Admixtures and hydraulic binders: design and use for performance in concrete repair

Transcription

1 Concrete Science and Engineering, Vol. 3, June 2001, pp UEF CONFERENCE PAPERS Admixtures and hydraulic binders: design and use for performance in concrete repair R. T. Coverdale, F. R. Goodwin, R. C. First and S. A. Farrington SKW-MBT Management, Inc Chagrin Blvd Cleveland, OH USA. ABSTRACT Material selection is of critical concern when designing a concrete repair. The materials selection process in repair is probably more important than in new construction, because demands on repair materials are higher in most cases. A general background on repair material selection is presented. Discussion focuses on important material properties and their relative importance in different types of repair. A review of hydraulic cement binders is presented that discusses the chemistry, properties, advantages and disadvantages, applications for different binder systems, and relative performance comparisons. A similar menu of topics is presented on a variety of admixtures, including water-reducing admixtures, accelerators, retarders, corrosion inhibitors, shrinkage-reducing admixtures, and foaming additives. The utilization of different binders and admixtures in repair material design is also discussed. 1. INTRODUCTION Concrete repair is a complex process. Concrete repair solutions involve an overall problem analysis and development of an effective repair strategy [1]. As a part of the repair strategy, selecting the appropriate material can be complicated, and appropriate choices can be critical to the ultimate success of the repair. Material selection depends on many factors, which can include the intended purpose of the repair, its performance requirements, and the installation requirements. Detailed discussions of material selection for repair have been published elsewhere [1, 2], however, some discussion and examples are included here as a context to describe the finer details involved with the selection of hydraulic binder materials for concrete repair. Once the performance and installation requirements of a repair material are determined, the designer can begin to determine what type of repair material should be specified. This paper presents information about hydraulic binders and admixtures and how they are used together to create repair materials with a variety of performance and handling properties. It is not always possible to have a repair material that meets all of the desired performance characteristics, so alternatives and compromises are also discussed. 2. MATERIAL PROPERTIES AND IMPORTANCE FOR REPAIR APPLICATIONS With damaged or deteriorated concrete, many decisions are required to determine how best to restore function to the concrete structure [1]. The first decision is whether to repair, replace, or protect the structure. If replacement is an option, cost, downtime, aesthetic duplications and constructability often prohibit the complete replacement of a structure. Protection of a structure extends its service life by covering and isolating the failure, mitigating the driving forces of the failure, and continued maintenance of the structure. Repair must address the cause of deterioration, and can include removal of the deteriorated section and replacement with an appropriate material. Repair applications often require materials that possess specific properties. As such, repair designers must be intimately aware of material properties in order to specify them appropriately. Four important properties of a repair material are bond strength, volume stability, modulus, and permeability. Strength and constructability need also be considered. Bond strength is required for the repair material to become an integral part of the structure. Volume stability is essential to maintain the integrity of the repair and keep it in place. Modulus determines the ISSN /01 RILEM Publications S.A.R.L. 110

2 Coverdale, Goodwin, First, Farrington ability of the repair material to accept load, resist impact, and resist cracking if restrained shrinkage occurs. In the absence of cracking, permeability determines the ingress of foreign substances, such as water, carbon dioxide, chlorides, and sulfates, into the repair. By optimizing bond strength, volume stability, modulus, and permeability for a given repair situation, durability is nearly always assured. Freeze/thaw resistance, tensile strength, and compressive strength are properties that result from optimizing for the repair situation, rather than the design criteria that is commonly used in new construction. 