Migrating to the use of low-carbon-footprint HPC using multi-sized fillers

Transcription

1 Migrating to the use of low-carbon-footprint HPC using multi-sized fillers Dr Johnny Ho PhD, MHKIE, MIEAust, CPEng, MIStructE, CEng School of Civil Engineering The University of Queensland The Saudi International Building and Constructions Technology Conference 11 May

2 Content Introduction to HPC Wet packing density and excess water Use of multi-sized fillers Theoretical packing model Three-tier design method Conclusions 2

3 INTRODUCTION TO HPC 3

4 Introduction In the past: Empirical mix design method. Why? - Simple/low performance criteria required - Low variety of ingredients (Cement) - Only water, no superplasticiser (SP) - Millions of combinations to satisfy requirement - Test, easily done at low cost - Why scientific mix design method? 4

5 Introduction Now for contemporary concrete:- - Demanding performance criteria required (eg. HPC, SCC) - Much more variety of ingredients (Cement, SCM, Fillers) - Chemical admixture (SP and slump retainer) - Less possible combinations because of the complexity - Test are time consuming + much more possible test combination - Empirical approach not cost effective - Need a scientific approach. How? By investigating the packing density of the mixed concrete (with mixing history accounted for) 5

6 Introduction HPC High performance concrete: High strength, high workability, high stability (segregation/dimensional), more durable, less permeable, low shrinkage, low heat, High strength concrete (HSC), self-levelling concrete, self-compacting concrete, self-consolidating concrete, low-heat, low-shrinkage concrete are all HPC. Advantages: - higher strength-to-weight ratio reduce construction and demolition waste, environmentally more friendly; cost effective solution; enlarge usable area, reduce carbon footprint (e.g. HSC) - dimensionally more stablised low-heat, low-shrinkage, low volumetric change 6

7 Introduction - More durable structures prolong design life, reduce cost or carbon consumption per year of design life, maintenance cost, burden of urban renewal, - More sustainable living reduced embodied energy/carbon content, less formwork, less compaction, transportation Structurally: - Higher strength (compressive, tensile, shear) - Increased structural ductility (albeit lower material ductility) - Higher stiffness and smaller elastic deflection - Reduced shrinkage and creep - In PC structures smaller prestress loss 7

8 Existing problems Problems: - Empirical trial-and-error mixing design procedure - Complexity in design mix incorporating SP - High cement content (> 300 kg/m 3 ; 10% human CO 2 emission) - Contradictory essential performance criteria Reasons: - No well defined essential performance criteria - No scientific method for developing HPC design mixes incorporating cementitious materials, non-cementitious fillers and SP - Codified method resulting unacceptably low packing density wrong and unscientific definition (agglomeration, van der Waals force) - Workability vs strength; workability vs low segregation 8

9 Existing problems Consequences: - Very difficult to achieve direct design solution of HPC mixes that can achieve all essential performance criteria through a single-step method - Design based on AS/NZS 2350:3 method of consistency cannot give satisfactory design with minimum void ratio (minimum water demand by penetration resistance in VICAT test) Recommendations: - Consistency should not be used for estimating the water demand of cement paste - Packing of ingredients under wet condition should be studied - Identify the fundamental fresh concrete properties - A systematic, scientific and one-step direct method is needed 9

10 Objectives of study To advocate the use wet packing density measurement of fresh concrete taking into account the effects of wet condition To use multi-sized fillers to improve concrete packing To investigate the effects of shape and forms of aggregates on concrete packing To develop a DEM packing model for evaluating the packing density Academic: Fundamental fresh concrete properties advocated for further study of fresh and hardened concrete performance Industrial: Reduce cost and improve strength, workability, durability, dimensional stability, segregation stability Social: Reduce carbon footprint; more sustainable living environment 10

11 WET PACKING DENSITY AND EXCESS WATER 11

12 Packing density and excess water Packing density is a measurement of total solid concentration in a particle system / mixture in unit volume. Therefore, the void ratio u (in unit volume) = 1 - For example, spherical mono-sized particles: - under random packing, packing density 0.6; - under organised packing (FCC or HCP) packing density 0.74; Blending solution: Successive packing of smaller mono-sized particles without crowding effect, packing density can reach = max. The respective void will be the smallest = u min = 1 - max 12

