Understanding refractory api 936 reading iv by charlie Chong

Understanding REFRACTORY For API936 Personnel Certification Examination Reading 4- ACI Committee 547 My Pre-exam Self Study Notes 26th September 2015

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Refractory for Steel Making

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http://www.industryphotographer.com.au/steel-mill-photographer/

Refractory for Steel Making

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Refractory for Steel Making

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Refractory for Petrochem

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BODY OF KNOWLEDGE FOR API936 REFRACTORY PERSONNEL CERTIFICATION EXAMINATION API certified 936 refractory personnel must have knowledge of installation, inspection, testing and repair of refractory linings. The API 936 Personnel Certification Examination is designed to identify applicants possessing the required knowledge. The examination consists of 75 multiple-choice questions; and runs for 4 hours; no reference information is permitted on the exam. The examination focuses on the content of API STD 936 and other referenced publications.

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REFERENCE PUBLICATIONS: A. API Publications:  API Standard 936; 3rd Edition, Nov 2008 - Refractory Installation Quality Control Guidelines - Inspection and Testing Monolithic Refractory Linings and Materials.

B. ACI (American Concrete Institute) Publications:  547R87 - State of the art report: Refractory Concrete  547.1R89 - State of the art report: Refractory plastic and Ramming Mixes

C. ASTM Publications:  C113-02 - Standard Test Method for Reheat Change of Refractory Brick  C133-97 - Standard Test Methods for Cold Crushing Strength and Modulus of Rupture of Refractories  C181-09 - Standard Test Method for Workability Index of Fireclay and High Alumina Plastic Refractories  C704-01 - Standard Test Method for Abrasion Resistance of Refractory Materials at Room Temperatures

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Todayâ&#x20AC;&#x2122;s Exam Result Releases (ASNT) 3 passes 1 flop. It is bad, not too bad.

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Todayâ&#x20AC;&#x2122;s Exam Result Releases (ASNT) 3 passes 1 flop. It is bad, not too bad as long you have

not give-up!

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Charlie Chong/ Fion Zhang

Exam Result Releases 3 passes 1 flop. It is bad, not too bad.

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The Magical Book of Refractory

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Fion Zhang at Shanghai 26th September 2015

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Video Time- shotcrete refractory

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https://www.youtube.com/watch?v=s81LE7XXZ4A&list=PLey7s_Oct4OK9-7tMIx5cp9-RjSdetDTq

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Reading IV Content Study note One: Refractory Concrete: Abstract of State-of-the-Art Report Reported by ACI Committee 547 (haphazard book)  Study note Two:  Study note Three:  Study note Four:

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Refractory Concrete: Abstract of State-ofthe-Art Report Reported by ACI Committee 547 (haphazardly Clauses)

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Refractory concretes are currently used in a wide variety of industrial applications where pyreprocessing and/or thermal containment is required. The service demands of these applications are becoming increasingly severe and this, combined with the constant demand for refractories with enhanced service life and more efficient means of installation, has resulted in an ever expanding refractory concrete technology. ACI Committee 547 has prepared this state-of-the-art report in order to 547R-5 meet the need for a better understanding of this relatively new technology. The report presents background information and perspective on the history and current status of the technology. Composition and proportioning methods are discussed together with a detailed review of the constituent ingredients. Emphasis is placed on proper procedures for the (mixing?) installation, curing, drying, and firing. The physical and engineering roperties of both normal weight and light weight refractory concretes are reported, as are stateof-the-art construction details and repair/maintenance techniques. Also included is an indepth review of a wide variety of applications together with the committeeâ&#x20AC;&#x2DC;s assessment of future needs and developments.

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Chapter 1- Introduction 1.1 Objective of report The objective of this report is to provide a source of information on the many facets of refractory concrete technology. The report is intended as a unified and objective source of information to aid the engineer or consumer in categorizing and evaluating monolithic refractory concrete technology and the many materials and processes available today. It is not intended to be a specification or standard, and should not be quoted or used for that purpose.

1.2 Scope of report Refractory concrete is concrete suitable for use at temperatures up to about 3400â °F (1870â °C). It consisted of a graded refractory aggregate bound by a suitable cementing medium. This report is concerned with refractory concrete in which the binding agent is a hydraulic cement, and does not consider concretes which use waterglass (sodium silicate), phosphoric acid, or phosphates as a principal cementing agent. It covers all facets of refractory concrete installation and use, including the properties of individual ingredients and concretes, placing techniques, methods of curing and firing, repair procedures, construction details, and current and future applications. Charlie Chong/ Fion Zhang

1.3 Nomenclature The following definitions are used in this report: ACID REFRACTORIES - Refractories containing a substantial amount of silica that may react chemically with basic refractories, basic slags, or basic fluxes at high temperatures. APPARENT POROSITY (ASTM C20) - The relationship of the volume of the open pores in a refractory specimen to its exterior volume, expressed as a percentage. BASIC REFRACTORIES - Refractories whose major constituent is lime, magnesia, or both, and which may react chemically with acid refractories, acid slags, or acid fluxes at high temperatures. (Commercial use of this term also includes refractories made of chrome ore or combinations of chrome ore and dead burned magnesite).

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CALCIUM ALUMINATE CEMENT - The product obtained by pulverizing clinker which consists of hydraulic calcium aluminates formed by fusing or sintering a suitably proportioned mixture of aluminous and calcareous materials. CASTABLE REFRACTORY - A proprietary packaged dry mixture of hydraulic cement and specially selected and proportioned refractory aggregates which, when mixed with water, will produce refractory concrete or mortar. CERAMIC BOND - The high strength bond which is developed between materials, such as calcium aluminate cement and refractory aggregates, as a result of thermochemical reactions which occur when the materials are subjected to elevated temperature. EXPLOSIVE SPALLING - A sudden spalling which occurs as the result of a build-up of steam pressure caused by too rapid heating on first firing. GROG â&#x20AC;&#x201C; Burned refractory material, usually calcined clay or crushed brick bats.

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HEAT RESISTANT CONCRETE - Any concrete which will not disintegrate when exposed to constant or cyclical heating at any temperature below which a ceramic bond is formed. HIGH ALUMINA CEMENT - See calcium aluminate cement. NEUTRAL REFRACTORIES - Refractories that are resistant to chemical attack by both acid and basic slags, refractories, or fluxes at high temperatures. REFRACTORY AGGREGATE - Materials having refractory properties which form a refractory body when bound into a conglomerate mass by a matrix. REFRACTORY CONCRETE â&#x20AC;&#x201C; Concrete which is suitable for use at high temperatures and contains hydraulic cement as the binding agent.

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SOFTENING TEMPERATURE - The temperature at which a refractory material begins to undergo permanent deformation under specified conditions. This term is more appropriately applied to glasses than to refractory concretes. THERMAL SHOCK - The exposure of a material or body to a rapid change in temperature which may have a deleterious effect.

