What is Prestressed Concrete?

What is Prestressed Concrete?

Prestressed Concrete is an architectural and structural material possessing great strength. The unique characteristics of prestressed concrete allow predetermined, engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads. This is accomplished by combining the the best properties of two quality materials: high strength concrete for compression and high tensile strength steel strands for tension.
Actually, prestressing is quite simple. High tensile strands are stretched between abutments at each end of long casting beds. Concrete is then poured into the forms encasing the strands. As the concrete sets, it bonds to the tensioned steel. When the concrete reaches a specific strength, the strands are released from the abutments. This compresses the concrete, arches the member, and creates a built in resistance to service loads.
Prestressed Concrete BeamPrestressed or pretensioned before it leaves the plant, a slight arch or camber is noticeable. Energy is stored in the unit by the action of the highly tensioned steel which places a high compression in the lower portion of the member. An upward force is thereby created which in effect relieves the beam of having to carry its own weight.
The upward force along the length of the beam counteracts the service loads applied to the member.
Ordinary Concrete BeamEven without a load, the ordinary concrete beam must carry its own considerable weight - this leaves only a portion of its strength available to resist added loads.
Under service loads, the bottom of the beam will develop hairline cracks.

prestressed concrete-2

Prestressed Concrete

Although prestressed concrete was patented by a San Francisco engineer in
1886, it did not emerge as an accepted building material until a half-century later. The shortage of steel in Europe after World War II coupled with technological advancements in high-strength concrete and steel made prestressed concrete the building material of choice during European post-war reconstruction. North America's first prestressed concrete structure, the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania, however, was not completed until 1951.

In conventional reinforced concrete, the high tensile strength of steel is combined with concrete's great compressive strength to form a structural material that is strong in both compression and tension. The principle behind prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service.

Prestressing removes a number of design limitations conventional concrete places on span and load and permits the building of roofs, floors, bridges, and walls with longer unsupported spans. This allows architects and engineers to design and build lighter and shallower concrete structures without sacrificing strength.

The principle behind prestressing is applied when a row of books is moved from place to place. Instead of stacking the books vertically and carrying them, the books may be moved in a horizontal position by applying pressure to the books at the end of the row. When sufficient pressure is applied, compressive stresses are induced throughout the entire row, and the whole row can be lifted and carried horizontally at once.

Compressive Strength Added

Compressive stresses are induced in prestressed concrete either by pretensioning or post-tensioning the steel reinforcement.
In pretensioning, the steel is stretched before the concrete is placed. High-strength steel tendons are placed between two abutments and stretched to 70 to 80 percent of their ultimate strength. Concrete is poured into molds around the tendons and allowed to cure. Once the concrete reaches the required strength, the stretching forces are released. As the steel reacts to regain its original length, the tensile stresses are translated into a compressive stress in the concrete. Typical products for pretensioned concrete are roof slabs, piles, poles, bridge girders, wall panels, and railroad ties.

In post-tensioning, the steel is stretched after the concrete hardens. Concrete is cast around, but not in contact with unstretched steel. In many cases, ducts are formed in the concrete unit using thin walled steel forms. Once the concrete has hardened to the required strength, the steel tendons are inserted and stretched against the ends of the unit and anchored off externally, placing the concrete into compression. Post-tensioned concrete is used for cast-in-place concrete and for bridges, large girders, floor slabs, shells, roofs, and pavements.

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Prestressed concrete has experienced greatest growth in the field of commercial buildings. For buildings such as shopping centers, prestressed concrete is an ideal choice because it provides the span length necessary for flexibility and alteration of the internal structure. Prestressed concrete is also used in school auditoriums, gymnasiums, and cafeterias because of its acoustical properties and its ability to provide long, open spaces. One of the most widespread uses of prestressed concrete is parking garages.

prestressed concrete

Prestressed concrete

Prestressed concrete is a method for overcoming the concrete's natural weakness in tension.[1][2] It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete.
Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.

