New Generation Admixtures Presented by: Graeme SMITH, Hennie van HEERDEN Sephaku Cement
Agenda • What are admixtures? • Why use them and current use • Admixture types to SANS 50934 • Plasticisers and Superplasticisers • SCC ; new generation concrete! • Air-Entrainers • Set control admixtures • Sprayed Concrete
Admixtures • Admixtures are Chemicals typically consisting of • 1 or 2 major constituents • + 2 or more minor constituents • Added to the concrete during mixing • Usually as an aqueous solution, < 40% active chemical • Quantity is very small • 0.2 to 5.0% Admixture by weight on cement • Active chemical usually less than 0.15% on the concrete • South African Standard – SANS 50934 :1 to 6 (adoption of EN 934);not yet a compulsory standard • Modify the properties of the concrete in the Plastic and / or Hardened state
Heading
Why use Admixtures? • Economic - Producer • Optimised mix design • Placing - Contractor • appropriate workability (consistence), compactability, cohesion, setting and strength development • Problem Solving – Designer / Engineer • concrete that will meet special needs for placing or performance • Durability - Owner • Ensuring that concrete fulfils its design requirements for the intended life of the structure • Sustainability - Everyone • helping to reduce the health and safety aspects of concrete during placing and environmental impact during its life cycle • www.sustainableconcrete.org.uk
Current Admixture Use • Admixtures are currently used in over 80% of all Ready Mixed and Precast Concrete • Admixtures are now a normal ingredient of concrete, together with Cement, Aggregate, Additions and Water- the 5 th ingredient! • Admixture sales by type for 2012
Admixture Types to SANS 50934 (EN 934) Normal Plasticisers (Water reducers) EN 934-2 T2 Super-plasticisers (High Range WR) EN 934-2 T3 Air Entraining EN 934-2 T4 Accelerating EN 934-2 T6 & 7 Retarding EN 934-2 T8 Water Resisting (Waterproofing) EN 934-2 T9 Mortar Admixtures EN 934-3 Grout Admixtures EN 934-4 Sprayed Concrete Admixtures EN 934-5
Other Admixture Types Corrosion Inhibiting Admixtures BS 8443 T4 Foamed Concrete Admixtures BS 8443 T7 Polymer Dispersion Admixtures Pumping Admixtures BS 8443 T5 Viscosity Modifying (Segregation Reducing) BS 8443 T6 Admixtures Semi-Dry concrete Admixtures BS 8443 Shrinkage Reducing Admixtures BS 8443 T3 Wash-water Recycling Admixtures Anti-Washout Underwater Concrete BS 8443 T2 Admixtures
Plasticisers & Superplasticisers • 80% of all Concrete Admixture Sales • Both types are essentially Particle Dispersants • The difference is in the dispersing power • But also in the secondary effects: • Retardation • Air entrainment • Also known as Water Reducers / High Range Water Reducers
Particle Dispersion • Dispersing Effect of Plasticisers & Superplasticisers cement flocs cement is uniformly dispersed reduce fluidity increasing fluidity add a plasticising admixture • Plasticiser or Super is adsorbed onto cement particle • These dispersant polymers repel other cement particles by electrostatic or steric repulsion • Repulsion stops cement from flocculating in clumps and trapping water
Dispersant Uses • Plasticise at = Cement content & W/C • Increased Consistence • Water Reduce to = Consistence • Increased Strength, • Reduced Permeability • Cement & Water Reduce to = W/C & Consistence • Lower Rands, lower water use, lower cementitous • Improved sustainability, lower embedded CO 2
Plasticisers vs Superplasticisers Plasticiser Superplasticiser Main constituent Lignosulphonate Melamine Polymers SMFC Selected Carbohydrates Naphthalene Polymers SNFC Polycarboxylate Ethers PCE Typical dose (40% 0.3 to 0.5% 0.6 to 1.5% Solution) Cost Plasticiser x 2 Not economic for cement cost savings Overdose Retardation Little effect till x 2 overdose Without water reduction, mix Air Entrainment may segregate Water reduction 8 to 12% 16 to 30% +
Slump Test
