ACI 318-99 no longer refers to Z factors or crack width formulae as in previous editions of the code. Instead, ACI 318-99 correlates bar spacing with clear concrete cover, indicating that following these guidelines will reduce crack widths at the concrete surface. Field investigations have found leakage at certain type cracks which exhibit widths of .23mm (0.009") or greater. Research and condition surveys completed by the author have found greater potential for concrete deterioration at cracks which extend to embedded reinforcement as compared with low slump, low water/cement ratio concrete having adequate cover over reinforcement. Current codes and standards present considerable variation with regard to recommended maximum crack widths to prevent leakage. Use of ACI 318-99 to design liquid or gas retaining structures could lead to designs that are not conservative, not durable and possibly unsafe, if preventing leakage is an important requirement for the particular facility.
{"title":"What is the Crack Width in Concrete Structures to Prevent leakage?","authors":"L. G. Mrazek","doi":"10.14359/10822","DOIUrl":"https://doi.org/10.14359/10822","url":null,"abstract":"ACI 318-99 no longer refers to Z factors or crack width formulae as in previous editions of the code. Instead, ACI 318-99 correlates bar spacing with clear concrete cover, indicating that following these guidelines will reduce crack widths at the concrete surface. Field investigations have found leakage at certain type cracks which exhibit widths of .23mm (0.009\") or greater. Research and condition surveys completed by the author have found greater potential for concrete deterioration at cracks which extend to embedded reinforcement as compared with low slump, low water/cement ratio concrete having adequate cover over reinforcement. Current codes and standards present considerable variation with regard to recommended maximum crack widths to prevent leakage. Use of ACI 318-99 to design liquid or gas retaining structures could lead to designs that are not conservative, not durable and possibly unsafe, if preventing leakage is an important requirement for the particular facility.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"13 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132220542","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Mirmiran, S. Kulkarni, Richard A. Miller, M. Hastak, B. Shahrooz, R. Castrodale
Precast prestressed girder bridges can be made continuous for live load if the deck and diaphragm are cast with sufficient positive and negative moment reinforcements. The continuity eliminates costly joints and enhances seismic performance, structural integrity and overall durability of the structure. If diaphragm is poured with sufficient negative moment reinforcement before the deck is cast, continuity may also apply to the dead load of the slab. Although, connection of the girders at the diaphragm varies from state to state, it generally consists of bent bars or bent strands. Also, a short length of the girder may be embedded into the diaphragm. The continuity connection is a doubly reinforced section, which requires a time-dependent analysis including differential shrinkage, creep due to prestressing and dead loads, and temperature effects. These time-dependent effects can result in considerable positive restraining moments at the supports, which can in turn crack the diaphragm or pull the girder out of the diaphragm. These positive moment cracks are not only unsightly, but may also result in durability issues for the bridge. Furthermore, it questions the integrity of the continuity connection. The paper examines the extent of positive moment cracking based on field observations, time-dependent analysis, and previous studies.
{"title":"Positive Moment Cracking in Diaphragms of Simple-Span Prestressed Girders Made Continuous","authors":"A. Mirmiran, S. Kulkarni, Richard A. Miller, M. Hastak, B. Shahrooz, R. Castrodale","doi":"10.14359/10816","DOIUrl":"https://doi.org/10.14359/10816","url":null,"abstract":"Precast prestressed girder bridges can be made continuous for live load if the deck and diaphragm are cast with sufficient positive and negative moment reinforcements. The continuity eliminates costly joints and enhances seismic performance, structural integrity and overall durability of the structure. If diaphragm is poured with sufficient negative moment reinforcement before the deck is cast, continuity may also apply to the dead load of the slab. Although, connection of the girders at the diaphragm varies from state to state, it generally consists of bent bars or bent strands. Also, a short length of the girder may be embedded into the diaphragm. The continuity connection is a doubly reinforced section, which requires a time-dependent analysis including differential shrinkage, creep due to prestressing and dead loads, and temperature effects. These time-dependent effects can result in considerable positive restraining moments at the supports, which can in turn crack the diaphragm or pull the girder out of the diaphragm. These positive moment cracks are not only unsightly, but may also result in durability issues for the bridge. Furthermore, it questions the integrity of the continuity connection. The paper examines the extent of positive moment cracking based on field observations, time-dependent analysis, and previous studies.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"84 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115657945","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Use of discrete fibers to reduce plastic shrinkage cracking of concrete is discussed. The results presented cover a wide range of fibers in terms of their material properties such as modulus of elasticity, diameter, lengths, and surface characteristics. Fiber contents used range from 0.45 kg/m3 to 60 kg/m3 and the matrix composition evaluated vary from mortar to concrete with normal and low density aggregates. The influence of fiber properties, fiber geometrics, volume fractions, and matrix compositions were evaluated for the crack reduction of concrete during the initial and final setting period. These cracks eventually influence the long-term durability of concrete. The results indicate that fibers provide a definite contribution to crack reduction and the major parameters that influence the crack reduction are: fiber count, geometry of the fiber, modulus of elasticity of the fiber, and fiber volume fraction. The fiber volume fraction needed for effective crack reduction ranges from 0.1 to 5%.
