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Home > Publications > International Concrete Abstracts Portal
The International Concrete Abstracts Portal is an ACI led collaboration with leading technical organizations from within the international concrete industry and offers the most comprehensive collection of published concrete abstracts.
Showing 1-5 of 14 Abstracts search results
August 1, 2001
E. G. Nawy
This paper presents the state-of-the art in the evaluation of the flexural crack width development and crack control of flexural cracks in reinforced and prestressed concrete structures It is based on extensive research over the past five decades in the United States and overseas in the area of macro-cracking in reinforced and prestressed concrete elements. Mitigation and control of cracking has become essential in order to maintain the integrity and aesthetics of concrete structures and their long-term durability performance. The trend is stronger than ever towards better utilization of concrete strength, use of higher strength concretes in the range of 12,000-20,000 psi and higher compressive strength, more prestressed concretes and increased uses of limit failure theories - all these trends require closer control of serviceability requirements of cracking and deflection behavior. The paper discusses and presents common expressions for the mitigation and control of cracking in reinforced concrete beams and thick one-way slabs, prestressed, pretensioned and post-tensioned flanged beams, reinforced concrete two-way action structural floor slabs and plates, and large diameter circular tanks In addition, recommendations are given for the maximum tolerable flexural crack widths in concrete elements based on the cumulative experience of many investigators over the past five decades. The expressions include the ACI 3 18-99 crack control provisions in reinforced concrete beams and one-way slabs, and the Concrete Euro Code 1999 for the design of concrete buildings.
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 untracked 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 0.00 10, 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 9Sth 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 AC1 expression for spacing of reinforcement near a surface in tension to treat the case of diagonal cracking.
L. G. Mrazek
AC1 3 18-99 no longer refers to Z factors or crack width formulae as in previous editions of the code. Instead, AC1 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 AC1 3 18-99 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.
R. J. Frosch
The AC1 building code has adopted a new design method for the control of flexural cracking. This design method is intended for structures containing steel reinforcement and not requiring specialized crack control procedures. It is the objective of this paper to explore the background for this method, highlight the assumptions, and develop design tools that can be applied for special design cases. This paper presents a summary of a physical model for cracking that was the basis for the new design method, illustrates the development and limitations of the design method, and develops design tools that are applicable for the control of cracking in structures requiring increased levels of crack control and in structures incorporating alternative reinforcement materials. Examples illustrating the use of the new design method as well as tools extending its applicability are presented.
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 ranges 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 geometries, 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 percent.
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