Click for next page ( 5


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 4
4 CHAPTER 1 Background 1.1 Problem Statement that crack width has been the most common measure used to quantify acceptable levels of cracks in reinforced concrete Precast/prestressed concrete girders are widely used in the structures. The majority of the cracking studies were con- United States for bridge construction. Longitudinal web cracks ducted to investigate flexural cracking in reinforced concrete have been observed during prestress transfer, particularly at beams. Flexural cracks are formed on the tension side of a the ends of girders. With the use of higher strength concrete, beam, typically at right angles to the reinforcing bars. They deeper girders, and significantly higher prestress forces, these largely depend on the concrete cover, level of stress in the steel cracks are becoming more prevalent and, in some cases, larger. reinforcement, and distribution of the reinforcement. The Reactions to these cracks have ranged from doing nothing to majority of the studies concentrated on providing informa- rejecting girders. Other reactions include debonding strands at tion on sources of cracking, factors affecting crack width, and the ends, reducing permissible prestress force, reducing allow- formulas used to estimate crack width. able compression stress at the time of transfer, injecting sealants Some information on cracking due to other effects--such into cracks, and coating the ends of girders with sealants. Clearly, as shrinkage, temperature, and alkali silica reaction--also was there is no consensus on the causes of the longitudinal cracking found in the literature. However, only a small amount of infor- and what level of longitudinal cracking is unacceptable. mation on the effects of web cracking due to prestress release Concerns regarding end zone cracks are based on the possi- in member ends was found. Web end cracking is most severe bility of having reduced structural capacity and future durabil- when the product is lifted off the bed. The cracks tend to get ity issues from strand corrosion. End cracks that run parallel smaller and sometimes totally disappear as the vertical gravity with and intersect with the prestressing strands, reflecting loads are introduced by superimposed loads and support reac- strand locations, could cause debonding. This would result in tion. When these cracks are diagonal, they are "normal" to that lengthening of the transfer and development lengths, which of the compression struts created by the shearing effects and, may consequently reduce the shear and flexural capacity. Open thus, are not additive to the principal tensile stresses due to cracks that travel along the strands and are exposed to chloride solutions may cause strand deterioration. Therefore, a thor- shear. When diaphragms are used, the most severe cracks at the ough understanding was needed to determine whether longi- member ends are partially enclosed in the diaphragm concrete. tudinal web cracks are of structural significance. If these cracks Thus, it appears to be logical to have less restrictive cracking are not structurally significant, an understanding of whether limitations on web end cracking than on conventionally rein- they reduce durability was required. Although published guid- forced concrete sections subject to flexure. ance exists regarding acceptance and repair criteria, these doc- uments needed validation. 1.2.1 Evolution of Permissible Crack Widths The reader should be aware that the expressions longitudinal web cracking and end zone cracking are synonymous, and they The evolution of, and recommendations for, permissible are used interchangeably in this report. crack widths developed between 1935 and 1970 can be traced from several references (16). A summary of the recommen- dations from these publications is compiled in Table 1.1. It 1.2 Control of Cracking should be noted that the majority of these recommendations in Concrete Structures were based on flexural cracking in beams. The statistical repre- Cracking of concrete structures has been the focus of sentation of these recommendations showed that the flexural researchers for decades. A review of the literature has shown crack width in beams, at 40 ksi tensile stress in reinforcement

OCR for page 4
5 Table 1.1. Permissible crack widths developed between 1935 and 1970.* Source Maximum Crack Conclusion Notes Width (in.) or Exposure Level N.J. Rengers, 1935, as 0.012 - Tolerable crack width Only one specimen tested. contained in Reference 1 0.0120.04 - Excessive crack width 0.0400.080 - Some corrosion danger Abeles, 1937, as 0.0120.016 - Present no danger of rusting Provided there are no special contained in Reference 1 chemical influences. Tremper, 1947, as 0.0050.050 - Cracks of fairly large widths in Sixty-four concrete blocks were contained in Reference 1 a sound concrete will not exposed to a marine environment for promote serious corrosion of 10 years. the reinforcement Brocard, 1957, as 0.004 - Corrosion was not appreciably The reinforcement consisted of thin contained in Reference 1 accelerated. walled steel tubes embedded in 0.024 - Corrosion rate increased by a concrete prisms. factor of 5 to 10. Engel and Leeuwen, 0.008 - Unprotected structures Recommendations are made from 1957, as contained in (external) investigations of structures existing Reference 1 0.012 - Protected structures (internal) for more than 15 years. Brice, 1957, as contained 0.004 - Severe exposure Flexural cracking in beams. in References 2 and 7 0.008 - Aggressive exposure 0.012 - Normal exposure Rusch, 1957, as 0.008 - Aggressive (salt water) Flexural cracking in beams. contained in References 2 0.012 - Normal exposure and 7 Etsen, 1957, as contained 0.0020.006 - Severe to aggressive Flexural cracking in beams. in References 2 and 7 0.0060.010 - Normal exposure (outside) 0.0100.014 - Normal exposure (inside) Voellmy, 1958, as 0.008 - No corrosion occurred Cracked beams were exposed to the contained in Reference 1 0.0080.020 - Slight corrosion at isolated atmosphere for 10 years. The regions locations varied from rural to > 0.020 - More localized corrosion industrial areas. Bertero, 1958, as 0.0010.006 - Exposure to seawater, smoke, These allowable crack widths apply contained in Reference 1 etc. to structural elements under 0.0100.014 - Indoor exposure permanent loading with 1-inch cover. Haas, 1959, as contained 0.008 - Exposed structures (external These values are applicable only in in Reference 1 environment) cases where the reinforcement is 0.012 - Protected structures (internal adequately covered and where the environment) loads are permanent. > 0.012 - Permissible in the absence of heating, humidity, and other aggressive conditions Shalon and Raphael, < 0.008 - Structures exposed to saline air 1959, as contained in - Exposed structures Reference 1 0.008 Hendrickson, as 0.010 - Acceptable limit for reinforced Crack widths smaller than 0.01 in. contained in Reference 1 concrete pipes may often close by means of autogenous healing and therefore present little danger of severe corrosive attack. ACI 1963 Building 0.010 - Exterior members Determined by tests on actual full- Code, Section 1508, as 0.015 - Interior members scale flexural members contained in Reference 1 CEB, 1964, as contained 0.004 - Interior or exterior, aggressive Flexural cracking in beams. in References 5 and 7 and watertight 0.008 - Aggressive 0.012 - Normal U.S. Bureau of Public DL Causes Compression and LL Causes Tension Flexural cracking in beams. Roads 0.008 - Seawater and seawater spray, (Maximum crack width alternate wetting and drying at steel level under 0.008 - Deicing chemicals, humidity service load), 1966, as 0.010 - Salt, air water and soil contained in References 6 0.012 - Air or protective membrane and 7 DL and LL Cause Tension 0.006 - Seawater and seawater spray, alternate wetting and drying 0.006 - Deicing chemicals, humidity 0.008 - Salt, air water and soil 0.010 - Air or protective membrane *Permissible crack widths provided in this table are taken from References 1 through 6.

