CONCRETEConcrete is a mixture of water, cement,sand, gravel, crushed rock, or otheraggregates. The aggregates (sand, gravel,crushed rock) are held together in a rocklikemass with a paste of cement and water.
As with most rocklike mass concrete has avery high compressive strength but have a.very low tensile strength. As a structuralmember, concrete can lie made to carrytensile stresses (as in beam in flexure). Inthis regard, it is necessary to provide steelbars to provide the tensile strength lackingin concrete. The composite member is calledreinforced concrete.
Aggregates used in concrete may be fineaggregates (usually sand) and coarseaggregates (usually gravel or crushed stone).Fine aggregates are those that passesthrough a No. 4 sieve (about 6 mm in size).Materials retained are coarse aggregates.
The nominal maximum sizes of coarseaggregate are specified in Section 5.3.3 ofNSCP. These are as follows: 1/5 thenarrowest dimension between sides offorms, 1/3 the depth of slabs, or 3/4 theminimum clear spacing between individualreinforcing bars or wires, bundles of bars, orprestressing tendons or ducts. Theselimitations may not be applied if, in thejudgment of the Engineer, workability andmethods of consolidation are stich thatconcrete can be placed without honeycombor voids
According to Section 5.3.4, water used inmixing concrete shall be clean and free frominjurious amounts of oils, acids, alkalis, salts,organic materials, or. Other substances thatmay be deleterious to concrete orreinforcement. Mixing water for prestressedconcrete or for concrete that will containaluminum embedments, including thatportion of mixing water contributed in theform of
free moisture on aggregates; shall notcontain deleterious amounts of chloride ion.:Non-potable (non-drinkable) water shall notbe used in concrete unless the following aresatisfied: (a) Selection of concreteproportions shall be based on concrete mixesusing water from the same source and (b)mortar test cubes made with non-potablemixing water shall have 7-day and 28-daystrengths equal to at least 90 percent ofstrengths of similar specimens made withpotable water.
Proportions ofmaterials for concrete shall be established to provide: (a) workability and consistencyto permit concrete to be worked readily into forms and around reinforcement underconditions of placement to be employed, without segregation or excessive bleeding,(b) Resistance to special exposures; and (c) conformance with strength testrequirements.
Concrete lighter in weight than ordinarysand-and-gravel concrete is used principallyto reduce dead load, or for thermalinsulation, nailability, or fill. Disadvantagesof lightweight structural concretes includehigher cost, need for more care in placing,greater porosity, and mote drying shrinkage,For a given percentage of cement, usuallythe lighter the concrete, the lower thestrength.
Concrete weighing up to about 60.5 kN/m3can be produced by using heavier than-ordinary aggregate. Theoretically, the upperlimit can be achieved with steel shot as fineaggregate and steel punching as coarseaggregate. The heavy concrete is usedprincipally in radiations shields andcounterweights.
Unlike steel and other materials, concretehas no definite modulus of elasticity. Itsvalue is dependent on the characteristics ofcement and aggregates used, age of concreteand strengths.
Ec= wc 1.5 0.043 √f’c (in MPa) Eq 1-1Where:f’c = the 28-day compressive strength of concrete in MPa,wc = the unit weight on concrete. in kg/m3.For normal weight concrete:Ec = 4700√f’cModulus of elasticity Es for nonprestressedreinforcement:.Es=200,000 MPa
Depending on the mix (specially the water-cement ratio) and the time and quality ofcuring, compressive strengths of concretecan be obtained up to 97 MPa or more.Commercial production of concrete withordinary aggregates is usually in the 21 to 83MPa range with the most common ranges forcast-in-place buildings from 21 to 41 MPa. Onthe other hand, precast and prestressedapplications often expect strengths of 27.6to 55.1 Mpa.
The 28-day compressive strength of concretecan be estimated from the 7-day strength bya formula proposed by W.A Sater:S28 = S7 + 2.5√S7 Eq. 1.2Where:S28 = 28-day compressive strength in MPaS7 = 78-day compressive strength in MPaThe stress-strain ·diagram for concrete of aspecified compressive strength is a curvedline as shown in Figure 1.1. Maximum stressis reached at a strain of 0.002 mm/ mm,after which the curve descends.
Concrete strength is influenced chiefly bythe water-cement ratio; the higher thisratio, the lower the strength. In fact, therelationship is approximately linear whenexpressed in terms of C/W, the ratio ofcement to water by weight. For a workablemix, without the use of water reducingadmixtures: S28 = 18.61 C – 5.24 Eq. 1-3 W
With the absence of any required data,concrete proportions shall be based onwater-cement ratio, limits in Table 1.3, ifapproved by the engineer. ·
Required average compressive strength f’crused as the basis for selection of concreteproportion shall be the larger of Eq. 1-4 orEq. 1-5 using a standard deviation calculatedin accordance with Sec. 18.104.22.168.1 or Sec.22.214.171.124.2 of the Code.f’cr = f’c + 1.34 s Eq. 1-4orf’cr = f’c + 2.33s – 3.5 Eq. 1-4Where s = standard deviation, MPa
Metal reinforcement in concrete shall bedeformed, except that plain reinforcement bepermitted for spirals or tendons; andreinforcement consisting of structural steel,steel pipe, or steel tubing. Reinforcing bars tobe welded _shall be indicated on the drawingsand welding procedure to be used shall bespecified. PNS reinforcing bar specifications shallbe supplemented to require a report of materialproperties necessary to conform to weldingprocedures specified in "Structural Welding Code- Reinforcing Steel" · (PNS/AWS D 1.4) of theAmerican Welding society and/ or "Welding ofReinforcing Bars (PNS/ A5- 1554) of thePhilippines National Standard.