Bonded post-tensioned concrete
Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around a plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element. A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete member itself. When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-stressing is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge.The advantages of this system over unbonded post-tensioning are:
Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.
Tendons can be easily 'weaved' allowing a more efficient design approach.
Higher ultimate strength due to bond generated between the strand and concrete.
No long term issues with maintaining the integrity of the anchor/dead end.

Unbonded post-tensioned concrete
Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are:
The ability to individually adjust cables based on poor field conditions (For example: shifting a group of 4 cables around an opening by placing 2 to either side).
The procedure of post-stress grouting is eliminated.
The ability to de-stress the tendons before attempting repair work.
Picture number one (below) shows rolls of post-tensioning (PT) cables with the holding end anchors displayed. The holding end anchors are fastened to rebar placed above and below the cable and buried in the concrete locking that end. Pictures numbered two, three and four shows a series of black pulling end anchors from the rear along the floor edge form. Rebar is placed above and below the cable both in front and behind the face of the pulling end anchor. The above and below placement of the rebar can be seen in picture number three and the placement of the rebar in front and behind can be seen in picture number four. The blue cable seen in picture number four is electrical conduit. Picture number five shows the plastic sheathing stripped from the ends of the post-tensioning cables before placement through the pulling end anchors. Picture number six shows the post-tensioning cables in place for concrete pouring. The plastic sheathing has been removed from the end of the cable and the cable has been pushed through the black pulling end anchor attached to the inside of the concrete floor side form. The greased cable can be seen protruding from the concrete floor side form. Pictures seven and eight show the post-tensioning cables protruding from the poured concrete floor. After the concrete floor has been poured and has set for about a week, the cable ends will be pulled with a hydraulic jack, shown in picture number nine, until it is stretched to achieve the specified tension.

Pre-tensioned concrete

Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Pre-tensioned elements may be balcony elements, lintels, floor slabs, beams or foundation piles. An innovative bridge construction method using pre-stressing is described in Stressed ribbon bridge.

photo from site

photo from the site

on 9-1-2009

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هذه الصور توضح لكم بنطونات محلية تم تصنيعها في المشروع لاستخدامها في اعمال الجسر في حوض النهر كبنطونات خدمية لسير العاملين ونقل المواد الى مكان تنفيذ الفقرة للجسر كأن يكون الدعامة وقبعة الدعامة وغيرها وتصنع هذه البنطونات من البراميل الفارغة وتربط بواسطة فريم حديدي يجمعها كلها ليتم تشكيلها بهذه الصورة بعد ان يتم تسطيحها بالبليت حسب المتوفر ويتم عمل محجر حديدي لها للحماية وبالمناسبة هذه البنطونات مستخدمة منذ قديم الزمان في السودان لنقل البضائع في النهر

ارجو الاطلاع والاستفادة مع تحياتي

PRECAST REINFORCED CONCRETE PILES

PRECAST REINFORCED CONCRETE PILES
Precast reinforced concrete piles are constructed of conventional reinforced concrete with internal reinforcement consisting of a cage made up of longitudinal bars and lateral or tie steel. The piles are usually in the form of square with sections ranging form about 250 mm to about 450 mm and a maximum section length of up to about 20 m. For the reason of saving weight, long piles would manufacture with a hollow interior in hexagonal, octagonal or circular sections. Maximum allowable axial load is up to about 1000 kN. The lengths of pile sections are often dictated by practical considerations.
The use of the main longitudinal bars is provided to prevent the bending moments induced when the pile is lifted from its casting to the stacking area. On the other hand, lateral steel in the form of hoops and links resist shattering or splitting of the pile during driving.
Sometimes, it may be necessary to extend the pile by casting a length on to it in-situ . In such situations, the concrete at the top of the original pile should be stripped to expose 200 mm of the reinforcement. The new longitudinal reinforcement is then joined to the original reinforcement by full penetration butt weld.
Where welding is not practical, the original longitudinal reinforcement should be exposed to a distance of 40 times the bar diameter, and the new reinforcement overlapped for this length. The extension is then cast in-situ. Also, it can use epoxy mortar with dowels to connect the piles.