Elkem Microsilica for Superior Concrete. How to improve strength and durability of concrete

Transcription

1 Elkem Microsilica for Superior Concrete How to improve strength and durability of concrete

2 ELKEM MATERIALS GIVES YOU ADVICE ON THE USE OF ELKEM MICROSILICA. Elkem Microsilica is a mineral additive that improves the properties of both fresh and hardened concrete. Elkem Materials has long experience in the use of Elkem Microsilica in concrete, and can offer a total package to the user. This package includes advice and guidance in the manufacture of concrete for various purposes and in the necessary equipment, as well as training in the use of Elkem Microsilica and technical assistance in the development of new applications. The purpose of this publication is to introduce benefits of Elkem Microsilica and to give general guidance for its proper use as an additive. BACKGROUND. Elkem Microsilica is a pozzolan. Pozzolans have been used for ages. Classical Greeks and Romans used cements made from volcanic ash that had pozzolanic properties. The Coliseum is just one example of the durability and lasting strength of such ancient concrete structures. Elkem Microsilica, because of its fineness and high reactivity, is a particularly valuable pozzolanic material. Thus it finds widespread use in the production of quality concrete. THE PRODUCTION OF ELKEM MICROSILICA. Elkem Microsilica is manufactured in silicon and ferro-silicon smelting plants. Elkem Microsilica has a bulk density of kg/m 3 from the furnace. In this form the material is difficult to handle, store and transport. Therefore it is usually processed further by densification or by slurrification. In the densification process, the bulk density of the material increases to more than double the original value. The other main form of delivery is as a water-based slurry, where, at 50% dry content, one cubic meter of slurry will contain about 700 kg Elkem Microsilica. ELKEM MICROSILICA 2

3 THIS IS ELKEM MICROSILICA. Elkem Microsilica contains 85-98% amorphous silicon dioxide (SiO2) in the form of microscopic spherical particles. The average diameter of these particles is micron; this is about the same as particles in cigarette smoke. A consequence of the diminutive particle size is that Elkem Microsilica has a specific surface area which is about 100 times that of cement. The large surface area and the high content of amorphous silicon dioxide gives Elkem Microsilica superior pozzolanic properties. Since the silicon dioxide in Elkem Microsilica is present in an amorphous, not a crystalline state, Elkem Microsilica is not considered to be hazardous to health. CEMENT GRAIN GRAVEL SAND FILLER MICROSILICA CEMENT WATER PASTE AGGREGATES WHAT IS CONCRETE? Concrete is the most widely used construction material today. About 1 m 3 /year is used for every man, woman and child on the planet. This universal acceptance follows from its basic properties: it is easy to shape, it is readily available locally, and it has strength and durability. Concrete contains cement, water, aggregates (sand and gravel), chemical admixtures and mineral additives. Elkem Microsilica is a mineral additive that gives superior properties to concrete. ADMIXTURES A B THE HYDRATION PROCESS WITHOUT ELKEM MICROSILICA. Hydration of Portland cement is the reaction between cement and water which makes calcium silicate hydrates (CSH). CSH is the gel or binder that gives concrete its strength. Calcium hydroxide forms as a by-product in the hydration process. Calcium hydroxide (15-25% of hydration products) is a weak and soluble material that does not contribute to the strength or the durability of the concrete. C A-Portland cement B-Water C-CSH D-Calcium hydroxide D 3

4 ELKEM MICROSILICA IN THE HYDRATION PROCESS. Elkem Microsilica reacts with the calcium hydroxide that forms during the hydration of cement. This pozzolanic reaction creates an increased amount of CSH binder. Thus the content of weak calcium hydroxide decreases, while the content of CSH, which provides strength and durability, increases. This is one of the main effects of Elkem Microsilica. A B AGGREGATE ELKEM MICROSILICA ELKEM MICROSILICA FILLS THE GAPS IN CONCRETE. One of the basic principles of concrete technology is the need for proper packing of the particles. The purpose is to obtain the smallest possible volume of empty or water-filled space in the material. The tiny particles of Elkem Microsilica help the particle distribution and the packing of the system. Very strong surface forces act between the particles of Elkem Microsilica. These forces may prevent proper dispersion of the particles in the fresh concrete. Accordingly, dispersing admixtures are value components of microsilicaconcrete. CEMENT WATER REDUCING ADMIXTURES. Good workability of a concrete often requires more water than needed for hydration of the cement. The excess water creates voids in the concrete. Water reducing admixtures make it possible to reduce the amount of excess water. These chemicals reduce the interparticle forces and make the particles more mobile and more densely packed. Such admixtures are essential to the manufacture of high performance concrete, including concrete with Elkem Microsilica. 4

