Prestressed Concrete PDF

PRESTRESSED CONCRETE PRE-TENSTIONED & POST TENSIONED C O N C R E T E S T RU C T U R E S PREPARED BY: KEN BENJAMIN T. CAMAO  Reinforced concrete Overview on RC: o - Concrete is strong in compression, weak in tension. o - Steel is strong in tensio...

PRESTRESSED CONCRETE PRE-TENSTIONED & POST TENSIONED C O N C R E T E S T RU C T U R E S PREPARED BY: KEN BENJAMIN T. CAMAO  Reinforced concrete Overview on RC: o - Concrete is strong in compression, weak in tension. o - Steel is strong in tension. o - Reinforced Concrete uses concrete to resist compression and to hold bars in position and uses steel to resist tension. o - Tensile strength of concrete is neglected. o - Reinforced Concrete beams allows crack under service load.  Reinforced concrete Overview on RC: o - In ordinary reinforced concrete, the Beam supports a load by developing compressive stresses at the top, but since the concrete cannot resist the tension at the bottom, it cracks o - Reinforcing steel bars are placed within this tension zone to resist the tension and control the cracking.  prestressed concrete Brief History: o - Internal stresses are induced to counteract external stresses. o - in 1904 it was introduced by Eugene Freyssinet, was formally patented in year 1928. o - to introduce permanent acting forces in concrete to resist elastic forces under loads, thus named “prestressing”. o Comparison of Reinforced concrete & prestressed concrete diagram Comparison of Reinforced concrete & prestressed concrete diagram  prestressed concrete Definition: o - Prestresed Concrete is defined as a concrete which reinforcing steel bars are stretched and anchored to compress it thus increases its resistance to stress. o Types of Prestressed Concrete: o 1. Pre-tensioning o 2. Post-tensioning  prestressed concrete Definition: o - Involving the application of forces to bend and compress a concrete element in order to counteract bending which comes from the structural load. o - The force applied is the tensioning of stretching of the steel component which usually in the form of high tensile strands, wires or bars.  HOW DO WE PRE- STRESS A MATERIAL?  TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S 1. MECHANICAL DEVICES 2. HYDRAULIC DEVICES 3. ELECTRICAL DEVICES 4. CHEMICAL DEVICES  TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S 1. MECHANICAL DEVICES The mechanical devices generally used include weights with or without lever transmission, geared transmission n conjunction with pulley blocks, screw jacks with or without gear devices and wire-winding machines. These devices are employed mainly for prestressing structural concrete components produced on a mass scale in factory.  TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S (MECHANICAL & HYDRAULIC WIRE TENSIONER, PULLEY BLOCKS)  TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S 2. HYDRAULIC DEVICES These are the simplest means of producing large prestressing force, extensively used as tensioning devices. TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S  TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S 3. ELECTRICAL DEVICES The wires are electrically heated and anchored before placing the concrete mould. This method is often referred to as thermo-prestressing and used for tensioning of steel wires and deformed bars.  TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S 4. CHEMICAL DEVICES Expanding cements are used and the degree of expansion is controlled by varying the curing condition. Since expansive action of cement.  TENSIONING DEVICES PRE-S T R E S S E D CONCRETE: P R O C E S S Expanding cement, also known as expansive cement, is a kind of Portland cement that produces slight volume expansion during the hydration and setting process. it is widely used in repair works. In the preparation of reinforced concrete, the expansion of the cement is utilized to stress the reinforcement.  prestressed concrete Common Applications of Prestressed Concrete: - Bridges - Slabs in buildings - Water Tank - Concrete Pile - Thin Shell Structures - Offshore Platform - Nuclear Power Plant - Repair and Rehabilitations  TYPES OF PRE-STRESSED CONCRETE  prestressed concrete PRE-S T R E S S E D CONCRETE: M E T H O D S There are two basic methods of applying pre-stress to a concrete member: 1. Pre-tensioning – most often used in factory situations 2. Post-tensioning – mostly executed on site or during site use.  prestressed concrete PRE-S T R E S S E D CONCRETE: M E T H O D S PRE-TENSION & POST-TENSION These two methods mainly differs in the method of stressing their components. A difference in the construction sequence and stress application.  