3. INORGANIC BINDER SYSTEMS Ordinary Portland cement (OPC) By far, the most compatible repair materials are those that are most chemically similar to the material being repaired. As such, Portland cement-based materials are most commonly used to repair Portland cement concrete. Portland cement hydration may be summarized as: C 3 S + C 2 S + C 3 A + C 4 AF + H C-S-H + CH + aluminate hydrates Portland cement-based systems possess the advantages of low cost, known technology, high durability in many applications, and general compatibility with the concrete being repaired. The disadvantages of this binder system include shrinkage, poor acid resistance, brittleness, slow strength development, and relatively high permeability unless modified with additives. Calcium aluminate cement (CAC) Another binder system commonly used in concrete repair materials is calcium aluminate cement. The hydration reaction may be summarized as: CA + 10H CAH 10 This material possesses rapid strength development, refractory properties, sulfate resistance, and allows application at lower temperatures. However, CAC is significantly higher in cost and more limited in supply than OPC, and CAC may undergo a phase change under certain conditions that results in increased porosity, cracking and reduced durability of the material. The hydration of blends of CAC and OPC can produce accelerated hardening and early strength development. Calcium sulfoaluminate systems (CSA) Another binding system is based on calcium sulfoaluminate reactions. Ettringite is the primary reaction product for this system, whose hydration reactions are summarized as: OPC + CAC + CS + H ettringite C 4 A 3 S + CS + C + H ettringite The components of this system may be blended using calcium sulfate, Portland cement, and calcium aluminate cement or produced by pyroprocessing sources of lime, alumina, and sulfate into a clinker that is then ground to a fine powder. The most common application for this material is as an addition to systems, where rapid strength development, reduced shrinkage, and lower cost are typical benefits. Reduced durability in wet environments, expansive reactions with Portland cement, and reduced corrosion protection for embedded steel can occur, depending upon the quantity of sulfate in the binder system. Alkali-activated pozzolan Under certain conditions, pozzolans can be activated by high ph to produce rapid setting binders [3]. Advantages of these systems include rapid strength development, low-temperature hardening and otherwise similar performance to Portland cement-based binders. High shrinkage and efflorescence may be associated with this type of binder. Magnesium phosphate cement (MPC) Another binder system is based on the hydraulic reaction of magnesia and phosphate species. This chemical reaction may be summarized as: MgO + NH 4 H 2 PO 4 + H 2 O NH 4 MgPO 4 6H 2 O (struvite) The advantages of this binder system include very low shrinkage, very rapid strength development, high bond strength, curing in air, and application at very low temperatures. Disadvantages include high cost, generation of ammonia gas during hydration and hardening, incompatibility with calcareous aggregates and carbonated concrete substrates due to the neutral ph of the material. The formation of dittmarite rather than struvite, resulting in reduced strength due to a volume decrease, can occur with hardening or exposure above 68 C (155 F). 4. REVIEW OF ADMIXTURES Water-reducing admixtures Water-reducing admixtures are more appropriately called dispersants, as they are added to disperse fine particles in a fluid matrix. Low efficiency dispersants are referred to as plasticizers, while higher efficiency dispersants may be referred to as superplasticizers or highrange water reducers. The addition of a dispersant allows hydraulic binders to be fluid at lower water-to-cement 111

3 Concrete Science and Engineering, Vol. 3, June 2001 ratios, preserving strength and durability. Dispersion of the fine particles is a complicated process that can include electrostatic repulsion, and/or steric hindrance to prevent agglomeration of the hydrating cement grains. Many reviews on the subject of dispersants have been published [4-8]. There are three general classes of dispersants that have found widespread use in OPC, CAC, CSA, and alkaliactivated pozzolan binders. Sodium or calcium salts of lignosulfonic acid are the first class of dispersants. These lignosulfonates are obtained during wood pulping operations [10]. Lignosulfonates have been used as dispersants in Portland cement concrete for more than sixty years. Their efficiency as dispersants is low compared to the other classes of dispersants. This is due in part to the fact that higher dosages tend to retard the setting behavior of s. Refined lignosulfonates have been developed that have greater dispersing ability, but these are specialty products that have not found wide acceptance. Sulfonated naphthalene formaldehyde condensate salts and sulfonated melamine formaldehyde condensate salts, as sodium or calcium salts, are the second class of dispersants. These