13 Packing density and excess water Successive packing by smaller particles can reduce the void and increase the packing density Use of multi-size aggregate to improve packing density 1. Reduction of cement paste volume due to smaller volume of gaps within aggregate skeleton Concrete composes of single-sized aggregate only Aggregates V v Air Water V a V w V Minimum cement paste required to fill up the gaps Paste coated on aggregate 2.Improving workability at the same paste volume due to formation of paste coating on aggregate surfaces Solid V s 13

14 Packing density and excess water So far, all codified packing is under dry condition Problems: (1) Inexact model of cement paste, mortar and concrete owing to wet condition; (2) Agglomeration effect; (3) Independent of mixing history / compaction / vibration / tapping / drumming Density measurement: By AS Bulk density measurement of aggregate resulting very low density measurement for fine to ultrafine particles (van der Waals force) Void measurement: Water demand measurement by AS/NZS Determination of normal consistency. No air void assumed and the minimum water demand = void of concrete/paste 14

15 Packing density and excess water VICAT apparatus No scientific correlation between penetration depth and void ratio (34 mm 1) = standard consistency = water absorption? 15

16 Packing density and excess water Packing density measurement in wet condition: During mixing process: Sufficient water that wets the surface of particles to enhance consolidation Increasing water content Pendular state Funicular state Capillary state Consolidation increases the rate of coalescence forming big granules Voids ratio Air Droplet Solid concentration a W/S ratio by volume 16

17 Packing density and excess water Solid concentration is measured in wet condition by: = V s V = M/V ρ w u w + ρ α R α + ρ β R β + ρ R M = Mass of concrete/paste (in a container) V = Volume of concrete/paste ( = volume of container) w = Density of water u w = W/solid ratio by volume = Density of R = Volumetric ratio of to solids By varying water ratios, maximum can be obtained = Maximum packing density = max =. Different ingredient ratios give different. 17

18 Packing density and excess water Excess water u w u w = u w u u w = water/solid ratio by volume u = minimum water needs to fill up the void (at capillary state) Excess water is the surplus that lubricates the mix and enhance workability (not just water content anymore!!) The more the excess water, the better is the workability performance. However, surface are should also be taken into account film thickness 18

19 Packing density and excess water The minimum water that needs to fill up the void is actually the void ratio when the capillary state just forms. This point is very close to the point of having maximum packing density in all circumstances. Hence, u can be approximated by the minimum void ratio at maximum packing density. At a given water content, maximising packing density of the mix with different ingredient proportions can reduce the void and hence maximising the excess water improve workability! The objective of designing concrete (and paste/mortar) is to maximise the packing density (aggregates/particles) by which both strength and workability can be improved at the same time! 19

20 USE OF FILLERS 20

21 Use of fillers Fillers can be use to fill up the interstitial void of concrete and improve packing density Criteria of fillers: 1. (Chemically) inert: no reaction with water, no cementitious property 2. Wide range of PSD 3. Abundant in nature - low carbon footprint 4. Readily available for use - Low cost 5. Can possibly be derived from waste materials Examples in this study: Limestone fines, super-fine sand Other possible fillers: Foundry sands; Dune sand; Shell fish; Bio-solid (dewatered and treated) 21

22 Use of fillers Limestone Size comparable to cement. Use as filler to replace cement and other SCM reduces paste 22

23 Use of fillers Limestone 23

24 Use of fillers Superfine sand Water washed high purity silica sand As filler to replace aggregates not CM materials 24

25 Use of fillers Foundry sands Foundry sand is high quality silica sand that is a by product from the production of both ferrous and nonferrous metal castings. Moulding sand Sub-angular to round shape - desirable from packing point of view. SG = PSD = 150 to 425 m As filler to replace aggregates not CM materials 25

26 Use of fillers Dune sands Some sand dune consists of very fine sand with PSD m Depends on the location and shapes of dune. The more complex the shape, the finer the sand (e.g. star or complex crescent PSD m). Compound or linear dune PSD m. Also depends on wind regime, downwind direction is finer. Kingdom of Saudi Arabia and Australia have got abundant availability of dune sands. 26

