CIV2226: Design of Concrete and Masonry Structures

Transcription

1 CIV2226: Design of Concrete and Masonry Structures Concrete Technology... 2 Concrete Mix Design... 2 Portland Cement... 4 Supplementary Cementitious Materials... 5 Concrete Aggregates... 6 Chemical Admixtures... 7 Properties of Fresh Concrete... 7 Segregation... 8 Bleeding... 8 Properties of Hardened Concrete... 8 Specification, Ordering and Supply of Concrete...13 Curing of Concrete...15 Durability of Concrete...15 Corrosion of Steel Reinforcement...17 Cracking and Crack Control...17 Reinforced Concrete Design...19 Strength Check...19 Serviceability Check/Design...22 Deflection...23 Strength Design in Bending...25 Strength Design in Shear...26 Column Design...27 Masonry...29 Masonry Terminology...29 Design for Robustness...32 Design for Compression...34 Design for Bending

2 Concrete Mix Design Concrete Technology Purpose Selecting the correct proportions of constituents of concrete: o Cement o Water o Supplementary Cementitious Materials o Coarse/Fine Aggregates o Chemical Admixtures Parameters Specified strength (MPa) and workability (slump mm) Workability The case with which fresh concrete can be handled, placed and compacted without excessive: Segregation (separation of fine aggregate/cement paste from course aggregate) Air voids Affected by: Grading, particle shape and proportions of aggregate Amount/qualities of cement/other cementitious material Free water content Cementitious paste content Presence of chemical admixtures Slump Adjusted by: Adjusting water content (positive relationship) Fine/coarse aggregate ratio (negative relationship) How to increase whilst maintaining strength? Add additional water to the mix while also increasing cement content for constant w/c. Strength The strength of the concrete is related to the free-water in the mix, and is not dependent on the absorption properties of the aggregates. Adjusted by: Adjusting w/c ratio W/c Water is necessary in the concrete mixture because it reacts with the cement in a series of chemical reactions termed hydration. Very little water needed to react with cement, however needs to be workable (not stiff) Additional water needed to coat fine/coarse aggregate particles with cement paste to achieve suitable workability Aggregate gradings The best proportion to use in a given mix will depend on the: Shape and the maximum size of the coarse aggregate Fineness modulus of the fine aggregate Chosen free w/c ratio Desired workability of the mix. 2

3 Coarse aggregate Crushed - higher strength - Specific gravity = 2.7 Uncrushed - lower strength - Specific gravity = 2.6 Fine aggregate Fineness Modulus Free water content Coarse and fine aggregates are porous and absorb water. o Aggregates that are soaked in a water tank will be fully saturated and surface wet o Oven-dried aggregates will be dry and prone to absorb water. W/c is needed to achieve strength; however laboratory mix will be affected by: w/c too high if wet aggregates are used because the aggregates will be fully saturated and the excess water will be introduced into the mix as additional mixing water o weaker concrete (and workability too high because of the excess water) w/c too low if dry aggregates are used because the aggregates will absorb some of the mixing water o higher strength but lower workability Saturated & Surface Dry Condition Saturated and Surface Dry (SSD) o Condition of aggregates where all pores of the aggregate are filled with water with no excess water on surface o Will neither absorb/add to the free water available in concrete mix for cement hydration Unsaturated o Absorbs free water until saturated o Increase water added to compensate for aggregate absorption of free water Surface wet condition o The excess water on the surface of the aggregate would be available for cement hydration o Water added to concrete mix should be reduced to compensate for aggregate surface water amount Wet Density of Concrete The density of the concrete measured during the trial mix should be checked against the estimated/assumed density during the mix design, and necessary adjustments should be made accordingly. Wet density of concrete = cement + free-water + total aggregate content o Fully compacted concrete (no air voids) Fineness modulus Measure of the fineness of the fine aggregate Computed by sieve analysis o Sum of the cumulative percentages retained on the sieves of the standard series, divided by 100 3

4 Portland Cement What is it? Dry powder of very fine particles Forms a paste when mixed with water o chemical reaction hydration, glue, coats all the aggregates together Hardens and forms a solid mass Manufacturing Clinker o Limestone o Clay/shale o Iron ore o Silica Final grading - Add ~ 5% gypsum o Shrinkage control (CaSO 4 : anhydrite) o Control flash set (CaSO 4.2H 2 O: di-hydrate) Components C = CaO; S = SiO 2 ; A = Al 2 O 3 Major Components 3CaO.SiO 2 ~ 55% (Tricalcium silicate, or alite ): C3S Light in colour Hardens quickly with evolution of heat Gives early strength Heat of hydration = 500 J/g 2CaO.SiO 2 ~ 15% (Dicalcium silicate, belite ): C2S Light in colour Hardens slowly Gives late strength Heat of hydration = 250 J/g 3CaO.Al 2 O 3 ~ 10% (Tricalcium aluminate): C3A Light in colour Sets quickly with evolution of heat Low strength Heat of hydration = 850 J/g 4CaO.Al 2 O 3.Fe 2 O 3 ~ 10% (Tetracalcium aluminoferrite): C4AF Dark in colour with little cementing value Heat of hydration = 400 J/g Other Components MgO, TiO 2 and Mn 2 O 3 Alkalis: K 2 O and Na 2 O Cement Types Type GP General Purpose Portland Cement Used for most forms of concrete construction Should specify where special properties aren t required (e.g. low heat of hydration) 5% mineral additions (fly ash, slag, limestone containing >80% calcium carbonate) Type HE High Early Strength Cement Used for concrete structures that must be function in a short amount of time 4

5 Economical to achieve rapid form and inventory turnover. Allows for additional reductions in labour costs, curing times, overhead Type LH Low Heat Cement Use where limitation of the heat of hydration (temperature rise in concrete) is necessary to avoid unacceptable thermal stresses (e.g. massive structures/thick structural elements) Peak temperature rise maximum = 23 o C (AS3972) Type SL Shrinkage Limited Cement Emphasis placed on drying shrinkage and crack control (e.g. road pavements/bridge structures) Portland or blended cement provided it meets drying shrinkage limit specified in AS 3972 T1 Type SR Sulfate Resisting Cement Tricalcium aluminate (C3A) content < 5% C3A is classified as SR Heat of Hydration Heat liberated (released) as the cement and water react Rate of heat liberation PARALLELS rate of strength increase o High during first 2-3 days post-mixing, then subsides appreciably Setting Initial set o Point at which paste stiffens o Time required for paste to cease being plastic and workable Final set o Point where paste may be regarded as a rigid solid o Begins to develop measurable strength Supplementary Cementitious Materials Primarily industrial waste materials o Used to partially replace Portland cement in concrete without significant additional processing Ground Granulated Blast Furnace Slag Waste product from iron/steel industry o Slag floats to top of furnace chilled with water (glassy)ground into fine powder Slag blended cement o Can replace 65% of PC - slow reaction, little cementitious properties o When ground into fine powder, reacts chemically with calcium hydroxide at ordinary temp to form compounds possessing cementitious properties Replacement of PC o Slow reaction - slow strength development (early-age strength) o Lower heat of hydration - reduces thermal cracking risk o Lower permeability - better long-term durability Fly Ash Waste by-product from bituminous coal-burning power stations o No additional processing required o Precipitated from exhaust gases o Can replace up to 30% of PC 5