prestressed concrete TYPE OF PRESTRESSED CONCRETE: PRE-TENSIONING “PRE-” “POST-” before after PRE-TENSIONED CONCRETE  Pre-TENSIONED concrete I. Pre-tensioning o In Pre-tension the tendon are tensioned against some abutments before the concrete is placed. o After the concrete hardened, the tension force is released. o The tendon tries to shrink back to the initial length but the concrete resists it through the bond between them, thus, compression force is induced in concrete. o Pretension is usually done with precast system. Pre-TENSIONED concrete Pre-TENSIONED concrete  Pre-TENSIONED concrete Pre-tensioned Concrete is usually fabricated away from the job site in a pre-stressing plant, whereas in post-tensioned pre-stressed concrete the application of stressing forces to the structure is done at the job site.  Pre-TENSIONED concrete The beams or elements are constructed on a stressing bed and stranded cable is place between two buttresses anchored to a stressing bed which holds the force in stretched cables. After stretching the steel with hydraulic jacks, concrete is placed in forms around the cables and allowed to harden. When the concrete reaches its sufficient strength, the pre- stress force is transferred to the concrete by bond when the steel strand at the ends of the beam is cut loose from buttresses. Pre-TENSIONED concrete Pre-TENSIONED concrete POST-TENSIONED CONCRETE BEAM POST-TENSIONED CONCRETE  Post-TENSIONED concrete II. Post-tensioning o Another technique for reinforcing concrete. o In Post-tensioning, the tendons are tensioned after the concrete has hardened. o Commonly, it uses metal or plastic ducts inside the concrete before casting.  Post-TENSIONED concrete II. Post-tensioning o Steel cables inside plastic ducts or sleeves are positioned in the forms before concrete is placed. Afterwards, once the concrete has gained strength, the cables are pulled tight and anchored against the outer edges of concrete.  Post-TENSIONED concrete II. Post-tensioning o After the concrete hardens, the tendons placed inside the duct are anchored against the concrete and then stressed. o Grout may be injected into the duct later. o Post-tensioning can be done either as pre-cast or cast-in- place.  prestressed concrete TYPE OF PRESTRESSED CONCRETE: POST-TENSIONING  prestressed concrete TYPE OF PRESTRESSED CONCRETE: POST-TENSIONING POST-TENSIONING PROCESS  prestressed concrete TYPE OF PRESTRESSED CONCRETE: POST-TENSIONING  prestressed concrete TYPE OF PRESTRESSED CONCRETE: POST-TENSIONING  prestressed concrete TYPE OF PRESTRESSED CONCRETE: POST-TENSIONING  prestressed concrete TYPE OF PRESTRESSED CONCRETE: POST-TENSIONING  prestressed concrete TYPE OF PRESTRESSED CONCRETE: POST-TENSIONING  ADVANTAGES VS. DISADVANTAGES  ADVANTAGES : Take full advantages of high strength concrete and high strength steel. Need less materials. Smaller and lighter structure. No cracks. Use the entire section to resist the load. Better corrosion resistance. Good for water tanks and nuclear plant. Very effective for deflection control Better shear resistance.  ADVANTAGES : Lower construction cost Thinner slabs, which are especially important in the high-rise buildings where floor thickness savings can translate into additional floors for the same or lower cost. Fewer joints since the distance that can be spanned by post-tensioned slabs exceeds that of reinforced construction with the same thickness. ADVANTAGES : Longer span lengths increase the usable unencumbered floor space in buildings and parking structures. Fewer joints lead to lower maintenance cost over the design life of the structure, since joints are the major focus of weakness in concrete buildings. DISADVANTAGES: Need higher quality materials More complex technically More expensive Harder to re-cycle DISADVANTAGES: The major problem with prestressed concrete is that it needs specialized construction machineries like jacks, anchorage, etc. Advanced technical knowledge and strict supervision is very important. DISADVANTAGES: For concrete pre-stressing, high tensile reinforcement bars are needed which costs greater that generally used steel reinforcement bars. Highly skilled labor is needed for pre -stressed concrete construction.  prestressed concrete Common Applications of Prestressed Concrete: - Bridges - Slabs in buildings - Water Tank - Concrete Pile - Thin Shell Structures - Offshore Platform - Nuclear Power Plant - Repair and Rehabilitations END.