Introduction to Prestressed Concrete Design PDF

INTRODUCTION CE163-2 INTRODUCTION PLAIN CONCRETE & REINFORCED CONCRETE PRESTRESSING PRINCIPLE AND PRESTRESSED CONCRETE REINFORCED CONCRETE VS PRESTRESSED CONCRETE 2 CONCRETE Concrete is a composite material composed of coarse aggregate (usually crushed...

INTRODUCTION CE163-2 INTRODUCTION PLAIN CONCRETE & REINFORCED CONCRETE PRESTRESSING PRINCIPLE AND PRESTRESSED CONCRETE REINFORCED CONCRETE VS PRESTRESSED CONCRETE 2 CONCRETE Concrete is a composite material composed of coarse aggregate (usually crushed stone or gravel), fine aggregate (sand), cement, admixtures, small air voids, and water. It is one of the most widely used construction materials in the world. The cement, when mixed with water, forms a paste that binds the aggregates into a solid mass, giving concrete its strength and durability. 3 PLAIN CONCRETE  A plain concrete is simply Plain Concrete composed of the hardened mixture of cement, water and aggregates as presented previously.  This is only suitable for non- structural elements, such as pavements where tensile forces are minimal. If plain concrete cannot withstand the tensile stresses, the failure will be abrupt and brittle. 4 REINFORCED CONCRETE Generally, concrete is strong in compression, but weak in tension. Reinforced Concrete  Its tensile strength varies from 8% to 14% of its compressive strength  Due to such low tensile capacity, flexural cracks develop at early stages of loading.  When properly designed, reinforcing bars absorbs the tensile stresses and eventually yields before the beam undergo total failure. 5 REINFORCED CONCRETE 6 PRESTRESSED CONCRETE  In order to reduce or prevent such cracks and excessive deflection from developing, a concentric or eccentric force is imposed in the longitudinal direction of the structural element.  This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical locations of the sections. 7 Such an imposed load is called PRESTRESSING FORCE Concentric Prestressing Force Eccentric Prestressing Force (coincides with the centroid) (off-centroid) 8 Prestressing Force A compressive force that prestress the sections along the span.  Applied prior to the application of the transverse gravity dead load and live loads or transient horizontal live loads. The beam undergoes significant "prestressing" or compression during production, occurring before experiencing any service loading. 9 Prestressing Force Type & Magnitude are determined by:  Type of System to be constructed Pretensioning Post-tensioning  Span Length  Slenderness desired 10 TYPES OF PRESTRESSING FORCE 11 Types of Prestressing Force Linear Prestressing  Prestressing force applied longitudinally along or parallel to the axis of the member 12 Types of Prestressing Force Circular Prestressing  Used in liquid containment tanks, pipes, and pressure reactor vessels.  The circumferential hoop or hugging stress on the cylindrical or spherical structure, neutralizes the tensile stresses at the outer fibers of the curvilinear surface caused by the internal contained pressure. 13 COMPARISON WITH REINFORCED CONCRETE PRESTRESSED VS REINFORCED 14 COMPARISON REINFORCED CONCRETE VS PRESTRESSED CONCRETE REINFORCED CONCRETE  Concrete in compression side is effective. It is assumed that the tensile strength of the concrete is negligible and disregarded.  Reinforced concrete must crack before the steel can significantly contribute to the strength and stiffness. Tensile forces resulting from the bending moments are resisted by the bond created in the reinforcement.  High Strength concrete is fairly considered depending on the usage. Tension zone in concrete 15 is negligible and disregarded COMPARISON REINFORCED CONCRETE VS PRESTRESSED CONCRETE PRESTRESSED CONCRETE  The entire section is effective.  Permanent Stresses are created BEFORE the full dead load and live loads are applied to eliminate or considerably reduce the net tensile stresses caused by these loads. It delays cracking, so that the service loads are carried primarily by uncracked concrete.  High strength concrete and steel is necessary to resist high stresses at the anchorages 16 END. 17