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1.6 Non-hydraulic refractory. The following discussion, while not pertinent to the main theme of the report, will be of some interest and use to the reader. 1.6.1 Refractory brick - High quality brick, known as firebrick, with unique chemical and physical properties is obtained by blending different types of clay and other ingredients and by varying both the method of processing and the burning temperatures. In addition to the many varieties of fireclay brick, have been developed: ■ high alumina, ■ insulating, ■ silica, ■ fused aggregate, and ■ basic firebrick

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Refractory brick remains a major construction material for applications in which heat containment and control is necessary and in many instances, is the only satisfactory solution to a specific problem. Brick has a number of disadvantages when compared to monolithic refractories. These disadvantages include:       

multiple joints, complicated anchoring, higher placement costs, more difficult repair procedures, the need to maintain expensive inventories of special or scarce items, a certain inflexibility in structural design, and higher fuel requirements during manufacture.

Keywords: Brick refractory Monolithic refractory

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1.6.2 Plastics and ramming mixes - Plastic refractories and ramming mixes are refractories which are tamped or rammed in place and are used for monolithic construction, for repair purposes, and for molding special shapes. These materials find extensive use in industry. They usually employ a clay, alumina, magnesite, chrome, silicon carbide, or graphite base, and are blended with a binder. ď Ž Heat setting mixes are likely to contain fireclay or phosphoric acid H3PO4 as a binder. ď Ž Air or cold-setting mixes generally contain fireclay and sodium silicate Na2O as the binder. Compared to ramming mixes, plastic refractories have higher moisture contents and therefore, higher plasticity.

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Plastics are generally placed without use of forms. With the exception of some specialized tabular alumina castables, plastics have a somewhat higher service limit than castable refractories. Their main disadvantages are greater shrinkage and crack development. Except for phosphate bonded materials cured above 600â °F (315â °C), plastics generally have lower cold and hot strengths than refractory concretes. In addition, plastics tend to have a relatively low strength zone on the cool side of the lining. Ramming mixes usually have higher density and less shrinkage than plastic refractories. With their low water content, they must be forced into place and require strong well-braced forms. Some of the dryer medium grind ramming mixes are suitable for gunning, and are used for patching and maintenance materials.

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1.6.4 Gunning mixes other than refractory concretes As used in this section, the term â&#x20AC;&#x153;gunning mixesâ&#x20AC;? does not refer to refractory concrete and should not be confused with gunned refractory materials which produce refractory concrete. Gunning mixes are mixtures of non-hydraulic setting ingredients which are installed hot or cold, usually by the shotcrete method. Gunning mixes generally have low rebound loss, are predominately used for patching or resurfacing brick or other refractories, have a strong internal bond, and exhibit excellent adhesion or bond to the existing refractory lining. They find extensive use in basic oxygen, electric arc and open hearth furnaces, among other applications.

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Chapter 2 - Criteria for refractory concrete selection 2.1 Introduction Refractory concrete is usually made with high alumina cement. It is not generally used as a structural material and its primary purpose is as a protective lining for steel, concrete or brick structures. Some of the destructive forces that refractory concretes withstand are abrasion, erosion. Physical considered a consumable material requiring replace, abuse, high temperatures, thermal shock, hot and molten metals, clinker, slag, alkalies, mild acid or replacement after an appropriate service life. acid fumes, expansion, contraction, carbon monoxide,and flame impingement. Refractory concretes are categorized as either normal weight or lightweight. The former are also referred to as “heavy refractory concretes” and the latter are often called “insulating refractory concretes.” Table 2.la shows the characteristics of a typical range of normal weight refractory concretes; Table 2.lb shows the characteristics of lightweight refractory concretes.

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2.2 Castables and field mixes Refractory concretes are usually prepared at the job site from materials supplied to the user in either of two ways: (1) prepackaged so-called â&#x20AC;&#x153;refractory castables;â&#x20AC;? (2) field mixes. Refractory castables are plant packaged mixes composed of ingredients that are weighed, blended and usually bagged in convenient sizes for shipping and handling. They require only mixing with water on the job to produce refractory concrete. Field mixes are made from material components which are proportioned and mixed on the site just prior to the addition of water.

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2.5 Load bearing considerations Most application designs of refractory concrete consider that there is a thermal gradient through the material with heat conducted from the hot face to the cold face. A cross section of the refractory will usually have a layer at the hot face that has a ceramic bond, an intermediate section with a weaker combination of ceramic and a partial hydraulic bond, and a cold face section that retains most of its hydraulic bond.

hot face that has a ceramic bond an intermediate section with a weaker combination of ceramic and a partial hydraulic bond cold face section that retains most of its hydraulic bond Charlie Chong/ Fion Zhang

Refractory concrete linings in this type of situation are usually well anchored and self-supporting. Castables containing high proportions of coarse aggregates produce refractory concrete with good load bearing characteristics. Certain types of refractory concrete tend to have low strengths in the intermediate temperature zones [1500⁰F -2250⁰F (820⁰C 1230⁰C)] and should not be subjected to excessive mechanical abuse or dead load. Generally, lightweight concretes designed for insulating purposes should not be subjected to impact, heavy loads, abrasion, erosion or other physical abuse. Normally, both the strength and the resistance to destructive forces decline as the bulk density of the refractory concrete decreases. There are a number of special refractory castables available which have better than average load-bearing capabilities and withstand abrasion much better than the standard types.

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2.7 Corrosion influences High temperature in combination with a corrosive environment can have a serious deleterious effect on both the concrete and the backup steel structure. Alkalies can effect the service life of refractory. Generally, the higher density, higher purity refracconcretes. The furnace charge can give off both alkalies (K2O) and the fuel sulfur compounds (SO2) as refractory concretes have better corrosion resistance than pores. These can penetrate into the pores of the refractory concrete and react; their reaction products the lower density, lower purity types. cool, solidify, and expand, sometimes causing the hot face of the refractory to peel or shear away. In certain applications, the refractory concrete is subjected to highly reducing conditions. Low-iron refractory concretes should be used for this type of application.

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2.10 Abrasion and erosion resistance Abrasion and erosion begin with the wearing away of the weakest matrix constituent, binder, leaving the coarse or hard aggregate to eventually fall away. A hard aggregate, a high modulus of rupture MOR, and high compressive strength at the hot face are necessary for good abrasion and erosion resistance in refractory concretes.

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Chapter 3 - Constituent ingredients 3.2 Binders The binders principally used in refractory concretes are calcium aluminate cements. However, ASTM type portland cements can be used in some refractory applications up to an approximate maximum of 2000â °F (1090â °C) with selected aggregates, if special precautions are taken to ensure a sound refractory concrete. Cyclic heating and cooling tends to disrupt portland cement concretes and adding a fine siliceous material to react with the calcium hydroxide, formed during hydration, is helpful in alleviating the problem. Calcium aluminate (high alumina) cements are commercially available hydraulic binders. They are specifically designed for use in monolithic refractory concrete construction.

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They are generally classified under three basic categories: ■ Low Purity, ■ Intermediate Purity, and ■ High Purity. This is a relative classification scheme and is based primarily on the total iron content of the cement. Binder selection is primarily based on the service temperature desired for the refractory concrete. Maximum service temperatures are extended with increasing Alumina, Al2O3 and decreasing iron contents (forming low temperature eutectic?) . Lower iron content binders are also beneficial in reducing carbon monoxide (CO) disintegration of concrete (Section 2.7). Comments: Lower Fe content ■ Reduce forming low temperature iron compound which go into liquid ■ Reduce CO disintegration (how?)

