2.1.8.5.2. Tension stiffening models in finite element method:
The phenomenon of concrete to carry tension even after cracking is represented in a finite element model by the use of tension stiffening. The tension stiffening effect improves the accuracy of the finite element (FE) models in representing cracks and, in some cases, improves the numerical stability of the solution. The tension stiffening effect must be estimated; it depends on such factors as the density of reinforcement, the quality of the bond between the rebar and the concrete, the relative size of the concrete aggregate compared to the rebar diameter, and the mesh. A reasonable starting point for relatively heavily reinforced concrete modeled with a fairly detailed mesh is to assume that the strain softening after failure reduces the stress linearly to zero at a total strain of about 10 times the strain at failure. The strain at failure in standard concretes is typically 10-4 (εcr), which suggests that tension stiffening that reduces the stress to zero at a total strain of about 10-3 is reasonable. This parameter should be calibrated before usage in any model.
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Tension stiffening in steel reinforced members was most commonly taken into account by either modifying the stress-strain response of the steel bar or by accounting for the tensile contribution of concrete after cracking by using a descending branch. The former 'tension stiffening strain' approach had been adopted by the CEB-FIP model code (CEB 1978, 1993) and the latter 'load-sharing' approach was first considered nearly a century ago by (Considere, 1899). The authors determined that tension stiffening was independent of concrete strength and reinforcement ratio as long as shrinkage was taken into account in the analysis of the member response. (Kharal,
Biomechanics is the overall topic that will be discussed throughout the presentation, this incorporates a brief discussion about water resistance, propulsion and buoyancy. Under lifesaving, appropriate techniques and the choice of stroke for the Timed Tow will be analyzed and justified.
According to major supplies, “Use of synthetic fibers for reinforcing concrete is continuously, increasing. The increase has been considerable since 1980, but slowed somewhat in 1990, a year of substantial construction cutbacks. Apparently the construction community believes there are advantages in the use of synthetic fibers in concrete.” (Schupack) Synthetic fibers are used to improve crack control in concrete. Some reports say that synthetic fiber reinforced concrete (SFRC) would replace welded wire fabric in many slab-on-grade applications. But in reality if the welded wire fabric is placed properly it controls crack width better than the synthetic fiber reinforced concrete. In a case study of the use of synthetic fibers in reinforced concrete, the following conclusions were obtained. No matter what concrete placing job is being done, there is no substitute for good concrete practices. The use of a low fiber volume mix will help control plastic shrinkage cracks and bleeding, but not give good cracking control once the crack forms. The synthetic fibers running through a crack have a poor bond providing no shear friction. Impact and toughness tests on synthetic fiber reinforced concrete imply less edge spalling will occur. The American Concrete Institute (ACI) conducted two studies on polypropylene fiber reinforced concrete, one dealing with plastic shrinkage cracking and the other on permeability characteristics. Plastic shrinkage cracking occurs when the surface water on the concrete evaporates faster than the bleed water reaches the surface of the concrete. It was determined by the plastic shrinking cracking study that polypropylene fibers helped reduce the total plastic shrinkage crack area on test panels. Also determined is that the screeding rate affects the total crack area in polypropylene reinforced concrete, while finishing operations showed no significant effects. This study also suggests the use of longer fibers (about 0.75in.) will produce less crack area. “Permeability plays an important role in long-term durability of concrete materials. Permeability of concrete generally refers to the rate at which particular aggressive substances (water, sulfates, chloride ions, etc.) can flow through the concrete.” (Soroushian) As discussed in the plastic shrinkage study that polypropylene fibers reduce cracking. Less cracking in the concrete surface that surface would be less permeable. In the permeability study, they concentrated on the effects of chloride and the permeability of the concrete. The results of this study concluded polypropylene fibers had little effects on chloride permeability of concrete. The polypropylene fibers only help reduce plastic shrinkage cracks.
In this paper we have studied compressive strength of concrete by replacing natural aggregate with recycled aggregate by 25%, 50% and 75%.This mixture formed in the sample of cubes of dimension 150x150x150mm3.
Aitcin P.C, “Cements of yesterday and today Concrete of tomorrow”, Cement and Concrete Research, Vol. 30, (2000), pp 1349 - 1359.
2) While many High Performance Concretes exhibits rapid stiffening and early strength gain, other’s may have long set times and low early strengths.
Thesis: In order to prevent the collapse of the Tacoma Narrows Bridge, all the static and dynamic strains should have been taken into account which would have involved proper consideration of effects of environmental conditions, building materials and design.
[20] BS Part 116: 1983. Testing Concrete and determination of compressive strength. British Standards Institution ;
In this report, we will introduce and illustrate on precast concrete, pre-stressed concrete, ready-mix concrete, reinforced concrete, terrazzo and Urbanite in details.
Now I would like to add examples of when Assef was the one who was discriminating against the Hazaras. There are several examples of that which lead not only to the tension between the Amir and Hassan. The first example would be the first time when Amir mentions him and also it is the first time in the book when Assef bullies both Hassan and Amir in Chapter 5. This is when both, Amir and Hassan decide to go climb the pomegranate tree and on their way they came across Assef and his friends who then teased Amir for being friends with Hassan, and in the end Hassan pulls the sling shot at Assef who then allows them to leave with a warning that he will get them in the future. The entire scene was not an example of the socail tension but it was
Concrete is used from the ground up in the construction of buildings. Foundations are made using concrete, and every from that point up can also employ concrete. As a result, the specifications of this concrete must be more stringent than the concrete used to support the aforementioned fence post, and with increased specifications comes more specialized installation practices. While Quikrete can be mixed and forgotten about, much of the work done for buildings requires much more careful monitoring over an extended period of time. In addition, sometimes even concrete is not strong enough on its own. As a result, sometimes concrete must be installed with a particular gauge of rebar installed in various spots in order to reinforce the concrete. These specifications are very specific, and it is an exact science. A contractor must be aware of these things ahead of time, and must be sure that they have the personnel and the training necessary to adequately install this
The continuous strain, which the concrete undergoes due to application of external loads is called……………………………………….
The concrete industries progress showed that the concrete Slab-on-Grade can be modified to be stiffer and have a higher tensile strength. It may resist the changes in soil due to swelling for example.
In figure (2.26), eight equations proposed by different researchers have been plotted for lightweight concrete, as shown in table (2.12). The experimental splitting tensile strength values are calculated as follows: Eq. (2.23), (2.24), (2.25) (2.26) for cube specimens and (2.27), (2.28), (2.29), (2.30) for cylinder specimens. The splitting tensile strength of lightweight concrete for Eq. (2.23) ranged from 2.8 – 3.5 MPa, as shown in figure (2.23).
In the design and construction of slabs and footings it is important to consider the durability of the finished product and devise ways in which to ensure the strength for which the slab was designed is maintained. Construction techniques and standards exist to ensure that all work is carried out to a specific level of quality to save the hassle of repairing prior mistakes. As with all business time efficiency is of paramount importance and sometimes contractors can overlook crucial elements of the design and supervision. Of course second only to efficiency is cost and usually this is of a high priority.
Mathematically, Hooke’s law states that F equals the displacement or extension length multiplies a constant k, or F = k∆l. F is the force in the spring which migh...