness Type of carbon fiber Extensile strength
0.17mm T-700-12K ≥3500MPa
Tensile elastic modulus Extension at break Weight per area
220 Gpa 1.8% 200g/m2
Cloth width
100mm
Different from CFRP bar material, CFRP cloth is thin and wide. So exerting pre-stress on CFRP cloth, its stretching offset must be overcome. The stretching test adopts the device designed by writers. The device is proceeded by Liuzhou Ouweimu Mechanism Limited Liability Company. Photos in Chart 1 and 2 respectively display the destroy instance of part of CFRP cloth resulted from non-uniform tension. Photo in Chart 2 displays the effect of overcoming non-uniform tension. Photo in Chart 3 displays the scene before loading when three CFRP are stretched and imbedded in concrete slab.



Photo 1                       Photo 2
Chart 1 destroy instance of VFRP due to non-uniform stretching

Chart 2 Photo of CFRP thanks to uniform stretching
Chart 3 the effect when CFRP cloth is stretched and imbedded in concrete.

    According to the performance parameters of material provided by CFRP manufacturer, after two tests of stretching, CFRP cloth pre-stress can be imbedded into concrete slab. The slab's size is 2040×600×80mm,and it adopts two strips of CFRP cloth as pre-stress stuff. It has 4 constructional bars of φ6 in portrait direction. In the transverse direction, it has constructional bars with a space of 190mm between.


Chart 4 Shop drawing of imbedding CFRP cloth pre-stress into concrete slab.

The test shows that the CFRP cloth's tension is 2117 Mpa, about 0.6 of tensile strength, and the final effective tension is 1529 Mpa, about 0.45 of tensile strength. It has passed the middle third point load beam test to effective span of 2000mm. Photo in Chart 5 displays the destroy scene when static loading CFRP cloth pre-stress into slab.



Chart 5 Photo of CFRP cloth pre-stress slab's structure when loading.


4.Conclusion and prospects
1)The research concentrates on the prestressing of CFRP and the properties of concrete structures. To achieve this aim, we have to find a effective method to adopt CFRP to increase prestressing.
2)After twice extension trial and the third extension when the final construction of prestressing concrete slab, it is proved viable to increase prestressing on CFRP.
3)The regular CFRP as a prestressing material is stick, which indicates the problem of anchor. With the material, it's great news for the concrete component prefabricate industry that they cn increase self friction without anchors.l
4)In concern of the problems of the single direction anti-drag and the difficulties in construction of the whole piece CFRP, the interleaving of CFRP lists can implement the effect of the traditional reinforcing steeling bar networks.
5)To increase the prestressing with the material, positive crossing of lists can add prestressing on two directions. This method is not favor only to the construction of concrete but also to the stick of CFRP and concrete, which assures the CFRP and concrete support together.
6)With CFRP as prestressing adder, there is no similar protection width requirement under traditional steeling bar construction. Multi flat material is possible up to request. But reasonable configure with regular reinforce concrete.
The combination of CFRP and prestressing techniques used in flat structure can take full advantage of the properties of CFRP, prestressing techniques and concrete slab. In concern with the inconvenient introduced from integrate of piece and slab, high capable concrete is necessary during the construction. CFRP is placed and extended after the irrigation of concrete base; another concrete flag is irrigated then.

5 The way ahead
   To use the CFRP in the concrete structure, there's still much to research, among which is the research of the calculation method of the CFRP piece behavior on prestressing, the support properties and destroy mechanism. It's necessary to extend the research on the followings.
(1)The relate research on CFRP prestressing including more reasonable prestressing method, the design and machining of prestressing equipments, the calculation and monitor of prestressing and relate calculation on steeling bars.
(2) The experimental research on the CFRP prestressing support power. The behavior properties and calculate method on flat support.
(3)Rational design and construction techniques research is favor to the application of CFRP in material structure and feasibility of the CFRP in other construction.

Thanks
  Liuzhou Ohm Machinery for their extension equipments.
  Shanghai Yichang Carbon Fabric for their CFRP


Reference
[1] Zheng Hong, Dai Ziqiang, Zhao Tong, "The application of CFRP in concrete structure, 1991(10):13-14,37
[2] Gu Lugong, The foreign development and application of CFRP on concrete structures, 2001,32(5):310-313
[3] Yu Qing, The properties of FRP and it's use in civil engineering, Ha Erbing university of Construction stress, 2000,33(6):26-30
[4] CFRP test method, national standard of china,GB/T 1447---1983
[5] Dolan C . FRP development in the United States [A].Fibre-reinforced-lastic reinforcement for concrete structures;properties and applications[C]. Amsterdam,Elsevier,1993
[6] Clark J L. Alternative materials for the reinforcement and prestressing of concrete[M]. Blackie Academic and Profession,1993
[7]"Composite Constructin-Conventional and Innovative"Conference Report. International Conference, Innsbruck,Austria,Austuia, Aeptember 16-18,1997.
[8] Fiber enforced composite bridge, Wei Guangsheng translate, Foreign highway,2001.2:21-23


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