Module 2 - Prestressed Concrete PDF

ARB 426 BUILDING TECHNOLOGY 5: PRE-STRESSED CONCRETE Ar. Jaerica Mae S. Espeleta, uap, rMP REGISTERED AND LICENSED ARCHITECT AND MASTER PLUMBER LEAVE YOUR CAMERA OPEN DURING CLASS SESSION WE’LL START AT 3:10 PM  PRESTRESS: P...

ARB 426 BUILDING TECHNOLOGY 5: PRE-STRESSED CONCRETE Ar. Jaerica Mae S. Espeleta, uap, rMP REGISTERED AND LICENSED ARCHITECT AND MASTER PLUMBER LEAVE YOUR CAMERA OPEN DURING CLASS SESSION WE’LL START AT 3:10 PM  PRESTRESS: Prestress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete. The pre-compression applied (may be axial or eccentric) will induce the compressive stress below the neutral axis or as a whole of the beam c/s. Resulting either no tension or compression. IN SIMPLER TERMS: Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.  MATERIALS USED FOR PRE- STRESSING 1. WIRES – Prestressing wire is a single unit made of steel. 2. STRANDS – Two, three or seven wires are wound to form a prestressing strand. 3. TENDON – A group of strands or wires are wound to form a prestressing tendon. 4. CABLE - A group of tendons form a prestressing cable. 5. BARS - A tendon can be made up of a single steel bar. The diameter of a bar is much larger 6. than that of a wire CONCRETE is good in compression but weak in tension. CONCRETE is good in compression but weak in tension. CONCRETE is good in compression but weak in tension. BRITTLE FAILURE DUCTILE FAILURE PRE-STRESSED CONCRETE also known as active reinforcement. HISTORY AND DEELOPMENT OF PRESTRESSED CONCRETE: A prestressed concrete structure is different from a conventional reinforced concrete structure due to the application of an initial load on the structure prior to its use. The initial load or ‘prestress’ is applied to enable the structure to counteract the stresses arising during its service period. The prestressing of a structure is not the only instance of prestressing. The concept of prestressing existed before the applications in concrete. Two examples of prestressing before the development of prestressed concrete are provided. Force-fitting of metal bands on wooden barrels: The metal bands induce a state of initial hoop compression, to counteract the hoop tension caused by filling of liquid in the barrels. Pre-tensioning the spokes in a bicycle wheel: The pre-tension of a spoke in a bicycle wheel is applied to such an extent that there will always be a residual tension in the spoke. That tension in spoke will nullify the applied compression.  NATURE OF CONCRETE- STEEL INTERFACE 1. BONDED TENDON - When there is adequate bond between the prestressing tendon and concrete, it is called a bonded tendon. Pre-tensioned and grouted post-tensioned tendons are bonded tendons. 2. UNBONDED TENDON When there is no bond between the prestressing tendon and concrete, it is called unbonded tendon. When grout is not applied after post-tensioning, the tendon is an unbonded tendon. STAGES OF LOADING The analysis of prestressed members can be different for the different stages of loading. The stages of loading are as follows. 1. Initial: It can be subdivided into two stages. a) During Tensioning of steel b) At transfer of prestress to concrete 2. Intermediate: This includes the loads during transportation of the prestressed members. 3. Final: It can be subdivided into two stage. a) At service, during operation b) At ultimate, during extreme events. ADVANTAGES OF PRE-STRESSING 1. Section remains uncracked under service loads. a) Reduction of steel corrosion – increase in durability b) Full section is utilized Higher moment of intertia (higher stiffness) Less deformations (improved serviceability) c) Increase in shear capacity d) Suitable for use in pressure vessels, liquid retaining structures. e) Improved performance (resilience) under dynamic and fatigue loading. 2. High span-to-depth ratios Larger spans possible with prestressing (bridges, buildings with large column-free spaces) Typical values of span-to-depth ratios in slabs are given below. For the same span, less depth compared to RC member. Reduction in self weight More aesthetic appeal due to slender sections More economical sections. ADVANTAGES OF PRE-STRESSING 3. Suitable for precast construction The advantages of precast construction are as follows. Rapid construction Better quality control Reduced maintenance Suitable for repetitive construction Multiple use of formwork ⇒ Reduction of formwork Availability of standard shapes. The following figure shows the common types of precast sections. ADVANTAGES OF PRE-STRESSING 4. The cross-section is utilized more efficiently in pre-stressed concrete as compared to reinforced concrete. 5. Pre-stressed concrete allows for a longer span. 6. Pre-stressed concrete members offer more resistance against shear force. 7. Considering same depth of concrete member, a pre-stressed concrete member is stifferthan the reinforced concrete member under working loads. 8. The use of higher strength concrete and high strength steel results in smaller cross section. LIMITATIONS OF PRESTRESSING: 1. Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete. 2. The use of high strength materials is costly. 3. There is an additional cost in auxiliary equipment. 4. There is need for quality control and inspection. 5. Prestressed concrete sections are less fire resistant.  