Structure II Course Code: ARCH 209 Dr. Aeid A. Abdulrazeg Statics:The Next Generation Mehta & Danielson Lecture Notes for Sections 4.1-4.4

Properties of Reinforced Concrete Reinforced Concrete is one of the principal materials used in many civil engineering applications. Applications: Construction of buildings, retaining walls, foundations, water retaining structures, highways and bridges. It is a composite material, consisting of steel reinforcing bars embedded in a hardened concrete matrix.

Concrete: Highly in compressive strength but weak in tensile strength Concrete: Highly in compressive strength but weak in tensile strength. The tensile strength of concrete is about 10% of its compressive strength. Reinforcement (steel): Highly in tensile strength but weak in compressive strength. By providing steel bars in the zones within a concrete member which will subjected to tensile stresses, an economical structural material can be produced through its composite action.

In addition, the concrete provides corrosion protection and fire resistance to the embedded steel reinforcing bars.

Property Concrete Steel Strength in tension Poor Good (at least 100 times the tensile strength of concrete) Strength in compression Good Very Good (at least 10 times the strength of concrete) but slender bars will buckle Strength in shear Fair Durability Corrodes if unprotected Fire resistance Poor – suffers rapid loss of strength at high temperatures Service life Good but requires more maintenance

Composite action The tensile strength of concrete is only 10% of its compressive strength. Reinforced concrete structures are designed on the assumption that concrete is not designed to resist any tensile forces. Reinforcement is designed to carry the tensile forces, which are transferred by bond between concrete and steel. If the bond is not adequate, the reinforcing bars will just slip within the concrete and there will be not a composite action. Reinforcing bars are ribbed or twisted for extra grip.

Stress-strain relations - concrete Concrete is variable material, having wide range of strengths and stress-strain curves. The ultimate strain for most structural concrete is approximately 0.0035, irrespective of strength of concrete. Concrete generally increases its strength with age.

Stress-strain relations - Steel Mild steel behaves as an elastic material up to yield. After yield point there is sudden increase in strain with no change in stress. After yield point, mild steel becomes a plastic material and strain increases rapidly up to the ultimate value. High-yield steel does not have a definite yield point but show a more gradual change from elastic to plastic behavior. Both materials have similar slope of elastic region with Es = 200 kN/mm2 approx. The specified strength used in design is based on the yield stress for mild steel. For high-yield steel the strength is based on a specified proof stress. A 0.2% proof stress is used for high-yield steel.

Stress-strain relations - Steel Removal of load within the plastic range would result in stress-strain diagram following a line approximately parallel to loading portion – see line BC in figure below. The steel will be left with permanent strain AC, which is known as slip. If steel is again loaded, the stress-strain diagram will follow the unloading curve until it almost reaches the original stress at B and then it will curve in the direction of the first loading. This action is called strain hardening or work hardening.

Reinforcement strength Hot-rolled mild-steel bars (fy = 250N/mm2) These bars usually have smooth surface so that the bond with the concrete is by adhesion only. They can be bent easily, so they are often used where small radius bends are necessary, such as for links in narrow beams or columns, but their availability and usage is becoming less common. High-yield bars (fy = 460N/mm2) These are manufactured either in the form of a twisted square (Type 1) or as circular cross-section with a ribbed surface (Type 2) or plain bars. Type 2 is most commonly used as reinforcing bars while Type 1 has inferior bond characteristics. All high yield bars exhibit tension cracking when bent through small radius, therefore, the radius of the bend should not be less than three times the nominal bar size.

Reinforcement strength

Steel: Forms of Prestressing Steel The terms commonly used in prestressed concrete are explained. The terms are placed in groups as per usage. Forms of Prestressing Steel Wires Prestressing wire is a single unit made of steel. Strands Two, three or seven wires are wound to form a prestressing strand. Steel:

Tendon A group of strands or wires are wound to form a prestressing tendon. Cable A group of tendons form a prestressing cable. Bars A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire.

Method of Prestressing There are two different ways of prestressing, pre-tensioning and post-tensioning. Moreover, there are mainly two ways in which to place the reinforcement, inside the concrete or outside as external reinforcement

1. Pre-tensioning Pre-tensioning is when the cables are stressed prior to casting of the concrete. The cables remain stressed until the concrete has cured and then they are released or cut. The cables can be bonded in two ways, to the concrete only or with a mechanical anchorage. Pre-tensioned cables are used for structures with the reinforcement inside the structure

1. Pre-tensioning

1. Pre-tensioning

2. Post-tensioning Post-tensioning is when the cables are stressed on an existing structural element, for example after the concrete has cured if it is a concrete structure. The cables can be located both inside and outside of the structure. Here a mechanical anchorage in the end must be used to hold the cables in place and to keep the prestress active.

2. Post-tensioning

Advanages of Prestressing   The use of prestressed concrete offers distinct advantages over ordinary reinforced listed as follows: General advantages: Prestressing minimises the effect of cracks in concrete elements by holding the concrete in compression. Prestressing allows reduced beam depths to be achieved for equivalent design strengths. Prestressed concrete is resilient and will recover from the effects of a greater degree of overload than any other structural material. If the member is subject to overload, cracks, which may develop, will close up on removal of the overload. Prestressing enables both entire structural elements and structures to be formed from a number of precast units, e.g. Segmented and Modular Construction. Lighter elements permit the use of longer spanning members with a high strength to weight characteristic.

The ability to control deflections in prestressed beams and slabs permits longer spans to be achieved. Prestressing permits a more efficient usage of steel and enables the economic use of high tensile steels and high strength concrete. Cost advantages of Prestressing • Prestressed concrete can provide significant cost advantages over structural steel sections or ordinary reinforced concrete.  

• Prestressing needs skilled technology. Hence, it is not as common as Limitations of Prestressing Although prestressing has advantages, some aspects need to be carefully addressed. • Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete. • The use of high strength materials is costly. • There is additional cost in auxiliary equipments. • There is need for quality control and inspection. Statics:The Next Generation Mehta & Danielson Lecture Notes for Sections 4.1-4.4