5 ORDEMs CEMENT FILLER FINE AGGREGATE CEMENT with Elkem Microsilica FILLER FINE AGGREGATE AGGREGATES. Concrete contains up to 70% aggregates by volume. Consequently, the quality and properties of the aggregates are of major importance. Aggregates are divided into fine and coarse fractions. Occasionally filler materials are also used. This is material with a maximum grain size of mm. The small particle size of Elkem Microsilica reduces the need for fine aggregate and filler. COARSE AGGREGATE COARSE AGGREGATE PROGRESSO ELKEM MATERIALS FOR FREEDOM OF CHOICE AND SECURE SUPPLY. Elkem Materials has production facilities at many sites around the world. Distribution facilities for Elkem Microsilica have been established in several locations. Our experience shows that the slurry forms of Elkem Microsilica have several benefits in transport, dispensing and effect in concrete. For some users, other forms of the product may be more appropriate. Accordingly, Elkem Microsilica is also available as powder: densified or undensified. All products may be shipped in a variety of packaging, adapted to the need of the customer. Several production and distribution facilities offer the user the freedom of choice and secure supply. ELKEM MICROSILICA: STORAGE AND HANDLING. Elkem Materials can provide help in the design of practical solutions for the storage and handling of Elkem Microsilica. 5

6 MIXING. Elkem Microsilica is available as a slurry or as a dry powder. When using slurry, the mixing time required is roughly that used for ordinary concrete without microsilica. For dry, densified powder the mixing time should be increased, but the need for additional mixing will vary with mix design and the actual mixer used. CLEANING OF EQUIPMENT. The use of Elkem Microsilica can make concrete more cohesive and sticky. This can make the concrete more difficult to remove during cleaning of mixing and other handling equipment. Good housekeeping in the production plant will assure less wear on the equipment and effective blending of the ingredients of the concrete. The mixer needs careful cleaning frequently, during breaks as well as at the end of the shift. The use of form release agents will reduce the labor required to maintain a clean operation. WORKABILITY. The workability of concrete often improves when the slump increases. Since addition of microsilica will give fresh concrete higher shear strength, microsilica concrete can appear stiffer and more difficult to move at a given slump. However, microsilica also decreases viscosity, so that at given slump, microsilica concrete will flow more easily. If concrete with Elkem Microsilica is to behave as normal concrete, one should increase the slump slightly (with admixtures). This can be done because Elkem Microsilica provides a stable fresh concrete with much reduced tendency to separation and segregation. In self-compacting concrete and concrete for pumping, the use of Elkem Microsilica gives improved robustness and stability; therefore improved performance. 6

7 ELKEM MICROSILICA PROVIDES REDUCED SETTLING AND MORE HOMOGENOUS CONCRETE. A layer of water will normally form on the horizontal surface of concrete without Elkem Microsilica. This is the consequence of settlement in the concrete and water rising to the surface. This phenomenon is commonly referred to as bleeding. The bleed-water will often transport fine particles such as cement and filler, and form a slurry on the surface of the concrete. After the slurry has dried, this represents a weak layer on the surface of the concrete. Too wet or excessively vibrated concrete can segregate. The aggregates sink so that the distribution of particles in the concrete is irregular. (Bleeding also increases.) Elkem Microsilica prevents both bleeding and segregation. This is mainly due to the strong internal forces caused by the large surface of the particles in Elkem Microsilica, and partly due to improved particle packing. ELKEM MICROSILICA MINIMIZES BLEEDING UNDER LARGE AGGREGATE PARTICLES. Bleeding and settling under large grains of aggregate are common in concrete. The bleeding is reduced or eliminated by the use of Elkem Microsilica. This gives improved bond between paste and aggregate as well as a less permeable concrete. ELKEM MICROSILICA PREVENTS BLEEDING UNDER REINFORCEMENT. Settlement cracks can sometimes be observed over the reinforcing steel in reinforced concrete due to bleeding and settling (as with large grains of aggregate). Elkem Microsilica reduces bleeding. This gives improved bond and better protection against corrosion of the reinforcement. 7

