Chemical Admixtures for Concrete: An Overview

Transcription

1 Chemical Admixtures for Concrete: An Overview Deepak Kanitkar GM Technology and Business Development Chembond Chemicals Limited (Construction Chemicals Division) Use of Chemical Admixtures in Concrete One of the most important and critical ingredient of modern concrete is the chemical admixture. The introduction of admixtures have changed the way we can work with the cement concrete. As the Power s Equation states, Vp=100w/ c The Porosity of concrete is inversely proportional to W/B ratio. The first and most important role of chemical admixtures is to reduce the water : binder ratio. Specifications and Test Methods IS 9103, ASTM C-494, ASTM C 1017, BS 5075 (withdrawn) and BS EN 934 part 1, 2, 6 with supporting test methods are commonly used, for classifying and testing chemical admixtures for concrete. There are different classes of admixtures, based on their effect on concrete properties. ASTM standard C-494 classifies admixtures in 8 types from A to G and S. Apart from reducing the mixing water, chemical admixtures have various functions such as slump retention, set retardation or acceleration, strength acceleration and more. Each type needs to meet different criteria, specified in test specifications. Moreover, there are other types like air entraining admixtures as per ASTM C-260, integral water proofing admixtures meeting the water impermeability requirements as per DIN 1048, anti- wash out admixtures, corrosion inhibitors, shrinkage reducing admixtures, foaming agents, corrosion inhibiting admixtures and shotcrete accelerators. Polycarboxylate Ether Based Admixtures Chemical admixtures are based on various chemistries. Water reduction greatly depends on the type of chemistry a formulator uses, to design the admixture. As we know, the best water reduction is achieved using PCE based admixtures. This is a result of very efficient dispersion of the binder which PCEs offer. The effectiveness of PCEs is more evident when the W/B ratio goes below The versatile chemistry of PCE polymers, ensures their use in almost every Admixtures mix design. SCC has been mostly associated with PCE based admixtures. Along with the excellent water reduction, they also produce flowing concrete without segregation. In a properly designed SCC mix using a PCE based admixture, there is either no need for VMAs or their usage could be limited. Now a days modified PCE based admixtures are available which also offer good rheology control. One of the challenges in earlier PCEs was the increased stickiness of the concrete mix, now there are some molecules which offer excellent reduction in stickiness. This is often necessary while designing very high strength mixes which have a tight water cement ratio and high fines. There are other issues associated with PCEs such as, rapid loss of slump, dosage and temperature sensitivity, sensitivity to moisture content. Due to such factors concrete producers generally avoid the use of PCE based admixtures in lower strength mix designs. Modified PCEs are useful in meeting the requirements of lower strength mixes. Often, due to use of manufactured sand or stone dust, there are issues with regard to slump retention or segregation, these need to be addressed by smart formulations of blended PCE molecules. Shrinkage Reducing Admixtures As described earlier apart from the traditional use as water reducing agents and slump retainers, admixtures are nowadays used for more functional roles. Shrinkage reducing admixtures is an example. These are very effective in reducing cracks caused due to drying and autogenous shrinkage, also known as selfdesiccation. They act on very fine capillaries with diameters between 2.5 to 50 nm in diameter, by reducing surface tension within pore solution. This helps in preventing collapse of capillary walls, thereby reducing cracking. Their use in heavy duty industrial floors, enables increased the spans and reduction in the requirement for number of joints. SRAs are mainly based on Ethylene and Propylene Glycol derivatives. While using SRAs, one must consider their effect on final compressive strength and air entrainment. In general, at same W/B ratio, 10-15% reduction in final compressive strengths, have been observed. It is imperative that by adjusting the W/B ratio, one can actually maintain the desired compressive strengths. 68 The Masterbuilder - July

