Concrete structure and its durability analysis

Abstract. A building in the long-term use process， with the effect of internal or external， artificial or natural factors， with the time went by， these will occur material aging and structural damage， this is an irreversible process， this kind of damage accumulation will lead to structural performance degradation， bearing capacity decline， durability reduce. For new buildings， if do the structure performance degradation research in the beginning of completion， it can not only master its structure member performance degradation， reveal the potential dangers， and at the same time can also according to the results of the study formulating the reasonable use and maintenance program， improve the safety performance of the building and prolong the use fixed number of year.

Key words： Concrete structure； durability analysis； Concrete carbonization； material levels

Introductions

Concrete durability refers to ability that the concrete can resist various damage factor in long-term effects， the so-called structure durability refers to the structure in design， construction， use and normal maintenance condition， in the provision of time， structure component performance degradation is over time but still can satisfy reserve function ability and durability can also be defined as structure in the chemical， biological or other adverse factors， during the predetermined period of time， the material properties of the deterioration did not make structure appeared unacceptable failure probability， or refers to the structure in the requirements of the target use period， do not need to spend a lot of money reinforcement but can ensure ability of safety and applicability.

1. Influence factors of concrete durability

The main structure of tunnel concrete durability is pointing to in a certain environmental effect and expected maintenance and use condition， the main structure of the tunnel at a predetermined period can maintain its minimum performance requirements of the function. This function not only includes the structure of the security， and more reflected in the applicability. There are many factors influencing main structure of the tunnel concrete durability， mainly displays in the following aspects.

1.1 Concrete carbonization

Concrete carbonization is also known as the concrete neutralization， all contact with air concrete surface is in carbonization process. This is because of the interaction between carbon dioxide in the air and cement stone in alkaline substance make its composition， microstructure and performance change， a complicated physical and chemical process which lead to the use of functional decline. Concrete carbonization has no damage effect to concrete itself， the main harm is because of concrete alkaline reduced resulting in high alkali environment of dense oxide film formed on the surface of steel and passive film destroyed， concrete lose the protective effect to reinforcement， lead to concrete corrosion of reinforcement. At the same time， concrete carbonation will also aggravate shrinkage of concrete， and may lead to concrete cracks and structural damage. Therefore， the concrete carbonization is closely related to the durability of the concrete structure， it is important index to measure reinforced concrete structure reliability.

1.2 Concrete freeze-thaw damage

At present， most of the tunnel body structure concrete is made of cement and coarse and fine aggregate composition of the capillary porous material. In the mix concrete， in order to get the necessary workability， join more the mixing water than cement hydration water， this part of the excess water with the form of free water occluded in concrete to form interconnected pores， and it has a certain volume， and some cement hydrated to form the gel hole. In the cold tunnel， wool stoma free water is the main factors lead to concrete frost， when water freezes the volume expands， concrete internal structure is damaged. When concrete in full water state， water in wool stoma is freezing， and gelling hole in the water is too cold， make the gelling hole-water enter into the wool stoma ice interface penetration， thus wool stoma produce seepage pressure. water in gelling hole go to capillary permeability make the volume of ice further expansion in wool stoma .When concrete in full water state frost， the capillary wall subject to both inflation and seepage pressure， when pressure exceeds concrete tensile strength， concrete cracking.

1.3 Material levels

Material durability research mainly start from factors that influencing the durability of structure， and analyzes the failure mechanism under the action of single factor or more factors， to establish the durability damage prediction model and empirical formula， realize process from the qualitative analysis to quantitative analysis. From material subject level to see， concrete structure durability research including concrete and reinforced aspects： concrete durability degradation performance form basically has concrete freezing and thawing， concrete suffered from aggressive chemical corrosion， concrete carbonation， alkali aggregate reaction， fresh water dissolution， etc； for reinforcement， basically is to study the corrosion of reinforcement. In material disciplines， at present， the structural durability research is more on the concrete freezing and thawing， carbonization and the corrosion of reinforcement.

2. endurance life prediction

2.1 Analysis and forecast the life of the structure with the theoretical

Durable life predictions of component life prediction research mainly is based on component failure mechanism research， and determine its failure criterion and life key， use theoretical analysis， the experimental results and mathematical method forecast the life of the structure. The service life of concrete structure prediction include two aspects： one is at the specified environment conditions， according to the known material durability of structure durability to design， make structure has scheduled service life； The second is according to the existing structure situation， hypothesized future structure service life. Concrete structure service life refers to the time that the structure from start to use to structure reach the destroy limit state； Residual life is structure in the current and in not add repair or normal maintenance and normal use conditions， year of structure may continue to use.

2.2 Structural concrete permeability of nondestructive testing

Permeability is one of the basic properties of porous materials； it reflects the material internal pore size， quantity， distribution and connectivity and so on. Concrete is a kind of porous， in every scale multiphase heterogeneous accord with material， its permeability refers to the gas， liquid or ion by pressure， the chemical potential or electric fields in the concrete penetration， diffusion or migration ease. This index is measuring ability of concrete against a variety of media intrusion.

2.3 Structure durability of concrete nondestructive testing technology

At present China's sustained economic growth has put forward more and more highly demand for infrastructure safety and durability， concrete structure design ideas start transform from a single "strength design" to "based on the performance design"， and durability based on performance design is an important part. This change also contributed to the structure of the nondestructive testing should add durability testing requirements. Carry out structural concrete durability of nondestructive testing has two aspects of meaning： the new structure testing can assess the design and construction of the durability of the measures taken in whether implemented， such as the structure durability index of the acceptance， can timely find all kinds of causes of the durability of the defects， provides the basis for effective repair for structure before it was put to use； The durability of the steel structure damage detection analysis can comprehensive evaluation structure durability situation， lay the foundation for the durability repair plan formulation and structure residual life prediction.

