It is a minor problem of concrete and can be ignored. cracking in the interior of dams. Disclaimer 8. Broadly speaking, all other factors which are affecting the water/cement ratio are also affecting the creep. In recent past the importance of influence of temperature on creep has assumed greater importance due to the use of concrete in nuclear pressure vessels as well as in other structures as bridges, multistory buildings etc. This process is known as hydration under sustained load. Loss of pre-stress due to creep of concrete of pre-stressed concrete structures (beams), is well known and suitable provisions should be made to prevent the loss of pre stress in the design itself. To find out the magnitude, of the creep the value of the drying shrinkage is subtracted from the total deformation of the loaded member. Creep property of concrete will be useful in all concrete structures to reduce This size effect is expressed in terms of the volume/surface ratio of concrete member as shown in Fig.15.14. ... Factors Affecting Concrete Strength - Duration: 4:32. Factors Influencing the Creep of Concrete: Creep has been found to be affected by the following factors: 1. The creep of concrete increases the deflection of reinforced beams and in some cases it may be a critical consideration in design, the deflection due to creep increases with time. Concrete Creep is affected by the humidity and the drying condition of the environment. Hence creep is not a completely reversible phenomenon. This is true more specially for low water/cement ratio concrete. In the direct stress tests on concrete a direct relation of proportionality between the creep and the applied load has been found to exist, but the upper limit is not certain. After about 25 years, sand stone shows greatest creep while lime stone shows lowest creep. Influence of Age: Age at which a concrete member is loaded will have a predominant effect on the magnitude of creep. It is really the paste Along with this phenomenon the process of hydration also goes on side by side producing more gel, which heals up the fine cracks produced by creep and shrinkage. The creep results in a gradual transfer of load from concrete to reinforcement. Aggregate undergoes very little creep. A typical testing device is shown in Fig. Loss of prestress due to creep of concrete in prestressed concrete structure. The creep of concrete has been found approximately a linear function of stress upto 30 to 40% of its strength. He observed that after 18 months under load at a relative humidity of 65% the maximum creep was five times that of the minimum creep. Due to the progressive hydration the micro cracks formed due to any reason are healed up. Usually creep is determined by measuring the change with time in the strain of a specimen of concrete subjected to a constant stress and stored under appropriate conditions. In more practical terms, creep is the increased strain or deformation of a structural element under a constant load. The migration of liquid from the gel pores due to creep may develop shrinkage to some extent. Fineness of cement affects the strength development at early ages and thus influences the creep. Some authors have suggested 50% instead of 55%. If a loaded concrete member is kept in atmosphere and subjected to shrinkage, the deformation in the member will be developed from three different causes, namely elastic deformation, dry shrinkage and creep deformation. The instantaneously recovery is followed by a gradual decrease in strain called creep recovery as shown in Fig.15.7. As a general rule, it can be said that creep of structural quality light weight aggregate concrete is about the same as that of concrete made with normal aggregate. Creep has been found to be affected by the following factors: It should be noted that really it is the cement paste in concrete which undergoes creep. Higher the modulus elasticity of aggregate, the greater the restraint offered by the aggregate to the potential creep of the cement paste i.e. 3. The creep of concrete cured at high temperature is smaller due to the fact that the strength of concrete is higher when cured at high temperature than when cured at normal temperature before heating and loading. When all such water has been removed, creep is reduced greatly and becomes equal to that of pre dried concrete. Effect of Mineral Composition of Aggregate: Though it is not possible to make a general statement about the magnitude of creep of concrete made with different types of aggregates, however it has been observed that after 20 years’ storage at a relative humidity of 50%, concrete made with sand stone aggregate showed creep more than twice that of concrete made with lime stone. The modulus of elasticity of aggregate is one of the important factors influencing creep. The higher creep of concrete made with light weight aggregates is due to the lower modulus of elasticity of light weight aggregate. In case of statically indeterminate structures and column and beam junctions creep may relieve the stress concentration induced by shrinkage, temperatures changes or movement The aggregate influences the creep of concrete through the restraining effect on the magnitude of the creep. But later studies have shown that the increase in creep beyond twenty 20 years under load is small and as a guide following values may be assumed: 1. For calculation purposes, often it is assumed that ultimate creep is 4/3 times of 1 year creep. Creep can be defined as the increase in strain under a sustained