How to Improve the Workability of Concrete?

Through experimental comparison, the addition of cellulose ether can significantly improve the workability of ordinary concrete and improve the pumpability of pumpable concrete. Incorporation of cellulose ether will reduce the strength of concrete.

Key words: cellulose ether; concrete workability; pumpability

1.Introduction

With the continuous development of society, the demand for commercial concrete is increasing. After more than ten years of rapid development, commercial concrete has entered a relatively mature stage. Various commercial concrete basically meet the requirements of various projects. However, in actual work, we found that when using pumped concrete, often due to reasons such as poor workability of concrete and unstable sand rate, the pump truck will be blocked, and a lot of time and manpower will be wasted on the construction site and mixing station, which will even affect the project. the quality of. Especially for low-grade concrete, its workability and pumpability are worse, it is more unstable, and the probability of pipe plugging and bursting is higher. Usually, increasing the sand rate and increasing the cementitious material can improve the above situation, but it also improves the concrete quality. material cost. In previous studies, it was found that the addition of cellulose ether to the foamed concrete will produce a large number of closed small air bubbles in the mixture, which increases the fluidity of the concrete, improves the collapse retention, and at the same time plays a role in water retention and retardation in the cement mortar. Therefore, adding cellulose ether to ordinary concrete should have a similar effect. Next, through experiments, under the premise of constant mix ratio, a small amount of cellulose ether is added to observe the performance of the mixture, measure the wet bulk density, and test the compressive strength of concrete 28d. The following is the process and results of the experiment.

2. Experiment

2.1 Test raw materials

(1) The cement is Yufeng brand PO42.5 cement.

(2) The active mineral admixtures used are Laibin Power Plant Class II fly ash and Yufeng S75 class mineral powder.

(3) The fine aggregate is limestone machine-made sand produced by Guangxi Yufeng Concrete Co., Ltd., with a fineness modulus of 2.9.

(4) Coarse aggregate is 5-25 mm continuous graded limestone produced by Yufeng Blasting Company.

(5) The water reducer is polycarboxylate high-efficiency water reducer AF-CB produced by Nanning Nengbo Company.

(6) The cellulose ether is HPMC produced by Kima Chemical Co.,Ltd, with a viscosity of 200,000.

2.2 Test method and test process

(1) Under the premise that the water-binder ratio and sand ratio are consistent, conduct tests with different mixing ratios, measure the slump, time-lapse collapse, and expansion of the new mixture, measure the bulk density of each sample, and observe the mixing ratio. The working performance of the material and make a record.

(2) After the slump loss test for 1 hour, the mixture of each sample was re-mixed evenly and loaded into 2 groups respectively, and cured for 7 days and 28 days under standard conditions.

(3) When the 7d group reaches the age, conduct a breaking test to obtain the relationship between the dosage and the 7d strength, and find out the dosage value x with good work performance and high strength.

(4) Use the dosage x to conduct concrete tests with different labels, and compare the strength of the corresponding empty samples. Find out how much the concrete strength of different grades is affected by cellulose ether.

2.3 Test results and analysis

(1) During the experiment, observe the state and performance of the new mixture of samples with different dosages, and take pictures for records. In addition, the state and working performance description of each sample of the new mixture is also recorded.

Combining the state and performance of the new mixture of samples with different dosages and the description of the state and properties of the new mixture, it can be found that the blank group without cellulose ether has general workability, bleeding and poor encapsulation . When cellulose ether was added, all samples had no bleeding phenomenon, and the workability was significantly improved. Except for the E sample, the other three groups had good fluidity, large expansion, and were easy to pump and construct. When the dosage reaches about 1‰, the mixture becomes viscous, the degree of expansion decreases, and the fluidity is average. Therefore, the dosage is 0.2‰～0.6‰, which can greatly improve the working performance and pumpability.

(2) During the experiment, the bulk density of the mixture was measured, and it broke after 28 days, and some rules were obtained.

It can be seen from the relationship between bulk density/strength and bulk density/strength of the new mixture and the dosage that the bulk density of the fresh mixture decreases as the dosage of cellulose ether increases. The compressive strength also decreased with the increase of cellulose ether content. It is consistent with the foam concrete studied by Yuan Wei.

(3) Through experiments, it is found that the dosage can be selected as 0.2‰, which can not only obtain good working performance, but also have relatively small strength loss. Then, design experiment C15, C25, C30, C35 4 groups of blank and 4 groups respectively mixed with 0.2‰ cellulose ether.

Observe the working performance of the new mixture and compare it with the blank sample. Then install the mold for standard curing, and break the mold for 28 days to obtain strength.

During the experiment, it was found that the workability of the new mixture samples mixed with cellulose ether has been greatly improved, and there will be no segregation or bleeding at all. However, the relatively low-grade mixtures of C15, C20, and C25 in the blank sample are easy to segregate and bleed because of the relatively small amount of ash. C30 and above grades have also improved. It can be seen from the data in the comparison of the strength of different labels mixed with 2‰ cellulose ether and the blank sample that the strength of concrete is reduced to a certain extent when the cellulose ether is added, and the magnitude of the strength decline increases with the increase of the label.

3. Experimental conclusion

(1) Adding cellulose ether can improve the workability of low-grade concrete and improve pumpability.

(2) With the addition of cellulose ether, the bulk density of concrete decreases, and the larger the amount, the smaller the bulk density.

(3) Incorporating cellulose ether will reduce the strength of concrete, and with the increase of the content, the degree of reduction will increase.

(4) Adding cellulose ether will reduce the strength of concrete, and with the increase of the grade, the magnitude of the decrease will increase, so it is not suitable for use in higher grade concrete.

(5) Adding cellulose ether can be used to improve the workability of C15, C20, and C25, and the effect is ideal, while the loss of strength is not large. The pumping process can greatly reduce the chance of pipe blockage and improve work efficiency.