Increased Workability/Consistence • Add the plasticiser / superplasticiser to the concrete . • W/C does not change so strength and durability are unchanged, only consistence increases • slump increased from 50 to 120 mm for site use • Normal Plasticiser • 180 to 220 mm slump for flowing concrete and congested reinforcement • Mid range or Superplasticiser • Pump mix to prevent segregation • Self Compacting Concrete for self levelling concrete, very congested reinforcement, high quality surfaces, difficult shapes, H&S -vibration free (reduces noise), no vibration white finger and back problems. • PCE Superplasticiser needed
Low Slump Concrete
High Flow but not quite Self- Compacting
High Flow but not Self-Compacting
Use of high consistence concrete • Pre 1970 – most concrete 50 to 120mm slump, occasionally 180mm • 1970 – 1995 up to 220mm slump • Based on SMF or SNF Superplasticisers • required little vibration or compaction • 1990 – Japan introduces SCC • 1995 – Europe experiments with SCC • 2003 to date – SCC use increasing • Widely used in Precast • Low noise • Early strength • Excellent surface finish • Used on site mainly for special applications: • Difficult access • Difficult shapes • Improved surface finish
Self-Compacting Concrete • SCC resulted from the development of PolyCarboxylate Ether (PCE) Superplasticisers. • SCC requires a combination of: • High fluidity • Resistance to segregation • Resistance to blocking in congested reinforcement • Earlier Superplasticisers could not consistently produce concrete that exhibited all these properties. • SCC requires greater control on constituent quality and on production control so is not appropriate for all concrete.
SCC mix design • High powder content • PC + ggbs, fly ash, limestone fines etc. 450 to 600 kg/m3 • PCE Superplasticiser • Sand 45 to 55% of aggregate • Coarse aggregate normally <16mm • Viscosity Modifying Admixture may be needed • Mix design must 100% consistently achieve: • Good flow • No segregation • Good passing ability through reinforcement /no blocking • Appropriate viscosity / speed of flow • SCC mix design requires an in depth knowledge of concrete mix design and materials as well as consistently good constituents • SCC should only be purchased from companies with a proven track record for consistent and high quality supply of this type of concrete.
Consistence by Slump Flow • Most SCC will have a slump flow of 660 – 750 mm • A J-Ring or L-Box should not significantly reduce the flow and there should be no height step from inside to outside the bars. • Coarse aggregate should be visible at the surface and right up to the outer edges • There should be no indication of bleed or grout separation even if left to stand.
Self-Compacting Concrete
Precast – High flow & No Vibration
Site – High flow & No Compaction
High flow and good passing ability
Precast – Surface Detail
Surface Quality
Other SANS 50934-2 Admixtures • Air Entrainers • Set Control • Accelerators • Retarders • Water Resistance – (Waterproofers)
Air Entertainment • 1% Air reduces strength by about 5% • Air entrainment: • Provides Freeze Thaw resistance. Is a requirement for highway pavements • Air increases the apparent paste volume, increasing cohesion, reducing settlement and can significantly reduce bleed • In low fines mixes and semi dry concrete it lubricates the mix, improving compaction
Freeze Thaw Resistance • Occurs when saturated concrete is subjected to Freeze Thaw action. • Water in the capillaries freezes, generating pressure and causing the surface to spall • Aggravated if de-icing salts are used • De-icing salts can increase the rate of freezing within the concrete • Can result in osmotic pressure due to concentration gradients • Entrained Air: • Does not fill with water in saturated concrete due to capillary action • Relieves the pressure if water in the capillaries start to freeze • Bubble spacing needs to be small (< 0.2 mm spacing factor) • Bubble size typically less than 0.3 mm
Freeze-Thaw
Cohesion/Bleed reduction
Accelerating Admixtures • Acceleration of Set or Strength? • Most types do one or other not both. • • Sprayed Concrete accelerators • very fast set, used in Mining/Tunnelling applications • Calcium Chloride • accelerates both set and strength. • very cost effective • enhances corrosion of embedded steel • Other Accelerators • Less effective than Calcium chloride • Usually accelerate Set or Strength but not both
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