Civil Engineering Materials

Civil Engineering Materials Department of Civil, Structural and Environmental Engineering Trinity College Dublin Dr. Roger P. West and Mr Peter Flynn

Section A: Concrete • A1 Basic Materials: • A2 Fresh Concrete Properties: • A3 Hardened Concrete Properties: • A4 Concrete Mix Design: • A5 Reinforced Concrete: • A6 Pre-stressed Concrete:

Reinforced Concrete: • What is it? • How does it work? • What are the applications? • What is there to be aware of? • What are the alternatives to the conventional systems?

Reinforced Concrete

Reinforced Concrete • What is Reinforced Concrete (RC)? : • Combination of concrete and steel (normally) • Acting compositely • Produces a strong, durable and versatile building material • Features best properties of each material

Reinforced Concrete • Concrete tends to fail in brittle manner (suddenly, without warning – not good for structures!!) • Reinforcing steel takes high tensile loads • When it takes a large enough load, it yields and becomes plastic (that is, stretches considerably under little increase in load • Known as a ductile material

Reinforced Concrete • In summary: • Materials are complementary! • Steel provides tensile strength and some shear strength • Concrete provides compressive strength and protects steel to give durability and fire resistance

Reinforced ConcreteHow Does it Work? • Simply supported beam – apply a load • Bottom surface in tension at beam centre • Top surface in compression

Reinforced ConcreteHow Does it Work? • Two span beam – apply a load in each span • Bottom surface in tension at beam centre span • Top surface in compression at beam centre span • Situation reversed at central support – tension top and compression bottom

Reinforced ConcreteHow Does it Work? • Consider failure mechanism of plain concrete • If sufficient load is applied, beam will fail suddenly by cracking at the location of maximum tension

Reinforced ConcreteHow Does it Work? • Construct a composite beam of concrete and reinforcing steel • If steel bars are located near bottom face (where tension is), the beam can take a much higher load before failing • Concrete resists tension on top and steel resists tension at bottom

Reinforced ConcreteHow Does it Work? • Bond: • In order to achieve composite action, steel and concrete must act together to transfer tension in the concrete into the steel • Bonding to round bars using cement paste is one method (“gluing” to surface) • Provide additional bond by having ribs in the bars

Reinforced ConcreteHow 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.

Reinforced ConcreteHow 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 failure • Need to provide vertical reinforcement to bridge the cracks – these are known as shear links • Have to provide additional longitudinal steel to hold top end of links in place; nominal size bars called “hangers”

Reinforced ConcreteHow Does it Work? • Reinforcement Cages: • Useful to prefabricate stable reinforcement cage and drop into location • Example below is for a continuous beam with two spans • Note additional links near support (biggest shear force) • Significant tensile (longitudinal) steel mid-span and over supports

Reinforced ConcreteHow 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

Reinforced ConcreteHow 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 component • Beam then fails suddenly by concrete failure in compression (before steel becomes plastic) – brittle failure • Code rules mitigate against this failure method

Reinforced ConcreteHow Does it Work? • Columns: • Concrete section in compression but also some moment – moment generates tension in part of the section • Reinforcement provided to take the tension but will also be required to take compression • Now links provided to restrain slender compression elements (vertical reinforcement) to prevent buckling. Spacing of links must be such as to prevent this.

Beams Slabs Columns Culverts Portal frames Bridge beams Piled foundations Roof structures Cladding Towers Silos Reservoirs and tanks Sculptures ???? Reinforced ConcreteApplications

Reinforced ConcreteApplications • Reinforcement generally in form of bars or mesh • Round 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 cracking • All reinforcement should be covered by CARES certificate to certify quality and standard of reinforcement

Reinforced ConcreteApplications • Reinforcement drawings specify bar type, size, location and centres/ number off • Bending schedules prepared for each drawing – show fabrication details for bars • These can be input directly into cutting and bending machines to produce required steel • All steel ID tagged before going to site using a bar code

Reinforced ConcreteApplications • Installation on site carried out by steel fixers • Bars are located according to reinforcement drawing • Cover blocks and spacers used to maintain adequate cover at perimeter of concrete section • Prefabrication used for beams more than slabs

Reinforced Concrete Applications • Continuity Reinforcement: • Used typically to permit vertical elements to continue up without casting horizontal elements • Continuity reinforcement is cast in • Can be bent into position after formwork is struck

Reinforced ConcreteAlternatives to Steel Reinforcement Bars • Steel fibres: • Lengths up to 60mm • Aspect ratio (length : diameter) up to 80 • Generally added to concrete truck on site to specified dosing rate • Increase “toughness” of section • Bridge micro-cracks in concrete • Generally used in ground bearing slabs • Good at resisting shrinkage cracks

Reinforced ConcreteVariations/Alternatives to Steel Reinforcement Bars • Plastic fibres: • Modify fresh concrete properties • Increase cohesion and reduce bleeding • Reduces plastic shrinkage cracking in ground bearing slabs • Not really an alternative to reinforcement to take tensile forces; can be used in addition

Reinforced ConcreteVariations/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

Reinforced ConcreteVariations/Alternatives to Steel Reinforcement Bars • Epoxy Coated Reinforcement: • Potential alternative to stainless steel • Bars need special handling to prevent damage to protective epoxy coating • Not often recommended

Reinforced ConcreteVariations/Alternatives to Steel Reinforcement Bars • Fibre reinforced plastic (FRP) reinforcement: • Glass fibre reinforcing bar available • Generally has to be manufactured to order • Materials behave elastically with brittle failure – design needs to cater for this • Applications: EMI considerations, cuttable concrete, severe aggressive environments.

Reinforced ConcreteVariations/Alternatives to Steel Reinforcement Bars • Fibre reinforced plastic (FRP) reinforcement (cont.) • Can also be used as an external strengthening system • Bond carbon fibre/ epoxy plate to concrete on tension face • Can wrap circular columns for compression enhancement

Testing new carbon fibre wrappings

Civil Engineering Materials