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3.3 Aggregates The maximum service temperatures of selected aggregates mixed with appropriate calcium aluminate cements are listed in Table 3.3a. These maximum temperatures are based on optimum conditions of binder and aggregate. Thermal properties of aggregates, such as volume change (expansion, shrinkage or crystalline inversion) and decomposition, can affect these maximum temperatures, along with the chemical composition of both aggregate and binder and the reactivity between these mix constituents. Comment: inversion [1]: A change in crystal form without change in chemical composition; as for example, the change from low quartz to high-quartz, or the change from quartz to cristobalite.

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Temperature stability of the aggregate determines the maximum service conditions below approximately 2400⁰F (1320⁰C). Therefore, any type of calcium aluminate cement can be used at these temperatures. For conditions above 2400⁰F (1320⁰C), binder purity also becomes a design factor. Generally, the low purity binder can be used with proper aggregates up to 2700⁰F (1480⁰C), intermediate purity to 3000⁰F (1650⁰C) and high purity to 3400⁰F (1870⁰C). Aggregate gradation is an important consideration in designing refractory concrete. Table 3.3b provides suggested guidelines for nominal maximum size and grading of refractory aggregates.

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For refractory mix designs a 1:3 or 1:4 by bulk volume dry basis cement: aggregate mix is generally used to satisfy typical applications. In certain cases the ratio may change from as low as 1:2 to as high as 1:6, with the latter being used for lightweight concretes. Within the range of normal usage, increasing the cement content will provide higher strength development. However, increased cement content may also result in increased shrinkage. A higher aggregate content will increase insulating or refractory properties, depending on the type of aggregate selected for the mix. Combinations of various aggregates can be made to secure the desirable properties of each. 3.3.1 Lightweight aggregates - Perlite, expanded shale, expanded fireclay, and bubble alumina are the more commonly used lightweight aggregate for commercial insulating concretes.

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TABLE 3.3a- Maximum service temperature of selected aggregates mixed with calcium aluminate cements under optimum conditions

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TABLE 3.3b- Aggregate grading

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3.4 Effects of extraneous materials Extraneous materials commonly associated with portland cements, either as admixtures or as contaminants from equipment or surrounding conditions, may behave differently when used with calcium aluminate cement mixes. Many castables contain proprietary additions which may be adversely affected by field admixtures.

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Chapter 4 - Composition and proportioning 4.1 Introduction In designing mixes, refractory concretes are not only defined by density but also by operating temperature. Refractory concretes fall into three subclasses based on service temperature ranges.  The first sub class is “ceramically- bonded concrete,” defined as concrete in which the cement binder and the fine aggregate particles react thermochemically to form a bond. This bond is referred to as the ceramic bond and may occur at temperatures as low as 1650⁰F (900⁰C).  The second subclass is “heat resistant concrete,” defined as concrete in which the cement has dehydrated but has not formed a ceramic bond.  The third category is concrete which still has some hydraulic bond when heated but performs satisfactorily under cyclic conditions.

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4.3 Field mixes 4.3.1 Ceramically bonded concrete - The ceramic bond can be formed at temperatures as low as 1650â °F (900â °C). To aid formation of the ceramic bond, concretes operating above this temperature should have 10-15 percent of the aggregate passing a No. 100 sieve. Most field insulating concretes are made with presoaked aggregate. Since the specified proportions are based on dry materials, the actual batch mixes may require correction.

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4.3.2 Heat resistant concrete - This concrete is generally used in the range 930⁰F (500⁰C) to 1650⁰F (900⁰C). Many coarse aggregates are unsuitable for use as refractory aggregates because they contain quartz, which has a large volume change at 1065 ⁰F (575⁰C) . 4.4 Water content A majority of the aggregates used in refractory and heat resistant concretes have high water absorbency. For this reason specific water/cement ratios are generally not used in developing mix designs. Instead, water requirements are arrived at by periodically conducting a “ball-in-hand” test (ASTM C860). This test is illustrated in Fig. 4.4. The correct water content is that which will provide a placeable, rather than a pourable, mix. When using well-soaked aggregates, it may be necessary to add little or no water at the mixer. It is sometimes found that a mixture which appears fairly stiff when discharged from the mixer will yield excess water as the concrete is placed.

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Chapter 5 - Installation 5.1 Introduction Regardless of the quality of the refractory cement, aggregate, and/or castable, and regardless of the research devoted to the selection of correct materials for a specific application, maximum service life will not be obtained unless the refractory concrete is installed properly. The most frequently used methods of installing refractory concretes are casting and shotcreting.

5.2 Casting 5.2.1 Mixing - Proper mixing of castables is of primary importance. Care should be taken to avoid mixing previously hydrated material into fresh refractory concrete. Mixers, tools and transporting equipment used previously with portland or other type cement concretes must be cleaned prior to mixing. Remains of lime, plaster, or portland cement will induce flash set and will lower refractoriness.

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Generally, paddle mixers are used for small to medium size jobs involving calcium aluminate cement concretes. In a paddle mixer, normal weight refractory concretes should be mixed for about 2 to 4 min. Refractory concretes of less than 60 lbs/cu ft (960 kg/m3) density should be mixed no longer than necessary to insure thorough wetting. This precaution is necessary because the lightweight aggregate may break-up during the mixing action and reduce the effectiveness of the concrete as a heat insulator. Refractory concretes in the 75 to 90 lb/cu ft (1200-1400 kg/m3) range should be mixed for approximately 2 to 5 min. Because working time may be short, all castables should be cast immediately after mixing.

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5.2.3 Mixing and curing temperature - Mixing and curing temperature can affect the type of hydrates formed in set concrete. A castable develops its hydraulic bond because of chemical reactions between the calcium aluminate cement and water. To get the maximum benefits from these chemical reactions, it is preferable to form the stable C3AH6 during the initial curing period. The relative amount of C3AH6 formed versus metastable CAH10 and C2AH8 can be directly related to the temperature at which the chemical reactions take place. Note: C = CaO, A = Al2O3, H = H2O. = H2O.

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Recent work illustrates the significant impact of mixing and curing temperatures on strength properties. Fig. 5.2.3 shows the flexural strength of a tabular alumina, high purity cement castable plotted as a function of mixing and curing temperatures. It can be seen that the strength developed after mixing and curing at 85⁰F (30⁰C) and drying at 230⁰F (110⁰C) is nearly twice that of the concrete mixed and cured at 60⁰F (15⁰C) and dried at 230⁰F (110⁰C). Explosive spalling of high purity cement concretes can occur when casting and curing temperatures below 70⁰F (21⁰C) are used. Thus, a refractory concrete containing a high purity cement should be cast or cured above 70⁰F (21⁰C). This spalling phenomenon is less likely to occur with low or intermediate purity cement binders.

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Explosive spalling of high purity cement concretes can occur when casting and curing temperatures below 70⁰F (21⁰C) are used. Thus, a refractory concrete containing a high purity cement should be cast or cured above 70⁰F (21⁰C). This spalling phenomenon is less likely to occur with low or intermediate purity cement binders.