OCR for page 4
6 Table 1.2. Tolerable crack widths in reinforced concrete structures (7). Exposure Condition Tolerable Crack Width, in. (mm) 1. Water-retaining structures (excluding non-pressure pipes) 0.004 (0.10) 2. Seawater and seawater spray, wetting and drying 0.006 (0.15) 3. Deicing chemicals 0.007 (0.18) 4. Humidity, moist air, soil 0.012 (0.30) 5. Dry air or protective membrane 0.016 (0.41) bars, ranged from 0.0025 to 0.016 in, with the majority of the In 1975, CEB Eurocode No.2 (8) developed limits for cracks results ranging from 0.005 to 0.010 in. developed in beams under flexure and concrete members In the early 1970s, Committee 224 of the American Con- under direct tension, see Table 1.3. crete Institute published the first edition of the ACI 224 The limits given in Table 1.3 were developed based on envi- report, Control of Cracking in Concrete Structures, which give ronmental criteria. A summary of the CEB procedure to principal causes of cracking in reinforced/prestressed concrete check bar spacing to control the crack width can be found and recommended crack control criteria and procedures (7). in Reference 9. In this paper, the author recommended that Since then, the report has undergone several revisions. The the maximum crack width be limited to 0.008 in. to avoid report discusses many possible sources of cracking, such as any concerns by casual observers and the public. shrinkage cracking, flexural cracking, tension cracking, and In 1983, the PCI Committee on Quality Control Perfor- end zone cracking on prestressed concrete members. The mance Criteria developed a report on Fabrication and Shipment report gives the following guidelines, shown in Table 1.2, for Cracks in Prestressed Hollow-Core Slabs and Double Tees (10). tolerable crack widths at the tensile face of reinforced concrete The report provides a collection of various cracks that may structures for typical conditions. The report recommends that occur in hollow-core slabs and double tees during casting, these values of crack width are not always a reliable indication stripping, or shipping. The objectives of the report are to of steel corrosion and deterioration of concrete to be expected. help precast producers and design engineers identify possi- The report states that engineering judgment should be exer- ble sources of cracking and make decisions on the accept- cised and other factors, such as concrete cover, should be taken ability of the product. The report recognizes end-of-beam into consideration to revise these values. Although the report cracking as follows: does not give any guidelines on tolerable crack size specifi- cally for end zone cracking in pretensioned members, it can For hollow-core slabs the report provides two types of web be interpreted from the report that the limits presented in cracking that may occur at prestress release due to the burst- Table 1.2 are applicable to all types of cracks regardless of their ing forces. The first type is above the strands and the second source. The report states the importance of proper design type is at or near the strands, as shown in Figure 1.1(a) of the bursting reinforcement, and that the first row of the and 1.1(b), respectively. The report states that the crack bursting reinforcement should be placed as close as possible width of the first type can range from a hairline up to 0.25 in. to the member end and the rest should be distributed over a (6.3 mm). However, it does not provide a crack width for certain distance. the second type. The report states that these cracks can Table 1.3. Tolerable crack widths in reinforced concrete structures (8). Maximum Crack Width 90th Percentile at Extreme Tensile of the Maximum Exposure Appearance Fiber of the Concrete Crack Width Section (in.) (in.) Severe: Corrosive gasses or soils 0.0012 0.004 Corrosive industrial or maritime Difficult to environment see with the Moderate: naked eye Running water 0.0160 0.008 Inclement weather without aggressive gasses Mild: Conditions where high humidity is Easily 0.0200 0.012 reached for a short period in any one visible year