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Specific Aspects

In document Sa Prefab Bridges 02 (Page 46-53)

The integral advantage of the combination precast + high quality materials require the clarification of many specific regulations. Between them should be appointed:

* II.1. Values of γc and γf to apply in the case of precast members.

* II.2. Contribution of the concrete to the strength to shear at the interface in function of the fatigue level.

* II.3. Influence of the form and position of the connection reinforcement for shear at the interface.

* II.4. Strength of compressed struts at shear, especially with HPC.

* II.5. Transfer length of tendons with HPC.

As in any other structural typology, the development of prefabricated bridges is hindered, in a way, by the need to solve problems via research, which delays the extension of the use of such solutions.

In our case, attention should be drawn to the following.

a) Obviously, in the case of the manufacture of bending members, the fundamental part of prefabricated bridges, prefabrication affords higher quality than cast-in-situ execution in many respects.

- Cutting, bending and placing reinforcement bars and tendons.

- Low pouring height together with very powerful concrete compaction procedures.

- Extraordinarily efficient curing systems.

All this means, in short, that the quality of the members, in particular bending members, is much higher in prefabrication than in most cases of cast-in-situ execution.

Since, broadly speaking, under the deterministic approach the bending safety factor is expressed by the equation

f s

s kγ γ

ε =

where k is a coefficient that varies very little from one in the case of prestressing, the only way to establish a suitable procedure to reward the highest prefabrication quality is, obviously, to make it truly dependent on the load factor

γ

f by means of the quality control to which the structure is subject.

The idea frequently invoked to simply reduce the

γ

c coefficient entails a more psychological than actual reward. The reduction of the

γ

c coefficient, indeed, only rewards cases as columns or where compression on struts is limited in the event of shear, but in all other respects its importance for bending members is essentially negligible. It is only by

b) The limitation on compressed struts had evolved pessimistically in recent years in the Model Code as well as in the successive versions of the Eurocodes, both with respect to the regulations dealing with concrete in general and those relating to precast concrete in particular.

Recent research (1) has shown that such reductions were unwarranted: Fortunately, new proposals to revise Eurocode EC-2 have re-established a more realistic solution.

c) The anchoring conditions for prestressed tendons, especially in high strength concrete, need to be analysed in greater depth, not only to establish the transfer length but to provide for a more accurate evaluation of the distribution of stress throughout the transfer zone to improve shear stress studies in these areas of members.

The shear strength of conection with “in situ” slab is 25% to 33% higher in b) than in a) Fig. 6.1.1.

d) The shear interface problem calls for supplementary research in two directions. On the one hand the use of normal strength cast-in-situ concrete in conjunction with high strength prefabricated members will become more and more frequent, which places special demands on engineering for shear at the interface.

Secondly, the present forms of computing the contribution of the connection reinforcement between cast-in-situ and precast concrete take account of the ratio of the area of reinforcement only. However, the way these bars are bent and placed has a substantial effect on shear strength and should be lent more attention in future (Fig. 6.1.1)

6.2. Details

6.3. Durability

6.3.1. Introduction

Durability is the capability of a structure, assembly or component to maintain minimum performance and adequate levels of stability and serviceability over at least a specified time under the influence of degradation factors with anticipated maintenance but without excessive unforeseen maintenance.

The requirement of an adequately durable structure is met if, throughout its required life, a structure fulfils its function with respect to serviceability, strength and stability without significant loss of utility.

Durability of concrete bridges is mainly governed by the quality of materials. The quality of the concrete, the bearings, the joints and the sealing of the surface are the essential parts which together determin the length of the service life.

At present the codes do not explicitly ask for the definition of the required lifetime of a structure. Generally the overall durability, as defined above, depends of the intended use of the structure together with load specifications.

In some countries maintenance programmes are set up for bridges. These programs form part of the requirements for durability. They are regulated in specific codes other than design codes and aim at the minimisation of maintenance during life time of the structures. Thorough knowledge of degradation mechanism in a structure which could develop to future failure is essential to secure serviceablity and to reduce cost due to developing faults.

Durability may be affected both by direct actions and also by consequential indirect effects inherent in the performance of the structure (e.g. deformations, cracking, water absorption, etc.). The possible significance of both direct and indirect effects is to be considered.

The load-bearing capacity of bridges can be violated by the degradation of concrete and reinforcement. Therefore bridges (and all other structures) must be designed in a way that the minimum safety level is secured during the intended service life despite degradation and ageing of materials.

As long as systematic durability design methods are still missing and common rules are not yet available as design tools, the general approach is to observe border conditions to improve performance of the structure and its components to avoid unexpected and premature failure.

These stipulations are met when specific design rules, special quality demands for materials, material compositions, working conditions, structural dimensions etc. are observed.

Durability calculations which could result in the use of modified safety factors in combination with traditional mechanical design will be developed in the future [1]. Until these methods are available durability design has to ensure a high standard of all parameters which could influence service life of the structure in general.

