TECHNIQUES TO RESTORE ORIGINAL STRENGTH OF STRUCTURES

The techniques are described below among with other restoration measures. Small cracks


If the cracks are reasonably small (opening width=0.075mm), the technique to restore the original tensile strength of the cracked element is by pressure injection of epoxy. The procedure is as follows.

The external surfaces are cleaned of non-structural materials and plastic ports are placed among the surface of the cracks on both sides of the member and are secured in place with an epoxy sealant. The centre to centre spacing of these ports may be approximately equal to the thickness of the element. After the sealant has cured, a low viscosity epoxy resin is injected into one port at a time, beginning at the lowest part of the crack in case it is vertical or at one end of the crack in case it is horizontal.
The resin is injected till it is seen flowing from the opposite sides of the member at the corresponding port or from the next higher port on the same side of the member. The injection port should be closed at this stage and injection equipment moved to next port and so on.
The small the crack, the higher is the pressure or more closely spaced should be the ports so as to obtain the complete penetration of the epoxy material throughout the depth and width of member. Larger cracks will permit larger port spacing, depending upon width of the member. This technique is appropriate for all types of structural elements- beams, columns, walls and floor units in masonry as well as concrete structures. Two items should however be taken care of in such type of repair:
In the case of loss of bond between reinforcement bar and concrete, if the concrete, if the concrete adjacent to the bar has been pulverised to a very fine powder, this powder will dam the epoxy from saturating the region. So it should be cleaned properly by air or water pressure prior to injection of epoxy.It has been stated that cracks smaller than 0.75mm may be difficult pressure inject. So cracks smaller than this should not be repaired by this method.Large cracks and crushed concrete




For cracks wider than about 6mm or for regions in which the concrete or masonry has crushed, a treatment other than injection is indicated. The following procedure may be adopted:
The loose material is removed and replaced with expansive cement mortar, quick setting cement or gypsum cement mortar.Where found necessary, additional shear of flexural reinforcement is provided in the region of repairs. This reinforcement could be covered by mortar to give further strength as well as protection to the reinforcement.In areas of very severe damage, replacement of the member or portion of member can be carried out. In case of damage to walls and floor diaphragms, steel mesh could be provided on the outside of the surface and nailed or bolted to the wall. Then it may be covered or plastered or micro-concrete.Fractured, excessively yielded and buckled reinforcement




In the case of severely damaged reinforced concrete member, it is possible that the reinforcement would have buckled, or elongated or excessively yielding may have occurred. This element can be repaired by replacing the old portion of steel with new steel using butt welding or lap welding.
Splicing by overlapping will be risky. If repair has to be made without removal of the existing steel, the best approach would depend upon the space available in the original member. Additional stirrup ties are to be added in the damaged portion before concreting so as to confine the concrete and enclose the longitudinal bars to prevent their buckling in future.
In some cases it may be necessary to anchor additional steel into existing concrete. A common technique for providing the anchorage uses the following procedure:
A hole larger than the bar is drilled. The hole is filled with epoxy, expanding cement, or other strength grouting material. The bar is pushed into the place and held there until the grout has set.


Fractured wooden members and joints
Since wood is an easily workable material, it will be easy to restore the strength of wooden members, beams, columns, struts and ties by splicing additional material. The weathered or rotten wood should first be removed. Nails, wood screws or steel bolts will be most convenient as connectors. It will be advisable to use straps to cover all such splices and joints so as to keep them tight and stiff.

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Posted in: ORIGINAL,RESTORE,STRENGTH,STRUCTURES,TECHNIQUES

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Tougher, lighter wind turbine blade developed: Polyurethane reinforced with carbon nanotubes

                                                                                                                 

Efforts to build larger wind turbines able to capture more energy from the air are stymied by the weight of blades. A Case Western Reserve University researcher has built a prototype blade that is substantially lighter and eight times tougher and more durable than currently used blade materials.


Marcio Loos, a post-doctoral researcher in the Department of Macromolecular Science and Engineering, works with colleagues at Case Western Reserve, and investigators from Bayer MaterialScience in Pittsburgh, and Molded Fiber Glass Co. in Ashtabula, Ohio, comparing the properties of new materials with the current standards used in blade manufacturing.

On his own, Loos went to the lab on weekends and built the world's first polyurethane blade reinforced with carbon nanotubes. He wanted to be sure the composite that was scoring best on preliminary tests could be molded into the right shape and maintain properties.

Using a small commercial blade as a template, he manufactured a 29-inch blade that is substantially lighter, more rigid and tougher.

"The idea behind all this is the need to develop stronger and lighter materials which will enable manufacturing of blades for larger rotors," Loos said.

That's an industry goal.

In order to achieve the expansion expected in the market for wind energy, turbines need a bigger share of the wind. But, simply building larger blades isn't a smart answer.

The heavier the blades, the more wind is needed to turn the rotor. That means less energy is captured. And the more the blades flex in the wind, the more they lose the optimal shape for catching moving air, so, even less energy is captured.

