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1、REINFORCED CONCRETE,by Vincent McKinnon e-mail: ,SYLLABUS,1Materials Concrete & Reinforcement 2Beams 3Slabs 4Columns 5Walls 6Bases & Foundations,Types of Cement,OPC the most common Rapid hardening Portland Cement Low heat Portland Cement Sulphate resisting Portland Cement,Aggregates,Course aggregate
2、 Fine aggregate,Water,Water/cement ratio Minimum is 0.23 weight of water Normally, ranges from 0.45 0.6,Admixtures,Hardening/setting accelerators or retarders Water reducing to improve workability Air-entraining improve damage resistance Superplasticisers for complicated sections,REINFORCED CONCRETE
3、 DESIGN,Reinforced concrete is a composite material of steel bars embedded in a hardened concrete mix. Concrete, assisted by the steel carries the compressive forces whilst the steel resists the tensile forces.,CONCRETE & REINFORCEMENT,Concrete itself is a composite material. The dry mix consists of
4、 cement together with course and fine aggregates. Water is added and this reacts with the cement which hardens and binds the aggregate into the concrete matrix which sticks or bonds onto the reinforcing bars.,CONCRETE & REINFORCEMENT,Knowledge of the properties and an understanding of the behavior o
5、f concrete is an important factor in the design process.,Cement,Ordinary Portland cement is the commonest type in use. raw materials of OPC: lime, silica, alumina and iron oxide. These constituents are crushed and blended in the correct proportions and then burnt in a rotary kiln. The clinker is the
6、n cooled, mixed with gypsum and ground to a fine powder to produce cement. The main chemical compounds in cement are calcium silicates and aluminates.,Aggregates,Aggregates are classed into the following two sizes: course aggregate gravel or crushed rock greater than 5mm in size fine aggregate sand,
7、 less than 5mm in size,Aggregates,Aggregates should be chemically inert, clean, hard and durable. alkali-silica reaction: Some aggregates containing silica may react with alkalis in the cement causing the concrete to disintegrate. the presence of chlorides in the aggregate eg salt in marine sands wi
8、ll cause corrosion of the steel reinforcement. Excessive amounts of sulphate will also cause the concrete to disintegrate.,Water,The water to cement ratio: most important factor in affecting concrete strength. For full hydration, cement absorbs 0.23 of its weight of water. But this amount of water w
9、ould produce a very dry mix and so extra water is therefore required to improve workability. The actual water/cement ratio used is generally from 0.45 0.6.,Admixtures,Admixtures are substances added to concrete mixes in very small amounts in order to improve certain properties by their chemical or p
10、hysical effects.,Types of admixture,hardening or setting accelerators or retarders water-reducing which gives an increase in workability with a lower water/cement ratio air-entraining which increases resistance to damage from freezing or thawing superplasticisers which are used in complicated sectio
11、ns,Concrete Mix Design,two types of mixes are used: Design Mix. Strength forms an essential part of the requirement for compliance Prescribed Mix. Here, proportions of the constituents needed to give the required strength and workability are specified. Strength testing is not required.,CONCRETE PROP
12、ERTIES,The COMPRESSIVE strength is the most important property of concrete. The characteristic strength is measured by the 28 day cube strength. standard cubes of 100mm or 150mm are crushed to determine the strength. Typical strength values are 30 N/mm2 or 40 N/mm2. These are the 28 day cube strengt
13、h.,CONCRETE PROPERTIES,TENSILE strength This is normally assumed to be around 10% of the concretes compressive strength. So, for a grade 30 concrete ie one whos compressive strength is 30 N/mm2, the tensile strength would be assumed to be 3 N/mm2 and for a grade 40 concrete, it would be assumed to b
14、e 4 N/mm2. The tensile strength is measured by loading a concrete cylinder across a diameter as shown in the diagram (see Dia 1).,SHEAR,A vertical shear force in a beam causes complimentary shear stresses and diagonal tensile and compressive forces of the same magnitude to occur. This is shown in th
15、e next diagram (see Dia 2) where the stresses are shown in a small element near the neutral axis. The maximum shear stress at the neutral axis of the section is vmax = 1.5*V/(b*h) You will note an important point about the shear strength of a section. It depends upon the amount of reinforcement pres
16、ent, on the grade of concrete and upon the sections depth.,MODULUS OF ELASTICITY,The short term stress-strain curve for concrete in compression is also shown in Dia 1. This is known as the short term value of Youngs Modulus. (See also Dia 3).,CREEP,This is the gradual increase in strain with time in
17、 a member subjected to prolonged stress. When loaded, it is observed that the creep strain is much larger than the elastic strain. If the specimen is unloaded, there is an immediate elastic recovery and a slower recovery in the strain due to creep. Both amounts of recovery are much less than the ori