21⁰C

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Fig. 5.2.3 - Flexural strength of tabular alumina, high purity cement castable (ASTM C268)

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Fig. 5.2.3 - Flexural strength of tabular alumina (MOF?), high purity cement castable (ASTM C268)

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5.2.4 Transporting - Other than shotcreting and pumping, the techniques for transporting refractory concretes are similar to those used for portland cement concrete. Some calcium aluminate cement binders have a shorter placing time available.

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5.3 Shotcreting Shotcreting of refractory concrete is particularly effective where, (1) forms are impractical, (2) access is difficult, (3) thin layers and/or variable thicknesses are required, or (4) normal casting techniques cannot be employed. Note: from the aforementioned, it sounds that casting is the preferred method? 5.3.1 Equipment - There are two basic types of shotcrete methods: dry-mix and wet-mix. â&#x2013; The drymix method conveys the aggregate and binder pneumatically to the nozzle in an essentially dry state where water is added in a spray. â&#x2013; The wet-mix method conveys the aggregate, binder and a predetermined amount of water, either pneumatically or under pressure, to the nozzle where compressed air is used to increase the velocity of impact. The dry method, though it produces greater rebound, is the most suitable and recommended technique for shotcreting refractory concrete. An exception is the recommended use of a wet-mix gun for hot patching.

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The dry method, though it produces greater rebound, is the most suitable and recommended technique for shotcreting refractory concrete. An exception is the recommended use of a wet-mix gun for hot patching.

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5.3.2 Installation - To ensure a uniform covering free of laminations and with minimum rebound, the nozzleman should move the nozzle in a small circular orbit and where possible, maintain the flow from a 3- 4 ft (0.9-1.2 m) distance at right angles to the receiving surface. The shotcrete should be left in its asplaced state. If for some reason scraping or finishing is required, the absolute minimum should be done so as to avoid breaking the bond or creating surface cracks. Shotcreting of refractory concretes can increase the in-place density and result in other changes in the physical properties. This effect is more pronounced in lower density castables, and must be taken into account when specifying thicknesses and material quantities for insulating applications. The user should be aware that certain aspects of portland cement concrete shotcrete practice do not apply to refractory shotcrete.

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5.4 Pumping and extruding Certain refractory concretes can be installed with positive displacement pumps in conjunction with rigid or flexible pipelines. The design of the mix is critical, and special attention must be given to the absorptive characteristics and sizing of the aggregate. Some applicators use the term â&#x20AC;&#x153;extrudingâ&#x20AC;? to describethe conveying and placing of refractory concrete at velocities that are very low or close to zero on exit from the pipeline. When extruding, mixing of the refractory castable and water can be done internally or externally depending on type of extruding device.

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5.5 Pneumatic gun casting Pneumatic gun casting, or gun casting, is a relatively new technique for casting concrete and is finding increased uses for refractory concrete. Conventional dry shotcrete equipment and procedures are utilized with the exception that an energy reducing device is attached to the nozzle body in place of the standard shotcrete nozzle tip.

5.8 Finishing Surface finishing or rubbing of refractory concretes should be kept at a minimum. Use of a steel trowel should be avoided, and the final surface can be lightly screeded to grade but should not be worked in any manner.

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Shotcreting

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Shotcreting

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Chapter 6 - Curing, drying, firing 6.1 Introduction Refractory concrete should be properly cured for at least the first 24 hr. Following this curing it should be dried at 220â °F (105â °C), and then heated slowly until the combined water has been removed before heating at a more rapid rate.

6.2 Bond mechanisms Calcium aluminate cements have anhydrous mineral phases which react with water to form alumina gel and crystalline compounds which function as a binder for the concrete. The hydration of these cements (Fig. 6.2) is exothermic. The rate of the chemical reaction is relatively fast. For all practical purposes, calcium aluminate concretes will develop full strength within 24 hr of mixing. The total drying shrinkage of calcium aluminate cement concretes in air, is comparable to that of portland cement concrete. In order to provide for complete hydration, and to control drying shrinkage, special attention must be given to the curing of refractory concretes.

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Fig. 6.2 - Hydration reaction products of calcium aluminates

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The Wiki: Phase diagram of calcium aluminates present in the anhydrous calcium aluminate cement before hydration. 2570

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https://en.wikipedia.org/wiki/Calcium_aluminate_cements

6.3 Curing The temperature of hardening calcium cement rises rapidly. If the exposed surfaces are not kept damp, the cement on the surface may dry out before it can be properly hydrated. The application of curing water prevents the surface from becoming dry and furnishes water for hydration. In addition, the evaporation has a cooling effect which helps to dissipate the heat of hydration. Conversion of the high alumina cement hydrates, which occurs if the cement is allowed to develop excessive heat, does not present the same problem in refractory concretes that it does in high alumina cement concretes used for structural purposes. It has been shown that if refractory concrete is fully converted by allowing it to harden in hot water and then heated to 2500⁰F (1370⁰C), the fired strength is equal to that obtained for well cured concrete. When possible, however, refractory concrete should be kept cool by appropriate curing under 210⁰F (99⁰C) for two reasons:

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ď Ž The entire refractory concrete structure does not usually reach the maximum service temperature, and the higher cold strengths obtained by good curing may be useful in the cooler portions of the refractory. ď Ž If the temperature within the concrete reaches a high level during hardening, the thermal stresses produced during cooling may be sufficient to cause cracking.

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Curing should start as soon as the surface is firm. Under normal atmospheric temperatures, this will occur within 4 to 10 hr after mixing the concrete. The concrete should be kept moist for 24 hr by covering with wet burlap, by fine spraying or by using a curing membrane. Alternate wetting and drying can be detrimental to the cure of the concrete. When using a curing membrane, the compound should contain a resin and not a wax base, and should be applied to the surface as soon as possible after placing and screeding. The reason for discouraging the use of wax is that a hot surface will melt the wax, causing it to be absorbed into the concrete, breaking the membrane.

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6.4 Drying The large amount of free water in the refractory concrete necessitates a drying period before exposure to operating temperatures. Otherwise, the formation of steam may lead to explosive spalling during firing.

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6.5 Firing Following drying of the refractory concrete, the first heat-up should be at a reasonably slow rate. A typical firing schedule, for a 9 in. (22.9 cm) thick lining, consists of applying a slow heat by gradually bringing the temperature up to 220⁰F (105⁰C), and holding for at least 6 hr. The temperature is then raised at a rate of 50-100⁰F (10⁰C - 40⁰C) per hr up to 1000⁰F (540⁰C) and again held for at least 6 hr. The first hold is to allow remaining free water to evaporate, and the second hold is to eliminate the combined water without danger of spalling. Beyond 1900⁰F (540⁰C), the temperature of the refractory concrete can be raised more rapidly. Calcining of the green concrete into a refractory structure will take place between 1600⁰F (820⁰C) and 2500⁰F (1370⁰C). Wall thickness and mix variations may require somewhat different rates of heating, but the hold temperatures should remain at least 6 hr. If steam is observed during heat-up, the temperature should be held until steam is no longer visible.