Lighter, stiffer blades enable maximum energy and production.

"Results of mechanical testing for the carbon nanotube reinforced polyurethane show that this material outperforms the currently used resins for wind blades applications," said Ica Manas-Zloczower, professor of macromolecular science and engineering and associate dean in the Case School of Engineering.

Loos is working in the Manas-Zloczower lab where she and Chemical Engineering Professor Donald L. Feke, a vice provost at the university, serve as advisors on the project.

In a comparison of reinforcing materials, the researchers found carbon nanotubes are lighter per unit of volume than carbon fiber and aluminum and had more than 5 times the tensile strength of carbon fiber and more than 60 times that of aluminum.

Fatigue testing showed the reinforced polyurethane composite lasts about eight times longer than epoxy reinforced with fiberglass. The new material was also about eight times tougher in delamination fracture tests.

The performance in each test was even better when compared to vinyl ester reinforced with fiberglass, another material used to make blades.

The new composite also has shown fracture growth rates at a fraction of the rates found for traditional epoxy and vinyl ester composites.

Loos and the rest of the team are continuing to test for the optimal conditions for the stable dispersion of nanotubes, the best distribution within the polyurethane and methods to make that happen.

The functional prototype blades built by Loos, which were used to turn a 400-watt turbine, will be stored in our laboratory, Manas-Zloczower said. "They will be used to emphasize the significant potential of carbon nanotube reinforced polyurethane systems for use in the next generation of wind turbine blades."


Disclaimer: Views expressed in this article do not necessarily reflect those of Royal Civilizers or its staff.

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Posted in: blade,carbon,developed,lighter,nanotubes,Polyurethane,reinforced,Tougher,turbine

Shoring, Underpinning and Scaffolding

  

             Shoring is a general term used in construction to describe the process of supporting a structure in order to prevent collapse so that construction can proceed. The phrase can also be used as a noun to refer to the materials used in the process. Underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure.


Scaffolding is a temporary frame used to support people and material in the construction or repair of buildings and other large structures.

                    

Shoring is used to support the beams and floors in a building while a column or wall is removed. In this situation vertical supports are used as a temporary replacement for the building columns or walls.

Trenches – During excavation, shoring systems provide safety for workers in a trench and speed excavation. In this case, shoring should not be confused with shielding. Shoring is designed to prevent collapse where shielding is only designed to protect workers when collapses occur. concrete structures shoring, in this case also referred to as falsework, provides temporary support until the concrete becomes hard and achieves the desired strength to support loads.

Shoring Techniques

Raking Shore :
Raking Shores consist of one or more timbers sloping between the face of the structure to be supported and the ground. The most effective support is given if the raker meets the wall at an angle of 60 to 70 degrees. A wall-plate is typically used to increase the area of support.

Hydraulic Shoring :
Hydraulic shoring is the use of hydraulic pistons that can be pumped outward until they press up against the trench walls. They are typically combined with steel plate or plywood, either being 1-1/8? thick plywood, or special heavy Finland Form (FINFORM) 7/8? thick.

Beam and Plate :
Beam and Plate steel I-beams are driven into the ground and steel plates are slid in amongst them. A similar method that uses wood planks is called soldier boarding. Hydraulics tend to be faster and easier; the other methods tend to be used for longer term applications or larger excavations.

Soil Nailing :
Soil nailing is a technique in which soil slopes, excavations or retaining walls are reinforced by the insertion of relatively slender elements – normally steel reinforcing bars. The bars are usually installed into a pre-drilled hole and then grouted into place or drilled and grouted simultaneously. They are usually installed untensioned at a slight downward inclination. A rigid or flexible facing (often sprayed concrete) or isolated soil nail heads may be used at the surface.

Continuous Flight Augering :
Continuous Flight Augering (CFA) is a method used to create concrete piles to support soil so that excavation can take place nearby. A Continuous Flight Augering drill is used to excavate a hole and concrete is injected through a hollow shaft under pressure as the auger is extracted. This creates a continuous pile without ever leaving an open hole.

Underpinning :
Underpinning may be necessary for a variety of reasons:
* The original foundation is simply not strong or stable enough, e.g. due to decay of wooden piles under the foundation.
* The usage of the structure has changed.
* The properties of the soil supporting the foundation may have changed (possibly through subsidence) or were mischaracterized during planning.
* The construction of nearby structures necessitates the excavation of soil supporting existing foundations.
* It is more economical, due to land price or otherwise, to work on the present structure’s foundation than to build a new one.
Underpinning is accomplished by extending the foundation in depth or in breadth so it either rests on a stronger soil stratum or distributes its load across a greater area. Use of micropiles and jet grouting are common methods in underpinning. An alternative to underpinning is the strengthening of the soil by the introduction of a grout. All of these processes are generally expensive and elaborate.

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Posted in: Scaffolding,Shoring,Underpinning

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