8 PROVIDE PROTECTION AGAINST PLASTIC SHRINKAGE. When water evaporates from the concrete surface at a higher rate than it can be renewed from the concrete underneath, the surface dries. The suction of water creates strong capillary forces that act on the material and attempt to pull the particles closer together. If the tensile strength of the concrete is unable to resist these forces (as will normally be the case in fresh concrete) the consequence will be cracks that run from the surface to the interior of the concrete. Since this shrinkage and cracking process takes place while the concrete is in a plastic state, the phenomenon is called plastic shrinkage. A layer of bleed-water on the surface of fresh concrete may protect the concrete against harmful drying. Since Elkem Microsilica prevents bleeding, concrete with microsilica may be subject to plastic shrinkage. The practice which eliminates plastic shrinkage is the same for all concretes: proper curing, such as curing compound, water spray, cover with plastic sheeting and hessian or ponding. DEGREE OF CURING CONCRETE SETS no Elkem Microsilica with Elkem Microsilica CURING DEVELOPMENT OF STRENGTH. Curing of concrete begins after the initial set. The rate of strength gain is initially large and decreases after the first few days. As a rule of thumb, the concrete reaches 80% of its final strength after 28 days curing in water at 20 degrees C. For a given 28-day strength, concrete with Elkem Microsilica may display a somewhat reduced rate of strength development just after pouring. However, after less than 2 weeks, the rate of strength gain will be higher than in concrete without microsilica. The relations between the rates of strength development depend on several factors. These factors include concrete quality, type and amount of cement and curing temperature. 1 hour 5 h 1 day 28 d HEAT no Elkem Microsilica with Elkem Microsilica HEAT OF HYDRATION. The hydration of cement (its reaction with water) is a process that releases heat. The pozzolanic reaction between Elkem Microsilica, calcium hydroxide and water also generates heat in such a fashion that the hydration of 1 kg. of microsilica usually gives about the same heat as the hydration of 1 kg of cement. Thus, for the same characteristic strength (cement replacement at 1 kg microsilica to 3 kg of cement), concrete with Elkem Microsilica will develop less total heat than concrete without microsilica. This effect will reduce temperature loads in massive structures, but it also means that concrete with microsilica needs more attention if poured in winter. TIME 8

9 30 20 C EFFECT OF CURING TEMPERATURE. The rate of strength gain depends on the curing temperature. For a given 28-day strength (i.e. cement replacement by 1 part Elkem Microsilica for 3 parts cement), microsilica concrete will be more influenced by the curing temperature than ordinary concrete. Also, concrete with Elkem Microsilica responds well to curing at elevated temperatures, but more care must be taken in winter to ensure adequate strength of the concrete before removal of the formwork. STRENGTH no Elkem Microsilica with Elkem Microsilica CONCRETING AT LOW OR ELEVATED TEMPERATURES. For a given target strength (cement replacement, see above), concrete with Elkem Microsilica will develop less internal heat than concrete without microsilica. The rate at which the properties of concrete develop is also more sensitive to temperature if the concrete contains Elkem Microsilica. Therefore concrete construction at low temperatures using Elkem Microsilica as cement replacement should include: - Heating of the microsilica-concrete at the batching plant; and/or - Added insulation to protect the fresh microsilica-concrete against heat loss. Fresh concrete always needs protection in the form of insulation during winter; this is particularly important for concrete that contains Elkem Microsilica. At high curing temperatures, the effect is the opposite; concrete with silica fume will gain strength at a higher rate than comparable conventional concrete. ELKEM MICROSILICA INCREASES STRENGTH AND REDUCES PERMEABILITY. Elkem Microsilica improves particle packing in the fresh concrete, and the pozzolanic reaction leads to a replacement of weak calcium hydroxide by strong CSH-gel. These two effects give concrete with Elkem Microsilica improved strength and permeability. Elkem Microsilica also improves the interface between paste and aggregate; a region where in conventional concrete channeling often occurs and calcium hydroxide tends to accumulate. 9

