CIVI321 Engineering Materials Chapter 5: Chemical Admixtures PDF

2024-02-14 CIVI321 ENGINEERING MATERIALS Chapter #5: Chemical admixture A.M. Soliman, PhD, P.Eng Assistant Professor Department of Building, Civil & Environmental Engineering (BCEE) 1 Admixtures Chemical admixtures are the ingredients in concrete other than Portland cement, water, and aggregate that...

2024-02-14 CIVI321 ENGINEERING MATERIALS Chapter #5: Chemical admixture A.M. Soliman, PhD, P.Eng Assistant Professor Department of Building, Civil & Environmental Engineering (BCEE) 1 Admixtures Chemical admixtures are the ingredients in concrete other than Portland cement, water, and aggregate that are added to the mix. 2 1 2024-02-14 Why….Admixtures ?  Modify the properties of hardened concrete  Accelerating admixtures (Increase Early strength)  Water-reducing admixtures (Reduce water content)  Reduce concrete construction cost  Accelerating admixtures (Saving time)  Water-reducing admixtures (Increase Workability)  Overcome emergencies during concrete operations  Accelerating admixtures (Cold weather casting)  Water-reducing admixtures (Narrow cross-sections) 3 Other Admixtures Air Entrainment shrinkage reducers Corrosion inhibitors ASR inhibitors 4 2 2024-02-14 Accelerating Admixtures 5 1 Accelerating Admixtures Cement C3S C2S C 3A C4AF Water Hydration Products H 2O CSH CH + + Heat Others CSH is the binder (glue) responsible for strength 6 3 2024-02-14 Accelerating Admixtures Cement Water Hydration Products H 2O CSH CH + C3S C2S C 3A C4AF + Heat Others Accelerator CSH strength Accelerate the hydration rate: Early strength and setting 7 Accelerating Admixtures Cement C3S C2S Water Hydration Products H 2O CSH CH + C 3A C4AF + Heat Others Accelerator CSH Heat strength 8 4 2024-02-14 Accelerating Admixtures Cement + The faster the hydration reaction Water Hydration Products Initial Setting + Heat Initial Setting The higher the strength The shorter the setting time 9 Accelerators Used in cold weather to accelerate construction. Reduce: 1) Curing time 2) Formwork removal time 10 5 2024-02-14 Accelerators Types: Alkali hydroxides organic compounds Sodium chloride …etc. Silicates Calcium nitrate, Calcium chloride Table salt Calcium chloride (since 1873) (Availability, low cost, predictable performance) 11 Calcium Chloride It accelerates Cement hydration Faster hardening and strength gain. The exact mechanism of this catalytic effect is not well understood Credit: Ramashandran 12 6 2024-02-14 Effect on Microstructure Change from fibrous structure of CSH to spherulite or lace-like structure is reported by researchers Credit Ramashandran 13 Effect on Fresh properties Increases the workability. Reduces significantly both initial and final setting time) Vicat, ASTM C 191 Reduces both bleeding rate and bleeding capacity 14 7 2024-02-14 Effect on Fresh properties Increases the workability. Reduces significantly both initial and final setting time) Vicat, ASTM C 191 ? Reduces both bleeding rate and bleeding capacity 15 16 8 2024-02-14 Water Cement Silica Fume SF Hydration product Cement Hydration product Pure Cement Hydration 17 Effect on Mechanical Properties Accelerates compressive strength gain at early stage. CSH Heat strength Most literature indicates that the long-term strength is lower than that of a non-accelerated control mixture. 18 9 2024-02-14 19 Effect on Durability Chlorides in CaCl2 increase corrosion of steel. CaCl2 usually banned in pre-stressed concrete. 20 10 2024-02-14 Effect on Durability Drying shrinkage of concrete increases Shrinkage cracking when CaCl2 is used. Freeeze /thaw Damage Alkali aggregate expansion is increased by addition of CaCl2 Addition of CaCl2 leads to lower durability under sulfate attack 21 Non-Chloride Accelerators Because of risk of corrosion due to CaCl2 , there is continuous effort to find an alternative Many organic and inorganic compounds suggested: aluminates, sulfates, formates, thiosulfates, nitrates, carbonates, formaldehyde, etc., but experience and research on these admixtures is limited. 22 11 2024-02-14 Retarding admixture 23 Retarding Admixtures Delay setting or hardening rate for: Hot-weather concreting Need longer setting time – Difficult placements – Special finishing processes Sugar 35 Temperature (ºC) 32 29 26 Admixture Dosage 23 (ASTM C 494 or AASHTO M 194, Type B) 20 Time (Hrs) 24 12 2024-02-14 Reduction in Workability at Various Temperatures 25 Reduction in Workability at Various Temperatures 23oC 100 125 60 min. 50 min. 50 mm 50 mm 32oC+ retarder 32oC 110 20 min. 32oC 23oC 50 mm 26 13 2024-02-14 Accelerators Retarding Calcium chloride Table salt Non-Chloride CSH Heat Setting Durability Drying shrinkage Corrosion Frost resistance Non-Chloride Sulfate attack Strength Alkali aggregate expansion Early Later 27 Water Reducer Admixtures 28 14 2024-02-14 Water Reducer Admixtures 29 Water Reducer Admixtures ADD MORE WATER 