materials are synthetic polymers that have been used as dispersants in Portland cement concrete for more than thirty years. The polymers provide dispersant efficiency that is higher than that provided by lignosulfonates. Performance is affected by the molecular weight of the polymer and the cation present. Polycarboxylate salts are the third class of dispersants. Polycarboxylate salts, also synthetic polymers, provide the highest level of dispersant efficiency through a combination of electrostatic repulsion and steric hindrance [10, 11]. These materials have been developed as dispersants for Portland cement concrete in the last decade. Accelerating admixtures Accelerating admixtures provide quicker setting and/or quicker early strength development to binders. Their usage further allows binders to set and develop strength normally under cold weather conditions. Accelerating admixtures are generally grouped into chloride containing and non-chloride admixtures, as chloride ions promote the corrosion of steel reinforcement. In OPC systems, calcium chloride is the most potent accelerator. Examples of non-chloride accelerating chemistries include calcium salts of nitrate, nitrite, formate, thiocyanate, and thiosulfate. In CAC systems, lithium salts of hydroxide, carbonate, sulfate, and nitrate are the most potent accelerators, while potassium and sodium salts of hydroxide, carbonate, and sulfate are also effective. Retarding admixtures Retarding admixtures provide slower setting and/or slower strength development to binders. Their usage further allows binders to set and develop strength normally under hot weather conditions. While many materials can be used to retard OPC systems, the most widely used are gluconic acid and its salts, sugars and phosphonates. In CAC systems, chloride salts (except lithium chloride), citric acid, and boric acid are used as retarders. Boric acid and borate salts are effective retarders in MPC systems. Thickening agents Thickening agents can be used to increase the viscosity of a binder. This is important for imparting thixotropic properties to trowel-applied repair mortars as well as preventing segregation in highly fluid grouts. Thickening agents include gelling materials like cellulose, polyacrylates, and gums, inorganic materials like bentonite, and finely divided materials with high surface area like silica fume. Corrosion-inhibiting admixtures Corrosion inhibitors are used to prolong the time before steel reinforcement degrades from exposure to air, moisture, and any chloride ions present. There are two general categories of inhibitors that are widely used today: anodic and mixed anodic/cathodic inhibitors [7]. Anodic inhibitors interfere with the corrosion mechanism at the anode. Calcium nitrite is the most widely used anodic inhibitor, and also acts as an accelerator in OPC systems. Mixed anodic/cathodic inhibitors interfere with the corrosion mechanism at both the cathode and anode, generally through a film-forming mechanism. Organic inhibitors that are used are based on combinations of amines and esters are generally classified as mixed anodic/cathodic inhibitors. Shrinkage-reducing admixtures In OPC and related binding systems, the main hydration products are amorphous in nature, and shrinkage and cracking that occur from drying over time can be deleterious to the repair. Traditionally, materials that cause an expansion reaction such as ettringite formation are designed into the binder. This is more accurately described as shrinkage compensation. Integral admixtures can also be used to reduce the level of drying shrinkage in OPC systems [7]. These materials, typically glycol ethers, reduce the surface tension of the pore solution; the reduction of drying shrinkage may be linked to this reduction in surface tension [12]. 112

4 Coverdale, Goodwin, First, Farrington Foaming additives Foaming additives, including air-entraining agents, are used in binders to protect the binder from the effects of freezing and thawing of integral water, increase the fluidity of the binder, and control the strength of the binder. Foaming agents that have been used include those naturally derived and synthetic, including wood rosin derivatives, lignosulfonates, proteins, fatty acids, and detergents. Alternatively, proprietary defoaming materials may be added to binders to increase the density, alter the surface texture, or alter the air-void structure. Densifying agents Densifying agents may be defoamers that are used to minimize entrapped air, heavy weight aggregates to dampen vibrations or provide radiation shielding, and pozzolans that reduce the porosity of the binding matrix. Some heavy weight aggregates such as metallic particles can also increase the impact and abrasion resistance of mortars. Polymers Polymer additives are used for a variety of purposes in mortar