18、ginal strains under load.,CREEP,The main factors affecting creep strain are, the concrete mix and strength, the type of aggregate, the curing, the ambient relative humidity and the magnitude and duration of the sustained loading.,SHRINKAGE,Shrinkage, or drying shrinkage is the contraction which occu
19、rs in concrete when it dries and hardens. The aggregate type and content are the most important factors affecting shrinkage. The larger the size of aggregate, the lower is the shrinkage. At the same time, the lower the workability and water/cement ratio the lower is the shrinkage.,REINFORCEMENT,Rein
20、forcing bars are produced in two grades hot rolled mild steel bars have a yield strength of fy = 250 N/mm2. Hot rolled or cold worked bars have a yield strength of fy = 460 N/mm2. Steel fabric is made from cold drawn steel wires to form a mesh. It has a yield strength of fy = 460 N/mm2.,REINFORCEMEN
21、T,The stress-strain curve for steel is shown in Dia 3. The Youngs Modulus or the Modulus of Elasticity for steel is 200 kN/mm2. The behaviour in tension and compression is assumed to be the same. Mild steel bars are produced as round bars. But high yield steel bars are produced as deformed bars with
22、 transverse ribs.,COVER,The cover (including links) to a reinforcing bar will depend upon the exposure conditions. These are as shown in the table of Dia 4. Notice also the exposure conditions which, as you will note, range from mild to extreme. For severe exposures in a marine environment, 75mm is
23、considered appropriate,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,The British code (BS8110) for reinforced concrete design states that the aim of the design is the achievement of an acceptable probability that the structure will perform satisfactorily throughout its lifetime. It must carry the appl
24、ied loads safely, it must not deform excessively and it must have adequate resistance to effects of misuse and fire,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,The criterion for a safe design is that the structure should not become unfit for use ie that it should not reach a limit state during its d
25、esign life. This is achieved in particular by designing the structure to ensure that it does not reach :-,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,the ultimate limit state where the whole structure or its elements should not collapse, overturn or buckle when subjected to design loads the servicea
26、bility limit state where the structure should not become unfit for use due to excessive cracking, deflection or vibration.,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,The structure must also be durable ie it must not deteriorate or be damaged excessively by the action of substances coming into conta
27、ct with it. For reinforced concrete structures the normal practice is to design for the ultimate limit state and then to check for serviceability and to take all necessary precautions to ensure durability.,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,Ultimate Limit State The structure must be designe
28、d to carry the most severe combination of loads to which it is subjected. The sections of the element must be capable of resisting the axial loads, the shears and moments derived from the analysis. The design is made for ultimate loads and design strengths of materials with partial factors applied t
29、o the loads and material strengths.,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,Stability The code of practice also states that the layout should be such as to give a stable and robust structure. It stresses that the engineer responsible for the overall stability should ensure compatibility of desig
30、n and details of parts and components.,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,Overall stability of a structure is provided by shear walls, lift shafts, staircases and rigid frame action or a combination of these means. The structure should be such as to transmit all loads, dead, imposed and win
31、d safely to the foundations.,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,Robustness The code states that the planning and design of the structure should be such that damage to a small area or failure of a single element should not cause the collapse of a major part of the structure. This means that
32、the design should be resistant to progressive collapse.,2LIMIT STATE DESIGN AND STRUCTURAL ANALYSIS,Serviceability Limit State The main serviceability limit state provisions relate to:- Deflection The deformation of the structure should not adversely affect its appearance or efficiency. Deflections
33、can be calculated, but in normal cases, span-to-effective depth ratios can be used to check compliance with requirements Cracking Crack widths can be calculated, but in normal cases cracking can be controlled by adhering to detailing rules with regard to bar spacing in zones where the concrete is in