10 ELKEM MICROSILICA MEANS INCREASED COMPRESSIVE STRENGTH. If the cement and water contents of a given concrete are constant, the addition of Elkem Microsilica means a significant increase in compressive strength. (In a cement replacement application, Elkem Microsilica may replace some of the cement for a given target 28-day strength) TENSILE STRENGTH. The tensile strength of concrete will be influenced by the presence of weak zones, such as bleed-pockets under grains of aggregates. In concrete with Elkem Microsilica the number of such areas will be reduced. However, as a general rule of thumb, microsilica concrete has the same relation between compressive strength and other mechanical properties as does conventional concrete. CORROSION RISK CHLORIDE PENETRATION WITH MICROSILICA ELECTRICAL CONDUCTIVITY RATE OF CORROSION NO MICROSILICA ELKEM MICROSILICA PROTECTS AGAINST CORROSION OF REINFORCEMENT. The corrosion process has two distinct stages: initiation and propagation (growth). Initiation means that the protective passive layer on the steel surface is destroyed; the two common causes are chloride penetration and carbonation of the concrete. Elkem Microsilica in concrete gives a more closed pore structure and the concrete is much less permeable than concrete without microsilica. For the process of corrosion initiation, this means that microsilica concrete is more resistant to the ingress of chlorides that can promote corrosion. Elkem Microsilica has no particular influence on the rate of carbonation. Regardless of the quality of the concrete, corrosion initiation will often take place. One example is reinforcement in the bottom of cracks. In the propagation stage, the rate of corrosion is normally governed by oxygen transport or internal electrical resistance in the concrete. Concrete with Elkem Microsilica has an internal electrical resistance that is several times that of conventional concrete. The increase in electrical resistance means a corresponding reduction in the possible rate of corrosion. 10

11 ELKEM MICROSILICA FOR REDUCED PERMEABILITY. A concrete with low permeability is more durable than a more permeable one. The use of Elkem Microsilica gives very low permeability. Concrete with Elkem Microsilica is therefore particularly suited to aggressive environments. These include sewage conduits and treatment plants, agricultural applications, industrial environments, marine or coastal structures and structures exposed to chlorides, such as bridges or parking structures. ELKEM MICROSILICA IMPROVES THE FROST RESISTANCE OF CONCRETE. Freezing water in the interior of concrete can expand and cause disruption of the concrete. In the presence of salt on the concrete surface, the danger of frost damage is particularly severe. Frost damage is countered by the use of entrained air. The air voids provide relief for water escaping from water-filled pores where the water is freezing. There are two reasons for good frost resistance in properly proportioned concrete with Elkem Microsilica: The stability of the fresh concrete promotes good distribution of the entrained air and prevents the disruption of the air-void system during transportation and handling of the concrete. The low permeability of the concrete can prevent water from entering the concrete.! Note that structural detailing is of outstanding importance in preventing frost attack. Proper drainage prevents ponding on the concrete surface. Ponding means that the concrete will become saturated sooner or later. In saturated concrete frost damage is likely. FROST DAMAGE NUMBER OF FROST CYCLES no Microsilica w Microsilica ELKEM MICROSILICA PROMOTES RESISTANCE AGAINST CHEMICAL ATTACK. Most forms of chemical attack on concrete are typified either by leaching of calcium hydroxide or by ingress of harmful, disruptive substances, such as sulfates or nitrates. Since Elkem Microsilica reduces the amount of soluble calcium hydroxide, the risk of leaching is reduced. The addition of Elkem Microsilica reduces the permeability of concrete; this retards the arrival of deleterious substances to the interior of the concrete. Consequently, concrete with Elkem microsilica will possess superior resistance to chemical attack. 11