{"title":"Use of Fibers for Plastic Shrinkage Crack Reduction in Concrete","authors":"P. Balaguru","doi":"10.14359/10819","DOIUrl":"https://doi.org/10.14359/10819","url":null,"abstract":"Use of discrete fibers to reduce plastic shrinkage cracking of concrete is discussed. The results presented cover a wide range of fibers in terms of their material properties such as modulus of elasticity, diameter, lengths, and surface characteristics. Fiber contents used range from 0.45 kg/m3 to 60 kg/m3 and the matrix composition evaluated vary from mortar to concrete with normal and low density aggregates. The influence of fiber properties, fiber geometrics, volume fractions, and matrix compositions were evaluated for the crack reduction of concrete during the initial and final setting period. These cracks eventually influence the long-term durability of concrete. The results indicate that fibers provide a definite contribution to crack reduction and the major parameters that influence the crack reduction are: fiber count, geometry of the fiber, modulus of elasticity of the fiber, and fiber volume fraction. The fiber volume fraction needed for effective crack reduction ranges from 0.1 to 5%.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"38 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126670966","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A case history is presented in which a laser-screeded slab showed more cracks than would be expected in conventional strip-cast slabs. It was determined that laser-screeded slabs are more sensitive to early-age, thermal cracking than strip-cast slabs because of extra restraint provided by fixed-edge boundary conditions. Among the possible solutions is closer spacing of contraction joints and fog curing during the first day.
{"title":"Early-Age Thermal Cracking in Laser-Screeded Concrete Slabs","authors":"H. Haynes","doi":"10.14359/10813","DOIUrl":"https://doi.org/10.14359/10813","url":null,"abstract":"A case history is presented in which a laser-screeded slab showed more cracks than would be expected in conventional strip-cast slabs. It was determined that laser-screeded slabs are more sensitive to early-age, thermal cracking than strip-cast slabs because of extra restraint provided by fixed-edge boundary conditions. Among the possible solutions is closer spacing of contraction joints and fog curing during the first day.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"19 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126793790","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A new technology was developed for controlling diagonal shear cracking in earthquake resistant concrete bridge columns. The technology consists of external prestressing columns in transverse direction by means of individual hoops consisting of prestressing strands and specially designed anchors. It is especially suitable for seismic retrofitting circular bridge columns against shear. The technology was verified through experimental investigation of shear deficient columns. Three full-scale columns were tested under constant axial compression and incrementally increasing lateral deformation reversals. The results, presented and discussed in the paper, indicate that the technique can control diagonal cracking, thereby suppressing shear failure and promoting flexural behavior. A design procedure is also presented for seismic retrofit of shear-deficient columns.
{"title":"Mitigation of Seismic Induced Diagonal Cracks in Concrete Columns by External Prestressing","authors":"M. Saatcioglu","doi":"10.14359/10820","DOIUrl":"https://doi.org/10.14359/10820","url":null,"abstract":"A new technology was developed for controlling diagonal shear cracking in earthquake resistant concrete bridge columns. The technology consists of external prestressing columns in transverse direction by means of individual hoops consisting of prestressing strands and specially designed anchors. It is especially suitable for seismic retrofitting circular bridge columns against shear. The technology was verified through experimental investigation of shear deficient columns. Three full-scale columns were tested under constant axial compression and incrementally increasing lateral deformation reversals. The results, presented and discussed in the paper, indicate that the technique can control diagonal cracking, thereby suppressing shear failure and promoting flexural behavior. A design procedure is also presented for seismic retrofit of shear-deficient columns.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"4 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125557278","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Cracking in concrete repair systems is one of the truly critical phenomena of repair pathology responsible for corrosion, deterioration and failure. The problem of repair cracking has become widespread not only with respect to severe environments which are intensifying restrained volume change stresses but also with respect to repairs in relatively benign environments. Cracking accelerates the penetration of aggressive substances into the concrete and repair material from the exterior environment which in turn aggravates any one or a number of various mechanisms of deterioration. Moisture transport mechanism in the repaired structures is a tool for transferring an outer standard environment into an inner environment, and from one inner environment (existing substrate) into another (repair material). The crack resistance of concrete repair is bearing on three equally important "elephants": (1) design details and specifications; (2) repair materials; (3) in-situ workmanship and quality control. This study demonstrates that the properties of cementitious repair materials have to be engineered for dimensional compatibility with existing concrete to improve their resistance to cracking. How good should the cementitious composite material used for repair of existing concrete structures be? How good is good enough. The paper summarized the factors involved and approaches taken when selecting cementitious repair materials. Performance criteria are presented for the selection of dimensionally compatible repair materials and standard material data sheet protocols. The recommended approach can enable material quality improvement, more accurate service life prediction, and satisfactory performance of repaired concrete structures during their intended service life.