2 The shrinkage could to some extent be determined from an equation derived by Tomita et al - (sh) = W X-4.758E (x10-6) - Where W, X and E corresponds to unit contents 3 (kg/m ) of water, SRA and expansive additives respectively. - This suggests that SRA are more than 4 times as effective than the expansive additives or the effective dosage of SRAs is only 20-25% that of the expansive additives. Corrosion Inhibiting Admixtures Preventing or rather delaying corrosion of steel in concrete is achieved by at least three methods. Use of corrosion inhibiting concrete admixtures, CP systems and protective covering or penetrative treatments. The most important factors are chloride ingress and carbonation, apart from sulphates and other corrosive contaminants. CIAs are mainly based on inorganic chemicals such calcium Nitrate / Nitrite or bipolar acting agents based on amino alcohols and Amino polycarboxylates. Various products are available in the market and are in most of the cases equally effective. The mechanisms are different, so as the dosages. Internal Curing Agents Use of Internal curing agents for concrete are also gathering momentum. They act by providing additional moisture in concrete for a more effective hydration of the cement and reduced selfdesiccation. Internal curing means the introduction of a curing agent into concrete to provides this additional moisture. Two major methods currently available. - Use of saturated porous lightweight aggregate (LWA) in order to supply an internal source of water. - Super-absorbent polymer (SAP) particles can absorb a very large quantity of water during concrete mixing and form large inclusions containing free water, thus preventing self-desiccation during cement hydration. For optimum performance, the internal curing agent should possess high water absorption capacity and high water desorption rates. Admixtures For Sprayed Concrete or Shotcrete Most tunneling work will never be complete without the use of gunniting or the sprayed concrete. Spraying concrete is a very sensitive application. The nozzlemens job will not be easy without the use of an accelerating admixture. Typically, quick setting is as critical as controlling the reheology of sprayed concrete. Accelerators combine both these requirement, thus reducing the rebound to a great extent and achieving quick setting. There are more and more applications which could be discussed. Admixtures for making pervious concrete, colorants for decorative concrete, admixtures used for stopping the rejected concrete from setting for a period as long as 24hrs are available. Concrete Mix Proportioning Science and Art What is mix design? Concrete proportioning or designing of a concrete mix, is an art more than science. Although it involves a lot of statistics and material science, more depends on the actual feel of the concrete during the mixing and placing stages. In general concrete mix has 4 main constituents viz. cementitious materials, aggregates, water and chemical admixtures. More precisely, concrete consists of coarse aggregates which are bound by a mortar made of a paste of hydraulic binders mixed with fine aggregates. Mix proportioning involves, physical formulation of these ingredients to achieve certain properties. Main criteria remains that of mechanical properties but without achieving the fresh concrete performance, mechanical properties can never really be attained. In order to achieve the desired level of workability and a cohesiveness concrete mix, the gradation as well as granularity of both coarse and fine aggregates, cement content, water to binder ratio and use of a suitable chemical admixture, play a vital role. While doing this, apart from getting the desired performance, one has to also remember economy and properties of available resources, including water. Ingredients Properties Cement Strength Grade Type Fineness Alkalinity - - Fine Agregates Zone or Gradation Shape Absorption Reactivity Density Mineralogy Coarse Aggregates Maximum Size and Gradation Shape / Crushing method Absorption Reactivity Density Mineralogy Admixtures Type Chemistry Dosage Pozolanic additions Mineralogy / Chemical Composition Ingredients and their important properties Particle size Dosage The Masterbuilder - July

