Experimental Study on Impermeability of Recycled Concrete

Recycled concrete is a kind of concrete which is constructed by crushing and removing the building waste and concrete blocks and mixing them according to a certain proportion after grading. In this study, the applicability of recycled concrete is studied only in terms of impermeability.


Introduction
Under the background of protecting environment, saving resources and harmonious development of human and nature, recycled concrete is bred.Recycled concrete is the construction waste to be broken, classified according to a certain proportion, used for coarse aggregate of concrete, so as to replace the original natural aggregate.There are many factors affecting the concrete, the applicability of recycled concrete is studied only in the aspect of impermeability.

Experimental ideas
The micro factor that affects the impermeability of concrete is cracks in the concrete.To enhance the impermeability of recycled concrete, it is necessary to reduce the internal cracks.The influencing factors of the impermeability of recycled concrete can be studied by means of the control variable method, such as sand ratio, replacement rate, water cement ratio, etc.A gradient test designed for influencing factors under the conditions of the specification.
Calculation formula of impermeability grade: 2、According to the calculation of anti permeability grade, if the maximum water pressure value H is even, then the calculated S value is odd, at this time S-1 is the impermeability grade, for example: H=0.8MPa, S=7, then the impermeability grade is S 6 .
The anti permeability level take calculation formula of impermeability coefficient: K: relative impermeability coefficient, unit cm/s; D: average water penetration height, unit cm; H: the pressure of water is expressed as the height of water, unit cm; T: constant pressure time, unit s; a: water absorption rate of recycled concrete.
Water absorption formula: m 0 : The quality of non absorbent, unit g; m: quality after water absorption unit g.Notes: Impermeability grade and impermeability coefficient are different, impermeability coefficient is used in hydraulic concrete, relative permeability coefficient is calculated by reference seepage height.In this test, the anti -permeability instrument is a step-up anti-seepage instrument, so the formula of the impermeability grade is selected.

Materials selection
Choose suitable construction waste as the test coarse aggregate, because the waste concrete requires the machine to break it, artificial fragmentation can not be achieved.In addition, the manufacturing of asphalt materials and sintered brick materials of concrete, under the same condition, the impermeability of sintered brick materials better than asphalt brick materials, also the strength of concrete and asphalt material is shortage, so this experiment selects sintered brick as coarse aggregate of recycled concrete.

experimental dates
The recycled concrete strength grade in this experiment is C30, the pressure increase v be equal to 0.0125Mpa/h, initial pressure 0.3MPa, the impermeability grade is S 4 , a total of five specimens of each test group, each group of 3 specimens, each gradient experiments were performed two times.Test results are shown in table 1 and table 2. Comparison of the results of the two tests is shown in figure 1-figure 4.  From the water cement ratio data, it can be seen that when the other conditions are unchanged, when the water cement ratio is 0.4, the seepage resistance pressure is the largest.With the increase of water cement ratio, the resistance to seepage pressure decreases continuously.Therefore, the water cement ratio is controlled at about 0.4, which can improve the impermeability of recycled concrete.
2. From the sand rate data, it is found that when the sand rate is about 32%, the impermeability pressure of recycled concrete is the maximum.
3. From the data of substitution rate, it can be seen that when the substitution rate is about 50%, the maximum impermeability pressure is observed.Therefore, adding 50% natural aggregate to the waste aggregate can not only improve the impermeability of recycled concrete, but also save the cost effectively.
4. From the data of the fly ash, it can be seen that the pressure decreases the fly ash is greater than 30%.Therefore, when the content of fly ash is about 30%, the impermeability of recycled concrete can be appropriately improved.
The test shows that the impermeability of recycled concrete can be improved by controlling the water cement ratio, sand rate, replacement rate of recycled aggregate and adding admixtures.But because of the particularity of recycled concrete itself, the crack inside the aggregate can not be avoided.Therefore, it is suggested that from the macroscopic and microcosmic two angles, it is a new direction to study how to remedy the cracks produced by recycled aggregate effectively.

Figure 2 Figure 4
Figure 2 sand rate test results of water cement ratio

Table 1
Water cement ratio and sand rate, impermeability gradient test results

Table 2
Test results of substitution rate and content of fly ash against seepage gradient

Water cement ratio Average pressure/Mpa the first time the second time Figure 1
Comparison of gradient test results of water cement ratio