27 Use of fillers Test programmes Different combinations of concrete mixes designed and tested. Control concrete: (1) Cement; (2) C + fly ash; (3) C + silica fume Tested concrete: (1) 5% vol. replacement of CM by LS (2) 2.5% vol. replacement of CM by LS + 2.5% replacement by superfine sand Measurement: Slump flow; L-Box; Segregation; 28 th day strength 27

28 Use of fillers Design mixes table 28

29 Use of fillers Concrete performance and carbon footprint tables 29

30 Use of fillers Photos LSSF1: Cement+LS+SFS (5% conc repl) LS1: Cement+LS(15% CM repl) 30

31 Use of fillers Strength and Flow Vs packing density Flow vs packing density 28 th day strength vs Packing density 31

32 THEORETICAL PACKING MODEL 32

33 Theoretical packing model A theoretical packing model has been developed by UQ/Civil Soil Group based on C++ Sequential packing algorithm takes into account particle size distribution, volume of container and chosen porosity Model established can be sent to DEM for fabric force / compaction analysis Another DEM model created by UNSW angularity accounted with capillary force Later stages: more water, particle immersed in water/sp, dynamic properties of fluid and solid needs to modify Maximum packing density determined 33

34 Theoretical packing model Some packing models generated: 34

35 Theoretical packing model Packing algorithm 35

36 THREE-TIER DESIGN METHODOLOGY 36

37 Three-tier design methodology Three tiers: paste, mortar; and concrete mixes Governing factors at each tier to be determined Design of HPC can be more systematic and scientific Easier to find out the critical factors at each tier than in whole mix Allow a optimised concrete mix design to be produced 37

38 Three-tier design methodology Films thickness - Water film thickness - Excess water to surface area ratio - Representing average thickness of the water coating the particles - Dominant factor governing the (workability) performance 38

39 Three-tier design methodology Water film thickness: = u w A = u w A α R α + A β R β + A R A = Total surface area of particle A = Specified surface area of (surface area / volume of ) R = Volumetric ratio of to CM materials Excesss water Solid surface of solids 39

40 Three-tier design methodology Paste film thickness (for fine aggregates > 75 m) Fine aggregate particle Paste trapped inside the voids Paste films coating fine aggregate particles Paste film thickness Role of paste film thickness in mortar Mortar film thickness (for aggregates > 1.2 mm) Coarse aggregate particle Mortar trapped inside the voids Mortar films coating coarse aggregate particles Mortar film thickness Role of mortar film thickness in concrete mix 40

41 CONCLUSIONS 41

42 Conclusions Low-carbon-footprint concrete reduces the cost of production and creates a more sustainable living environment. Low-paste concrete improves dimensional stability and durability of concrete. Packing density (and film thickness) are the fundamental properties of fresh concrete governing the strength, workability and durability of concrete. Low-carbon-footprint of concrete can be produced by partial fly ash replacement. And if higher strength concrete is needed, silica fume of max 15% vol replacement can be used. Low-carbon-footprint and low-paste concrete (down from 0.25) can be achieved by improving the packing density of concrete (>0.8) with LS+SFS replacement (5% - 10% conc vol) 42

43 Conclusions With the concept of packing density and fillers application, using waste materials (treated) in concrete is now possible, e.g. shellfish (limestone), foundry sands, bio-solid (dewatered and treated) Durability of low paste concrete needs to be quantitatively studied. So far minimum size of fillers is limited to that comparable to cement. To further reduce the carbon footprint, nano-fillers can be used to increase the CM replacement. Existing problems: Cost is high; low workability (or high dosage of SP needed); water absorption; large scale production; quality control Future of concrete: (low cement + some CM + water 25%) + (fillers: nano, micro and macro scales + aggregates 80%) 43

44 Research team and collaborators Research Team Academic Institutions: University of Queensland / Civil University of NSW / Materials Science and Engineering University of Hong Kong / Civil Industrial sectors: Cement Australia Cement, SCM, filler Xypex Silica fume BASF Superplasticiser and other chemical admixtures Boral Concrete Coarse and fine aggregates Sibelco Limestone and washed high purity sand Southern Pacific Sand Foundry sands Dune sand 44

45 THANK YOU! 45