Li Ruoyu smiled: "Okay, I believe in you." Xiao Qin, you continue to talk, Xiao Li, you will continue to calculate. However, just now the water-cement ratio calculation, you have to take into account, now we have added fly ash, so we also need to take out the standard of fly ash application, the current use of fly ash excess coefficient substitution method, you come according to the standard! ”

In fact, the excess coefficient substitution method was later cancelled because of its unreasonableness, and a new method was adopted to calculate the measured strength of cement with an influence coefficient of fly ash or other admixtures.

Li Liangfeng hurriedly opened the 1991 edition of the "Technical Regulations for the Application of Fly Ash Concrete" and selected the excess coefficient in accordance with the above regulations.

It is very clear in the regulations that in order to achieve the purpose of the same strength of concrete and benchmark concrete after being mixed with fly ash, fly ash is replaced by an excessive amount, and the amount of fly ash is equal to the mass of the substituted cement multiplied by the excess substitution coefficient of fly ash. The better the quality of fly ash, the smaller the substitution coefficient, usually the over-substitution coefficient of grade I ash is 1.0~1.4, grade II ash is 1.2~1.7, and grade III ash is 1.5~2.0.

Li Liangfeng's choice of excess coefficient is 1.2, and most people should take the median value, which is a conservative approach.

Li Ruoyu asked: "Xiao Li, do you know how this excess coefficient came about?" ”

Li Liangfeng said seriously: "This should also be the conclusion of a large number of tests!" The fly ash we use is a high-quality first-class fly ash, and it should be possible to take an excess coefficient of 1.2! ”

Li Ruoyu smiled: "Well, as a newcomer, it's right to be conservative. However, we have to take into account another point, that is, we are now designing C15 concrete, generally speaking, its amount of cementitious material, if fly ash is not added, it means that the amount of cement will be relatively small, generally less than 280 kg, in this case. The lack of cementitious material will lead to poor working performance of concrete, especially for pumping concrete, which is not quite in line with the requirements. Therefore. When we design the mix ratio, we should consider increasing the amount of cementitious material appropriately, so we hope that the fly ash is as much as possible, because the price of fly ash is low, so when taking the excess coefficient. You can take a little larger, for example, I take the upper limit of 1.4, so that the amount of fly ash will be more, and the overall cementitious material will be a little more, so as to make the low-strength concrete work performance more ideal. Do you understand? ”

Li Liangfeng nodded and said, "Well, I know." It turns out that the design mix ratio is really not a trivial matter. ”

Qin Haijuan smiled: "That's right! If conditions permit, we can also use 32.5 ordinary cement, or secondary fly ash to prepare and produce low-strength concrete, so that the total cementitious material will be more, and the slurry volume of concrete will be richer. The working performance can meet the requirements of pumping. Another aspect of concrete mixed with fly ash is also to improve the durability of concrete, but the incorporation of fly ash will reduce the amount of cement and thus affect the strength of concrete, in order to achieve the purpose of equal strength of concrete and benchmark concrete after being mixed with fly ash, fly ash is replaced by an excess amount, and the amount of fly ash is equal to the mass of substituted cement multiplied by the fly ash excess coefficient. The better the quality of fly ash, the smaller the substitution coefficient, and the quality of fly ash with low grade is lower than that of fly ash with high grade, so the excess coefficient is large. Well, you just follow what Mr. Li just said. Take the excess coefficient of 1.4, and when you notice the intensity level above C30, this excess coefficient can be reduced a little. ”

Li Liangfeng nodded and continued to calculate.

The water-cement ratio is calculated to be 0.71. Although Li Liangfeng didn't understand whether this result was ideal, he couldn't see it now, and he only knew it by continuing to calculate.

According to the concrete mix design regulations, the next step is to determine the benchmark water consumption of concrete, of course, this water consumption is determined by guò test, and the second is selected from the table in the regulations. Experimental certainty is, of course, unrealistic. Now there is no empirical number jù, only from the table.

Qin Haijuan guided beside him again: "Xiao Li, pay attention to this water selection. The first is to determine the amount of water used in unilateral concrete without admixtures. This should be selected from the table according to the concrete slump, the type of aggregate and the maximum particle size of the aggregate that we have determined now. ”

Li Liangfeng nodded and said, "Well, Director Qin, I understand." Now our concrete slump requirement is 120 mm, and the coarse aggregate is crushed stone, and the maximum particle size is 25 mm. According to the number jù in the table, the maximum value is only 90 mm slump, and if it is larger, it will be increased on this basis, and for every 20 mm of slump, 5 kg of water will be added, and I think it should be possible to add 10 kg of water. The benchmark water consumption of 90 mm slump concrete corresponds to the maximum particle size of 25 mm of gravel, and the value is 210 kg, so the benchmark water consumption of the mix design is 220 kg, I don't know if I calculated it correctly? ”

Qin Haijuan smiled and said, "Exactly right! Well, you also need to consider the addition of admixtures, and the water consumption at this time needs to be calculated according to the water reduction rate. ”

Li Liangfeng continued: "Well, now our admixture, according to the number provided by the manufacturer, the water reduction rate is 15%, so the final water consumption is 220 times 0.85, and the result is 187 kilograms, right?" ”

Qin Haijuan nodded and said, "There are two different opinions now. One is your current calculation method, and the other is also taking into account the water content in the admixture, now we are using a liquid superplasticizer, which contains about 80% of the water, and this admixture manufacturer provides a number of jù. Therefore, if you want to be precise, you also need to multiply the amount of admixture by the water content of the admixture, for example, if we use 5 kg of admixture, there will be 4 kg of water in it, which still has a relatively large impact on the real water-cement ratio. Therefore, let's use a more precise method to calculate, so that the result will be more accurate. ”

After speaking, Qin Haijuan looked at Li Ruoyu and Xie Zili again, meaning to ask for their opinions.

Xie Zili hadn't spoken yet, Li Ruoyu smiled and said, "Director Qin is right!" We do technical work, or to keep improving, don't underestimate the water contained in the admixture, you want to use 5 kg of admixture, there is 4 kg of water inside, in this way, the impact on the water-cement ratio is still greater. Take C15 as an example, if you do not deduct the water content in the admixture, then the actual water consumption is 191 kg, according to the results of the previous calculation, 187 kg of water consumption, water-cement ratio of 0.71, cement consumption of 255 kg. However, without deducting the water in the admixture, the actual water-cement ratio is divided by 191 by 255, and the result is 0.75, which is 0.04 more than the calculated water-cement ratio, which has a greater impact on the concrete strength results, almost 2 megapascals. (To be continued.) )