3 Air in concrete is also a critical factor. Controlling the amount of air in the mix can to a certain extent be controlled during mix designing stage. There are two classes of concrete viz normal (Non Air Entrained) concrete and air entrained concrete. The amount of allowed air varies considerably in both types. Air entrainment is achieved through the use of air entraining admixtures. More recently, it is also known that Self Compacting Concrete (SCC) requires a different approach, than these two types. Essentials of a Mix Design Following are typically the most important factors, which need to be taken into account while deciding the mix proportions. - Characteristic Strength - Air content - Workability requirements - Climatic conditions / exposure conditions. - Handling conditions / Automation - Durability - Economy - Aesthetics and appearance We will take a look on, how some of the above factors help in arriving at a desired mix proportion. 1. Characteristic strength: Compressive strength of a mix is typically the most important mechanical characteristic. Once we know the strength requirements, it becomes simple to select the grade of cement, water content or W/B ratio as well as fine aggregate content. It is well known that reducing the W/B ratio, directly increases the compressive strength. 2. Air Content: Air in concrete is not always desirable. In non airentrained concrete it is desirable to have an air content less than 1 % more than the control mix or a maximum 2.5% by its volume. A little air helps in getting better slump and finishing of the concrete. As the air content increases, it starts decreasing the strength and simultaneously increase the permeability. We need to take this air into consideration, as the density and yield depend on the percentage of air in the mix. 3. Workability: It is the ability of fresh concrete which allows the placement and finishing at a particular point after mixing. The slump as it is called is measured using a slump cone at mixing point and at the point and time of placement. Depending on the actual application and placement technique, the value of slump can vary between almost zero for a pavement grade roller compacted concrete to a flowable consistency for the SCC. Apart from this point measure, the slump or workability may be required to be retained, up to a certain period of time. So, the required workability at a given time needs to be considered, while designing a mix. 4. Climatic conditions and Exposure: Whether the concreting is taking place in winter or summer matters a lot. It is also imperative to understand the variations during the day and nights. Under water placement, needs anti washout admixtures and set accelerators. We have to also consider the exposure conditions. If there is a chance of moderate to heavy exposure to sulphates, one needs to consider the use of sulphate resistant cement or pozolanic additions such as fumed silica. in case of chloride based environment, use of corrosion inhibitors based on calcium nitrite or bipolar types, may be necessary. Industrial environment, may necessitate use of hydrophobic agents / corrosion inhibitor, along with low permeability mixes. 5. Handling conditions / Automation: Whether the concrete is hand placed, pumped, roller compacted or placed in moulds at a precast facility, will mean a different approach to the mix design. Each method has different requirement of consistency, viscosity and slump retention. 6. Durability: Air entrained concrete in Freeze Thaw zones, use of pozolans in marine environment, low permeability concrete in areas with heavy rain fall, use of internal curing agents or Shrinkage Reducing Admixtures in concretes prone to self- desiccation, are some of the examples, how the durability needs to be accounted at mix design stage. 