Width development and crack properties

The relationship between the load and the deformation of the beams is shown in Figure 6. It can be seen that the relationship between the load and the compressive strain of the concrete in the beams can be divided into three parts including: elastic deformation ( when the load is less than the load, cracks appear), then the deformation sustaining zone and finally the failure zone. This result is consistent with the theory of calculation and design of reinforced concrete beams according to TCVN 5574:2012. The relationship between load and tensile strain of concrete types is linear when the load is lower than the load on which cracks appear. At load when cracks appear, the tensile strain of beams using reinforced concrete is about 117 μm, larger than those of CI and II beams (97 μm and 95 μm, respectively), but the tensile strain of concrete beams when using CKDXK7% (BTCLTC-XK7%) is much larger (about 130 μm). This is because the CLBTTC particles always have many defects (cracks, voids), so the tensile strength of BTCLTC decreases sharply [17,20]. However, the use of alkaline slag binder did not improve the tensile strain of the BTCLTC beam.

Cracks often appear in the tensile zone of reinforced concrete beams and when the tensile stress in this region exceeds the tensile strength of the concrete. As the load increases, the cracks grow with height. After that, inclined cracks began to appear and the number of cracks also increased. Figure 7 shows the relationship between flexural loads and the width of cracks appearing in the beam, the value shown is the average value of the width of the first cracks of the test beams. The increased load will increase the crack width according to an exponential approximation. With the same load level, the crack width of reinforced concrete beams is much larger than that of reinforced concrete beams (DCI and DCII) [22].

Figure 8 visually shows the shape and crack distribution on beams using CLTN and CLBTTC at breaking loads. From the crack diagram, it is shown that most of the cracks occur in the pure flexural zone of the beam (in the flexural load region). At first, the cracks appear perpendicular to the beam axis, then the crack height gradually increases and changes inclination angle rapidly and becomes inclined crack. The cracks in the reinforced concrete beams are usually longer and farther apart than the cracks in the control beams (DCI, CCII). Some small cracks can link together into large cracks and accelerate the destruction of the concrete compressive area of ​​the beam. Moreover, the number of cracks in the reinforced concrete beams is also higher than that of the control beams. This result is also consistent with previous studies of Arezoumandi and Knaack [7,10]. The use of CKDXK7% contributed to limiting the crack width development and reducing the number of cracks in the reinforced concrete beams, but not significantly (Figure 8).

4. Conclusion

From the research results, some conclusions can be drawn as follows:
Recycled concrete aggregates replace natural aggregates, significantly reducing the mechanical properties of concrete. However, the alkaline slag binder that completely replaces cement has the ability to significantly improve the mechanical properties of concrete, but the number of cracks and the crack width are not significantly improved.

Recycled aggregate concrete using an alkaline slag binder has the same bending behavior as a natural aggregate concrete beam using cement and the failure occurs at a load state causing yielding of the longitudinal reinforcement. . The crack appearing load and the destructive load of recycled aggregate concrete beams are 25 and 10% lower than those of normal concrete beams. However, the flexural mechanical behavior of the reinforced concrete beams has been significantly improved and is equivalent to the control reinforced concrete beams when using alkaline slag binders to completely replace Portland cement.

Simultaneous use of recycled aggregate from concrete waste to replace natural aggregate, combined with alkaline slag binder (product from industrial waste of iron and steel industry) to replace Portland cement, Not only can it create concrete with quality equivalent to natural aggregate cement concrete in terms of bearing capacity, but also contribute to minimizing the environmental impacts caused by the concrete and cement production process. caused by traditional bamboo shoots.


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2. Hansen T.C. (1992), “Demolition and Reuse of Concrete and Masonry: recycling of demolished concrete, recycling of masonry rubble, and localized cutting by blasting of concrete”, RILEM report 6, E & EN Spon, London.
3. Kien T.T, Thanh T.L, Lu V. P. (2013), “Recycling construction demolition waste in the world and in Vietnam”, Ed. Soutsos Marios et al., The international Conference on Sustainable Built Environment for Now and the Future, 26-27 March 2013, Construction publishing house, Hanoi, Vietnam, 247-256.

4. Jianzhuang X. et. al. (2012), “An overview of study on recycled aggregate concrete in China (1996-2011)”, Construction and Building Materials, 31:364-383.

5. Yagishita F., Sano M., Yamada M. (1994), “Behavior of reinforced concrete beams containing coarse recycled aggregate”, Demolition and reuse of concr

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