8 Reinforced Concrete Property Concrete Steel Good Strength in tension Strength in compressionStrength in shearDurabilityFire ResistanceConcretePoorGoodFairSteelGoodGood, but slender bars buckleCorrodes if unprotectedPoor – rapid loss of strength at high T
9 Reinforced ConcreteConcrete tends to fail in brittle manner (suddenly, without warning – not good for structures!!)Reinforcing steel takes high tensile loadsWhen it takes a large enough load, it yields and becomes plastic (that is, stretches considerably under little increase in loadKnown as a ductile material
11 Reinforced Concrete In summary: Materials are complementary! Steel provides tensile strength and some shear strengthConcrete provides compressive strength and protects steel to give durability and fire resistance
12 Reinforced Concrete How Does it Work? Simply supported beam – apply a loadBottom surface in tension at beam centreTop surface in compression
13 Reinforced Concrete How Does it Work? Two span beam – apply a load in each spanBottom surface in tension at beam centre spanTop surface in compression at beam centre spanSituation reversed at central support – tension top and compression bottom
14 Reinforced Concrete How Does it Work? Consider failure mechanism of plain concreteIf sufficient load is applied, beam will fail suddenly by cracking at the location of maximum tension
15 Reinforced Concrete How Does it Work? Construct a composite beam of concrete and reinforcing steelIf steel bars are located near bottom face (where tension is), the beam can take a much higher load before failingConcrete resists tension on top and steel resists tension at bottom
16 Reinforced Concrete How Does it Work? Bond:In order to achieve composite action, steel and concrete must act together to transfer tension in the concrete into the steelBonding to round bars using cement paste is one method (“gluing” to surface)Provide additional bond by having ribs in the bars
17 Reinforced Concrete How Does it Work? Anchorage:Bending bars (to an L or U shape) at the end of the span provides better anchorage (longer length over which to transfer tension)If beam is long, normal to use two reinforcement bars overlapped sufficiently to develop full anchorage – thus they act as one bar.
20 Reinforced Concrete How Does it Work? Shear:If such a beam were tested, failure would probably occur due to diagonal cracking near supports (despite presence of ductile steel)This is known as shear failure – another dangerous form of brittle failureNeed to provide vertical reinforcement to bridge the cracks – these are known as shear linksHave to provide additional longitudinal steel to hold top end of links in place; nominal size bars called “hangers”
23 Reinforced Concrete How Does it Work? Reinforcement Cages:Useful to prefabricate stable reinforcement cage and drop into locationExample below is for a continuous beam with two spansNote additional links near support (biggest shear force)Significant tensile (longitudinal) steel mid-span and over supports
25 Reinforced Concrete How Does it Work? Under-reinforced Beams:Suppose increase load on beam until failure:If steel weaker than concrete, steel will yield and stretch significantly – ductile failure and plenty of warning (through cracking on the bottom surface)Preferred design condition because of this warning
26 Reinforced Concrete How Does it Work? Over-reinforced Beams:Suppose increase load on beam until failure:If additional steel is provided to make beam stronger, could lead to concrete becoming the weaker componentBeam then fails suddenly by concrete failure in compression (before steel becomes plastic) – brittle failureCode rules mitigate against this failure method
27 Reinforced Concrete How Does it Work? Columns:Concrete section in compression but also some moment – moment generates tension in part of the sectionReinforcement provided to take the tension but will also be required to take compressionNow links provided to restrain slender compression elements (vertical reinforcement) to prevent buckling. Spacing of links must be such as to prevent this.
30 Reinforced Concrete Applications Reinforcement generally in form of bars or meshRound bars – Y bars (Yield stress = 250MPa)Ribbed bars – T bars (Yield stress = 460MPa)Mesh reinforcement – small diameter bars welded at regular centres, usually to prevent shrinkage crackingAll reinforcement should be covered by CARES certificate to certify quality and standard of reinforcement
31 Reinforced Concrete Applications Reinforcement drawings specify bar type, size, location and centres/ number offBending schedules prepared for each drawing – show fabrication details for barsThese can be input directly into cutting and bending machines to produce required steelAll steel ID tagged before going to site using a bar code
32 Reinforced Concrete Applications Installation on site carried out by steel fixersBars are located according to reinforcement drawingCover blocks and spacers used to maintain adequate cover at perimeter of concrete sectionPrefabrication used for beams more than slabs
36 Reinforced Concrete Applications Continuity Reinforcement:Used typically to permit vertical elements to continue up without casting horizontal elementsContinuity reinforcement is cast inCan be bent into position after formwork is struck
37 Reinforced Concrete Alternatives to Steel Reinforcement Bars Steel fibres:Lengths up to 60mmAspect ratio (length : diameter) up to 80Generally added to concrete truck on site to specified dosing rateIncrease “toughness” of sectionBridge micro-cracks in concreteGenerally used in ground bearingslabsGood at resisting shrinkage cracks
38 Reinforced Concrete Variations/Alternatives to Steel Reinforcement Bars Plastic fibres:Modify fresh concrete propertiesIncrease cohesion and reduce bleedingReduces plastic shrinkage cracking in ground bearing slabsNot really an alternative to reinforcement to take tensile forces; can be used in addition
39 Reinforced Concrete Variations/Alternatives to Steel Reinforcement Bars Stainless Steel Reinforcement:Used in high chloride areas where carbon steel reinforcement would be subject to corrosion€€€€!!Only very specialist uses
40 Reinforced Concrete Variations/Alternatives to Steel Reinforcement Bars Epoxy Coated Reinforcement:Potential alternative to stainless steelBars need special handling to prevent damage to protective epoxy coatingNot often recommended
41 Reinforced Concrete Variations/Alternatives to Steel Reinforcement Bars Fibre reinforced plastic (FRP) reinforcement:Glass fibre reinforcing bar availableGenerally has to be manufactured to orderMaterials behave elastically with brittle failure – design needs to cater for thisApplications: EMI considerations, cuttable concrete, severe aggressive environments.
42 Reinforced Concrete Variations/Alternatives to Steel Reinforcement Bars Fibre reinforced plastic (FRP) reinforcement (cont.)Can also be used as an external strengthening systemBond carbon fibre/ epoxy plate to concrete on tension faceCan wrap circular columns for compression enhancement