To ensure an adequately durable structure, the following inter-related factors are to be considered:

* Degradation Factors

* Influence Of Environmental Conditions

* Composition, Properties And Performance Of The Materials

* Shape Of Members And The Structural Detailing

* Quality Of Workmanship, And Level Of Control

* Particular Protective Measures

* Maintenance During The Intended Life.

6.3.2. Degradation Factors

The following factors may have negative long-term effects on the load-bearing capacity of concrete structures:

For reinforced concrete, corrosion protection for any type of reinforcement is provided by taking care of the following aspects:

Concrete Stress conditions: Compliance e.g. with the requirements contained in the clause 4.4.1 of the ENV 1992-1-1.

Excessive compressive stresses in concrete may promote the formation of cracks.

Steel stress conditions: Compliance e.g. with the requirements in the clause 4.4.2 of the ENV 1992-1-1 results in limitation of cracks under SLS.

To avoid complex crack-width calculations the following simplification is often used:

Concrete stresses calculated at the uncracked section must not exceed the value σb:

3 2

In cross-sections in reinforced concrete (e.g. webs or base plates for transverse flexural loading) the transverse flexural-tensile stresses (calculated in accordance as uncracked section) may not exceed the values of σb either.

6.3.3. Influence of Environmental Conditions

Environment, in this context, means those chemical and physical actions, to which the structure as a whole, the individual elements, and the concrete itself is exposed, and which results in effects not included in the loading conditions considered in structural design.

Durability parameters:

* depth of deterioration of concrete and corrosion of reinforcement

* concrete cover

* diameter of rebars

* Factors to be taken into account:

* strength of concrete

design and detailing, standards of workmanship and construction, and intended maintenance regimes - to produce the required level of intended maintenance regimes - to produce the required level of performance for the structure throughout its service life.

Standard values can generally be increased by an allowance (∆h) for tolerances, which for precast elements may be assumed in the range 0 ≤ ∆h ≤ 5 mm depending on the standard of workmanship and quality control.

In some countries the values for concrete cover may be reduced when prefabrication in a factory is ensured.

6.3.5. Shape of Members and Structural Detailing

Early in the design process, the effects and possible significance of the environmental conditions is to be considered in relation to the durability requirements.

Reference should be made to the design criteria in point 3 above and to the requirements for concrete cover to reinforcement in point 4.

Other factors to be considered in design and detailing, in order to achieve the required level of performance, should include the following aspects:

the adoption of a structural form which will minimise the uptake of water or exposure to moisture.

the size, shape and design details of exposed elements or structures should be such as to promote good drainage and to avoid run off or standing pools of water. Care should be taken to minimise any cracks that may collect or transfer water. In the presence of cracks across a complete section, additional protective measures (coated bars, coatings, etc.)may be necessary.

for the drain of the deck slab surface, the sealing surface must be detailed carefully.

attention in design and detailing, to the different aspects of indirect effects.

Expansion joints between bridge decks can seriously effect the long term durability of the structure. They can lead to serious corrosion of the r.c. elements of the deck and substructure due to leakage of salt solution through the joints.

Several methods of eliminating joints in continuous decks are acceptable.

6.3.6. Quality of Workmanship and Level of Control

The standard of workmanship on site shall be such as to ensure that the required durability of the structure will be obtained. The combination of materials and procedures used in making, placing and curing the concrete shall be such as to achieve satisfactory resistance to aggressive media for both concrete and steel.

During construction, adequate measures shall be taken, by means of supervision and quality control, to ensure that the required properties of the materials and standards of workmanship are achieved.

Quality Control:

An other important factor to ensure the obtaining of the required durability is the quality control in the different phases of the construction.

7. Protective Measures

In addition to all the facts that must be considered to ensure the required durability, other protective measures should also be taken into account.

Sealing: Joints between precast elements must be sealed.

Tendons placed in sheaths or ducts in the concrete, couplers and anchorage device shall be protected against corrosion.

Should the delay between stressing and grouting exceed the time permitted, then protection of the tendons shall continue until grouting takes place.

Where temporary protection is provided, the material used shall have an approval document and shall not have a deleterious effect on the prestressing steel or on the cement grout.

Written instructions shall be provided for the site or the works for the preparation and execution of the grouting.

It frost is liable to occur, measures shall be taken to prevent freezing of water in sheaths which are not yet grouted. After a period of frost, the sheaths should be free from ice before grouting is started.

Tendons may be protected by materials based on bitumen, epoxy resins, rubber, etc., provided that there are not detrimental effects on bond, fire resistance, and other essential properties.

6.3.8. Control and Maintenance of the Completed Structure.

A planned control programme should specify the control measures (inspections) to be carried out in service where long term compliance with the basic requirements for the project is not adequately ensured.

All the information required for the structure’s utilisation in service and its maintenance should be made available to the person who assumes responsibility for the complete structure.

In document Sa Prefab Bridges 02 (Page 46-53)

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