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typical firing schedule, for a 9 in. (22.9 cm) thick lining, consists of applying a slow heat by gradually bringing the temperature up to 220⁰F (105⁰C), and holding for at least 6 hr. The temperature is then raised at a rate of 50-100⁰F (10 - 40⁰C) per hr up to 1000⁰F (540⁰C) and again held for at least 6 hr. The first hold is to allow remaining free water to evaporate, and the second hold is to eliminate the combined water without danger of spalling. Beyond 1900⁰F (540⁰C), the temperature of the refractory concrete can be raised more rapidly. Calcining of the green concrete into a refractory structure will take place between 1600⁰F (820⁰C) and 2500⁰F (1370⁰C)..

Temperature

820⁰C ~ 1370⁰C@6hrs

540⁰C@6hrs 105⁰C@6hrs

dT/dt = 10⁰C - 40⁰C/hr

Time

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Chapter 7 - Properties of Normal Weight Refractory Concretes 7.1 Introduction There are various physical properties and tests which are standard in the refractory industry and these are usually provided in the material specifications. Table 2.1a is an example of typical data for normal weight refractory concrete.

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7.2 Maximum service temperature The recommended maximum service temperature will normally assume that the castable will be used in a clean, oxidizing atmosphere, such as is present when firing with natural gas. The maximum service temperature is usually determined as the point above which excessive shrinkage will take place. It is about 150-200⁰F (70-90⁰C) below the actual softening point of the concrete. If fuel has solid impurities, such as in coals or heavy fuel oils, or if the solids or dust in the process contact the refractory, the maximum permissible service temperature will usually be considerably reduced. Solid impurities can react with the concrete and produce compounds of lower melting point which melt and run. This is generally referred to as slagging. The lower softening point thus represents a limit for the operating temperature. Slag forming reactions usually do not occur below about 2500⁰F (1320⁰C) except in the presence of alkalies where reactions can occur in the 1900-2000⁰F (1040-1090⁰C) range. A reducing atmosphere can lower the melting point and hence the maximum operating temperature by 100-200⁰F (40-90⁰C) if sufficient quantities of iron compounds are present in the refractory.

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7.4 Shrinkage and expansion In discussing shrinkage and expansion of a refractory concrete, it is important to define the distinction between: â&#x2013; the independent effects of permanent shrinkage or â&#x2013; expansion and reversible thermal expansion. Permanent change is determined by measuring a specimen at room temperature, heating it to a specified temperature, cooling to room temperature, and remeasuring it. The difference between the two measurements is the permanent change, which occurs during the first heating cycle. Subsequent heating to the same or lower temperature will have little or no additional effect on the permanent change. Heating to a higher temperature may cause some additional permanent change. Reversible thermal expansion of a specimen which has been previously stabilized against further permanent change, is the dimensional change as a specimen is heated. Upon cooling, the specimen contracts to its original size. At any given temperature, the net dimensional change of a refractory concrete is the sum of the reversible expansion and the permanent shrinkage corresponding to the highest temperature to which the castable has been heated. Charlie Chong/ Fion Zhang

At any given temperature, the net dimensional change of a refractory concrete is the sum of the reversible expansion and the permanent shrinkage corresponding to the highest temperature to which the castable has been heated. Previous higher reversible expansion temperature

â&#x2C6;&#x2020;T

permanent shrinkage corresponded to +â&#x2C6;&#x2020;T

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New higher temperature

7.4.1 Permanent shrinkage and expansion - The initial heating of a refractory concrete usually causes shrinkage. At higher temperatures permanent expansion can occur. This effect, which varies with the maximum temperature attained, must be considered with reversible thermal expansion when calculating the net expansion (or shrinkage) at service temperature. The ASTM rating of castables is based on no more than 1.5 percent permanent linear shrinkage occurring at prescribed temperatures (ASTM C64 and C401). Most normal weight refractory concretes will have less than 0.5 percent permanent linear shrinkage after firing at 2000â °F (1090â °C). The permanent change appears as cracks after the first firing. These cracks will generally be about 2-3 ft (0.6-0.9 m) on centers, and may vary, depending on the concrete thickness and the anchor spacing. Usually, the width of the cracks at room temperature is partly dependent on the permanent shrinkage. Normally, the cracks will be tightly closed at operating temperatures. Such cracking, which may start during drying, is to be expected and will not adversely affect the service performance of the refractory.

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7.4.2 Reversible thermal expansion - The reversible thermal expansion of most refractory concretes is approximately 3 x 10-6 in./in./⁰F (5 x 10-6 cm/cm/⁰C However, the expansion coefficient may be as high as 4 x 10-6 in./in./⁰F (7 x 10-6 cm/cm/⁰C) for high alumina concretes and to 5 x 10-6 in./in./⁰F (9 x 10-6 cm/cm/⁰C) for chrome castables. Reversible thermal expansion: ■ most refractory concretes: 3 x 10-6 in./in./⁰F (5 x 10-6 cm/cm/⁰C ■ high alumina concretes: 4 x 10-6 in./in./⁰F (7 x 10-6 cm/cm/⁰C) ■ chrome castables: 5 x 10-6 in./in./⁰F (9 x 10-6 cm/cm/⁰C) Fig. 7.4.2 shows typical length changes due to permanent shrinkage and reversible expansion.

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Fig. 7.4.2 - Net thermal expansion of a typical refractory concrete

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7.5 Strength 7.5.1 Modulus of rupture - Modulus of rupture is measured by means of a flexure test and is considered as a measure of tensile strength (ASTM C268). The extreme fiber tensile strength calculated from this test will be 50 to 100 percent higher than the tensile strength derived from a straight pull test. Typical modulus of rupture values are 300 to 1500 psi (2.07-10.4 MPa). Shotcreting can increase modulus of rupture values by up to 50 percent. Fig. 7.5 shows typical trends of modulus of rupture strength versus temperature. 7.5.2 Cold compressive strength (crushing) – The test is ordinarily run on 9 x 4½ x 2½ in. (22.9 x 11.4 x 6.4 cm) specimens 9” straights in brick terminology with pressure applied to the smallest. surface (ASTM C133). Failure in this test is generally due to shear. Crushing strengths vary from 1000 to 8000 psi (6.9 to 55.2 MPa). Typically, compressive strengths are three to four times (3 ~ 4 X) greater than modulus of rupture values.

4½ 2½ Charlie Chong/ Fion Zhang

Fig. 7.5 - Effect of temperature on modulus of rupture

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7.6 Thermal conductivity For normal weight refractory concretes, thermal conductivity tends to vary with density. Typical values (k factors) range from about 5 Btu-in./sq ft -hr-⁰F (72 W-cm/m2-⁰C) for 120 pcf (1920 kg/m3) material to about 10 Btu-in./sq ft– hr-⁰F (144 W-cm/m2-⁰C) for 160 pcf (2560 kg/m3) material. There is usually an increase in thermal conductivity with temperature. 120 pcf: 160 pcf:

5 Btu-in./sq ft -hr-⁰F 10 Btu-in./sq ft–hr-⁰F

7.10 Specific heat The specific heat of a refractory concrete increases with temperature from about 0.20 Btu/lb/⁰F (837 J/ kg-⁰C) at 100⁰F (40⁰C) to about 0.29 Btu/lb/⁰F (1210 J/ kg-⁰C) at 2500⁰F (1370⁰C). This can vary plus or minus 0.025 units, depending on the aggregate. at 100⁰F: at 2500⁰F: Charlie Chong/ Fion Zhang

0.20 Btu/lb/⁰F 0.29 Btu/lb/⁰F

Chapter 8 - Properties of lightweight refractory concretes 8.1 Introduction Refractory concretes are widely used as insulating materials. They have a wide range of densities (20 to 100 pcf (320 to 1600 kg/m3) and can be formulated to have high maximum service temperatures and relatively high strengths. This often allows the use of these materials as single component, exposed service linings. Table 2.1b shows physical property values for typical lightweight refractory concretes.

