With the development of the construction industry, commercial concrete as a building material is also bound to experience product updates to meet the current technical requirements of high strength, high performance, durability and high level pumping. At present, the quality of cement, sand, stone and fly ash as raw materials for commercial concrete has not changed after years of use, but the water reducing agent has undergone many product changes, so the best way to improve the quality of concrete products is to use a better water reducing agent, and as the third generation of high-performance water reducing agent, polycarboxylic acid water reducing agent has naturally become the first choice of commercial concrete mixing plants. As the third generation of high performance water reducing agent, polycarboxylic acid water reducing agent has become the first choice of commercial concrete mixing plant.

　　I. Characteristics of polycarboxylic acid water reducing agent

　　In terms of commercial concrete, polycarboxylic acid water reducing agent has the following characteristics compared with naphthalene water reducing agent: 1, the ideal water reduction rate can be achieved at a lower dose; 2, the strength growth effect is large; 3, good fluidity and slump retention, less impact on the setting time of concrete, can meet the requirements of concrete pumping in long-distance, super high-rise buildings; 4, good adaptability to cement; 5, the volume of concrete good stability and good control of the cracking situation of the cast-in-place slab [1].

　　At present, the concrete construction of domestic high-speed railway and other lines are using polycarboxylic acid system water reducing agent, and achieved good results, but there are big differences between commercial concrete mixing plant and these special mixing plant in raw material procurement, management operation, etc., which lead to various problems when using polycarboxylic acid water reducing agent in commercial concrete mixing plant. This paper discusses the problems of polycarboxylic acid water reducing agent in commercial concrete mixing plant.

　　II. Common problems and control points of polycarboxylic acid water reducing agent in the application of commercial concrete

　　(A) Large slump loss

　　1.1 Small initial slump and expansion

　　Polycarboxylic acid water reducing agent is mainly used in the production of large slump and large flow rate concrete, but in the process of commercial concrete use, the slump requirement range covers 160mm to 220mm due to the different casting parts, and in the actual production and supply process, there will be unstable slump loss. In response to the above problems, tests were conducted by using the same raw materials, adjusting the sand rate and changing the initial slump, with the following results.

　　From the data, it is obvious that the difference of slump loss by time, indicating that the slump loss of polycarboxylic acid water reducing agent is very serious when the slump is less than 200mm and the extension is less than 550mm.

　　1.2 Control points

　　1) During the normal production and construction of concrete, try to ensure that the slump is greater than 200mm and the expansion is greater than 550mm to ensure the workability of the concrete when it arrives at the site; 2) When producing concrete with slump less than 200mm and expansion less than 550mm, the vehicle should carry an appropriate amount of the same type of water reducing agent for backup.

　　2.1 Site vehicles stalled while waiting

　　In the actual construction, many times encountered a continuous issue of concrete slump by time loss inconsistent situation, through the occurrence of this problem vehicle statistics, found that such problems occur mostly in external rental vehicles, through further understanding of the site situation, such problems occur in the vehicle are waiting for unloading stalled situation.

　　It was finally concluded that for concrete mixed with polycarboxylic acid water reducing agent, if the tanker stalled while waiting, it would cause further increase of slump loss.

　　2.2 Control points: It is strictly forbidden to turn off the engine while the vehicle is waiting for unloading.

　　(II) Large fluctuation of workability

　　1.1 Stability now poor

　　Good adaptability to cement as a major advantage of polycarboxylic acid water reducing agent, its adaptability to cement is better than other water reducing agents, but because of the many varieties of cement in China, the admixture is complex, prone to poor stability [2].

　　The poor stability of concrete referred to in this paper is mainly manifested in the case of concrete in good condition after leaving the machine, but after standing for a period of time, there is water secretion, or in good condition after leaving the machine and in good standing, but after entering the concrete transport truck and continuing to mix, there is serious water secretion and even segregation. The occurrence of such problems makes concrete quality control more difficult and concrete workability indexes unpredictable. Through the test summary of concrete raw materials with such problems, the follow-up observation of concrete quality, excluding the case of poor stability caused by poor adaptation of cement, is mainly due to the following two reasons.

　　1) Insufficient mixing. In the railroad concrete mixing station, concrete mixing time has strict requirements, even if the workload is large, it must ensure that the concrete mixing time should not be less than 2 min [3], in the national standard "Concrete Admixture Application Technical Specification" (GB50119-2013) also provides for the mixing time when mixing concrete with polycarboxylic acid system high-performance water reducing agent. The commercial concrete mixing plant will choose to sacrifice the mixing time to ensure the continuous supply of concrete when the task volume increases.

　　2) Poor coarse aggregate grading. Commercial concrete mixing plants generally use 5-31.5mm coarse aggregate, which is also prone to poor concrete stability when the coarse aggregate grading is poor and the proportion of the largest particle size increases.

