fly ash for concrete block manufacturing
The manufacturing of concrete masonry units uses a dry, harsh concrete mixture compacted into molds with great mechanical energy. When demolded, these units maintain their shape during handling and transportation into a curing environment. Curing methods consist of the high pressure, high temperature autoclave, or the atmospheric pressure, high temperature kiln. The use of high quality fly ash has become accepted practice in the industry.
F
ly ash improves block manufacturing in two basic ways. It gives producers the strength required and, at the same time, the added plasticity that fly ash contributes (reported by Belot, 1976) to the relatively harsh block mixes assures improved finish and texture; better mold life, and better, sharper corners. Additional benefits of fly ash in block include reduced permeability and shrinkage, increased durability and virtual elimination of efflorescence.Fly Ash Chemical Activity.
Fly ash is produced by burning powdered coal to generate electricity. Fly ash is a chemically active, finely divided mineral product high in silica, alumina and iron. Fly ash that has been burned in the process of manufacturing (in the same sense that portland cement clinker is “burned”) seeks lime. One hundred pounds of portland cement usually liberates from 12 to 20 pounds or more of free lime (calcium hydroxide) during hydration. Fly ash then chemically reacts with this free lime to form additional stable cementitious compounds. The formation of insoluble cementing compounds is accelerated and can be secured in a matter of hours in the steam curing cycle of the concrete products plant (autoclave or atmospheric).Steam Curing.
Autoclave curing, though not as common as in the past, is still used to manufacture high quality masonry units. Concrete masonry units cured in high-pressure autoclaves show early strength equivalent to that of 28-day moist-cured strength and reduction in volume change in drying (Hope 1981). The process uses temperatures of 275° to 375°F (135° to 275°C) and pressures of 75 to 170 psi (0.52 to 1.17MPa). These conditions allow for the use of fly ash as a cement replacement up to 35 percent for Class C and 30 percent for Class F fly ashes. Particular care should be taken to insure that the fly ash meets the soundness requirement of ASTM C618, indicated in Note C, Table 2 especially where the fly ash will constitute more than 20 percent of the total cementitious material.Low-pressure steam curing is usually performed in insulated kilns at elevated temperatures, the exact emperature used being a function of the materials and operation of the specific plant. This process allows for the use of fly ash as a cement replacement up to 35 percent for Class C and 25 percent for Class F fly ash. Tests with 25 percent Class F fly ash were successful with a curing temperature above 160°F (71°C) and indicate that drying shrinkage of low pressure steam-cured concrete units can be reduced by the addition of fly ash. Accelerated curing techniques allow for a period of preset before the concrete products are subjected to elevated temperatures. The preset period may lengthen slightly where cement is replaced with fly ash and if so, it must be allowed for.
Tests for resistance to freezing and thawing of concrete masonry units containing fly ash indicate that such units, in general, could be expected to perform well in vertical wall construction. For the more severe condition of horizontal exposure, a minimum compressive strength of 3,000 psi (21MPa) based upon the net area of the unit is recommended when normal weight aggregates are used. This is true whether fly ash is used or not. Air-entrainment is not practical at the extremely low or zero slumps used for concrete block. It could be applicable to slump block or quarry tile. To provide adequate freezing and thawing durability for units made with slump concrete, air-entrainment is needed (Redmond 1969). Acceptance by the engineering profession and most code bodies to use concrete masonry units for high-strength, high-rise, load-bearing construction is increasing. To meet this demand, block producers find it necessary to produce both light and normal weight units testing 3,500 psi net area (1,860 gross area assuming 53 percent solid units) and 5,000 psi net area (2,650 gross area), respectively. The 1,860 psi gross area strength units are known as high strength block and those of 2,650 psi gross area strength are known as extra high strength block.
Trial Mixes. Proportioning mixtures for the manufacture of concrete masonry units is not an exact science. Conditions may vary widely from plant to plant. When proportioning mixtures, concrete producers should check the grading and types of aggregates, cements, equipment, and kiln temperatures, and then adjust trial batches with various amounts of fly ash to achieve specific technical or economic objectives (Valore 1970). For assistance in this regard, the reader is referred to Siliceous Fines in the Cementing Medium of Steam Cured Concrete Masonry Units”, a 1967 publication by the National Concrete Masonry Association.
Debonding of FRP in bending: Simplified model and
experimental validation
Joan R. Casas
*, Jordi PascualSchool of Civil Engineering, Technical University of Catalonia (UPC), Bridge Engineering, Jordi Girona 1-3, Modulo C1, Campus Norte, 08034 Barcelona, Spain
Received 2 August 2005; received in revised form 15 January 2006; accepted 31 May 2006
Available online 2 October 2006
Abstract
Reinforced concrete members strengthened in bending by bonding of surface-mounted fiber-reinforced polymer (FRP) may present several failure modes: failure of material (reinforcing steel, concrete and composite material) or failure of the interface between concreteadhesive or adhesive-FRP. Nevertheless, experience gained from testing confirms that in most cases delamination prevails over the other possible rupture modes. Delamination in FRP strengthened sections is difficult to model because it involves multiple parameters such as FRP stiffness, adhesive material properties, presence of cracks in concrete, among others. A simplified model to predict strain of FRP at failure is presented in this paper. The experimental validation is presented as well. With this purpose carbon FRP and aramid strengthened specimens and large scale bridge models are considered. The types of bridge models tested include externally-prestressed segmental box-girder and monolithic RC continuous girders. Based on the results of the proposed model, an equation for the prediction of the ultimate force per unit width in the FRP to prevent FRP debonding is proposed. The equation has been experimentally checked with beams of small and large size, representative of real structures. In comparison to other available models, the equation is very simple to apply.