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8.4 Shrinkage and expansion The reversible thermal expansion of lightweight concretes will vary from 2.5 x 10-6 to 3.5 x 10-6 in./in./â °F (4.5 x l0-6cm/cm/â °C) Because of compensating permanent shrinkage, the thermal expansion of lightweight refractory concrete is normally insignificant and is usually ignored in the design of lightweight refractory concrete systems.

8.5 Strength Strengths of lightweight refractory concrete are measured by both a modulus of rupture and a crushing test. 8.5.1 Modulus of rupture - Typical values range from approximately 50 (0.3 MPa) to 400 psi (2.8 MPa).

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8.6 Thermal conductivity Thermal conductivity is one of the most important physical properties of a lightweight refractory concrete and is controlled primarily by the density of the concrete. For hydraulically bonded, alumina-silica concretes, a usable correlation exists between concrete density [after drying at 230⁰F (110⁰C)] and the thermal conductivity (k factor). Typically, the thermal conductivity for insulating concretes ranges from 1 to 4 Btu-in./sq ft-hr-⁰F (0.1 to 0.6 W/M2-C). 8.6.2 Cold compressive strength (crushing) – Cold crushing strengths vary from 200-500 psi (1.4-3.5 MPa) for lightweight refractory concretes with densities up to 50 pcf (800 kg/m3). For materials having densities in the 75100 pcf (1200-1600 kg/m3) range, the cold crushing strength varies from 1000- 2500 psi (6.9-17-3 MPa). Table 8.5.1 shows the difference between the cold and hot modulus of rupture for a typical 2800⁰F (1540⁰C) lightweight refractory concrete.

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TABLE 8.5.1 - Hot and cold modulus of rupture of a 2800 â °F (1538 â °C) lightweight refractory concrete containing expanded fireclay aggregate

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TABLE 8.5.1 - Hot and cold modulus of rupture of a 2800 â °F (1538 â °C) lightweight refractory concrete containing expanded fireclay aggregate

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8.10 Specific Heat The specific heat of a lightweight refractory concrete is approximately the same as that of normal weight concrete. The range is from 0.2 Btu/lb/⁰F (837 J/kg-⁰Cl at 100⁰F (40⁰C) to approximately 0.3 Btu/lb/⁰F (1255 J/kg-⁰C) at 2500⁰F (1370⁰C).

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Chapter 9 - Construction details 9.1 Introduction Construction details are an important ingredient in the successful application of refractory concrete. Proper design details and careful implementation are essential, and parameters such as support structure integrity, forms, anchors, and construction joints have a major influence on the overall quality and performance of refractory concrete installations.

9.2 Support structure Normally, refractory concrete is permanently supported by a back-up structure. The support material is usually bolted or welded steel which, prior to installation of the refractory concrete, should be checked to ensure that there is no warpage and that all joints are structurally sound and tight.

9.3 Forms Both metal and wood forms are used for refractory concrete.

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9.4 Anchors An anchor is a device used to hold refractory concrete in a stable position while counteracting the effects of dead loads, thermal stressing and cycling, and mechanical vibration. Anchors and anchoring systems are not designed to function as reinforcement. Anchors are produced as alloy steel rods or castings, and prefired refractory ceramic shapes. The requirements of a particular installation will determine the type and positioning of anchors. Typical factors to be considered are: unit size, wall thickness, number of refractory concrete components, area of application, and service temperature.

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9.4.1 Metal anchors The most frequently used metal anchors are V-clips, studs, and castings. However, in special applications, welded wire fabric, hex steel and chain link fencing are used. Generally, metal anchors are extended from the cold face for 2/3 to 3/4 of the lining thickness and are staggered to avoid formation of planes of weakness. Metal V-clips, stud anchors and castings are available in carbon steel, Type 304 stainless alloy, Type 310 stainless alloy, and other suitable alloys. The choice of material depends on the temperature to which the anchors will be exposed.  Carbon steel can be used for anchor temperatures of up to 1000⁰F (540⁰C).  Type 304 stainless is suitable for anchor temperatures of up to 1800⁰F (980⁰C) and  Type 310 stainless is adequate up to 2000⁰F (1095⁰C). Depending on the grade of alloy,  alloy steel castings can sustain a maximum temperature of between 1500⁰F (815⁰C) and 2000⁰F (1095⁰C).  ceramic anchors are available with maximum service temperature ratings of up to 3200⁰F (1760⁰C). (see next clause) Charlie Chong/ Fion Zhang

9.4.2 Pre-fired refractory anchors (ceramic anchors) â&#x20AC;&#x201C; The principal use of ceramic anchors is to anchor refractory plastic, rather than refractory concrete. However, ceramic anchors are used in areas where refractory concrete is subjected to high service temperature. In addition, they are sometimes used as a substitute for metal anchors where concrete thicknesses are 9 in. (230 mm), or greater. Ceramic anchors usually are composed of refractory aggregates, clays, and binders. They are mechanically pressed into shapes which provide for attachment to either the wall or roof and are ribbed to aid in securing the refractory concrete. Ceramic anchors are pre-fired at elevated temperature to provide a strong, dense structure. Depending on the composition, service conditions, and other factors, ceramic anchors are available with maximum service temperature ratings of up to 3200â °F (1760â °C). Ceramic anchors are attached to structural wall or roof supports by bolts and/or metal support castings. In order to minimize the tendency of the refractory concrete to sheet spall, the hot face of the ceramic anchor should extend to the hot face of the refractory concrete.

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9.4.6.1 Thin single component linings. Plain metal chain link fencing is often used to anchor single component linings, less than 2 in. (50 mm) thick, composed of lightweight or medium weight refractory concrete and exposed to low to moderate mechanical stresses and/or service temperatures. 9.4.5.2 Single component linings up to 9 in. (230 mm) thick. Normally, single component linings 2 in. (50 mm) to 9 in. (230 mm) thick, composed entirely of lightweight, medium weight or normal weight refractory concrete, and exposed to moderate stresses and service temperatures use metal anchors. 9.4.5.3 Single component linings greater than 9 in. (230 mm) thick. Normal weight refractory concrete linings, greater than 9 in. (230 mm) thick, utilize either ceramic or metal anchors. The type of anchor chosen will depend on the operating parameters.