Deformed, reinforcing bars shall conform tothe standards specified in Section 126.96.36.199 ofN$CP. Deformed reinforcing bars with aspecified yield strength fy exceeding 415MPa shall be permitted, provided fy shall bethe stress corresponding to a strain of 0.35percent and the bars otherwise conform.t6one of the ASTM and PNS specifications listedin Sec. 188.8.131.52.1.
Plain bars for spiral reinforcement shallconform to the specification listed in Section184.108.40.206.1 of NSCP. For wire with specifiedyield strength fy exceeding, 415 Mpa, fyshall be the stress corresponding to a strainof 0.35 percent if the yield strengthspecified in the design exceeds 415 MPa.
According to Section 5.7~6 of NSCP, theminimum clear spacing between parallel barsin a layer should be db but not less than 25mm. Where parallel reinforcement is placedin two or more layers, bars in the upperlayers should be placed directly above barsin the bottom layer with clear distancebetween layers not less than 25 mm. Inspirally reinforced or tied reinforcedcompression members, clear distancebetween longitudinal bars shall be not lessthan 1.5db nor 40mm.
Groups of parallel reinforcing bars bundledin contact to act as a unit shall be limited tofour in any .one bundle. Bundled bars shall.be enclosed within stirrups or ties and barslarger than 32 mm shall not be bundled inbeams. The individual bars within a bundleterminated within the span of flexuralmembers should terminate at differentpoints with at least 40db stagger. Sincespacing limitations and minimum concretecover of most members are based on a singlebar diameter db, bundled bars shall betreated as a single bar of a diameter derivedfrom the equivalent total area.
Diameter of single bar equivalent to bundledbars according to NSCP to be used forspacing limitation and concrete cover.
Precast concrete (Manufactured Under PlantConditions). The following minimumconcrete shall be provided forreinforcement:
The following minimum concrete cover shall·he provided for prestressed. Andnonprestressed reinforcement, ducts andend fittings.
For bundled bars, the minimum concretecover shall be equal to the equivalentdiameter of the bundle, but need not begreater than 50 mm, except for concretecast against and permanently exposed toearth, the minimum cover shall be 75 mm.
The term standard hook refers to one. of thefollowing:(a) 180°bend plus 4db extension but not less than65 mm at free end,(b) 90° bend plus 12db extension, at free end ofbar,(c) For stirrups and tie hooks: (1)16 mm bar and smaller, 90°· bend plus 6db extension at free end of bar, or (2) 20 mm and25 mm bar, 90° bend plus 6db extension at free end of bar, o (3)25 mm bar and smaller, 135° bend plus 6db , extension at free end of bar.
The diameter of bend measured on theinside of the bar, other than for stirrup andties in sizes 1O mm through 15 mm shall notbe less than the following:(a) 6db for 10 mm to 25 mm bar,(b) 8db for 28 mm to 32 mm bar, and(c) 10 db for 36 mm bar.
The inside diameter of bend of stirrups andties shall not be less than 4db for 16mm barand mm bar and smaller. For bars larger than16 mm, the diameter of bend shall be inaccordance with the previous paragraph.
The most important and most critical task ofan engineer is the determination of the loadsthat can be applied to a structure during itslife, and the worst possible combination ofthese loads that might occur simultaneously.Loads on a structure may be classified asdead loads or live loads.
Dead loads are loads of constant magnitudethat remain in one position. This consistsmainly of the weight of the structure andother permanent attachments.
Live loads are loads that may change inmagnitude and position. Live loads that moveunder their own power are called movingloads. Other live loads are those caused bywind, rain, earthquakes, soils, andtemperature changes. Wind and earthquakeloads are called lateral loads.
Live loads may be applied only to the floor orroof under consideration, and the far ends ofcolumns built integrally with the structuremay be considered fixed. It is permitted bythe code to assume the followingarrangement of live loads: a) Factored dead load on all spans withfull factored live load on two adjacent spans,and (b) Factored dead load on all spanswith full factored live load on alternatespans.
Notes:1. In all figures shown, the wind comes fromthe left.2. In the formula for pressure coefficient onthe windward slope:(a) e is the angle of slope with the horizontalin degrees;(b) The wind force is a pressure if coefficientis positive;(c) The wind force is suction if coefficient isnegative.
Structure and structural members should bedesigned to have design strengths at allsections at least equal to required strengthscalculated for the factored loads and forcesin any combination of loads.