请教翻译

ABSTRACT
The reinforced concrete shell structures are among the most exposed structures to erecting faults and errors, to geometry small miscalculations and concrete pouring. The main source of errors comes from the great differences between their thickness and curvature radiuses of their median surface. Due to these differences all the aspects related to erection of shell structures (such as concrete composition, concrete pouring, formwork stiffness and geometry, etc) affect exponentially the structural quality. Another sensitive feature of shell structures ius the magnitude of the surfaces they cover without intermediate supports. This aspect leads to significant and critical reductions in the structural safety when a small fault or error occurs. The present contribution deals several aspects assocoated to structural assesment and structural rehabilitation techniques from, both, the structural and technological points of view.
Keywords: Concrete shell, rehabilitation, structural strengthening, carbon fibres, steel lamellas.
1. INTRODUCTION
Increased loading requirements, change of servicebility and structural deterioration contribute to the increasing requirement to strengthen structures of all types.
Structures made of reinforced concrete, need a special approach to strengtheninig R/C structural strengthening may be efficiently achieved using modern retrofitting techniques and materials.
One important responsibility of structural engineers is to identify and fix problems of the deterioration of aging concrete structures. A common problem is the cracking of R/C elements. Cracks that appear around the reinforcement expose most of the steel bars causing the bars to rust. However, the shel structures which are made of concrete are vulnerable to corrosion induced physical damages. Concrete and steel embedded in it will progressively deteriorate when exposed for long periods to environment actions.
In earthquake resistant design, shell type structures can be used to undertake lateral loads and make up a shells have been accepted as an alternatives to moment resistant frames and they proved to be able to respond to the seismic design requirements of concrete structures.
The R/C shells are structural strehngth elements of curve formes with a very small ratio between the thickness and the main 1and 2 curvature radiuses of their median surface. This ratio is as small as from 0.001 to 0.05.
The structural forms are, ususlly, inferred such that the stresses in every point are located in the tangent plane to the shell, i.e., the membrane theory is observed.
In many cases of open shells, the coincidence of the pressure surface and the real geometry form is not possible either due to some serviceability or erecting constraints. The consequence is a state of supplementary bending that, in to many cases, are undertaken through incorrect reinforcing or even erroneously computed leading to structural degrading and deformations.
There are, also, other typical sources of structural deformations and degrading of the shell concrete: foundation problems, corrosive environment (sulphides, chlorides), concrete quality, erection faults. The structural mechanical state has to be assesed by evaluating the degree of deterioration (through conducting nondestructive testing of concrete such as ultrasonic pulse velocity, rebound hammer test, chloride penetrati on test and corrosion of reinforcements). On the basis of the assesing report a repairing scheme will be elaborated. For each individual shell structure the degradation state has to be evaluated in a assesing report mentioning the nature of the degradations, their locations, the frequence of their appearance, the available strength and stability capacity [1], [2], [3].
If the assesing report concludes that the global or local carrying capacity dose not mach the state limits
requirements, structural rehabilitation or retrofitting measures have to be taken and implemented. In most of the cases, the rehabilitation or retrofitting solutions are, financially, more convenient than a new structure. Nowadays, many such structures have been included in the list of architectural or historical buildings and, consequently, they are protected from being demolished. More than this, these buildings have to be strengthen to extend their life time.
The rehabilitation, retrofitting and strengthening of shell structures require typical solutions and they are recommended in the literature [4], [5], [6], [7], [8].
IASS 2005
In time, several theoretical solutions become, also, practical techniques of strengthening shell type structures:
Increasing the bearing capacity to stress resultants (M, T, N) via:
Changing the mechanical properties of the cross sections, changing the structural mechanical model (by external nonadherent prestessing, changing the hinges into fixed ends, etc.).
Decreasing the own weight through:
Removing the old type hidro and thermo isolations layers that, usually, have a high weight,
Removing or reducing the weights suspended or hanging from the shell structure.
