Experimental study on antifreeze performance of recycled concrete mixed with hydroxypropyl methylcellulose

Abstract: Recycled concrete can effectively utilize waste concrete, but its frost resistance is far inferior to that of ordinary concrete. Hydroxypropylmethylcellulose (HPMC) is a white powdery substance, chemically stable and inactive. By changing the replacement rate of HPMC and recycled coarse aggregate in recycled concrete, the effects of different replacement rates on the mass loss rate and relative dynamic elastic modulus of recycled concrete were explored, and then the frost resistance of recycled concrete was obtained. It can be obtained through experiments: within a certain range, HPMC can enhance the frost resistance of recycled concrete to a certain extent, and the enhancement effect is the best when the cellulose content reaches 0.6%; however, as the content of recycled coarse aggregate continues The frost resistance of recycled concrete decreases.

Key words: hydroxypropyl methylcellulose; mass loss rate; relative dynamic elastic modulus; recycled concrete

After entering the 21st century, the accelerated pace of urbanization has led to the rapid development of the world's construction industry. The use of concrete, the most commonly used building material, has risen rapidly. At the same time, the amount of discarded concrete has also increased year by year. showing a steady growth trend. The impact of the development of concrete technology on recycling has not ceased since the existence of concrete. Research on recycled concrete is mainly for two purposes, one is to improve the performance of concrete, and the other is to utilize solid waste, energy saving and environmental protection. Therefore, the development and research of recycled concrete will bring obvious social benefits, promote the green development of the construction industry, and play a major role in the sustainable development of society. Frost resistance is one of the durability indicators of concrete, and a large number of scholars at home and abroad have done a lot of research in this area. Nagatakia et al. found that the relative dynamic elastic modulus and mass loss rate of recycled concrete were much lower than ordinary concrete, indicating that its frost resistance was not as good as that of ordinary concrete; Zhang Leishun et al. found that the strength loss index can be used to measure the frost resistance of concrete, while Reducing the water-cement ratio can improve the strength of concrete; Wang Wuxiang et al. believe that there are a lot of pores between recycled aggregates, and the water in these pores freezes at low temperature, which increases the quality of concrete and improves the frost resistance of recycled concrete.

1. Test plan

1.1 Raw materials

(1) Cement: 42.5# Ordinary Portland cement is produced by Anhui Zhonglian Cement Co., Ltd.;

(2) Fine aggregate: ordinary high-quality river sand with a fineness modulus of 2.9, a moisture content of 4.2%, an apparent density of 2710 kg/m³, and a good gradation. The sand rate is 40%;

(3) Coarse aggregate: The natural coarse aggregate is made of crushed stone with a clean surface and low mud content, with a maximum particle size of 40 mm. The waste coarse aggregate is obtained from the laboratory of Suzhou Engineering Quality Inspection Center with an initial strength of C30. waste concrete. Put the waste concrete test block into the jaw crusher for crushing, and under the action of the sieving machine, select the recycled coarse aggregate with a particle size range of 5-25 mm.

(4) Fiber: Hydroxypropylmethylcellulose (HPMC) produced by KIMA CHEMICAL CO.,LTD.

(5) Water: All use ordinary tap water.

1.2 Mix ratio design

The amount of coarse and fine aggregate, cement, water and other materials required for the test is obtained through the calculation formula of ordinary concrete mix ratio. Because natural crushed stone has a small water absorption rate, the water absorption rate of natural crushed stone is not within the scope of our test this time. This test is based on the mix ratio of C30 concrete, the sand rate is 40%, the water-cement ratio is 0.5, and the replacement rate of recycled coarse aggregate is 0%, 20%, 40%, 60%, respectively.

The replacement rates of HPMC were 0%, 0.3%, 0.6%, and 0.9%, respectively, and there were a total of 16 groups of specimens, with 3 specimens in each group and a total of 48 specimens.

1.3 Specimen preparation and maintenance

Various materials are calculated and added to the forced mixer according to the mixing ratio. Because HPMC swells into a jelly in cold water, in order to prevent the cellulose in the matrix concrete from agglomerating, it is usually used to dry mix first and then add water. Put cement, sand and stones into the mixer in sequence, and pre-mix for 2 minutes. Then sprinkle the cellulose powder evenly in the pre-stirred mixture, sprinkle the powder while stirring, and stir for 5 minutes. Finally, add water for wet mixing, and put it into the test mold after the mixing is completed. Use a spatula to insert and tamp the sample from the outside to ensure that the height of the mixture is higher than the test mold by a certain distance. Then take a vibrating operation, which needs to be carried out on a vibrating table. When mud is generated on the surface, the vibrating is completed. In order to ensure the flatness of the sample, it is necessary to smooth the concrete surface with a small trowel. Then place the samples in an environment of (20 ± 2) ℃ for 1d, after which they are numbered and demoulded and subjected to standard curing for 28d.