formulations, including improving the workability, providing water repellency, lowering the elastic modulus, improving resistance to chemical attack, and improving bond to the substrate. Hydrophobes are a subclass of polymer additives, used primarily for imparting water repellency to mortar. Polymer additives include liquid latexes that form a film at the surface of the mortar such as such as vinyl acetate and acrylics, redispersible latexes, and water-soluble polymers with water retentive properties, including cellulosics and polyacrylamides. Other polymer additives are liquid resins that include epoxies and polyurethanes. Mineral admixtures Mineral admixtures are used to provide a number of benefits to binder systems that contain calcium hydroxide, by way of the pozzolanic reaction. These benefits include long term strength development, porosity and permeability reduction, and increased resistance to chemical attack. In all binder systems, mineral admixtures can be used as fillers, and some allow for reduced water content due to the spherical nature of the particles. Mineral admixtures that can react pozzolanically are lowcalcium fly ash, silica fume, and metakaolin. High-calcium fly ash and ground granulated blastfurnace slag are pozzolans that also have cementing properties. Fibers The addition of fibers can provide benefits to both binders and mortars, including reduced drying shrinkage, improved toughness, and cohesion. Steel, natural, and synthetic fibers are used in binding systems. Synthetic fibers include polyethylene, polypropylene, and polyolefin. Apart from the composition, the factors that are important for fiber selection are fiber geometry, fiber length, and material quality. 5. MATERIALS SELECTION PROCESS The materials selection process involves identifying the project objectives, determining the required material properties, and selecting repair materials. This process can involve making informed compromises. As discussed earlier, bond, volume stability, permeability and modulus are important properties of any repair material. Moreover, rate of strength gain and handling properties can affect the constructability and duration of a given repair project. It is often difficult for any single repair material to satisfy all of these requirements. The important design criteria for each of the main properties are discussed following. Bond While adequate substrate surface preparation is paramount to achieving high bond strength, reducing the viscosity of the repair material, incorporating an expansive material, or incorporating polymers may also help. Volume stability A repair mortar designed specifically for low shrinkage would contain a high aggregate content, an expansion mechanism and a shrinkage-reducing agent. The material would require a low water-to-cement ratio and would exhibit a low heat of hydration. Good placement and curing practices are always necessary to promote low shrinkage. Modulus Designing a repair material for high modulus generally requires a high binder content, the addition of pozzolans, and a low water-to-cement ratio. Polymer modification can be used to reduce the modulus of a repair mortar. The choice of aggregate can also affect modulus. Permeability Designing a repair material for low permeability is similar to designing for a high modulus: using pozzolans and a low water-to-cement ratio. Polymer modification and incorporation of other hydrophobic materials can also be used to reduce permeability. 113

5 Concrete Science and Engineering, Vol. 3, June 2001 Strength Designing for strength can require a choice between early age strength or ultimate strength. High early-age strength often demands a rapid-setting binder system along with set-controlling additives. Binder content plays a role in determining ultimate strength of a mortar. For high ultimate strength, slow early-age strength development may be required. Considerations Bond, volume stability, modulus, permeability, and strength are properties that must be factored into the design criteria for a repair material. Another important aspect concerns the ease of application, or constructability, of a repair mortar. Some issues that must be considered are material rheology, working and/or setting time, and rate of strength gain. Consideration of all of these properties can lead to a set of compromises. For example, it is difficult to design for very low shrinkage and very high strength. Alternatively, constructability may suffer as a result of designing for very high early strength/rapid hardening. Some of these compromises can best be illustrated by comparing a few typical mortar formulations designed for applications that require different properties. The first example in Table 1 compares formulations for structural and surface repair mortars. The primary differences between the two formulas are that the