I. INTRODUCTION

With the development of the construction industry, as the building materials of the commercial concrete will also experience the renewal of products to meet the current high strength, high performance, durability and high level pumping technology requirements. At present, as the raw material of concrete cement, sand, stone, fly ash, etc., its quality after many years of use, and no qualitative changes, but the water reducer has experienced a number of product replacement, so the best way to improve the quality of concrete products is the use of better performance of Superplasticizer, As the third generation of High-performance water-reducing agent Polycarboxylate Superplasticizer naturally become the first choice of concrete mixing plant.

Second, the characteristics of polycarboxylate Superplasticizer

In the case of commercial concrete, polycarboxylate Superplasticizer is compared with naphthalene series Water-reducing Agent, has the following characteristics: 1, in the lower dosage can achieve the ideal water reduction rate, 2, strong growth effect, 3, with good liquidity and slump, the impact on the concrete setting time is small, can meet the long distance, super high-rise building concrete pumping requirements 4, good adaptability to cement, 5, the concrete volume stability is good, has the very good control function to the cast-in-place slab surface crack condition [1].

At present, the domestic high-speed train and other lines of concrete construction are using polycarboxylic superplasticizer, and achieved very good results, but the commercial concrete mixing station and these special mixing stations in the procurement of raw materials, management, and other aspects of the major differences, resulting in the use of concrete mixing plant polycarboxylate Superplasticizer, there will be a variety of problems, In this paper, the problems of Polycarboxylate Superplasticizer in the use of commercial concrete mixing station are discussed.

Common problems and control points of Polycarboxylate Superplasticizer in the application of commercial concrete

(a) The slump loss is big

1.1 Initial slump, small expansion

Polycarboxylate Superplasticizer is mainly used in the production of large slump and large fluidity concrete, but in the process of using the concrete, the slump requirement covers 160mm to 220mm and the slump loss is unstable in the actual production and supply process. In view of the above problems, through the use of the same raw materials, the sand rate adjustment to change the initial slump test, the results are as follows:

It can be seen from the data that the slump losses are different, which shows that the slump loss is very serious when the Polycarboxylate water-reducing agent is less than 200mm and the expansion degree is lower than 550mm.

1.2 Control Points

1 in the normal production and construction of concrete because as far as possible to ensure that the slump is greater than 200mm, the expansion is greater than 550mm to ensure the work of concrete arrival at the site, 2 in the production slump is less than 200mm, the expansion of less than the 550mm of concrete, the car should carry the appropriate amount of water

2.1 Field vehicles stall while waiting

In the actual construction, many times the continuous loss of concrete slump in the case of inconsistent losses, by statistics on the number of trains appearing in this problem, it is found that these problems occur in the external leasing vehicles, and through the further understanding of the situation, the vehicles with such problems have to wait for unloading.

Finally concluded that the use of poly-carboxylic acid water reducer mixed with the concrete, the tanker if the waiting for flameout, will cause a further increase in slump loss.

2.2 Control points: vehicles in the waiting for unloading process is strictly prohibited.

(ii) High volatility of work

1.1 Stable Current Difference

Good adaptability to cement as a major advantage of Polycarboxylate Superplasticizer, its adaptability to cement is better than other water reducer, but because of China's many varieties of cement, admixture complex, easy to appear stable poor situation [2].

The stability difference of concrete in this paper is mainly manifested in good state after the concrete is out of the machine, but the situation of bleeding after resting for a period of time, or the state of good after the machine, and the static state is good, but after entering the concrete transport vehicle to continue to stir the serious bleeding, even segregation. The appearance of such problems makes the quality control of concrete more difficult, and the concrete working index cannot be predicted. Through the inspection of the concrete raw materials with such problems, the follow-up observation of concrete quality and the poor stability caused by the poor adaptability of cement are mainly due to the following two reasons:

1 stirring is not sufficient. Concrete mixing station in the railway, concrete mixer time has strict requirements, even if the task volume is large, it is necessary to ensure that the concrete mixing time should not be less than 2min[3], in the GB "concrete admixture Application Technology Specification" The mixing time for the use of Polycarboxylic high performance water-reducing agent (gb50119-2013) is also stipulated. In order to ensure the continuous supply of concrete, the concrete mixing station will choose the way of sacrificing stirring time.

2 Coarse aggregate grading difference. 5-31.5mm coarse aggregate is generally used in the concrete mixing station, the coarse aggregate gradation is not good, and the maximum particle size ratio increases, the concrete stability becomes worse.