12 ALKALIS WATER reactive aggregates alkali-silica gel ELKEM MICROSILICA AS A MEASURE AGAINST ALKALI-SILICA REACTION (ASR). Alkalis in solution and as solids are normal constituents of hardened concrete. If the aggregates contain reactive silica, these alkalis can, in the presence of water, form a gel by reaction with the silica. The gel can cause internal expansion which is deleterious to the concrete. Elkem Microsilica promotes reduced permeability and effectively binds the alkalis in the hardened concrete. Thus Elkem Microsilica eliminates the risk of alkali-silica attack. In Iceland, ASR was an enormous problem, however all concrete since 1979 has contained microsilica, and since then, ASR has not been a problem. expansion, cracking and concrete distress DRYING SHRINKAGE. Concrete is a porous material that adjusts to the ambient humidity. Changes in moisture content influence internal forces of attraction in the material. Cured concrete will therefore shrink and swell as it exchanges moisture with the environment. Drying shrinkage is the reduction in volume that is the consequence of drying of the concrete. It is usually distinguished between reversible and irreversible shrinkage. The extent of drying shrinkage depends very much on the water content of the concrete, and there will not be any significant difference in behavior between concretes with or without Elkem Microsilica. ELKEM MICROSILICA IMPROVES BOND. Elkem Microsilica reduces settling and bleeding. In addition, the packing density of particles improves, including the wall effect on the interface between small and large particles. These factors mean improved bond to embedded materials and substrate, such as old concrete or rock. 12

13 STRENGTH Removal of formwork REMOVAL OF FORMWORK. If Elkem Microsilica is added to an existing mix design, the strength necessary for removal of formwork will be gained earlier than for the original concrete without microsilica. If Elkem Microsilica replaces some of the cement, the user should be aware that concrete with microsilica has somewhat reduced very early strength (24 hrs). This is particularly the case when the fresh concrete is cold when poured. The rate of strength development can be increased by the use of rapid-hardening cement, by heat treatment and by the use of insulation to prevent heat loss from the concrete. no Elkem Microsilica with Elkem Microsilica TIME ELKEM MICROSILICA MEANS LESS PATCHING. By using Elkem Microsilica, the slump of concrete can be increased significantly without increasing the risk of segregation and bleeding. This means improved compaction of the fresh concrete and accordingly less repair after removal of the formwork. Joints between pours: Bleeding will often cause the formation of a weak layer on the surface of a horizontal joint of concrete. This layer must be removed before the next pour can be made. Elkem Microsilica reduces the bleeding to such a degree as to prevent the formation of this weak layer. VACUUM TREATMENT. Vacuum treatment of a concrete surface removes water from the concrete surface and the nearby interior of the concrete. This treatment provides locally improved compaction and reduction of the water/cement ratio at and near the surface. Since bleeding is negligible in concrete with Elkem Microsilica, such concrete does not need and should not be exposed to vacuum treatment. Additionally, since fines can cause clogging and other problems in the vacuum process, concrete that is to be vacuum-treated should not have a high content of fines. This is another reason why concrete with microsilica is not suited to vacuum treatment. 13