　　1.2 Control points: 1) in the concrete production process due to strict control of mixing time; 2) to ensure the coarse aggregate grading, control the maximum particle size, or abandon continuous grading, the use of double-graded compounding.

　　2.1 After pumping, the concrete appears delamination, segregation or poor fluidity

　　With the rise in the number of layers of the main body and the extension of the pumping pipeline, a greater degree of flow is required, but in the production and construction process, there is often a good concrete and ease before pumping, but after pumping to the operating area, there is delamination, segregation or poor liquidity, mainly for the structure of mortar, net slurry floating, coarse aggregate sinking, and in serious cases, even the loss of slurry. The main reason for this problem is due to the use of polycarboxylic acid water reducing agent mixed concrete in the liquidity of the larger, the viscosity of its slurry dropped sharply, can not maintain the original state under the action of higher pressure.

　　2.2 Control points: when constructing in higher floors or pumping pipeline with longer distance, the concrete sand rate should be increased appropriately and the maximum particle size of coarse aggregates should be controlled.

　　3.1 The viscosity of the prepared concrete is large

　　In the process of continuous production of concrete, there is a sudden increase in concrete viscosity, poor fluidity, through the summary of such cases, mainly concentrated in the water-cement ratio of less than 0.4 in ready-mixed concrete, in the emergence of such problems, coarse and fine aggregates have not changed significantly, water reducing agent has not been the same batch, cement for the same manufacturer, the same batch, the most likely problem is concentrated in the external mixer fly ash. Due to the current increasing demand for commercial concrete, the amount of fly ash has also increased, resulting in a tight supply of fly ash, some suppliers in the supply of fly ash, Ⅱ grade fly ash mixed with Ⅲ grade fly ash, and commercial mix enterprises in the sampling of a slight negligence will lead to such fly ash for production.

　　In the laboratory conditions, using the same mix ratio, different grades of fly ash for trial mixing, the results are clear at a glance.

　　It is obvious from the above table that the slump and expansion severely decrease as the grade of fly ash decreases.

　　3.2 Control points: When mixing concrete with water-cement ratio less than 0.4, the quality of fly ash should be strictly controlled, while the amount of water-reducing agent admixture should be increased appropriately.

　　(C) hardened concrete surface problems

　　1.1 Cracks appear on the surface of cast-in-place slab

　　One of the major advantages of polycarboxylic acid water reducing agent is that the concrete mixed with it has good volume stability and can better control the cracks on the surface of cast-in-place concrete, but in the actual use process, there are still cracks on the surface of the slab. Through a period of follow-up observation, in the same site, the same construction conditions, the same construction technology under the cast concrete slab surface, in the case of good concrete compatibility, even if the slump of concrete into the mold is slightly larger, there will be no cracks in the slab surface after hardening, so it is not because of cracks caused by the large thickness of the surface paste after hardening.

　　Through the continuous tracking of a site 2-10 floor slab construction, combined with the video and image data when the slab surface cracks, raw material testing records for analysis found that when the overall coarse aggregate particle size in the concrete is large or fine aggregate containing increased stone and resulting in poor aggregate grading, it is very easy to cause cracks in the slab surface, the main reason is that when the aggregate grading is poor, it is easier to appear water secretion, segregation and other situations.

　　1.2 Control points: polycarboxylic acid water reducing agent is very sensitive to aggregate grading, so try to control aggregate grading, and use double-graded coarse aggregate as far as possible under the condition.

　　2.1 Vertical members are prone to honeycomb pockmarks

　　In the use of polycarboxylic acid water reducing agent for concrete vertical members, especially the height of the larger vertical members of the construction, occasionally after the removal of the formwork, the vertical members of individual parts of the surface finish is not enough, the situation of honeycomb surface. Initially, the construction party and the commercial concrete mixing plant considered that it was because some of the formwork was not cleaned up, but the phenomenon still existed when pouring by improving the quality of the formwork.

　　By viewing the relevant technical data, combined with the actual construction, it was found that such problems occurred mostly in the concrete worker handover operation, mainly because the concrete was not timely pounded, or the pounding degree was poor.

　　2.2 Control points: Since polycarboxylic acid water reducing agent has certain air-entraining capacity, concrete mixed with polycarboxylic acid is timely and fully pounded after entering the mold to avoid honeycomb pitting.

　　Conclusion

　　By understanding the control points of the above problems, most of the control points are focused on the control of raw material quality.

　　With the gradual standardization of the concrete industry, the increasing competition, coupled with the increasing demand for high-strength and high-performance concrete in the construction industry, polycarboxylic acid water reducing agent will definitely become the trend of water reducing agent development in the future, and commercial concrete enterprises must improve the procurement quality of concrete raw materials, strengthen the testing of concrete raw materials, and ensure the quality of concrete products.