constant stress, taking into account other time dependent deformations not associated with stress viz., shrinkage, swelling and thermal deformations. Some concrete accelerator cause concrete creep. The aggregates in the increasing order of creep are basalt, quartz, gravel, marble, granite and sand stone. Thus creep is useful for concrete structures. Broadly speaking, all other factors which are affecting the water/cement ratio are also affecting the creep. lesser the creep Fig.15.9. Uploader Agreement. The usual normal weight aggregates are not liable to creep under the stresses existing in concrete. In order to calculate the magnitude of creep in a member subjected to drying a companion specimen is always placed at the same temperature and relative humidity condition and the drying shrinkage of the un-loaded specimen is determined. Above the limit of proportionality, creep increases with the increase in stress at an increasing rate and above a certain stress/strength ratio, creep produces time failure. In reinforced concrete beams, creep increases the deflection with time and may be a There will be gradual transfer of stress from concrete to reinforcement. Generally this strain is smaller than the initial elastic strain due to the increase in the modulus of elasticity with age. In reinforced concrete columns creep property of concrete has been found useful. Concrete creep is defined as: deformation of structure under sustained load. After the removal of stress, the strain is reduced immediately. conditions at the interior and surface, creep is harmful and by itself may be a cause of What is said above is not a very accurate statement At temperature below 20°C (68°F) creep decreases till the formation of ice which causes an increase in creep, but below the ice point creep decreases again. Thus it can be said that, the creep, the shrinkage and slip deformation cause deformation and fine or micro cracks at the discontinuities. of support. Image Guidelines 4. The amount of paste content and its quality is one of the It has been observed that the creep with fine-nest cement was greatest at first, but after 1000 days it became least due to the high gain of strength. If the sustained load is removed after some time, the strain decreases immediately by an amount equal to the elastic strain. It has been observed that for a given type of concrete, the creep decreases as the age at the time of application of load increases as the strength increases with age Fig.15.12. Report a Violation 11. A research program was planned to investigate the significance of different factors affecting creep of concrete. If a concrete member is subjected to a drying condition, it will also undergo continuous shrinkage process. 1. Measurement 4. For the sake of simplicity, it is assumed that shrinkage of concrete does not affect the creep in addition to load. This can be easily understood from the fact that the However, the aggregate influences the creep of Higher the porosity, lower the modulus of elasticity, hence higher the creep. 15.18. In all concrete structures, creep reduces internal stresses due to non-uniform or restrained shrinkage, so that there is a reduction in cracking. Under compressive stress the creep measurement is associated with shrinkage of concrete. Effect of Relative Humidity of Ambient Air: It has been observed that for a given concrete, lower the relative humidity of surrounding air, higher the creep. Various factors affecting the concrete durability are discussed below. Terms of Service 7. Account Disable 12. The role of aggregate in concrete is that of restraint. age also influences the magnitude of creep. Content Filtration 6. Thus with in the wide range of mixes, creep is inversely proportional to the strength of concrete at the age of application of the load. Magnitude 5. Save my name, email, and website in this browser for the next time I comment. The recent research has indicated that weight of aggregate has no effect on creep properties. If creep after 1 year under load is taken as unity, then the average value of creep at later ages may be assumed as follows: These values show that ultimate creep is 1.36 times the one year creep. Under such complex situation the structure undergoes creep, drying shrinkage and also experiences micro cracks. Creep does not necessarily cause concrete to fail or break apart. that increase in temperature does not take place in the interior of mass concrete structure. Thus the values of constants a and b can be determined easily from the curve. A durable concrete is one that performs satisfactorily under expected exposure conditions for specified life of the structure. 15.11. 15.19. Creep is factored in when concrete structures are designed. Under load elastic deformation takes place immediately. Creep effects may also be seen from another point of view. The creep of concrete of different strengths is shown in Table 15.3 below: The linear function between the creep and stress/strength at different relative humidity is shown in Fig. Thus creep is reckoned from the initial elastic strain as given by secant modulus of elasticity at the age of loading. This rapid recovery is then followed by a continuous decrease in strain, called creep recovery that is a part of total creep strain suffered by the concrete. Definition of Creep 2. Generally it is assumed that creep tends to a limiting value after an infinite time under load. most important factors influencing creep.
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