14 ELKEM MICROSILICA FOR BETTER SHOTCRETE. In Norway Elkem Microsilica has been used in shotcrete for many years. The reason is that Elkem Microsilica enhances important qualities of the shotcrete, such as: Improved internal cohesion so that the concrete becomes more sticky and gives improved adhesion, both to base and to underlying shotcrete. This means: -Reduced rebound by a factor of The thickness of the layer in each pass can be increased, up to 200 mm or more. -The use of fibers is easier. Since thicker layers can be established and still adhere, the amount of accelerating admixtures can be reduced. This means that for a given strength, the content of cement need not be increased to compensate for the effect of the accelerating admixture. The compressive strength is improved. The concrete is easier to pump. The enhanced properties that Elkem Microsilica provides are particularly well utilized in shotcrete technology. These benefits apply in both the wet and the dry processes. ELKEM MICROSILICA FOR UNDERWATER CONCRETING. For concreting jobs under water, the internal cohesion of a concrete with Elkem Microsilica means that it has significant reduction in tendency to wash-out. Since the workability and slump can be increased, the tremie pipe can be sunk further into the in-place concrete than when placing conventional underwater concrete. This furthers improved filling of the form and limited risk of water intrusion. The displacement of water along the sides of the formwork improves and the top of the pour will be more level. QUALITY CONTROL OF CONCRETE WITH ELKEM MICROSILICA. The quality of concrete with Elkem Microsilica is maintained with the same tools and procedures as the quality of conventional concrete. 14

15 Elkem Materials The World s Leading Microsilica Supplier Elkem Materials markets Elkem Microsilica, an ultrafine powder created during the production of ferrosilicon and silicon metal. Microsilica is today used in prestigious projects throughout the world wherever concrete durability is the prime concern. Elkem Materials is part of Elkem ASA, and was set up in 1982 to market and promote microsilica into a variety of industries on a world-wide basis. One priority was that the products and services promoted should contribute to raising environmental standards within Elkem and the countries it operates in. This emphasis continues to be maintained as Elkem continuously focuses on: Excellence Innovation Quality With its advanced research and development facilities, Elkem Materials is committed to further improvements in new product technology and in developing new solutions and product improvements for our customers. This commitment is consistent with Elkem Materials role in protecting our environment, which will continue to remain a main focus of the company. Elkem A World Class Producer Elkem is one of Norway s largest industrial companies. Many of today s most essential technical devices and consumer goods have components which contain Elkem products. These products maintain the highest quality standards and are in demand in a steadily growing market. Elkem s main products are ferroalloys, silicon metal, aluminium and microsilica. Elkem s production facilities around the world provide the world market with a wide range of metals and materials through more than 15 sales offices in 12 countries. Elkem is one of the largest producers of ferroalloys in the world, particularly with respect to ferrosilicon. Elkem is a leading supplier of ferrochrome to the European producers of stainless steel, and is the western world s largest manufacturer of silicon metal to the electronics and chemical industries. Elkem s aluminium production plays a major role in a number of specialised markets in Europe. The company s operations are based on clean, replenishable hydroelectric power, and Elkem itself produces a substantial amount of energy.

16 El k em M a terials a t y our ser vice Elkem Materials is the world s largest supplier of microsilica and we are the leader in marketing and implementing microsilica technology worldwide. We also actively participate in international development of technical and quality standards. Elkem Materials offers a total package to our customers.this includes advice and guidance with respect to materials handling and technical assistance in the manufacture of high performance concrete for a variety of applications. To our customers Elkem Materials stands for a reliable supply of quality-assured Elkem Microsilica, either through our sales offices around the world or through local representatives. 50 years of research and practical use have led to Elkem Microsilica becoming a key component for the enhancement of concrete properties such as: improved pumpability less segregation increased strength reduced permeability to water and chlorides high sulfate resistance better protection of steel reinforcement. Elkem Microsilica is now used in more and more projects wherever durability is the prime concern. The world s most comprehensive internet site dedicated to microsilica concrete Disclaimer : The information given is based on many years of research and field experience and is accurate to the best knowledge of Elkem Materials. However, due to the numerous factors that can affect the performance of a concrete, with or without microsilica, Elkem Materials offers this information without guarantee and accepts no liability for any direct or indirect damage from its use. If further information or assistance is required, please contact your local representative or the office number provided below. Representative: Tel.: Fax: sales@spec-kw.com Elkem Microsilica is a registered trademark owned by Elkem Materials NORWEGIAN ACCREDITATION No. S03 NS-EN ISO 9001 CERTIFICATED FIRM Elkem ASA, Materials, P.O.Box 8126, Vaagsbygd, N-4675 Kristiansand Norway Switchboard: Telefax: microsilica.materials@elkem.no