1.4 Test method

The 150 mm × 150 mm × 150 mm concrete sample after standard curing for 28 days was subjected to rapid freezing and thawing treatment. After 25, 50, 75, and 100 freeze-thaw cycles, the relative dynamic elastic modulus and mass loss rate of the sample were measured respectively, so as to judge the compactness of the recycled concrete, and then evaluate the hydroxypropyl methyl fiber Frost resistance of plain recycled concrete.

2. Test results and analysis

2.1 The effect of recycled aggregate substitution rate on the freeze-thaw resistance of recycled concrete

(1) Mass loss rate

Regardless of whether HPMC was added or not, the mass loss rate showed a trend of first decreasing and then increasing with the increase of the number of freeze-thaw cycles, indicating that the fiber-reinforced concrete has a similar freeze-thaw law to ordinary concrete. In addition, the mass loss rate of recycled concrete added with HPMC began to increase after 50 freeze-thaw cycles, which is different from that of concrete without cellulose. It shows that the addition of HPMC can improve the compactness of concrete, which is beneficial to the initial freeze-thaw resistance of concrete.

(2) Relative dynamic elastic modulus

In the process of increasing the number of freeze-thaw cycles, the relative dynamic elastic modulus loss rates of the two kinds of concrete showed an upward trend. In addition, the relative dynamic elastic modulus loss rate of recycled concrete is generally higher than that of ordinary concrete, which may be related to larger pores and reduced compactness of recycled concrete. The incorporation of a certain amount of HPMC can reduce the loss of relative dynamic elastic modulus, indicating that HPMC can be filled into the pores of recycled concrete and improve the compactness of recycled concrete. By comparison, it can be seen that the amount of recycled aggregate in the initial freeze-thaw period is beneficial to the frost resistance of recycled concrete, and the amount of recycled aggregate in the later freeze-thaw period is not conducive to the frost resistance of recycled concrete. When the recycled aggregate content is 20% The antifreeze performance is almost the same as that of ordinary concrete. This may be due to the fact that the gaps between recycled aggregates are larger than those between natural crushed stones, resulting in higher water content of recycled concrete than ordinary concrete.

2.2 The effect of HPMC substitution rate on the freeze-thaw resistance of recycled concrete

ring

(1) Mass loss rate

Regardless of the content of HPMC, as the number of freeze-thaw cycles increased, the mass loss rate showed a trend of first decreasing and then increasing. For recycled concrete, the incorporation of HPMC improves the frost resistance of recycled concrete. When the content of HPMC is 0.6%, the overall mass loss rate is the lowest, indicating that when the content of HPMC is 0.6%, the frost resistance of recycled concrete is the best.

(2) Relative dynamic elastic modulus

The effect of HPMC on the loss rate of relative dynamic elastic modulus of recycled concrete is similar to that of recycled aggregate, and it shows an upward trend with the increase of the number of freeze-thaw cycles, and it is different from the effect of HPMC on the mass loss rate of recycled concrete. Overall, when the HPMC content of recycled concrete reaches 0.6%, the loss rate of relative dynamic elastic modulus is the lowest, and the frost resistance of recycled concrete is improved at this time.

The comparison shows that with the increase of HPMC content, the mass loss rate and relative dynamic elastic modulus loss rate both decrease first and then increase, reaching the extreme value when the content is 0.6%.

3. Conclusion

The influence of different recycled aggregate substitution rates and HPMC substitution rates on the mass loss rate and relative dynamic elastic modulus loss rate of recycled concrete mixed with hydroxypropyl methylcellulose can be obtained through experiments:

(1) The freeze-thaw resistance of recycled concrete is higher than that of ordinary concrete in the early freeze-thaw period, and the frost resistance of recycled concrete in the later freeze-thaw period is generally worse than that of ordinary concrete.

(2) Adding HPMC can improve the frost resistance of recycled concrete, and the frost resistance of recycled concrete is the best when the cellulose content reaches 0.6%.

(3) On the whole, the frost resistance of recycled concrete is not as good as that of ordinary concrete. It is recommended to control the amount of recycled aggregate at about 20%.