Reinforcement concrete and properties of matrial

University Institute of Engineering Civil Engineering Department www.cuchd.in

CHANDIGARH UNIVERSITY

DEPARTMENT OF CIVIL ENGINEERING DESIGN OF CONCRETE STRUCTURES-I

INTRODUCTION TO

Vikas KhandelwalAP, CIVIL ENGG DEPTT.

Concrete:• is a composite material composed mainly of water,

aggregate, and cement. Often, additives and reinforcements (such as rebar) are included in the mixture to achieve the desired physical properties of the finished material. When these ingredients are mixed together, they form a fluid mass that is easily moulded into shape. Over time, the cement forms a hard matrix which binds the rest of the ingredients together into a durable stone.

University Institute of Engineering Civil Engineering Department Vikas Khandelwal www.cuchd.in


https://en.wikipedia.org/wiki/Composite_material

https://en.wikipedia.org/wiki/Water

https://en.wikipedia.org/wiki/Construction_aggregate

https://en.wikipedia.org/wiki/Cement

https://en.wikipedia.org/wiki/Rebar

Reinforced concrete (RC)• is a composite material in which concrete's relatively low

tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength and/or ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets. Reinforcing schemes are generally designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and/or structural failure. Modern reinforced concrete can contain varied reinforcing materials made of steel, polymers or alternate composite material in conjunction with rebar or not.

University Institute of Engineering Civil Engineering Department Vikas Khandelwal (E3177) www.cuchd.in


https://en.wikipedia.org/wiki/Composite_material

https://en.wikipedia.org/wiki/Concrete

https://en.wikipedia.org/wiki/Ultimate_tensile_strength

https://en.wikipedia.org/wiki/Ductility

https://en.wikipedia.org/wiki/Rebar

https://en.wikipedia.org/wiki/Tension_(physics)

https://en.wikipedia.org/wiki/Stress_(mechanics)

https://en.wikipedia.org/wiki/Fracture_mechanics

Reinforcement concrete as human body:



RCC :



Properties of material:• Properties of concrete: • Plain concrete is prepared by mixing cement, sand (also known

as fine aggregate), gravel (also known as coarse aggregate) and water with specific proportions. Mineral admixtures may also be added to improve certain properties of concrete. Thus, the properties of concrete regarding its strength and deformations depend on the individual properties of cement, sand, gravel, water and admixtures. • In plastic state concrete should be:• 1. workable• 2. free from segregation• 3. bleeding



• (a) Characteristic strength property • Characteristic strength is defined as the strength

below which not more than five per cent of the test results are expected to fall. Concrete is graded on the basis of its characteristic compressive strength of 150 mm size cube at 28 days and expressed in N/mm2.



• b) Flexural strength (fcr)• The flexural and splitting tensile strengths are

obtained as described in IS 516 and IS 5816, • fcr = 0.7 N/MM2



d) Shrinkage of concrete • Shrinkage is the time dependent deformation, generally

compressive in nature. The constituents of concrete, size of the member and environmental conditions are the factors on which the total shrinkage of concrete depends. However, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing for a given humidity and temperature. The cement content, however, influences the total shrinkage of concrete to a lesser extent. The approximate value of the total shrinkage strain for design is taken as 0.0003 in the absence of test data .



• Shrinkage of concrete is influenced by • • Water/cement ratio • • Humidity and temperature of curing• • Humidity during the period of use • • Age of concrete at first loading • • Magnitude of stress and its duration • • Surface-volume ratio of the member



• f) Thermal expansion of concrete • The knowledge of thermal expansion of concrete is very important as it is

prepared and remains in service at a wide range of temperature in different countries having very hot or cold climates. Moreover, concrete will be having its effect of high temperature during fire. The coefficient of thermal expansion depends on the nature of cement, aggregate, cement content, relative humidity and size of the section. IS 456 stipulates (cl. 6.2.6) page:16 the values of coefficient of thermal expansion for concrete / oC for different types of aggregate.