structural repair mortar requires a high modulus and volume stability, while the surface repair material does not have to be as robust with respect to properties, but does require good finishability. Another good example contrasts two grout formulations, a high-grade precision machine grout and a general construction grout in Table 2. In the case of the grout compositions, the strict application and physical property requirements of the precision machine grout mandate the use of sophisticated shrinkage compensation mechanisms, additives, aggregate grading for high fluidity without segregation, and high binder content coupled with low water-to-cement Table 1 Typical formulations for structural and surface repair mortars Structural Repair Mortar Low water/binder ratio High binder content Thickening agent Polymer fibers High-range water reducing admixture Air-entraining agent Shrinkage-reducing admixture Aggregate graded for pumping/spraying High-grade precision machine grout Surface Repair Mortar Moderately low water/binder ratio Moderate binder content Water reducing admixture Expansive mechanism Polymer modifier Aggregate graded for finishing Table 2 Typical formulations for high-grade precision machine and general construction grouts Low water/binder ratio High binder content Thickening agent High-range water reducing admixture Densifier Early- and late-age expansion mechanisms Aggregate graded for high fluidity General construction grout Moderate water/binder ratio Moderate binder content Water reducing admixture Expansive mechanism ratio for high strength and modulus. The lesser requirements for the construction-grade grout afford the use a less sophisticated formula. While the two formulation comparisons above illustrate how a product designer can adjust for various property requirements, each of the formulations listed are based on Portland cement. When considering the relative applicability of various binder systems, one can find their use in various applications. An example of an application that finds many binder systems in various formulations is a dowel bar retrofit mortar. There are currently dowel bar mortars on the market that are based on magnesium phosphate cement, blended Portland and calcium aluminate cement, calcium sulfoaluminate cement, and alkaliactivated pozzolan binders. 6. SUMMARY Materials selection for concrete repair applications is a complicated process. A variety of hydraulic binder systems and admixtures are available for designing materials targeted toward various and specific repair strategies. Prioritization of desired material properties is required to successfully select an appropriate repair material. 114

6 Coverdale, Goodwin, First, Farrington REFERENCES [1] Emmons, P. H., 'Concrete Repair and Maintenance Illustrated', 1st. Edn. (R.S. Means, Kingston, 1993). [2] 'Guide for Selecting and Specifying Materials for Repair of Concrete Surfaces', Guideline No (International Concrete Repair Institute, Sterling, 1996). [3] Brook, J. W., Factor, D. F, Kinney, F. D., McCallen, C. L, and Young, A. M., 'Cementitious Binders', United States Patent 5,556,458 (1996). [4] Edmeades, R. M., and Hewlett, P. C., 'Cement admixtures', in 'Lea s Chemistry of Cement and Concrete', Fourth Edition (Arnold, London, 1998) [5] Collepardi, M. M., 'Water reducers/retarders', in 'Concrete Admixtures Handbook', Second Edition (Noyes Publications, Park Ridge, 1995) [6] Ramachandran, V. S. and Malhotra, V. M., 'Superplasticizers', in 'Concrete Admixtures Handbook', Second Edition (Noyes Publications, Park Ridge, 1995) [7] Rixom, R. and Mailvaganam, N., 'Chemical Admixtures for Concrete', Third Edition (E&F Spon, London, 1999). [8] Scrivener, K. L. and Capmas, A., 'Calcium aluminate cements', in 'Lea s Chemistry of Cement and Concrete', Fourth Edition (Arnold, London, 1998) 749. [9] Gargulak, J. D. and Lebo, S. E., 'Commercial use of lignin-based materials', in 'ACS Symposium Series 742 Lignin: Historical Biological, and Materials Perspectives' (American Chemical Society, Washington, 2000) [10] Shonaka, M., Kitagawa, K., Satoh, H., Izumi, T. and Mizunuma, T., 'Chemical structures and performance of new high-range water-reducing and air-entraining agents', in 'ACI SP 173 Superplasticizers and Other Chemical Admixtures in Concrete', Proceedings of the Fifth CANMET /ACI International Conference (American Concrete Institute, Farmington Hills, 1997) [11] Uchikawa, H., Hanehara, S. and Sawaki, D., 'The role of steric repulsive force in the dispersion of cement particles in fresh paste prepared with organic admixture', Cem. Con. Res. 27 (1) (1997) [12] Ai, H. and Young, J. F., 'Mechanisms of shrinkage reduction using a chemical admixture', in 'Proceedings of the 10th International Congress on the Chemistry of Cement', Gothenburg June 1997 (Amarkai AB and Congrex Göteborg AB, Göteborg, 1997) 3iii