7. Economy: The skill of a concrete technologist is in designing the best mix which meets optimum performance requirements within the allowable economy / cost. Low cost resources such as sand and gravel, need to be taken from the closest possible quarry. Major contribution to the cost of these materials, comes from freight and handling. Adjustment in gradation of these aggregates and minimizing the cement content will automatically result, in good economy of the mix. 8. Aesthetics and Appearance: Whether you require a plain finish or a texture, coloured concrete or a stamped finish, all such factors need to be considered during finalizing the mix design. Effect of ingredients on Properties of Concrete Cement Grade of cement decides maximum achievable compressive strength of a mix. Type of cement generally decides exposure conditions as well as water demand and compatibility with admixtures. Fineness of cement is important to decide water demand, initial workability, setting time as well as retention of slump. Alkalinity is responsible for both short term and long term performance parameters. IS Codes : 12269, 456, 8112 ASTM codes : C The Masterbuilder - July

4 Fine Aggregates Fineness modulus and gradation of sand plays an important part in deciding the workability and cohesiveness of concrete mixes. Shape determines the flow, segregation and bleed characteristics. Absorption value determines the amount of water needed for adjustment after deciding free water content. Reactivity if any mainly with alkali needs to be mitigated using proper means such as pozolans, Lithium Silicates etc. Density which is generally determined by mineralogy affects yield. Coarse Aggregates Size of aggregates decides the amount of mortar required to coat all coarse aggregates. It also decides the maximum thickness up to which the concrete can be cast. Heavy density aggregates are generally used in radiation shielding concrete. The shape and size of aggregates is also critical in achieving desired workability and cohesiveness. Coarse aggregates can have round, angular, or irregular shape. Rounded aggregates because of lower surface area will have lowest water demand and also have lowest mortar/paste requirement. Hence they will result in most economical mixes for concrete grades up to M35. However, for concrete grades of M40 and above the possibility of bond failure will tilt the balance in favour of angular aggregate with more surface area. Flaky and elongated coarse aggregate particles not only increase the water demand but also increase the tendency of segregation. Flakiness and elongation also reduce the flexural strength of concrete. IS Codes : IS 383 ASTM Codes : C 33 Admixtures Admixtures play a very important role in today s concrete industry. Faced with a lot of environment, space and resource constraints and add to that the challenges of the modern day structural requirements and design diversities, no concrete today can really be considered without the use of chemical admixtures. We are having a detailed discussion on this topic at the end of this article. IS Codes : IS 9103 ASTM Codes : C-494, C-1017, C-260 Pozolanic Materials Use of pozolanic materials has now become common. Both separate additions as well as blended cements are available. Microsilica (Fumed Silica), Fly ash, Slag, Meta Kaolin, Risk Husk Ash are predominant. Various specifications and test methods are used to determine the quality of these materials. IS Codes : IS 3812 ASTM codes : C-1240, C-311, C-441, C-618, C-989 There are different approaches to concrete mix design. We are showing a typical example of M-30 grade concrete, designed as per guidelines from IS : Here we have used KEM SUPLAST 128 UT which SNF based Concrete Super plasticizer conforming to ASTM C-494 type G and IS Mix Design Calculations as per IS 10262:2009 Stipulation for Proportioning Grade of concrete 30 Type of Cement OPC Type of mineral admixture Fly Ash Maximum nominal Size of aggregate 20mm Minimum cement content 320Kg/cum Maximum water- cement ratio 0.45 Type of exposure Severe Method of concrete placing Pumping Degree of Quality Control Very Good Type of aggregate Crushed Maximum cement(opc) content 450Kg/cum Chemical admixture type Superplasticizers Chemical Admixture brand KEM SUPLAST128 UT Test Data for Material A) Cement a) Cement brand used U/T OPC b) Specific Gravity of Cement 3.15 B) Fly Ash a) Fly ash brand used Dirk b) Specific Gravity of Fly Ash 2.15 C) Water a) Source Local Sieve Size % Passing % Passing Specification as per IS mm 20 mm 10 mm 20 mm Combined 100% 10 mm 20 mm 40 mm mm mm mm mm The Masterbuilder - July

