0 Introduction
At present, the superplasticizer testing specified in the national standard is to use cement, sand, stone and other materials specified in the standard, and the tested superplasticizer is formulated into concrete that meets the requirements of the standard, and then the prepared concrete is tested for various parameters. The benefit of this method of testing is that it guarantees a certain degree of fairness and makes the different superplasticizers comparable, as all standard materials are used. However, there are some irrationalities in this method, for example, the guiding significance for practical engineering application is not very direct, because the materials used in practical engineering are still different from standard materials. Air content is an important parameter for the detection of superplasticizers, because the air content will have a significant impact on the workability, durability and mechanical properties of concrete. In this paper, three batches of superplasticizer (a total of 46 samples) sampled from Yunnan Expressway were used as samples to analyze and discuss the effects of gas content on the water reduction rate, compressive strength ratio and 1h longitudinal variation of slump of concrete, and relevant suggestions were put forward for the detection methods of superplasticizers.
1. The relationship between the gas content of the superplasticizer and the water reduction rate
Figure 1 shows the scatter plot of the relationship between gas content and water reduction rate of 46 samples. As can be seen from the figure, the water reduction rate of the superplasticizer is roughly positively correlated with the gas content, that is, with the increase of the gas content, the water reduction rate of the superplasticizer will also increase. In addition, air bleed in concrete can also increase the cohesion of concrete and the lubrication between materials, improve the segregation and water leakage of concrete, and improve the workability of fresh concrete [1]. At the same time, proper air entrainment in concrete can also improve the frost resistance of concrete, and significantly improve the impermeability, chloride ion permeation resistance, and carbonization resistance of concrete [2]. However, if the air content is too large, it will also cause too many holes in the concrete, making the concrete not dense enough and reducing its strength. Therefore, the current standard GB8076-2008 "Concrete Admixtures" [3] stipulates that the upper limit of the gas content of high-performance superplasticizer is 6%.
2. The effect of the air content of the superplasticizer on the strength of concrete
Figure 2 shows the scatter plots of the compressive strength ratio vs. gas content of the 46 samples. It can be seen that the compressive strength ratios of 7D and 28D superplasticizer both show a decreasing trend with the increase of gas content. In order to analyze the relationship between the gas content and the compressive strength ratio more clearly, Fig. 3 selects 9 samples with the same water reduction rate of 32%, and it can be seen that under the same water reduction rate, the 7d and 28d compressive strength ratios of the superplasticizer also decrease with the increase of gas content. It is worth noting that even if the gas content of the superplasticizer exceeds the upper limit (6%) specified in the standard GB8076-2008, the compressive strength ratio of 7d and 28d can also meet the requirements of the standard.
3. The relationship between the air content of the superplasticizer and the change of concrete slump over time
Fig. 4 is the relationship between the gas content of the superplasticizer and the change of slump over time. It can be seen from the figure that when the air content increases, the slump of concrete has a tendency to increase over time for 1h. This may be due to the loss of the gas content of concrete over time [4,5]. We already know that the air content will improve the workability of concrete, so when the air content is lost, the slump of the concrete will also be correspondingly lost. It can also be seen from Figure 4 that, except for individual points, the slump of concrete can basically meet the requirements specified in the standard GB8076-2008 for 1h over time.
4. Suggestions for the detection of concrete superplasticizers
From the above analysis, it can be seen that the gas content of the superplasticizer will have different degrees of influence on the workability of concrete, the compressive strength ratio and the change of slump over time, but on the whole, the favorable aspects of the increase in air content seem to be greater than the disadvantages, especially considering the significant improvement effect of the air content on the workability and durability of concrete [6]. Moreover, in the previous experimental data, although the gas content of some samples exceeds the requirements specified in the standard, the strength can meet the requirements of the standard, and the slump 1h temporal variation is basically within the qualified range. In this case, it seems that there is something inappropriate to judge the failure of the superplasticizer only based on the unqualified gas content.
Further, although there are some feasibility points in the current standard for testing admixtures with standard materials, the superplasticizer is actually inseparable from other materials of concrete, and the actual performance of the superplasticizer can only be seen when the superplasticizer is placed in the applied concrete material. Therefore, it is suggested that the detection of admixtures should be carried out on the relevant parameters of the actual application of concrete mixed with the tested admixtures, that is, it is equivalent to a mix ratio verification, and the detection parameters obtained in this way may be more instructive for practical engineering applications than the parameters detected by standard materials.
5 Epilogue
Through the analysis of 46 superplasticizers sampled on Yunnan Expressway, the influence of the air content of the superplasticizer on the water reduction rate, compressive strength ratio and 1h longitudinal variation of slump of concrete was discussed. There are some unreasonable places in the current admixture testing standards, and it is suggested that the detection of admixtures should be the detection of the actual application of concrete mixed with the tested admixtures, rather than the detection of simple admixtures, so that it is more instructive for the actual engineering application.