• Workability • It is the property which determines the ease and homogeneity with which

concrete can be mixed, placed, compacted and finished. A workable concrete will not have any segregation or bleeding. Segregation causes large voids and hence concrete becomes less durable. Bleeding results in several small pores on the surface due to excess water coming up. Bleeding also makes concrete less durable. The degree of workability of concrete is classified from very low to very high with the corresponding value of slump in mm (cl. 7 of IS 456).



Workability of concrete:PLACING CONDITION DEGREE OF WORKABILITY SLUMP (mm)

Blinding concrete Very low Compaction factor : 0.75-0.85

Mass conc., lightly rcc section beam, slab,wall

low 25-75

Heavy RCC section slab, beam, column.

medium 50-10075-100

Pumped conc, in-situ piling high 100-150

Terime work Very high Ease of flow is good.



(c) Durability of concrete • A durable concrete performs satisfactorily in the working

environment during its anticipated exposure conditions during service. The durable concrete should have low permeability with adequate cement content, sufficient low free water/cement ratio and ensured complete compaction of concrete by adequate curing. For more information, please refer to cl. 8 of IS 456

• Factor influence the durability of conc.• 1. the environment• 2. cover to embedded steel.• 3. types and quality of material used.• 4. water/cement ratio of concrete.• 5. workmanship, to obtain full compaction and efficient curing.



• Properties of steel:• As mentioned earlier in sec. 1.2.2, steel is used as the reinforcing material in concrete to make it good in tension. Steel as such is good in

tension as well as in compression. Unlike concrete, steel reinforcement rods are produced in steel plants. Moreover, the reinforcing bars or rods are commercially available in some specific diameters. Normally, steel bars up to 12 mm in diameter are designated as bars which can be coiled for transportation. Bars more than 12 mm in diameter are termed as rods and they are transported in standard lengths.

• Like concrete, steel also has several types or grades. The four types of steel used in concrete structures as specified in cl. 5.6 of IS 456 are given below: • (i) Mild steel and medium tensile steel bars conforming to IS 432 (Part 1) • (ii) High yield strength deformed (HYSD) steel bars conforming to IS 1786 • (iii) Hard-drawn steel wire fabric conforming to IS 1566 • (iv) Structural steel conforming to Grade A of IS 2062. • Mild steel bars had been progressively replaced by HYSD bars and subsequently TMT bars are promoted in our country. The implications of adopting different kinds of

blended cement and reinforcing steel should be examined before adopting.



Steel bar FROM FIG-2 PLAIN BAR

–MILD STEEL BAR

DEFORMED-HYSD BARS

FIG: 1FIG:2



Stress-strain curves for reinforcement:



• The characteristic yield strength fy of steel is assumed as the minimum yield stress or 0.2 per cent of proof stress for steel having no definite yield point. The modulus of elasticity of steel is taken to be 2x105 N/mm2. • The two grades of cold worked bars used as steel

reinforcement are Fe 415 and Fe 500 with the values of fy as 415 N/mm2 and 500 N/mm2, respectively.



The grades of concrete: • designated by one letter M (for mix) and a number

from 10 to 80 indicating the characteristic compressive strength (fck) in N/mm2. • As per IS 456 , concrete has three groups as • ordinary concrete (M 10 to M 20), • standard concrete (M 25 to M 55) • high strength concrete (M 60 to M 80). The size of

specimen for determining characteristic strength may be different in different countries



Permissible Stresses in material:• Permissible Stresses in concrete:• The permissible stresses of concrete in bending

compression σcbc,• in direct compression σcc and• the average bond for plain bars in tension τbd are

given in Table 21 of IS 456 for different grades of concrete. Grade of concrete Direct tension σtd (N/mm2 ) Bending compression σcbc

(N/mm2 )Direct compression σcc (N/mm2 ) Average bond

τbd for plain bars in tension (N/mm2 )