{"title":"Cracks-Concrete Repair’s life Threatening Wounds","authors":"A. Vaysburd, R. Poston, J. E. McDonald","doi":"10.14359/10823","DOIUrl":"https://doi.org/10.14359/10823","url":null,"abstract":"Cracking in concrete repair systems is one of the truly critical phenomena of repair pathology responsible for corrosion, deterioration and failure. The problem of repair cracking has become widespread not only with respect to severe environments which are intensifying restrained volume change stresses but also with respect to repairs in relatively benign environments. Cracking accelerates the penetration of aggressive substances into the concrete and repair material from the exterior environment which in turn aggravates any one or a number of various mechanisms of deterioration. Moisture transport mechanism in the repaired structures is a tool for transferring an outer standard environment into an inner environment, and from one inner environment (existing substrate) into another (repair material). The crack resistance of concrete repair is bearing on three equally important \"elephants\": (1) design details and specifications; (2) repair materials; (3) in-situ workmanship and quality control. This study demonstrates that the properties of cementitious repair materials have to be engineered for dimensional compatibility with existing concrete to improve their resistance to cracking. How good should the cementitious composite material used for repair of existing concrete structures be? How good is good enough. The paper summarized the factors involved and approaches taken when selecting cementitious repair materials. Performance criteria are presented for the selection of dimensionally compatible repair materials and standard material data sheet protocols. The recommended approach can enable material quality improvement, more accurate service life prediction, and satisfactory performance of repaired concrete structures during their intended service life.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"26 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125656529","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"\"Design for Crack Control in Reinforced and Prestressed Beams ,Two-Way Slabs and Circular Tanks\"","authors":"E. Nawy","doi":"10.14359/10812","DOIUrl":"https://doi.org/10.14359/10812","url":null,"abstract":"","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"46 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128292452","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Shrinkage reducing admixtures (SRA's) are a new type of admixtures, which are effective in reducing the drying shrinkage of concrete. SRA performance has typically been evaluated on the basis of unrestrained drying shrinkage tests. However, it is usually the cracking performance of concrete when shrinkage is restrained that is of primary interest to the marketplace. The current paper presents an evaluation of SRA's based on several parameters: free shrinkage, tensile stresses which develop in a uniaxially restrained rig, and the sensitivity to cracking in such conditions. The positive influence of SRA's on all of these three parameters is demonstrated. A comparison is made between the effect of SRA and of low-volume, polypropylene fiber reinforcement. The latter is known to be effective in controlling early age plastic shrinkage cracking. The present data show that in the case of hardened concrete, after one day of curing, low volumes of fibers do not give any advantage, and it is in this range where the SRA is effective. Thus, the two types of additives can complement each other: the fibers are efficient in controlling plastic shrinkage cracking while the SRA can take over the role of crack control in the hardened concrete, where low volume-low modulus fibers are not effective.