2.4 Structure applicable scope

Through the structure bearing capacity and durability design that housing wall thickness is mainly composed of bearing capacity intensity control. Wall height is the main factor affecting wall stress. Housing not fill intermediate material， wall thickness is mainly composed of self-weight produce tensile stress control， in the housing structure internal longitudinal adding buttress can effectively resist section stress arising under self-weight； after the box filled with stone， wall thickness is mainly controlled by cross section intensity， in the bottom wall adding reinforcement， can effectively reduce the thickness of the wall.

3. Measures to improve the concrete durability

of maritime structures

3.1 Optimizing structure design

Marine structure type usually has high pile beam plate structure， thin wall structure and large volume structure. In structure， due to the structure section mutation， the ratio of surface area and volume， structure points seam too long don't adapt to the change of temperature， structure stiffness leads to horizontal displacement， uneven settlement and foundation constraint condition difference big and other reasons， all will lead to concrete cracks. So they should attach importance to optimizing structure design， and strive to avoid the above the influence of unfavorable factors. In addition， important structure and the difficult position of component repair， in the first kind of loads， also should control the concrete crack generation and development. For high piled wharf， even should reasonable arrangement of wharf surface breather hole， to decrease Marine atmospheric erosion.

3.2 Reasonable stratified concrete

In the concrete pouring process， if the thickness of layered casting is larger， after the coarse aggregate sinking， surface upwelling of mortar and cement paste content is higher， so as to increase the concrete non-uniform shrinkage， that is also one of the reasons cause concrete shrinkage crack produced. Therefore， reasonable stratified concrete and vibrated is the important measures to reduce concrete non-uniform shrinkage.

3.3 The Marine environment durability of concrete structure

At present the dry-wet circulation theory mainly combined with capillary adsorption theory and diffusion theory this two aspects to carry on research， but research results far from perfect， puts forward capillary adsorption theory and the superposition of diffusion theory， and through the numerical method to solve water total transmission， its damage failure rule and life prediction model is still needs further research.

Influence factors of concrete durability are numerous， each other mutual coupling， so the concrete construction life in evaluation and prediction is a significant and very difficult work， the establishment of various model most based on laboratory simulation， often have very big difference with the actual situations. In addition， for multiple environmental chemistry，under the action of mechanical coupling of concrete durability and its life prediction research is very limited， is still does not exist reliable mathematical model.

4. Regeneration concrete

4.1 The durability of regeneration concrete

With the acceleration of the urbanization process and the arrival of the old buildings service life， every year in our country demolition waste concrete quantity is very huge，and increasing year by year. Regenerated aggregate concrete， as building MSW recycled important way， is advantageous to solve building garbage discard landfill problem， and also is beneficial to reduce the aggregate origin ecological damage problem caused by the uncontrolled exploitation of natural sand and gravel. Therefore， using waste concrete as aggregate to product recycled concrete， it has strategic significance for resources recycling， and evolution environment and keep the sustainable concrete material application.

4.2 The recycled concrete chloride ion permeability

Chloride ion permeability test method can be roughly divided into three categories： natural diffusion method， the applied electric field accelerated diffusion method， pressure penetration method. Natural diffusion method needs a longer time， compared chloride ion accelerated diffusion method is rapid， simple， and is the most widely used method. Invented in 1981， its added electric is dc. This method are called Rapid Chloride ion penetration Test method （Rapid Chloride Permeability Test， referred to as RCPT） in the earliest， its principle is the ion in the electric field solution under the action of seepage velocity to speed up， make the Test time shortened. Because this method test time is short， in the laboratory has repeated applicability.

5. The durability of Songhua river highway bridge

concrete structure

5.1 Concrete carbonization test

Concrete carbonation depth greatly influenced by the concrete strength， ramp bridge pier， the bridge pier， the approach of the T beam and concrete box girder bridge the same age in the carbonation depth decreases in turn. The pier concrete carbonation depth， showing a clear upward trend with the growth of the carbonization time，but part of the bridge due to the use of high-strength concrete， carbonation depth value is extremely small and the variation is not obvious.

5.2 Carbonation test

Bridge water level changes in the main bridge piers in contact with the river frequently，the degree of concrete saturated water is relatively high and vulnerability to influenced by freezing and thawing cycles， In addition， the parts of the concrete in contact with air， will take place concrete carbonation. Consequently， take carbonation test after freezing and thawing research for Bridge water level changes in main bridge pier concrete. Concrete carbonation affected more obvious by the effect of freeze-thaw cycles， with the increase of the number of freezing and thawing， concrete carbonation depth increasing. After more than 100 times the number of freeze-thaw cycle， the concrete carbonation depth growth has significantly increased.

6. Summarise

On the analysis of a large number of document literature， germinated the idea of the design of concrete durability research expert system. First it is necessary to develop a system to improve a set of strengthening quick test method，so to be able to compare objectively reflect the concrete destruction in the use of environmental factors and their cross composite factors， and to long-term exposure tests by each other. Meanwhile it is necessary from basic theory to study the impact of the durability of the essential reason， cement - water cementations materials is an imbalance， open dissipative system and various compositions formed various micro-structure and pore structure of network systems， material movement follow in the thermodynamics of non-equilibrium state and non-homogeneous process dynamics basic theory， changes in the composition of the substance and structure are essential characteristic parameters of the material characterization durability decay， need to study from the relationship between the two.

References

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