ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297018
36 | ASPIRE , Winter 2012 F H WA F H WA C o n c r e t e i s a q u a s i - b r i t t l e m a t e r i a l with a low tensile strength. Applied loadings, deleterious chemical r eactions, and environmental effects can result in the development of tensile stresses in concrete. When these tensile stresses exceed the tensile strength, the concrete will crack. The extent and size of cracks have an effect on the performance of the bridge. However, the adverse effects of cracking can be minimized by proper selection of materials and proportions, attention to design and details, and quality control and quality assurance in fabrication and construction. This article outlines practices in control of concrete cracking to ensure better short- and long-term performance of bridges. Concrete can be used satisfactorily for an extended period of time without any significant loss of aesthetics, service life, safety, and serviceability. It is important to understand why cracks develop in bridges. Much of the cracking in concrete can be traced to volumetric instability or deleterious chemical reactions. The volume instability results from response to moisture, chemical, and thermal effects. External loading is responsible for generating the majority of the tensile stresses in a bridge. Table 1 Classification of Cracks provides basic information on the main causes of cracking in concrete. The impact of cracking on dura bility, especially corrosion, is detrimental to the performance of highway bridges. In particular, tidal exposures initiate dr y-wet cycles and provide a constant source of salts to enter the cracks, significantly exacerbating deterioration. Similarly, cracked concrete in contact with sulfate rich soil can lead to accelerated sulfate attack. S t u d i e s s h o w t h a t c r a c k w i d t h h a s a significant influence on the corrosion process. When the cracks are relatively small (< 0.04 in.), they have little impact on the corrosion process and the structural performance. However, larger cracks (> 0.04 in.) increase the corrosion rate and lead to poor structural performance. The LRFD Specifications T h e A A S H T O L R F D B r i d g e D e s i g n S p e c i f i c a t i o n s h a s p r o v i s i o n s fo r c r a c k control to assure serviceability, aesthetics, and economy. Article 3.4.1, Load Factors and Load Combinations, Service Limit States I, III, and IV are intended to control crack width and tension in reinforced concrete, prestressed concrete, and segmental concrete members. Article 126.96.36.199, Crack Control Reinforcement, is intended to control the width of cracks by redistribution of internal stresses using the strut-and-tie models for determining internal force effects. Article 188.8.131.52, Control of Cracking by Distribution of Reinforcement, is intended for the distribution of tension reinforcement to control flexural cracking. Article 184.108.40.206, Maximum Spacing of Transv erse Reinforcement, is intended to provide crack control related to shear and torsion. Article 5.10.8, Shrinkage and Temperature Reinforcement, is intended for the control of cracking due to shrinkage and temperature effects. Transportation Research Circular The Transportation Research Circular EC-107 (2006), Control of Cracking in Concrete: State of the Art, was prepared by the Transportation Research Board (TRB) Basic Research and Emerging Technologies Related to Concrete Committee (AFN 10). The circular discusses causes of cracking, testing, and ways to minimize stresses and strains that cause cracking in bridges and p a v e m e n t s . T h e m o s t c o m m o n c a u s e o f premature deterioration in concrete bridges and pav ement s may be attributed to the development of cracks. The reasons for cracking are identified in the circular with guidance for prevention and crack control in structural design and detailing, selection of materials, Control of Concrete Cracking in Bridges by M. Myint Lwin, Federal Highway Administration TABLE 1 Classiﬁcation of Cracks Type of Cracking Form of Crack primary Cause Time of Appearance Plastic settlement Over and aligned with reinforcement, subsidence under reinforcing bars Poor mixture design leading to excessive bleeding; excessive vibration 10 minutes to 3 hours Plastic shrinkage Diagonal or random Excessive early evapo- ration 30 minutes to 6 hours Thermal expansion and contraction Transverse Excessive heat generation; exces- sive temperature gradients 1 day to 2-3 weeks Drying shrinkage Transverse, pattern, or map cracking Excessive mixture wa- ter; inefﬁcient joints; large joint spacings Weeks to months Freezing and thawing Parallel to the surface of concrete Lack of proper air- void system; nondurable coarse aggregate After one or more winters Corrosion of rein- forcement Over reinforcement Inadequate cover; ingress of sufﬁcient chloride More than 2 years Alkali-aggregate reaction Pattern and longitu- dinal cracks parallel to the least restrained side Reactive aggregate plus alkali hydroxides plus moisture Typically more than 5 years, but weeks with a highly reactive material Sulfate attack Pattern Internal or external sulfates promoting the formation of ettringite 1 to 5 years Book_Win12.indb 36 12/29/11 11:12 AM