What s in the Mix. Aim of this presentation Overview of mix design Overview of constituents Cement Aggregates Coarse Fine.

Transcription

1 What s in The Mix

2 What s in the Mix Aim of this presentation Overview of mix design Overview of constituents Cement Aggregates Coarse Fine Water SCM s

3 Aim of a Mix Design Mix Design is a Scientific Art the aim of a good mix design is to Comply with Australian Standards Comply with expected concrete properties Comply with project specification Have a competitive materials cost Have an low risk of rejection / dispute

4 Typical M 3 of Shotcrete Weight (Kg) % By Weight % By Volume Cement Aggregate Water Aggregates = Coarse Aggregates and Fine Aggregates Coarse aggregates 4.75mm. Fine Aggregates < 4.75mm

5 Introduction to Mix design Majority of Batch plants in Australia use batch weighing Mix design is quoted in Kg s Material is Weighed in Kg s Concrete is always sold by Volume M 3 or Cubic Metre 1000 litres = 1M 3

6 1M 3 Must = 1 Cubic Metre

7 Factors affecting Mix Design Maximum aggregate size Grading of aggregate shape and surface texture of aggregate Aggregate absorption Relative density of aggregate Type of cement

8 Mix designs are site specific Dependant on available materials and site requirements Mix proportioning is dependant upon Specific Gravity (SG) & Particle distribution Cheap is not always cost effective Always consider placed cost.

9 Cement Use General Purpose Cement (Type GP) Composition of GP Cement % Portland Cement % Fly Ash, Slag or limestone 0 5% Minor additional constituents

10 Mix Design Methodology Determination of Water Content: Aggregate size and type (crushed or natural) and required slump will determine effective water requirement. Determination of Cement Content: f cm = f ck + 2S Compressive strength 40Mpa = Characteristic Strength of 48MPa Cement content is determined by Cement / Water = Af cm (A is a constant related to cement class and aggregate type) Crushed aggregate max size 10mm with high slump will require approx. 250l water SP can be used to reduce effective water requirement

11 Volume of Aggregate Determination of Total Volume of Aggregates V water (l/m3) = Water (kg/m3)/1 V cement (l/m3) = Cement (Kg/m3)/3.15 V solids = 1025-V water (common assumption in mix design that 1m3 of concrete = 1025 litres) V aggregates = V water - V cement (if Cement = 400kg and water = 180 litres) V aggregates = 1025 ((400/3.15)+180) = 1025 ( )= 718 litres

12 Proportioning of Aggregates The aim is to pack as much aggregate as possible into a M 3 To Achieve this you need to have an idea of the different sizes of your materials so you can Pack them properly.

13 Material Grading (Sieve Analysis)

14 Cumulative % Passing Shotcrete Mix Design Natural Stickiness Essential part of a shotcrete design 25% below 0.3mm Sieve Size (mm) Max Limit Min Limit Actual grading Min. Limit Max. Limit Actual Grading

15 Reduction of Voids

16 A look at voids in concrete aggregates 1 empty cubic meter box Voids in box = 1000 litres

17 A look at voids in concrete aggregates 1 cubic meter box filled with 1000L of 10mm aggregate Void % in aggregate = 40% Voids in box = 40% of 1000 L = 400 L

18 A look at voids in concrete aggregates 1 cubic meter box filled with 1000L of 20mm agg & filled with 400L of sand Void % in sand = 40% Voids in box = 40% of 400L = 160 litres The voids in the sand is filled with cement and water.

19 A look at voids in concrete aggregates Learnings More sand = more water & more cement (for the same strength) More sand = less coarse agg & more shrinkage Poor shaped aggregate = more sand required to fill voids

20 In the real world

21 Aggregates Aggregate occupies ~70-75% of volume of concrete, so its quality is of great importance. Aggregates may affect the following properties of concrete: Strength Durability Structural Performance Economy

22 Properties of Aggregate Bonding capability Strength Deleterious substances Soundness Alkali Silica and Alkali carbonate reactions Thermal properties Particle Shape and texture Specific Gravity Bulk density Porosity and Absorption Moisture content Bulking Sieve Analysis Bonding Capability how well does aggregate bond to cement paste. Strength UCS of Concrete can be limited by the UCS of the aggregate Deleterious substances Organic impurities, clay, salt contamination and unsound particles Soundness ability of aggregate to resist excessive changes in volume as a result of changes in physical conditions Alkali Silica and Alkali carbonate reactions- these are reactions between the cement paste and active silica or carbonates found in some aggregates. Thermal properties problematic if thermal expansion of aggregate differs by a great deal to that of the cement paste.

23 Particle shape and texture

24 Relative Density

25 Porosity and Absorption

26 Aggregates

27 Water 1. Total water ( T w ) = All water in the mix from all sources. 2. Free water (F w ) = all water in mix that is available to act with the cement 3. Absorbed water is water unavailable as it is absorbed by aggregate (A w ) 4. F w = T w - A w

28 Mix design & Water Mix designs are based on assumption that only free water will react with the cement. Free water will also determine the workability (pre admixtures) Free water is that water which is outside of the saturated surface dry condition (SSD) Beware, There might be free water in the aggregate stockpile.. Beware, The aggregate might be dry so water requirement might exceed that stipulated on mix design. Optimal W/C ratio range

29 When Batching you need to know Absorption rate of all Aggregates Moisture content of stock piles Water requirement for mix Make allowances for moisture in aggregate (reduce added water and increase aggregate by same amount)

30 Batching Concrete bowl should not be filled more than 65% of volume Concrete should be mixed until it is uniform in appearance and all the ingredients are evenly distributed. Mix as per bowl manufacture's recommendations If the blades of the mixer become worn or coated with hardened concrete, the mixing action will be less efficient.

31 Batching Typical Mixing guidelines 50 to 100 revolutions of the drum at the rate of rotation designated by the manufacturer as mixing speed typically revolutions per minute Transporting Agitating speed is usually about 2 revolutions per minute Bowl should not turn more than a total of 300 revolutions from start of mix to discharge. Australian Standards state that bowl manufacturer should specify mixing required by their bowl to get a consistent mix

32 Thank You