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Temperature and shrinkage effect on structural analysis -


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Temperature and shrinkage effect on structural analysis -

  1. 1. Ahsanullah University of Science & Technology Pre-stressed Concrete Sessional CE 416 Course Teacher: Munshi Galib Muktadir Lecturer Department of Civil Engineering
  2. 2. Presentation on
  3. 3. Presented by: Mohammed Shakib Rahman Roll No. :
  4. 4. Introduction: Influence of a different nature cause concrete, even when free of any external loading, to undergo deformations and volume changes. The most important of these are shrinkage and the effects of temperature variations.
  5. 5. Effect of Shrinkage We know that workable concrete mix contains more water than is needed for hydration. If the concrete is exposed to air, the large part of this free water evaporates in time. As the concrete dries, it shrinks in volume, probably due to the capillarity tension that develops in the water remaining in the concrete.
  6. 6. Effects: Shrinkage, which continues at a decreasing rate for several months, depending on the configuration of the member, is a detrimental property of concrete in several respects. When not adequately controlled, it will cause unsightly and often deleterious cracks, as in slabs, walls, etc. In structures that are statically indeterminate, it can cause large and harmful stress. In pre-stressed concrete it leads to partial loss of initial pre-stress.
  7. 7. Figure : Crack due to shrinkage.
  8. 8. Determination of the amount of final shrinkage: A key factor in determining the amount of final shrinkage is the unit water content of the fresh concrete. This is illustrated in figure 1: Figure 1
  9. 9. This figure shows that the amount of shrinkage for varying amounts of mixing water. The same aggregates were used for all tests, but in addition to and independently of the water content, the amount of cement was also varied from 376 to 1034 lb/yd3 of concrete. This very large variation of cement content causes a 20 to 30 percent variation in shrinkage strain for water content between 250 to 350 lb/yd3, the range used for most structural concretes.
  10. 10. Again an increase in aggregate can significantly decrease shrinkage. This is shown in figure 2: This figure compares the shrinkage of concretes with various aggregate contents with the shrinkage obtained for neat cement paste (cement and water alone). Figure 2
  11. 11. Value of final shrinkage for ordinary cements are generally on the order of 400*10-6 to 800*10-6, depending on the initial water content, ambient temperature and humidity conditions, and the nature of aggregate. Highly absorptive aggregates, such as some sandstones and slates, result in shrinkage values 2 and more times those obtained with less absorptive materials, such as granites and some lime-stones. Some light weight aggregates, in view of their great porosity, easily result in much large shrinkage values than ordinary concrete.
  12. 12. Long term studies shows that, for moist-cured concrete at any time t after the initial 7 days, shrinkage can be predicted satisfactorily by the equation : Where is the unit shrinkage strain at time t in days and is the ultimate value after a long period of time. Equation pertains to “standard” conditions that is for humidity not in excess of 40 percent and for an average thickness of member of 6 in, and is apply for both normal-weight and lightweight concretes. Modification factor are to be applied for nonstandard conditions, and separate equations are given for steam –cured members.
  13. 13. Control of shrinkage: It is evident that an effective means of reducing shrinkage involves both a reduction in water content and an increase in aggregate content. In addition, prolonged and careful curing is beneficial for shrinkage control. For structures in which a reduction in cracking is of particular importance, such as bridge decks, pavement slabs, and liquid storage tanks, the use of expansive cement concrete is appropriate. Of the three types of expansive cements produced, only type K is commercially available in the United States, it is about 20 percent more expensive than ordinary Portland cement.
  14. 14. Effect of temperature change Like most other materials, concrete expands with increasing temperature and contracts with decreasing temperature. The effects of such volume changes are similar to those caused by shrinkage that is temperature contraction can lead to objectionable cracking, particularly when superimposed on shrinkage. In indeterminate structures, deformation due to temperature changes can cause large and occasionally harmful stresses.
  15. 15. The coefficient of thermal expansion and contraction varies somewhat, depending upon the type of aggregate and richness of the mix. It is generally within the range of 4*10-6 to 7*10-6 per oF. A value of 5.5*10-6 is generally accepted as satisfactory for calculating stresses and deformations caused by temperature changes.
  16. 16. Figure : Crack due to temperature change.