M 20 2.8 7.0 5.0 0.8

M 25 3.2 8.5 6.0 0.9

M 30 3.6 10.0 8.0 1.0

M 35 4.0 11.5 9.0 1.1

M 40 4.4 13.0 10.0 1.2



• Permissible stresses in steel reinforcement

Type of stress in steel reinforcement Mild steel bars, Fe 250, (N/mm2 ) High yield strength deformed bars, Fe 415, (N/mm2 )

Tension σst or σss (a) up to and including 20 mm diameter (b) over 20 mm diameter

140130

230230

Compression in column bars σsc 130 190



• Working stress method • The method is designated as working stress method as the

loads for the design of structures are the service loads or the working loads. The failure of the structure will occur at a much higher load. The ratio of the failure loads to the working loads is the factor of safety. Accordingly, the stresses of concrete and steel in a structure designed by the working stress method are not allowed to exceed some specified values of stresses known as permissible stresses. permissible stresses are determined dividing the characteristic strength fck of the material by the respective factor of safety. The values of the factor of safety depend on the grade of the material and the type of stress.



• Assumptions for Design of Members by Working Stress Method • As mentioned earlier, the working stress method is based on

elastic theory, where the following assumptions are made, as specified in cl. B-1.3 of IS 456.

• (a) Plane sections before bending remain plane after bending.• (b) Normally, concrete is not considered for taking the tensile

stresses except otherwise specifically permitted. Therefore, all tensile stresses are taken up by reinforcement only.

• (c) The stress-strain relationship of steel and concrete is a straight line under working loads.

• (d) The modular ratio m has the value of 280/3σcbc, where σcbc is the permissible compressive stress in concrete due to bending in N/mm2 . The values of σcbc are given in Table 21 of IS 456. The modular ratio is explained in the next section.



• Modular Ratio m In the elastic theory, structures having different materials are made equivalent to one common material. In the reinforced concrete structure, concrete and reinforcing steel are, therefore, converted into one material. This is done by transformation using the modular ratio m which is the ratio of modulus of elasticity of steel and concrete. Thus, m = Es/Ec. where Es is the modulus of elasticity of steel which is 200000 N/mm2 . However, concrete has different moduli, as it is not a perfectly elastic material. The short-term modulus of concrete Ec = 5000 fck in N/mm2 , where fck is the characteristic strength of concrete. However, the short-term modulus does not take into account the effects of creep, shrinkage and other long-term effects. Accordingly, the modular ratio m is not computed as m = Es/Ec = 200000/(5000 ) The value of m,i.e., 280/3σcbc, partially takes into account long-term effects. This is also mentioned in the note of cl. B-1.3 of IS 456.



• Ultimate load method (ULM)• The method is based on the ultimate strength of reinforced

concrete at ultimate load is obtained by enhancing the service load by some factor called as load factor for giving a desired margin of safety .Hence the method is also referred to as the load factor method or the ultimate strength method. In the ULM, stress condition at the state of in pending collapse of the structure is analysed, thus using, the non-linear stress – strain curves of concrete and steel. The safely measure in the design is obtained by the use of proper load factor. The satisfactory strength performance at ultimate loads does not guarantee satisfactory strength performance at ultimate loads does not guarantee satisfactory serviceability performance at normal service loads



• Limit state method • The term “Limit states” is of continental origin where there are three limit

states - serviceability / crack opening / collapse. For reasons not very clear, in English literature limit state of collapse is termed as limit state. As mentioned in sec. 1.1.1, the semi-empirical limit state method of design has been found to be the best for the design of reinforced concrete members. More details of this method are explained in Module 3 (Lesson 4). However, because of its superiority to other two methods (see sections 2.3.2 and 2.3.3 of Lesson 3), IS 456:2000 has been thoroughly updated in its fourth revision in 2000 taking into consideration the rapid development in the field of concrete technology and incorporating important aspects like durability etc. This standard has put greater emphasis to limit state method of design by presenting it in a full section (section 5), while the working stress method has been given in Annex B of the same standard. Accordingly, structures or structural elements shall normally be designed by limit state method.



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