Australian Society for Concrete Pavements Concrete Mix Design Principles Michael van Koeverden
Good Concrete Mix Design: Understand requirements of the concrete. Identify if an existing mix may be suitable Understand the properties of your constituents, strengths and weaknesses- E.g. silt in sand etc.
Good Concrete Mix Design-Cont d: Understand the properties required for the application Identify how to achieve these Plastic and hardened properties. Proportion your mix design and provide relevant batch instruction to ensure your design is implemented correctly
Understanding requirements: Concrete requirements may be detailed in an Engineering specification or simply verbal-by phone: Plastic Property Requirements- Slump, Pumpability, Cohesion, Bleed Rate and Volume of Bleed Water, Setting Time etc. Hardened properties- Compressive Strength, Flexural Strength, Indirect Tensile Strength, Permeability, Drying Shrinkage, Density, Colour etc.
Identifying suitable existing mixes Similar Mixes which may have been used previously in similar applications may become the starting point for the required design.
Understand the properties of your constituents Binders: Cement: Type- SL,GP,GB etc Setting Time Bleed rate Fineness etc Cement Grain Fly Ash: LOI Fineness Consistency etc Ca(OH)2
Understand the properties of your constituents Binders: Ground Granulated Blast Furnace Slag (GGBFS): Neat Slag: Amorphous Silica
Understand the properties of your constituents Binder Interactions: Rate of gain of strength Bleed rate of combined binder Likely setting time Heat of hydration Pozzolanic behaviour of binder
Understand the properties of your constituents Min Binder Requirements: To Satisfy: Compressive Strength Flexural Strength Durability requirements Early Age vs Later Age strength
Understand the properties of your constituents Aggregates:- AS2758.1 Dimensional- Grading- AS1141.11- (washed for sands) Fine Fractions- Mat <75µm and <2µm- AS1141.12,.13 Particle Shape or Flakiness- AS1141.14 or AS1141.15 Strength & Durability- Wet/Dry AS1141.22 Los Angeles Abrasion- AS1141.23
Understand the properties of your constituents Aggregates:- AS2758.1 Sodium Sulfate Soundness- AS1141.24 Clay and Fine Silts in Natural Sands- AS1141.33 In organics- AS1141.34 Sugar- AS1141.35
Control charting of Individual Constituent Materials Coarse Aggregate Grading-20mm 100 90 % PASSING 80 70 60 50 40 30 20 10 0 01- Jan- 07- Jan- 13- Jan- 19- Jan- 25- Jan- 31- Jan- 06- Feb- 12- Feb- 18- Feb- 24- Feb- 01- Mar- 07- Mar- 13- Mar- 19- Mar- 25- Mar- 31- Mar- G9 = 26.5 C6 x 19C9 + D6 x 13.2 D9 + E6 x 9.5E9 + F6 x F9 =100% Sample Date
Dedicated Stockpiles- Grading / 400T 5 sample increments 5 sample increments 5 sample increments 5 sample increments 5 sample increments Lot 1-400T Lot 2-400T Lot 3-400T Lot 4-400T Lot 5-400T First 2000 T Aggregate stockpile Some Road Authorities require dedicated Aggregate stockpiles
Combined Grading Grading of each individual aggregate is important as it affects the total combined grading.
Mix Design depends on available materials and combination Sieve (mm) 20mm 10mm 7mm Man Sand Fine Sand 19.0 Zones of interaction 13.2 9.5 6.7 4.75 2.36 Shape ALD 1.18 Quantity 0.600 0.300 0.150 0.075 Clay etc Wash / Not wash Shape and Flow Cone Tests
Correct Water Content Water/Cementitious Ratio Design W/C vs Batched W/C Batched W/C may allow for water wash-in after truck mixing Water in admixtures normally included Batching to slump-batch & any other water on docket. Cementitious= Cement + other binders C o m p S t r e n g t h ( M P a ) 100 80 60 40 20 Concrete Water Cement Ratio vs Comp Strength 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 W/C Ratio
Understand the properties of your constituents Admixtures: WRRE- Water reducer Retarder- Reduces water and provides some small set retardation WRAC- Water reducer Accelerator- Reduces water and provides some small set acceleration. By increasing dose rate they reduce more water but do they also entrain some air? What does this do to set? Combination admixtures are cheap but may not always be the most appropriate?
Understand the properties of your constituents Admixtures: AEA- Air Entraining Admixture- At low dose effective means of controlling bleed, improving workability, slipformability, and Freeze-Thaw At higher dose can be catastrophic for: strength, durability, bond to reinforcement, cracking (no bleed) etc
Understand the properties of your constituents Superplasticisers: Naphthalene Formaldehyde (NF) Sulphonates (NFS) Beta Naphthalene Sulphonates- (BNS) Used in conventional precast or other applications Or polycarboxylate ethers (PCE)- most commonly used in super workable concretes.
Superplasticisers- How do they work?. negative negative negative negative negative 20 NFS and BFS- Electrostatic repulsion of negatively charged cement grains negative negative PCE- Electrostatic repulsion of negatively charged cement grains, plus steric hindrances of side chain fingers
Super-workable concrete mixes Slower strength gain if high % of fly ash, or ground slag are used. Little or no bleeding due to higher fines content and viscosity. Potential for increased plastic shrinkage cracking. Curing procedures are important, particularly on large, flat exposed areas. 21
Importance of Mixing Concrete Concrete is a composite material- relies on interaction between aggregates, binder, water admixtures to develop properties. Poor mixing = poor concrete The bond between aggregates and the cementitious paste are critical to develop concrete strength and resist cracking Uniformity of mixing ensures same concrete load after load. 22
Different concrete /Different Application Slip-formed concrete must be workable, with higher entrained air content generally- to avoid surface tearing during slip-forming, bleed issues? Is placed at lower slump as must stand up under own weight generally Surface quality and ride may be critical Line and level often critical, sometimes less manual surface finishing involved May be large surface area risk of cracking 23
Different concrete /Different Application Slab on Ground concrete Usually low entrained air content- as want some bleed to resist early age cracking Is supported by formwork that usually is in place overnight. Line and level not so critical, surface finish may not be critical- carpet/tilesexcept garage Consequences of cracking may not be critical, may be post tensioning in slab. 24
Concrete Mix Design - much more than proportioning materials to achieve a Compressive Strength. It is understanding a problem Researching and designing a solution tailored for the application Providing the same consistent solution load after load Thankyou 25