Manual ACI 231R-10: Report on Early-Age Cracking: Causes, Measurement and Mitigation

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The comparative result implies that cracking stress may be estimated using this equation and splitting strength even in early-age concrete. Table 4 gives the maximum strain of each test, which represents the tensile strain capacity of the concrete. The strain capacity develops according to hydration before the age of 2 days, and becomes constant after the age of 3 days. The difference may be caused by the restraint strain of concrete shrinkage by the embedded rebar. The restraint of concrete shrinkage induces the tensile pre-stress and the growth of micro cracks around the rebar.

Most of concrete structures have rebars near the surface to prevent cracking, so the strain capacity shown in the paper may be considered as conservative data for the design. The study evaluates the slip by integrating both the strains from the mid-span, which has a boundary condition of zero-slip.

That is to say, the sum of the two strains is the slip as given in Equation 5 and Figure Mimura et al. The assumption is appropriate for the range where the strain of the concrete is equal to the strain of the rebar. However, concrete strain near the bond end is not always equal to the strain of the rebar as illustrated in Figure Thus, this study considers the strain of concrete near the bond end for the estimation of slip.

Assuming a concrete element is a linear elastic to tensile stress, the mean strain of concrete can be estimated by using Equation 6. Therefore this study uses the tensile modulus obtained in the previous chapter instead of the compressive modulus. Equation 7 gives a definition of local bond stress. For simplification, this study assumes the deformed bar as a round bar having a nominal diameter of 13 mm. Yamao et al. Referring to the method in these investigations, the study estimates the bond stresses. As shown in Equation 7 and Figure 11 , the bond stress is estimated by differentiating the strain distribution of rebar.

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Based on the previous evaluation method, the present study also employs the standard bond stresses and the relative slip. In particular, the method using standard bond stress may be appropriate because the property of the early age concrete varies with time even in the same mix proportion. Figure 12 shows the relationship between the standard bond stress and the relative slip. The bond stress at the age of 0. Figure 12 shows that the bond stresses at the age of 1 day and 1.

This observation represents the insufficient bond of rebar in the unhardened concrete at a very early age. On the other hand, the bond behavior after the age of 2 days demonstrates almost the same curve as shown in Figure Thus, the bond behavior after this age may be predicted by the relationship for mature concrete.

A regression curve for the experiment is given in Equation 9. The coefficient a is 0. Figure 13 shows that the data obtained from the experiments can be expressed by using the empirical equation. The coefficient a represents the ultimate bond stress; it develops with age and becomes constant after the age of 2 days. Interestingly, the coefficient b of all the regression lines in the graph is approximately 0.

The empirical equations may be useful for predicting the crack width of early age concrete, but they are limited to the mix proportions and materials used in the study. Empirical equations to predict bond behaviors. The present study aimed to obtain the tensile properties of the concrete which is usually employed in construction of infrastructure in Japan.

Based on the uniaxial tension test using RC slender member, this paper describes the cracking stress and bond properties of early age concrete. The conclusions of the study are listed as follows:. Concrete strain can be estimated by using the strain of a rebar having a long embedment length.

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Tensile stress-strain responses of early age concrete are extremely linear. The empirical equation for mature concrete can be applied to the relationship between uniaxial tensile strength and splitting tensile strength even in early age concrete.

R Report on Early-Age Cracking: Causes, Measurement and Mitigation

Bond behaviors after the age of 2 days are almost equal, so an equation can be applied to this behavior. The empirical equation based on the test is proposed in the paper. National Center for Biotechnology Information , U. Journal List Materials Basel v. Materials Basel. Published online Aug 2.

Find articles by Isamu Yoshitake. Find articles by Wenbo Zhang. Author information Article notes Copyright and License information Disclaimer. Abstract The present study aims to obtain the tensile properties of early age concrete based on a uniaxial tension test employing RC slender members. Keywords: cracking, tensile stress, bonding. Introduction The mechanical properties of concrete vary according to the degree of hydration as the strength develops with the age of the concrete.

Experimental Program 2. Materials and Mix Proportion of Concrete The present study uses the materials shown in Table 1 for concrete. Table 1 Materials for concrete. Open in a separate window.

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Table 2 Mix proportion of concrete. Figure 1. Mechanical Properties of Concrete The uniaxial tension tests were conducted to obtain tensile properties of concrete at early age. Table 3 Tests and number of specimens. Age days 0. Figure 2. Figure 3. Uniaxial Tension Test Both ends of the rebar were attached to a loading jig, and each of the end conditions was rotatable.

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Figure 4. Cracking Behavior of Early Age Concrete 3. Evaluation Method for Stress of Concrete Figure 5 presents a typical strain distribution of the rebar embedded in the concrete. Figure 5. Strain distributions of the rebar embedded in the concrete at 7 days old. Figure 6.

Figure 7. Tensile Stress-Strain Response Figure 8 shows the tensile stress-strain responses at the ages of 1, 2 and 7 days. Figure 8. Most hundreds, currently, when looking about Agriculture and Food Production Revolution, 're Making about the Declarative knowledge of students that confederacy is.

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