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9.4.5.4 Roofs. Two types of anchor systems, internal and external, are used for single component roofs. The choice depends on roof thickness and on construction and design preferences. 9.4.5.5 Multicomponent linings. Multicomponent linings of 9 in. (230 mm) or less in thickness which are subjected to moderate service temperatures and mechanical stresses should employ metal anchors. Multicomponent linings of 9 in. (230 mm) or greater thickness, composed of a combination of lightweight or medium weight refractory concrete as back-up in conjunction with a normal weight refractory concrete, can use a combination of ceramic and metal anchors. With multicomponent shotcrete linings, the backup component is applied directly to the shell and provisions must be made either to protect the anchor (metal or ceramic) from rebound build-up, or to clean the anchor after placing of the back-up layer. Rebound build-up can destroy the grip between the heavy weight refractory concrete and the ceramic anchor.

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9.5 Reinforcement and metal embedment The use of steel as a reinforcement should be avoided. In general, the metal will cause cracking due to the differential expansion, caused by temperature or oxidation, between the metal and concrete. For the same reason heavy metal objects such as bolts, pipes, etc. should never be embedded in refractory concrete.

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9.6 Joints In cast installations, construction joints occur at the junction of walls and roofs or where large placements are broken into separate sections. Cold joints of this type will not bond and should be avoided where it is necessary to contain liquid or gases. It is often necessary to include a provision for expansion. Expansion joints can be formed by inserting materials such as wood, cardboard, expanded polystyrene or ceramic fiber in the appropriate location. Shotcrete installations require construction joints at transitions between materials, or when application must be curtailed due to shift changes or material supply. In these cases, the in situ refractory concrete should be trimmed back to produce a clean edge perpendicular to the shell. (not cascaded!) Expansion compensating materials are not generally inserted into this type of joint. If a joint edge is allowed to stand for a prolonged period of time (more than 4 hr), it should be thoroughly moistened before any new material is applied.

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Chapter 10 - Repair 10.1 Introduction Repair of refractory concrete should be considered only when economics dictate that cost and downtime do not justify complete replacement. Before undertaking a repair, an effort should be made to determine the cause of the previous failure. If possible, the design and/or construction details should be modified to reduce the possibility of a recurrence of failure and to prolong service life between repairs. Hot repair techniques are valuable for minimizing downtime and for extending an operating run until a scheduled shutdown. Hot repairs are especially suitable for temporary repairs of localized failures and hot spots.

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Refractory Repair

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http://www.rebl-refractories.com/refractory-issues/

Hot repairs are especially suitable for temporary repairs of localized failures and hot spots. (hot welding repair)

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http://www.fosbel.com/Industries/Glass/Ceramic_Welding.aspx

Hot repairs are especially suitable for temporary repairs of localized failures and hot spots. (hot welding repair)

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http://www.fosbel.com/Industries/Glass/Ceramic_Welding.aspx

10.2 Failure mechanisms Some of the phenomena that can cause failure are: (1) (2) (3) (4) (5) (6) (7)

Thermal stress and thermal shock; Exposure to excessive temperatures; Mechanical loading; Erosion and abrasion: Corrosive environments; Anchorage failures and Operational problems or upsets.

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10.3 Surface preparation When the installation to be repaired is made of mortar or concrete, it is important to prepare the surface of the old material so that a mechanical bond will be formed between it and the new refractory concrete. No significant chemical bond will be formed, and adhesion of the repair material must depend primarily on the mechanical bond. Preparation of the surface requires removal of all deteriorated or spalled materials and roughening of the exposed sound surface of the old concrete. In all cases, the chipping of old material must leave a flat base, and square shoulders approximately perpendicular to the hot face, completely around the perimeter of the repair section. If this is done properly, there is no need to chamfer the edges or provide fillets to walls and floors.

square shoulders approximately perpendicular to the hot face

hot face

flat base

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Once initial removal of loose concrete has been completed, the old refractory should be sounded with bars or hammers to make certain only sound material remains. Areas that were not chipped should be thoroughly sandblasted to remove any traces of soot, grease, oil or other substances that could interfere with the bond. Excess sand and loose debris must then be blown from the surface with compressed air. Particular care must be taken to remove any debris from around the anchors.

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10.4 Anchoring and bonding If possible, patches should be anchored with a minimum of two anchors which should be solidly attached to the shell. In cases where this is impossible, anchors should be solidly embedded in the old refractory. Ceramic anchors should extend to the hot face of the new refractory concrete. Otherwise, sheet spalling may occur. If metal anchors are used, they should be brought as close as possible to the hot face (3/4 or 2/3?) . The distance will depend on the metallurgy of the anchors and the thermal conductivity of the concrete. Where anchors are not practical, or repairs are shallow, mechanical bonding will be aided by cutting chases or keyways in a waffle pattern across the entire surface of the repair section and by slightly undercutting the existing refractory. In certain limited applications, where other means are not available, the bond may be improved by precoating the surface to be repaired with a bonding agent. When repairing refractory concrete with a similar cast-in-place material pre-wetting is required, and use of a neat calcium aluminate cement slurry may improve bonding.

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Where anchors are not practical, or repairs are shallow, mechanical bonding will be aided by cutting chases or keyways in a waffle pattern across the entire surface of the repair section and by slightly undercutting the existing refractory.

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Anchoring and bonding ď Ž Minimum 2 anchor bonded to cold face or embedded in underlying concrete ď Ž Where anchors are not practical, or repairs are shallow, mechanical bonding will be aided by cutting chases or keyways in a waffle pattern across the entire surface of the repair section and by slightly undercutting the existing refractory. ď Ž where other means are not available, the bond may be improved by precoating the surface to be repaired with a bonding agent.

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10.5 Repair materials A wide range of repair products is available for repairing refractory concrete. However, it is usually best to use a material similar to that being repaired. Refractory concrete is frequently used as a repair material and performs satisfactorily in many situations. Among the other available repair materials are the following: 1. Air setting mortars; 2. Phosphate-bonded and clay-based heat-setting mortars; 3. Steel-fiber reinforced refractory concrete; (Steel-fiber reinforced refractory concrete will generally exhibit superior resistance to cracking and abrasion. However, the fibers will not perform well if the temperatures to which they are exposed induce oxidation. If the conditions are such that the fiber-reinforced system is above the oxidizing, but below the melting temperature of the particular fibers being used, it is possible that they may still be utilized, depending on the temperature gradient through the concrete, the furnace atmosphere, the permeability of the concrete, the severity and frequency of temperature cycles, the exposure time at maximum temperature, and the mechanical loading.)

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4. Plastic refractories and ramming mixes; and 5. Hot repair materials. Some of the repair materials used for hot patching contain calcium aluminate cement as the principal binder, however, most do not. The latter utilize non-hydraulic and chemical binders (see Section 1.6.4). Since these materials are intended for temporary repairs, they may not have service life or properties equivalent to those in the original lining. While field mixes can be used for hot gunning, most applications use proprietary (prepackaged) materials which are specially designed for specific conditions of installation. Some manufacturers have designed special spray or gunning equipment and maintenance programs to install their hot repair materials on a planned basis.