1.2 Control points: 1 in the concrete production process due to strict control of mixing time, 2 to ensure coarse aggregate gradation, control the largest particle level, or give up continuous gradation, the use of two-stage matching for the compound.

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

With the increase of the number of main layer and the extension of pumping pipeline, the need for greater mobility, but in the production and construction process, often will appear before pumping concrete and workability good, but the pump sent to the operating area after delamination, segregation or liquidity changes, mainly in the structure of mortar, net slurry floating, coarse aggregate sinking, Even slurry loss occurs in severe cases. The main reason for this problem is that the slurry viscosity of the concrete which is mixed with polycarboxylic acid water reducer is very large, and it cannot keep the original state under the high pressure.

2.2 Control points: in the higher floors or the long distance of pumping pipeline construction, should be appropriate to improve the concrete sand rate, and control the largest size of coarse aggregate.

3.1 The concrete viscosity is big

In the process of continuous concrete production, the concrete viscosity increases suddenly, liquidity of the situation, through the summary of such a situation, mainly concentrated in the water gel ratio less than 0.4 of premixed concrete, in the emergence of such problems, coarse, fine aggregate did not occur a large change, water-reducing agent not the same batch, cement for the same factory, the same batch, The most likely problem is to concentrate on the outside mixture of fly ash. Due to the increasing demand for concrete, the consumption of fly ash also increases, which leads to a tight supply of fly ash, while some suppliers add Ⅲ-grade fly ash to Ⅱ-grade fly ash in the supply of fly ash, while the commercial mixed enterprises will cause such fly ash to be used for production.

In the laboratory conditions, using the same mix ratio, different grades of fly ash to carry out the test, the results at a glance.

It can be seen from the above table that with the decrease of the grade of fly ash, the slump and the expansion degree decrease seriously.

3.2 Control points: In the mixing water glue ratio is less than 0.4 of the concrete, we should strictly control the quality of fly ash, while the appropriate increase in the amount of water-reducing agent.

(iii) Hardened concrete surface problems

1.1 Cast-in-place Slab Surface cracks

One of the advantages of Polycarboxylate Superplasticizer is that it can control the cracks of cast-in-place concrete slab by using its mixed concrete with good volume stability, but it still exists in the actual use process. Through a period of follow-up observation, at the same site, the same construction conditions, the same construction technology pouring concrete slab surface, in the case of good concrete and workability, even if the concrete into the mold when the slump slightly larger, after the plate hardening will not appear cracks, so it is not because of hardened surface slurry thickness caused by large cracks.

Through continuous tracking of construction of 2-10 floors of a site, combined with the video and image data of the surface crack and the raw material inspection record, it is found that when the coarse aggregate size of the concrete is larger or the content of the fine aggregate is increased, the aggregate gradation is not good, so it is very easy to cause the cracks in the plate surface. The main reason is due to poor aggregate gradation, more prone to bleeding, segregation and so on.

1.2 Control points: Polycarboxylate Superplasticizer on the aggregate gradation is very sensitive, as far as possible to control the aggregate gradation, in conditions permitting, coarse aggregate as far as possible use of two-stage matching.

2.1 Vertical components prone to cellular pock

In the use of Polycarboxylate water-reducing agent for concrete vertical components, especially the high height of vertical components pouring construction, even if the template is removed, the vertical components of individual parts of the surface finish is not enough, the situation of honeycomb pock. Initial construction and commercial concrete mixing station are considered because some of the template is not clean, but by improving the quality of the template pouring, there is still this phenomenon.

By looking at the relevant technical data, combined with the construction practice, it is found that many of these problems occur in the handover of concrete works, mainly because the concrete is not in time vibration, or the degree of vibration is poor.

2.2 Control Points: Because Polycarboxylate Superplasticizer has a certain amount of gas, so the use of poly-carboxylic acid mixed with concrete in the mold after the timely, full vibration, to avoid the emergence of honeycomb pock.

Iv. Conclusion

Most of the control points are focused on the control of the quality of raw materials through the understanding of the control points of the above problems.

With the gradual standardization of the concrete industry, increasing competition, coupled with the increasing demand for high strength and high performance concrete in the construction industry, Polycarboxylate Superplasticizer will become the future trend of water reducer development, commercial concrete enterprises in such an environment must improve the quality of the procurement of concrete raw materials, Strengthen testing strength of concrete raw materials, ensure the quality of concrete products.