The present contribution deals with several strengthening techniques of concrete and reinforced concrete shells focusing on the structural and technological implications.
2. REHABILITATION VIA CARBON FIBRES REINFORCEMENT [11] [16]
The main applications of carbon fibres based composites in the construction industry over the last years have been bridge structures and storage tanks and more recently the CFRP pultruded plates and prepreg
plates have been used to increase the flexural and shear capacity of existing structures. CFRP plates are now being accepted in construction industry as an effective alternative to replacement and other traditional methods of strengthening. When the strengthening technique of R/C elements has to be decided, some properties of the CFRP type composite materials such as their low weight, their undisputed handleability, a large range of elastic moduli, high resistance to corrosion, high strength, availability in long lengths avoiding the need for lapping, and good fatigue, creep and fire resistance characteristics have to be taken in view. Recently, the prestressing technique of CFRP plates was successfully demonstrated.
The strengthening techniques based on carbon fibres are, relatively, new and, in the field of shell structures, two procedures have made their way from theory to practice:
Overconcreting or shotcreting the existing structure using a carbon fibres reinforced concrete,
Using carbon fibres reinforced strips adherent to the concrete shell sructure.
The concrete and mortars fabricated using carbon fibres have been in use, mainly since 1980. The carbon fibres are mainly PAN (polyacril nithril) type fibres with a very high tension strength (from 3,000 N/mm2 to 5,000N/mm2), while the Young modulus is ranging from 150,000 N/mm2 to 300,000 N/mm2, therefore closed to the values of the steel Young modulus.
These fibres are sensitive to repeated bending and a consequence of this action is the minimum thickness of the overconcreting layer (about 30 mm). Good results have been recently reported using PITCH type fibres (based on petrol and carbon) with a tensin strength from 450 N/mm2 to 800 N/mm2 and an elasticity module ranging from 26,000 N/mm2 to 56,000 N/mm2.
The carbon strips use mainly PAN type fibres. The fibres rate has to be at least 68%. The most used carbon strips are SIKA_CARBO_DUR strips produced in three different types (Table 1). The carbon strips stick to the concrete surface through the SIKADUR 30 adhesive.
Strengthening can, also, be achieved by bonding directly a singlelayer CFRP (Carbon Fibres Reinforced Polymers) laminates (strips) to the concrete surface. The producers have to present the results of laboratory testing results regarding the relevant mechanical properties and the design values of the required parameters. For several types of ME 423 Polymers & Polymer Composites products the main mechanical properties that have to be taken into account are presented in Table 2 [17].
From the structural analysis point of view, the carbon fibre repair system can be used to recover the initial elastic stiffness and to increase the yield load and ultimate flexural capacity of damaged shells. In strengthening applications to cilindrical reservoires, the carbon fibre sheets can be used to increase the precracked stiffness, the secant stiffness at yield and the cracking load, the yield load and the ultimate flexural capacity of undamaged shells.
In structural terms the following aspects and features have to be considered when the carbon fibres based strips are used for R/C shell type structures:
The carbon fibre reinforcement polymer (CFRP) sheets used as reinforcement behave as high strength unidirectional continuous carbon fibre tow sheets.
The CFRP sheets have to be bonded on the external face of the shell using an epoxy bonding product.
The elastic modulus in tension is around 230 GPA, the tensile strength reaches 3,480 MPa, while the ultimate strain is 1.5%.
The carbon fibre materials expose a linearly elastically behaviour down to collapse.
When the CFRP strips are used in retrofiting of R/C shell structures, the fibres of the sheets have to be oriented as the usual steel reinforcing bars.
The effective contribution of the CFRP strips to the flexural capacity can be increased by anchoring the CFRP sheets at the base ring or beam structure through a load transfer mechanism.
It is necessary to have a level concrete surface before the bonding of the carbon based fibres plates. Also, the surface should be cleaned of all particles such as dust or lubricants. To achieve this, many simple and efficient existing technologies can be employed to prepare the concrete surface.