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10.6 Repair techniques 10.6.2 Refractory concrete - When a refractory concrete is selected to effect repairs, the type of placement procedure must insure that the full thickness of the repair section is installed in as short a time as possible, preferably in a single lift. When refractory concrete is placed by the shotcrete method, certain precautions must be followed. The area being repaired must be delineated in advance so that the concrete can be shot to the full section depth or thickness before any layer develops an initial set. It is important that the refractory concrete be cured properly during the 24-hr period following placement (see Section 6.3). After the concrete has been moist-cured for 24 hr, drying and firing can be initiated (see Sections 6.4 and 6.5). Speeding up the moist-curing, drying and firing can result in a marked reduction in the physical properties and life of the repair.

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10.6.3 Plastic and ramming mixes - A refractory mortar coating may be used to improve bonding when repairing refractory concrete with a plastic or ramming mix. In order to achieve high density and prevent laminations, it is recommended that plastic refractories be installed by the pneumatic ramming method using a steel wedge-type head. The basic pattern of ramming should be to build up layers of plastic on top of the backing wall. The plastic is placed in strips and laid at right angles to the forms. It is important to angle the pneumatic rammer so that the strips are driven against the form, and sideways against the previously installed material. The repaired area should be trimmed to a rough surface for more uniform drying. Moisture escape holes should be made by inserting a 1/8 in. (3 mm) diameter pointed rod, approximately two-thirds of the depth of the material, on approximately 6 in. (150 mm) centers. In order to prevent formation of an outer skin, which can seal in moisture, a short period of forced drying of air-setting plastic refractories is desirable.

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Excessive temperature or direct flame impingement, which will seal the surface and prevent escape of moisture, must be avoided. The following heatcuring procedure has been found to give good results with plastic and ramming mixes: Remove all free moisture at a temperature of not over 250⁰F (120⁰C). Following removal of free and absorbed moisture, raise the temperature at a rate of 75-100⁰F (42-56⁰C) /hr until the desired operating temperature is reached. If steam is observed during heat-up, hold the present temperature until it stops. Whenever possible, repairs using plastic mixes should be carried out immediately prior to heat-up. A properly burned-in plastic will exhibit less cracking than a plastic exposed to lengthy air drying.

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Remove all free moisture at a temperature of not over 250⁰F (120⁰C). Following removal of free and absorbed moisture, raise the temperature at a rate of 75⁰F -100⁰F (42-56⁰C) /hr until the desired operating temperature is reached.

Temperature

Operating Temperature

dT/dt = 42⁰C - 56⁰C/hr 120⁰C

Time

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Temperature

820⁰C ~ 1370⁰C@6hrs

540⁰C@6hrs 105⁰C@6hrs

dT/dt = 10⁰C - 40⁰C/hr

Time

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10.6.4 Steel-fiber reinforced refractory concrete 10.6.4.1 Cast-in-place mixes. A problem with steel fibers is their tendency to â&#x20AC;&#x153;ball-upâ&#x20AC;?. Clusters of fibers can be broken up by hand feeding or shaking of the sieve before addition to the concrete mix. In some cases, vibration will tighten up the fiber clusters and it is not a recommended method of fiber dispersal. The addition of steel fibers tends to reduce the workability of the mix. Normally, this can be overcome by internal or external vibration. Use of additional water is not recommended since this will degrade cured strength and increase the porosity. 10.6.4.2 Shotcrete mixes. Steel fiber reinforced refractory concretes can be shot into place by either the wet or dry process. Fiber lengths approaching the internal diameter of the material hose or nozzle can be shot successfully. Because rebound of the fibers can be dangerous, the nozzleman and support crew should wear protective clothing when dry shooting with steel fibers.

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Steel Fiber Reinforced Concrete

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Steel Fiber Reinforced Concrete

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http://www.xpandrally.com/images-fibers-for-concrete.html

10.6.5 Hot repair procedures Hot repair procedures are based on standard shotcreting technology. However, because of the high temperatures, certain differences are necessary. Compared to normal shotcreting, the high temperatures require a specially designed nozzle and an excessive amount of water in the mix in order to insure proper delivery, impingement, compaction, and material retention. Hot shotcreting requires that the nozzleman and a helper stand outside the furnace and manually or mechanically manipulate an extended nozzle or “lance” within the furnace. Special ports or openings must be provided in the furnace for proper access. The length, size, and design of the nozzle depends on the furnace configuration, temperature, and type of application. In general, the best bonds are achieved when the vessel interior is a red or orange color (1500-1700⁰F (815-925 ⁰C). The refractory concrete repair must be allowed to heat-cure prior to placing the unit back in service. The length of time to accomplish this, although usually brief, will depend on the temperature at the time of repair, the type of material used for the repair, and the thickness of the installed material.

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Chapter 11- Applications 11.1 Introduction Refractory concretes are currently used in a wide variety of industrial applications where pyroprocessing or thermal containment is required. Because there are literally hundreds of refractory concretes available, it is impossible to discuss every composition and its application. Accordingly, only the more important applications, where refractory concretes have been used successfully, are reviewed. Included in the review are the following industries:

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a) Iron and steel (b)lNon-ferrous metal (c)lPetrochemical (d)lCeramic processing (e)lGlass (f) Steam power generation (g) Aerospace (h)lNuclear (i) Gas production (j) MHD power generation (k) Lightweight aggregate (l) Incinerator (m) Cement and lime

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Chapter 12 -New development and future use of refractory concrete 12.1 Introduction Traditionally, developments in the refractories industry have been closely related to the process industries, the primary customers for the product. In recent years, there have been marked changes in the production and use of refractories. A number of factors have contributed to these changes including: (a) development of new and improved industrial processes, (b) demand for higher temperatures and increased production rates associated with the above developments, (c) improvement in the quality of refractory products and increased use of different types of refractories, especially the monolithic castables and, (d) increased technical knowledge of the service behavior of refractory materials.

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With these technological advancements, investigations into the use of refractory concretes for special applications is increasing. Typical of these new and proposed applications are incinerators, coal gasification plants, chemical process plants, steel plants, and foundries.

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12.2 New developments 12.2.1 Steel fibers - The following potential advantages are offered by the use of steel-fiberreinforcement in monolithic construction: (a) improved flexural strength at ambient and elevated temperatures, (b) improved thermal and mechanical stress resistance, (c) improved thermal shock resistance, (d) improved spall resistance, and (e) improved load-carrying ability. However, degradation of the steel fibers at high temperature can occur under service conditions and, therefore, limit the full potential of these materials. Note: See References 197 through 205. 12.2.2 Shotcrete - The use of shotcrete for new refractory construction and for repairs is a rapidly growing field and successful results have been achieved in many applications. 12.2.3 Precast shapes - Increasingly, precast shapes are being used for special conditions and this trend will continue.

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12.3 Research requirements Unfortunately, selection and use of refractory concretes is still considered an art and, with a few exceptions, the properties of refractory concretes are not utilized in rational design schemes. In many instances, the wrong properties are being measured or the available data are not being used correctly. Future research efforts should be directed towards obtaining a better understanding of the behavior of refractory concretes under service conditions. Increased emphasis will be placed on elevated temperature properties and how they are influenced by such factors as proportioning, grading and compo sition. Areas of needed research include the following: (a) Dimensional stability (b) Chemical attack (c) Mechanical properties (d) Property measurements and tests (e) Process conditions (f) Rational design procedures

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Good Luck!

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Good Luck!

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