Making good concrete An overview of basics

Transcription

1 Concrete Pavements for Local Government 14 March 2016 Making good concrete An overview of basics John Hodgkinson Honorary Member ASCP

2 Two basic sources of standards/specifications AS1379 Specification and supply of concrete RMS R83 - written primarily for large highway projects - contains some significant differences from AS should not be used without informed review /modification

3 RMS Specification R83 Clause 1.1 Scope This specification is intended for the construction of road pavements in applications such as highways which are carrying substantial volumes of commercial road vehicles. It is unlikely to be applicable to other applications (such as industrial, commercial or residential pavements) without suitable modification.

4 Concrete for Local Government paving will usually be produced and supplied by readymix companies

5 Concrete is made in batches Mixed in delivery vehicle Each batch is a separate cycle Typical batch is about 4.6m 3 or 11 tonnes

6 Dry batch plant. Materials added to truck mounted mixer before mixing

7 Concrete is a combination of well graded aggregate bound with a cementitious paste A good concrete mix will have about + 82% weight combined aggregate * 12% weight cementitious 6% weight water (*coarse/fine approx 2:1 RMS approx 60%:40 %)

8 3 Basics in making good concrete Good concrete must have Right materials Right proportions Thoroughly mixed

9 The right materials Aggregates (coarse : fine) Cement and flyash Water Admixtures - accelerate/retard setting - reduce water - entrain air

10 The right proportions Aggregate bins Cement, flyash silos

11 Thorough mixing Good specifications refer to : concrete is a thoroughly mixed combination of.. Concrete must be thoroughly mixed..

12 .. otherwise it is just a bowl full of ingredients

13 Mixing readymix concrete ( readymix, transit, mobile, premix ) Mixed in the bowl Internal spiral flutes lift and drop To discharge, change rotation, concrete wound out

14 How long should a full load be mixed? Mixing bowl ID plate

15 Mixing time = 55 drum revs / rpm = 3-4 minutes ( If no ID plate RMS Spec n says mix for 3.5 minutes) This is NOT mixing and is NOT counted as mixing time

16 If docket has no evidence to prove mixing time remix on site for full mixing time? Where is mixing time shown on docket? If using RMS based Specifications, mixing time starts only after all ingredients are in the mixer, this includes any water later added on site

17 Slump test Measure of wetness or consistency Not necessarily measure of workability

18 Slump requirements Typical values Non-vehicular usage 80mm Vehicular traffic (if using RMS based Specification) - fixed form paving 50-70mm - slipform paving 15-50mm Slump control Very important Particularly vehicular pavements ride quality Uniform within and between loads

19 Adding water on site (if using RMS based Specification) Water can be added : But only within 40 minutes of completion of batching Total water should not exceed designed mix Concrete fully remixed Slump tested In presence of contractor s nominated representative

20 Adding water to concrete you would not add an egg to a cake mix without mixing it all the way through

21 Adding water Water is added in here, but it has to be mixed all the way through the concrete. Fully remix after adding water on site

22 Completion of batching (RMS Specification) Either : Time at which slump is adjusted in plant before delivery or 10 minutes after all ingredients added to mixer Whichever comes first Words completion of batching and time must be on docket

23 Why the 40 minute limit? The cement/water paste glues the aggregate to give concrete strength and durability The quality of the paste is very important What happens when water is added to cement.

24 Portland cement electron microscope

25 When water added to cement chemical reaction hydration Dormant period then crystal growth

26 Source: Prof. Kenneth C. Hover +

27 If water added after dormant period Velcro tennis balls float apart Cement paste crystals do not interlock with each other Cement paste quality reduced Concrete strength/durability reduced

28 No water added past here!!

29 You would not add water to a good wood working glue Same principle applies to making concrete

30 Wetting up concrete not a good idea For concrete designed for 30 MPa and a 60mm slump If slump increased to 90mm strength reduced to 22 Mpa 30 MPa 22 MPa 60 mm 90 mm

31 Making steel fibre concrete (not plastic fibres) Roundabouts Fibres must be located uniformly through concrete when batching mixing hauling placing paving Concrete must be very cohesive Higher paste content Cement content kg m -3 Conventional concrete kg m -3

32 A final word on concrete mixing An old saying that has stood the test of time Good concrete can be made from good Cement, aggregates, water and admixtures Bad concrete can be made from exactly the same materials Ensure thorough mixing