TYPES OF PRESTRESSING: 1. SOURCE OF PRESTRESSING FORCE – This classification is based on the method by which the prestressing force is generated. There are four sources of prestressing force: Mechanical, hydraulic, electrical and chemical. 2. PRE-TENSIONING OR POST-TENSIONING - This is the most important classification and is based on the sequence of casting the concrete and applying tension to the tendons. 3. LINEAR OR CIRCULAR PRESTRESSING - This classification is based on the shape of the member prestressed. 4. FULL, LIMITED OR PARTIAL PRESTRESSING - Based on the amount of prestressing force, three types of prestressing are defined. 5. UNIAXIAL, BIAXIAL OR MULTI-AXIAL PRESTRESSING - The classification is based on the directions of prestressing a member.  SOURCE OF PRESTRESSING FORCE: 1. HYDRAULIC PRESTRESSING: o This is the simplest type of prestressing, producing large prestressing forces. The hydraulic jack used for the tensioning of tendons, comprises of calibrated pressure gauges which directly indicate the magnitude of force developed during the tensioning. 2. MECHANICAL PRESTRESSING: o In this type of prestressing, the devices includes weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear drives and wire-winding machines. This type of prestressing is adopted for mass scale production 3. ELECTRICAL PRESTRESSING: o In this type of prestressing, the steel wires are electrically heated and anchored before placing concrete in the moulds. This type of prestressing is also known as thermo-electric prestressing.  EXTERNAL OR INTERNAL PRESTRESSING: 1. EXTERNAL PRESTRESSING: o When the prestressing is achieved by elements located outside the concrete, it is called external prestressing. The tendons can lie outside the member (for example in I-girders or walls) or inside the hollow space of a box girder. This technique is adopted in bridges and strengthening of buildings. In the following figure, the box girder of a bridge is prestressed with tendons that lie outside the concrete. Figure: External prestressing in circular tank  EXTERNAL OR INTERNAL PRESTRESSING: INTERNAL PRESTRESSING: o When the prestressing is achieved by elements located inside the concrete member (commonly, by embedded tendons), it is called internal prestressing. Most of the applications of prestressing are internal prestressing. In the following figure, concrete will be cast around the ducts for placing the tendons. PRE-TENSIONED CONCRETE is applying pressure to the reinforcement before it is put into service. Source: https://williams-works.com/prestressed-concrete-101/ POST-TENSIONED CONCRETE Stress is developed after the concrete is cured. LINEAR OR CIRCULAR PRESTRESSING: LINEAR PRESTRESSING: When the prestressed members are straight or flat, in the direction of prestressing, the prestressing is called linear prestressing. For example, prestressing of beams, piles, poles and slabs. The profile of the prestressing tendon may be curved. The following figure shows linearly prestressed railway sleepers. Figure: railway sleepers LINEAR OR CIRCULAR PRESTRESSING: CIRCULAR PRESTRESSING: When the prestressed members are curved, in the direction of prestressing, the prestressing is called circular prestressing. For example, circumferential prestressing of tanks, silos, pipes and similar structures. The following figure shows the containment structure for a nuclear reactor which is circularly prestressed. Figure: pre stressing of circular tank FULL, LIMITED, OR PARTIAL PRESTRESSING 1. FULL PRESTRESSING: o When the level of prestressing is such that no tensile stress is allowed in concrete under service loads, it is called Full Prestressing 2. LIMITED PRESTRESSING: o When the level of prestressing is such that the tensile stress under service loads is within the cracking stress of concrete, it is called Limited Prestressing. 3. PARTIAL PRESTRESSING: o When the level of prestressing is such that under tensile stresses due to service loads, the crack width is within the allowable limit, it is called Partial Prestressing UNIAXIAL, BIAXIAL, OR MULTIAXIAL PRESTRESSING 1. UNIAXIAL PRESTRESSING: o When the prestressing tendons are parallel to one axis, it is called Uniaxial Prestressing. For example, longitudinal prestressing of beams. 2. BIAXIAL PRESTRESSING: o When there are prestressing tendons parallel to two axes, it is called Biaxial Prestressing. 3. MULTIAXIAL PRESTRESSING: o When the prestressing tendons are parallel to more than two axes, it is called Multiaxial Prestressing. For example, prestressing of domes. DIFFERENCES OF PRESTRESSED CONCRETE OVER REINFORCED CONCRETE: CLASSIFICATION PRESTRESSED CONCRETE REINFORCED CONCRETE Passive (Stress depends on the external ROLE Active loads) STRESS Constant Varies SHEAR RESISTANCE More (load independent) Less (external load dependent) Less due to the eccentric prestressing More deflection due to passive DEFLECTIONS force that causes upward deflections reinforcements More durable due the use of high-grade DURABILITY Prone to cracking concrete Less because external stresses are counterbalanced by the internal stress Uneconomical for long span due to large DIMENSIONS induced by prestress (quantity of concrete quantity of concrete and steel is reduced by 30%, steel is reduced by about 60% to 70%) THANK YOU