5 IS Sieve % Passing Individual Combination Combine % Lower Limit Upper Limit R.Sand C.Sand Pan F.M= 3.17 Zone= ZoneI D) Admixture a) Type & brand Kemsuplast 128UT b) Specific Gravity of Admixture 1.24 c) % of dosage 1.2% E) Coarse Aggregate a)source Local Source b) Specific Gravity (SSD) 20mm 2.80 c) Specific Gravity (SSD) 10mm 2.78 d) Combine Sp. Gr e) Water absorption % 20mm 0.0% 10mm 0.0% f) Free surface moisture 20mm 0.0% 10mm 0. 0 % Coarse aggregate Sieve Analysis F) Fine Aggregate River Sand a) Source of River sand Local Source b) Specific Gravity of River Sand 2.65 c) Water absorption 0.0% d) Free moisture 0.0% Crushed Sand a) Source of Crushed sand Local Source b) Specific Gravity of Crushed Sand 2.68 c) Water absorption 0.0% d) Free moisture 0.0% e) Combine Sp gr of total F.A. 2.7 Combination of Fine Aggregate Fractions Design Calculation: Target Mean Strength X 5 fck+1.65x SD Selection of Water/Cement ratio From Table 5 of IS 456, maximum water-cement ratio for M30 =0.45 On trial experience base adopted water- cement ratio for this mix design is < 0.45 Hence OK Selection of Water content From table 5 of IS 456, maximum water content for 20m 50 mm slump for 20 mm MSA =186Kg/cum Estimated water content for 120 mm slump =208Kg/cum Since use of superplastisizer will reduce 15 20% of water & above on trial with present superplastisizer water reduction achieved is 17% So the water content I =173Kg/cum Calculation of Cement content in design mix Water-cement ratio =0.42 water content =173Kg/cum Total Cementitious content will be =410Kg/cum As per IS 456 minimum cementitious content is =320Kg/cum 410Kg/cum > 320Kg/cum Hence OK Total cement (OPC) content =308Kg/cum Fly Ash content % by weight of 25% =103Kg/cum I.S. Sieve Size in C.A % F.A% Total % Specifications 56.00% 44.00% passing Min Max The Masterbuilder - July

6 Proportion of volume of coarse aggregate& fine aggregates From table 3 of IS 10262(2009), Vol of CA corresponding to 20 mm size aggregate and FA (Zone I) for W/C ratio of 0.50 = In present case W/C ratio is 0.42 Therefore Vol of CA is required to be increase to decrease the FA content as the W/C ratio is lower by 0.08 the proportion of vol of CA is increased by 0.02 So corrected vol of CA=0.62 For Pumping Mix this value should be reduced by 10% So Total vol of CA =62 X 0.9 =0.56cum Total Vol of FA = = 0.44 cum Percentage of Fine aggregate and Coarse aggregate are arrived after conducting sieve analysis and combined grading as follows Percentage of C.A. To total aggregates =56% Combination ratio of 20mm:10mm =50:50 % Percentage of F.A. To total aggregates =44% Combination ratio of R.Sand : C.Sand =30:70 % Combination Of All Aggregates Fractions Fine Aggregates % =44% Coarse Aggregates % =56% Mix Calculations a) Volume of Concrete = 1cum b) Volume of Cement = Cement Qty./Sp.Gr x 1 /1000 = 0.098cum c)volume of Fly ash = Fly Ash Qty./Sp.Gr x Constituent Materials Material Cemmentitious Aggregates Material Dry wt Moist. W.A. SSD & Source kg/m 3 % % kg/m 3 OPC Fly Ash CA2: 20 MM CA1: 12 MM FA2: R.Sand FA1: C.Sand Admixture Kem Suplast 128 UT Water Local source Theoretical plastic density - = Water / Cement Ration = Test Results Concrete Temperature 27 o C Mix Cohesive Workability Initial Slump - Slump After 60 Min - Slump After 120 Min - Slump After 150 Min - Slump After 180 Min - Collapse Collapse 210 mm 180 mm 140 mm 1 /1000 =0.047cum d) Volume of = Admixture Qty./Sp.Gr x 1/1000 =0.004cum e) Volume of Water =0.173 f) Total cementitious material + water + admixture = 0.322cum g) Total volume of all aggregates =1-f =0.678cum h) Total quantity of coarse agg. =g x Vol of CA x Sp. Gr. Of C.A x 1000 =1062Kg/cum Quantity of 20mm =531Kg/cum Quantity of 10mm =531Kg/cum I) Total quantity of Fine agg. =g x Vol of FA x sp. Gr. Of F.A x 1000 =793Kg/cum Quantity of River Sand =238Kg/cum Quantity of Crushed Sand =555Kg/cum Compressive Strength Data Age Weight Density Load Strength N/MM 2 Days KG KG/Cum KN Strength Average % Note The above Mix is for guidelines purpose only, all the aggregates are considered as in SSD condition & their Sp. Gravity are considered according to materials available in Mumbai region. For practical purpose it is always advised to conduct the confirmatory trials at site conditions. Our representatives will be available for further advise if necessary. 74 The Masterbuilder - July