30 15 2024-02-14 Water Reducer Admixtures 31 Water Reducer Admixtures 32 16 Compressive Strength 2024-02-14  Strength 150-200 MPa 60-100 MPa 25-35 MPa NC HPC UHPC Water-cementing materials ratio Concrete Workability Designed Strength 33 Strength Workability Water-cement ratio 34 17 2024-02-14 Water Reducer Admixtures Main objective Reduce mixing water 35 Water Reducer Admixtures Main objective Reduce mixing water (5 %) Reduce mixing water : Reduce water-cement ratio 36 18 2024-02-14 Water Reducer Admixtures Main objective Reduce mixing water (5 %) Reduce mixing water : Reduce water-cement ratio Reduce cement content 37 38 19 2024-02-14 Mixing Mixing Casting Transporting Transporting Casting Finishing Finishing Shorter Slump Life 39 Water Reducer and Slump Loss WR L 100 75 mm 6 min. 40 min 6 min 40 20 2024-02-14 Water Reducer Admixtures 41 HOW 42 21 2024-02-14 Dispersion of cement particles without addition with water-reducing admixture (a) Flocculated (b) Dispersed 43 Chemical Composition The main compounds used in the manufacture of water reducers can be divided in 4 groups: 1) Lignosulfonates 2) Hydrocarboxylic acid 3) Carbohydrates 4) Other compounds 44 22 2024-02-14 1) Lignosulfonates 2) Hydrocarboxylic acid 3) Carbohydrates 4) Other compounds The most used raw material in the manufacture of water reducers. Waste liquor in the production of paper making Contain about 30% reducing sugars They tend to entrain air in concrete, 45 1) Lignosulfonates 2) Hydrocarboxylic acid 3) Carbohydrates 4) Other compounds Developed as water reducers in the 1950ies. Usually synthesized chemically. Act also as retarding agents. 46 23 2024-02-14 1) Lignosulfonates 2) Hydrocarboxylic acid 3) Carbohydrates 4) Other compounds Include natural products such as glucose and sucrose. Act also as retarding agents. 47 1) Lignosulfonates 2) Hydrocarboxylic acid 3) Carbohydrates 4) Other compounds 48 24 2024-02-14 Effects on Fresh Concrete Workability is improved, less cold joints and air pockets Most water reducers decrease the bleeding rate and capacity. Heat of hydration: Delay the rapid heat evolution 49 Effect on Hardened Concrete Early strength can be decreased due to retarding effect, but 28 day strength is increased beyond what could be expected from reduction of w/c ratio. Better dispersion of cement grains leads to better particle packing and higher degree of hydration at later ages. Water Cement 50 25 2024-02-14 Effect on Durability Permeability 51 Effect on Durability Increase frost durability (39%) Less expansion under sulfate attack Decrease chloride diffusion and enhance corrosion durability 52 26 2024-02-14 Mid-Range Water Reducing Admixtures Reduce water content 6% to 12% Reduce cement content Reduce water-cement ratio No retardation Improve placeability and finishability 53 High-Range Water-Reducing Admixtures ASTM C 494 or AASHTO M 194 Reduce Water content 12% to 30% Reduced W/C produces concrete with:  Compressive Strength > 70 MPa  Increased early strength gain  Reduced Cl ion penetration 54 27 2024-02-14 Plasticizers for Flowing Concrete Also known as ― Super-plasticizers ASTM C 1017 Type 1 ―Plasticizing Type 2 ― Plasticizing and Retarding Essentially High-Range Water Reducer 55 Plasticizers for Flowing Concrete Also known as ― Super-plasticizers ASTM C 1017  Produce flowing concrete with high slump (≥ 190 mm [7.5 in.] (Normal concrete 50-100 mm)  Reduce bleeding  Extended-slump-life plasticizer reduces slump loss. 56 28 2024-02-14 Credit Ramashandran Cement + water Cement + water + superplasticizer 57 Flowing Concrete 58 29 2024-02-14 59 Dispersion of cement particles without addition (a) Flocculated with water-reducing admixture (b) Dispersed 60 30 2024-02-14 Superplasticizer Dispersion Phenomena 1. Steric inhibition of particle-particle contact 61 2. Inducing electrostatic repulsion between particles 62 31 2024-02-14 3. Surface absorption Polymeric dispersants adsorb on cement grains, deflocculate agglomerated particles, release water entrapped, hence the water content could be reduced while maintaining workability. 63 4. Inhibition of surface hydration of cement particles 64 32 2024-02-14 4. Inhibition of surface hydration of cement particles 65 Cement - Superplasticizer Compatibility 66 33 2024-02-14 Cement Hydration Chemical reactions between cement and water Cement C 3S C 2S C 3A C4AF Hydration Products Water + + H 2O - CSH (Calcium silicate hydrated) - CH (Calcium hydroxide) C3A + Gypsum Heat Hydration Product 67 Cement superplasticizer compatibility 1) Influence of C3A content of cement C3A adsorb of sulphonate type superplasticizers. Hence, more superplasticizer is required to achieve a certain workability 68 34 2024-02-14 Cement superplasticizer compatibility 2) Influence of C3A/SO4 ratio C3A + Gypsum Hydration Product - Sulphates (SO4) React with C3A. - Sulphate phases in cement: anhydrate, hemi-hydrate or gypsum forms. - Sulphonated superplasticizers can retard the hemihydrate/gypsum conversion and react with C3A. 69 3) Influence of alkali content of cements 70 Flocculation is enhanced by alkalis Water reduction with superplasticizers is more achievable with low alkali content cements ? ? 