2006 Elsevier Ltd. All rights reserved.Keywords:
Fiber-reinforced polymer; Debonding; Bending; Strengthening; Concrete; Test; Delaminationبرای دریافت اصل مقاله از لینک زیر استفاده نمایید
Construction and Building Materials 21 (2007) 1940–1949
Design recommendations for the strengthening of reinforced concrete beams with externally bonded com
Design recommendations for the strengthening of reinforced concrete beams with externally bonded composite plates
Yung-Chih Wang
*, Kai HsuDepartment of Civil Engineering, National Central University, Jhongli, Taoyuan 32001, Taiwan
Abstract
A design approach for strengthening reinforced concrete beams with externally bonded fibre reinforced polymer (FRP) laminates is proposed. The use of staggered FRP laminate bonding to the tension face of the beam is suggested as an economical design. The FRP development length suggested in the guidelines is adopted. It is recommended that the FRP U-shaped strips be mechanically anchored so as to increase the longitudinal FRP bond strength and enhance the beam’s shear strength. With this design method failure due to debonding of the FRP laminate to the beam soffit can be avoided by the incorporation of FRP U-side strips. Recommendations based on the idea of modified shear friction are also given.
2007 Elsevier Ltd. All rights reserved.Keywords:
Design; Reinforced concrete beams; Retrofitting; Fibre reinforced polymer (FRP); Bending; Shearبرای جهت مطالعه اصل این مقاله از لینک زیر استفاده نمایید
Composite Structures xxx (2008) xxx–xxx
Aspects of behaviour of CFRP reinforced concrete beams in bending
Muhammad Masood Rafi
*, Ali Nadjai, Faris Ali, Didier TalamonaFire Safety Engineering Research & Technology Centre (FireSERT), University of Ulster at Jordanstown, Shore Road, Newtownabbey BT37 0QB, UK
Received 13 February 2006; received in revised form 24 July 2006; accepted 30 August 2006
Available online 18 October 2006
Abstract
The corrosion of steel poses a serious problem to the durability of reinforced concrete structures and fibre reinforced polymer (FRP) has emerged as a potential alternative material to the traditional steel. The results of a test series consisting of carbon FRP (CFRP) and steel bars reinforced concrete beams are reported in this paper. The results indicated that the behaviour of CFRP and steel reinforced beams was similar in many aspects. Both type of beams failed in their predicted modes of failure. The strength design method underestimated nominal moment capacity of CFRP reinforced beams. The deflection of CFRP reinforced beams was satisfactory at service load level, corresponding to theoretical load capacity. The deformability factor of CFRP reinforced beams was more than 6 indicating their ductile nature of failure.
2006 Elsevier Ltd. All rights reserved.Keywords:
Fibre reinforced polymer; Carbon FRP; Mode of failure; Concrete beams; Reinforcement; Moment; Deflection; Deformabilityبرای دریافت اصل مقاله از لینک زیر استفاده کنید
FRP-strengthened RC beams. I: review of debonding strength models
S.T. Smith, J.G. Teng *Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, People’s Republic of China
Received 11 April 2001; received in revised form 5 September 2001; accepted 12 September 2001
Abstract
Bonding of a fibre-reinforced polymer (FRP) plate to the tension face of a beam has become a popular flexural strengthening method in recent years. As a result, a large number of studies have been carried out in the last decade on the behaviour of these FRP-strengthened beams. Many of these studies reported premature failures by debonding of the FRP plate with or without the concrete cover attached. The most commonly reported debonding failure occurs at or near the plate end, by either separation of the concrete cover or interfacial debonding of the FRP plate from the RC beam. This and the companion paper are concerned with strength models for such plate end debonding failures. In this paper, a comprehensive review of existing plate debonding strength models is presented. Each model is summarised and classified into one of the three categories based on the approach taken, and its theoretical basis clarified. The review not only brings together for the first time all existing plate end debonding strength models into a unified framework for future reference, but also provides the necessary background information for them to be assessed in the companion paper using a large test database assembled by the authors from the published literature.
Kywords: Debonding; FRP; RC beams; Retrofitting; Strengthening; Strength models
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Engineering Structures 24 (2002) 385–395
Finite Element and Experimental Serviceability
Analysis of HSC Beams Strengthened with FRP Sheets
1Seyed Hamid Hashemi, 2Reza Rahgozar, 3Ali Akbar Maghsoudi
1PhD. Candidate and 2, 3 Assist. Professor, Department of Civil
Engineering,
Abstract:
The use of externally bonded composite sheets or laminates is by now a diffuse technique to strengthen existing RC structures. However, some aspects of flexural condition still need experimental and numerical analysis; furthermore, especially for serviceability checks, there is a lack of code provisions. In this paper six reinforced high strength concrete (HSC) beams strengthened with FRP sheets were fabricated and tested, the finite element (FE) models adopted by ANSYS was performed to examine the structural behavior of tested beams was performed. A comparison between the finite element analysis results and the experimental data available on the specimens is made and by using trial and error method, the finite element model was calibrated. Six under-reinforced concrete beams were fabricated and tested to failure. With the exception of the control beam, one or four layers of CFRP were applied to the specimens. The structural response throughout the loading regime was primarily captured in terms of the load deflection behavior. The load deflection plots obtained from numerical study show good agreement with the experimental results. The serviceability characteristics of the test beams were evaluated in terms of the crack width, deflection and stress in steel and concrete. The crack patterns in the beams are also presented.
Keywords: Finite Element Model, FRP, HSC, Serviceability
American Journal of Applied Sciences 4(9):725-735, 2007
ISSN 1546-9239
Science Publications 2007
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