{"title":"Crack Mitigation Effects of Shrinkage Reducing Admixtures","authors":"A. Bentur, M. Berke, T. Durning","doi":"10.14359/10818","DOIUrl":"https://doi.org/10.14359/10818","url":null,"abstract":"Shrinkage reducing admixtures (SRA's) are a new type of admixtures, which are effective in reducing the drying shrinkage of concrete. SRA performance has typically been evaluated on the basis of unrestrained drying shrinkage tests. However, it is usually the cracking performance of concrete when shrinkage is restrained that is of primary interest to the marketplace. The current paper presents an evaluation of SRA's based on several parameters: free shrinkage, tensile stresses which develop in a uniaxially restrained rig, and the sensitivity to cracking in such conditions. The positive influence of SRA's on all of these three parameters is demonstrated. A comparison is made between the effect of SRA and of low-volume, polypropylene fiber reinforcement. The latter is known to be effective in controlling early age plastic shrinkage cracking. The present data show that in the case of hardened concrete, after one day of curing, low volumes of fibers do not give any advantage, and it is in this range where the SRA is effective. Thus, the two types of additives can complement each other: the fibers are efficient in controlling plastic shrinkage cracking while the SRA can take over the role of crack control in the hardened concrete, where low volume-low modulus fibers are not effective.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"41 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132277147","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A number of fundamental concepts relevant to all types of cracking are examined. A tension stiffening relationship derived from first principles indicates that traditional empirical relationships include significant residual tension stresses from uncracked concrete. Service load crack strains should not be estimated using an empirical tension stiffening expression. While primary cracks continue to form up to strains of 0.0010, due to deformation of concrete between visible cracks, the minimum strain that should be used with the stable crack spacing is 0.0005. A magnification factor must be applied to crack spacings at smaller strains, or a minimum strain of 0.0005 used to estimate crack width. Test results indicate that the 95th percentile crack width is 2.0 times the average crack width. Procedures for diagonal crack inclination, spacing and width are reviewed, and a simplified expression for estimating diagonal crack widths is presented. Diagonal crack widths are generally larger than flexural crack widths in members with orthogonal reinforcement due to diagonal strains being larger than reinforcing bar strains. Current code requirements for side-face reinforcement were developed to control flexural cracking, and may not be adequate to control diagonal cracking in certain exposure conditions. The simplified expression for diagonal cracking was used to develop an expression for the maximum spacing of side face reinforcing bars to control flexural and diagonal cracking in large members. A design example illustrates the proposal. Finally, it is shown how the proposed methodology can be used to extend the current ACI expression for spacing of reinforcement near a surface in tension to treat the case of diagonal cracking.
{"title":"Diagonal Cracking and Diagonal Crack Control in Structural Concrete","authors":"P. Adebar","doi":"10.14359/10815","DOIUrl":"https://doi.org/10.14359/10815","url":null,"abstract":"A number of fundamental concepts relevant to all types of cracking are examined. A tension stiffening relationship derived from first principles indicates that traditional empirical relationships include significant residual tension stresses from uncracked concrete. Service load crack strains should not be estimated using an empirical tension stiffening expression. While primary cracks continue to form up to strains of 0.0010, due to deformation of concrete between visible cracks, the minimum strain that should be used with the stable crack spacing is 0.0005. A magnification factor must be applied to crack spacings at smaller strains, or a minimum strain of 0.0005 used to estimate crack width. Test results indicate that the 95th percentile crack width is 2.0 times the average crack width. Procedures for diagonal crack inclination, spacing and width are reviewed, and a simplified expression for estimating diagonal crack widths is presented. Diagonal crack widths are generally larger than flexural crack widths in members with orthogonal reinforcement due to diagonal strains being larger than reinforcing bar strains. Current code requirements for side-face reinforcement were developed to control flexural cracking, and may not be adequate to control diagonal cracking in certain exposure conditions. The simplified expression for diagonal cracking was used to develop an expression for the maximum spacing of side face reinforcing bars to control flexural and diagonal cracking in large members. A design example illustrates the proposal. Finally, it is shown how the proposed methodology can be used to extend the current ACI expression for spacing of reinforcement near a surface in tension to treat the case of diagonal cracking.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"57 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124747675","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper provides an outline of the provisions for design for cracking given in the current version of Eurocode 2: the Eurocode for the design of concrete structures. The basic theory underlying the clauses is derived, the content of the clauses themselves are outlined and the development of simplified detailing rules for the control of cracking is considered.
{"title":"Crack Control Provisions in the New Eurocode for the Design of Concrete Structures","authors":"A. Beeby","doi":"10.14359/10814","DOIUrl":"https://doi.org/10.14359/10814","url":null,"abstract":"This paper provides an outline of the provisions for design for cracking given in the current version of Eurocode 2: the Eurocode for the design of concrete structures. The basic theory underlying the clauses is derived, the content of the clauses themselves are outlined and the development of simplified detailing rules for the control of cracking is considered.","PeriodicalId":130124,"journal":{"name":"SP-204: Design and Construction Practices to Mitigate Cracking","volume":"21 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2001-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133023529","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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