34、 tension.,3.0CHARACTERISTIC AND DESIGN LOADS,Design Loads The characteristic or service loads are the actual loads the structure is deigned to carry. They are normally thought of as the maximum loads which will not be exceeded during the life of the structure. In statistical terms the characteristic
35、 loads have a 95% probability of not being exceeded.,3.0CHARACTERISTIC AND DESIGN LOADS,There are four characteristic loads which we are mostly concerned with:- The characteristic dead load Gk This is the self weight of the structure together with the weight of finishes, ceilings, services and parti
36、tions. The characteristic imposed load Qk This is caused by people, furniture and equipment on floors and snow on roofs. The characteristic wind load Wk This depends upon the location, the shape and the dimensions of the building.,3.0CHARACTERISTIC AND DESIGN LOADS,The characteristic earth load - En
37、The code says that earth loads are to be obtained in accordance with normal practice. The design loads are then obtained from the following equation:- Design Load = Fk*fWhere Fk = characteristic load and f = partial safety factor for loads.,3.0CHARACTERISTIC AND DESIGN LOADS,The partial safety facto
38、r takes account of :- Possible increases in loads Inaccurate assessments of the effects of loads Unforeseen stress distributions in members The importance of the limit state being considered (See Dia. 5),3.0CHARACTERISTIC AND DESIGN LOADS,Material Factors The characteristic strengths or grades of ma
39、terials are as follows:- Concrete, fcu is the 28 day strength in Newtons per square millimetre Reinforcement, fy is the yield or proof stress in Newtons per square millimetre,3.0CHARACTERISTIC AND DESIGN LOADS,Grades for concrete are:- Fcu = 30, 35, 40, 45 and 50. Grades for steel are:- Hot rolled m
40、ild steel, fy = 250 N/mm2. High yield steel, hot rolled or cold worked, fy = 460 N/mm2.,3.0CHARACTERISTIC AND DESIGN LOADS,The resistance of sections to applied stresses is based upon the design strength which is defined as:- Characteristic strength/partial factor of safety for materials = fk/m,Mini
41、mum rebar - tension,Mild steel bars Type R, Eg 2-R10For tension reinforcement in a beam, the minimum amount of mild steel is 0.24% ie 100*As/Ac = 0.24 High yield steel bars Type T, Eg 2-T25 For tension reinforcement, the minimum amount of HYS is 0.13% ie 100*As/Ac = 0.13,Minimum rebar - compression,
42、In compression, the minimum amount of reinforcement to be provided for both mild as well as high yield steel is 0.2%. Thus 100*Asc/Ac = 0.2 In the abbreviations, Ac = total concrete area and Asc = Area of steel in compression. As = area of steel in tension,Maximum area of reinforcement,The maximum a
43、rea of reinforcement in both tension as well as compression should not exceed 4% of the gross cross sectional area of the beam.,Reinforcement spacing,First of all, remember the cover requirements In the horizontal direction, the minimum spacing between bars should not be less than the maximum aggreg
44、ate size plus 5mm. Remember also to leave room for the vibrator.,Reinforcement spacing,Where there are 2 or more rows, the bars should be vertically in line and the vertical distance between bars should not be less than 2/3 times the maximum aggregate size. Note that bundles of bars are treated as a
45、 single bar of equivalent area. This spacing should ensure that the concrete can be properly compacted around the reinforcement.,Load factors,Load factors,The design load is the characteristic load multiplied by the load factor ie DL = Fk*partial safety factor for loads,Materials factor,Resistance o
46、f section,The resistance of sections to applied stresses is based upon the design strength which is defined as :- The characteristic strength divided by the partial safety factor for materials,Singly reinforced rectangular beams,Design Assumptions:- Plane sections remain plane- ie the strains in the
47、 concrete and in the reinforcement are derived assuming that the plane sections remain plane. The stresses in the concrete in compression are derived using the simplified stress block where the depth of the stress block is 0.9 times the depth to the neutral axis.,Singly reinforced rectangular beams,
48、The strain in the concrete at failure is 0.0035 The tensile strength of the concrete is ignored,Singly reinforced rectangular beams,Singly reinforced rectangular beams,Singly reinforced concrete beams,The stress in the concrete is:- 0.67*fcu/gm 0.67*fcu/1.5 0.45*fcu,Singly reinforced concrete beams,
49、The stress in the steel is:- fy/1.15 = 0.87*fy However, for internal compatibility, C = T,Singly reinforced concrete beams,Hence equating forces, Force in concrete 0.67*fcu*b*0.9*x/1.5 = 0.4*fcu*b*x Note the factor 0.67. This is relationship between the bending and the cube strength of concrete,Sing
50、ly reinforced concrete beams,Similarly the force in the steel fy*As/gm = 0.87*fy*As Now we introduce a new factor called the lever arm. Its symbol is z The lever arm is the distance between the centroid of the concrete in compression and the centroid of the steel in compression,Singly reinforced con