35 2024-02-14 Air-Entraining Admixtures 4 71 Air-Entraining Admixtures Air entrainment: The process by which small air bubbles are incorporated into concrete at the mixing stage and remain dispersed in concrete at the hard state. 72 36 2024-02-14 Air-Entraining Admixtures Process discovered accidentally in the late 1930’s when it was observed that concrete pavements in New York State made with certain cements were more durable than others (cements were ground with beef fat as a grinding aid and fish oil which apparently acted as air-entraining agents). Air entrainment is now essential for the durability of concrete under freezing-thawing cycles. 73 Mechanism of Action of AEA The air bubbles are generated by mixing (entrapped air). All what the AEA does is stabilize them (does not generate them). 74 37 2024-02-14 Frost Damage 75 76 38 2024-02-14 Scaled Concrete Surface 77 78 39 2024-02-14 Chemical Composition Air-entraining agents belong to a class of chemicals called surfactants (short for surface active substances) Surfacactant is material whose molecules adsorb strongly on air-water or air-solid interfaces. Their molecules have one polar end and one non-polar end. Detergents are one sub-group 79 AEA adsorbs to bubble surface with polar heads in the water phase. When bubbles approach each other they will repulse (not coalesce). 80 40 2024-02-14 81 82 41 2024-02-14 Effect of Air and Cement Content on Performance of Concrete in Sulfate Soil Without air Cement content With air 222 kg/m3 (375 lb/yd3) 306 kg/m3 (515 lb/yd3) 392 kg/m3 (660 lb/yd3) Type II-Cement 5 years exposure to sulfate soil 83 Most important parameter is air spacing For any concrete subjected to any freezing-thawing sequence, there exists a critical air spacing called spacing factor beyond which concrete deteriorates rapidly Optical microscope Spacing of air bubbles can now be measured using automated image analysis. 84 42 2024-02-14 Air-Void System Spacing factor (⎯L ):   An index related to the maximum distance of any point in the cement paste from the periphery of an air void. ASTM 457: less than 0.2 mm (0.008 in.) Specific surface (α):   The surface area of a quantity of air voids that have a total volume of one cubic inch. ASTM 457: 24 mm2/mm3 (600 in.2/in.3) or more 85 86 43 2024-02-14 Spacing Factor and Air Content 87 Factors Affecting Amount of AEA AEA dosage Zero Air Entraining Admixture 3 mL of Air Entraining Admixture 6 mL of Air Entraining Admixture 88 44 2024-02-14 Finely divided materials (silica fume, etc.) cause a drop in the EA: less water available for bubble formation + AEA is adsorbed on high surface area Type 10 Type 30 89 Coarse aggregate: max size increases, air requirement decreases. 90 45 2024-02-14 Fine aggregate: the higher the proportion of fine aggregate in the total aggregate the more air generated. 91 Higher temperature of concrete leads to lower air content and vise versa. Effect is more clear at higher slumps 92 46 2024-02-14 The higher the slump, the higher the air content (roughly 3” more slump, 1% more air, Until reach the maximum of 150 to 175 mm…Then drop) Any vibration will significantly reduce the air content 93 Proportioning Air Entrained Concrete Air causes increase in slump and decrease in strength. Less water is required, w/c ratio can be decreased to compensate for strength loss. For rich mixtures, the loss in f’c due to air is more significant than the gain due to w/c ratio reduction. Often the cement content needs to be increased to maintain equal f’c For lean mixtures, with lower f’c and higher w/c ratio, the weakening effect due to air is just about offset by reduction in w/c ratio, usually no additional cement needed for equal strength 94 47 2024-02-14 95 Effect of Entrained Air on Fresh Concrete Properties Increased 96 48 2024-02-14 Effect of Entrained Air on Hard-ened Concrete Properties 97 Other Chemical Admixtures 98 49 2024-02-14 Coloring Admixtures (Pigments) 99 Waterproofing Admixtures Those that reduce water permeability (under pressure) are called waterproofing agents (pozzolanic fillers, superplasticizers, etc.). Those that give water repellency are termed dampproofing admixtures (silicone based sealers, wax emulsions, etc.) 100 50 2024-02-14 Corrosion Inhibitors Control Corrosion of Steel Reinforcement Dosage dependent on anticipated chloride level 101 Alkali-Aggregate Reaction Inhibiting Admixtures Soluble salts of lithium, barium and sodium reduce expansion due to alkali-silica reaction Lithium salts and ferric chloride have been reported to reduce expansion due to alkali-carbonate reaction Treated Spraying 30% LiNO3 solution Untreated 102 51