Qin Haijuan thought for a while and said: "When it comes to how to determine the preparation strength of concrete, we have to talk about the guarantee rate of concrete strength. Well, under normal circumstances, it is not easy to ensure that the concrete produced by a mix ratio is 100% strong. Because there are too many factors that affect the final strength of concrete, we can only choose a more reasonable strength guarantee rate! ”

Li Liangfeng asked, "Well, what is the value of this strength guarantee rate?" ”

Qin Haijuan said with a smile: "This involves a statistic in mathematics, normal distribution, you should know." A low-probability event is usually defined as an event with a probability of less than 5% occurring, and it is considered nearly impossible for the event to occur in a single trial. This understanding is the starting point for inference. We need to understand this in the following two aspects. One is that the almost impossible occurrence here is for one trial, because there are many trials, and the event is of course very likely; Second, when we use the principle that a small probability event is almost impossible to make inferences, we also have a 5% chance of making mistakes. The same is true for the strength of concrete, we have selected a strength guarantee rate of 95%, which means that we allow 5% of the strength to not meet the standard value. ”

Li Liangfeng was curious: "Is it allowed to reach the standard value?" I thought that all concrete strength must reach the standard value, and the concrete must be considered qualified! Why is that? ”

Qin Haijuan said with a smile: "According to our large number of tests, the measured value of concrete strength will also show a normal distribution. The standard normal curve is a special normal distribution curve with a bell-shaped curve and symmetry on both sides. The axis of symmetry is the average strength, and the highest peak of the curve occurs here, indicating that the strength of the concrete is closest to its average strength and occurs the most, and as it moves away from the axis of symmetry, the probability of the strength measurement becomes smaller and smaller, and finally approaches zero. This means that if the intensity measurement is close to the average intensity, the smaller the standard deviation will be. It shows that the quality control of concrete is better. ”

Qin Haijuan paused and continued: "So, according to the results of the calculation, this is how the 1.645 in this formula comes from. This strength is guaranteed at 95%. If the strength guarantee rate is more than 99.9%, then this coefficient will be even greater, about 3. A 100% guarantee rate is almost impossible. You do the math, if you change 1.645 to 3, what is the strength of the C15 concrete! ”

Li Liangfeng hurriedly calculated it on the paper. Truly. After changing this coefficient to 3, then multiply 3 by the standard deviation of 4.0, and the result is 12, plus the standard value of 15, that is, when the formulation strength is 27 megapascals, the concrete strength guarantee rate can reach 99.9%.

Calculated with 1.645, the formulated strength is 21.6 megapascals, which is a 95% strength guarantee rate.

Li Liangfeng quickly finished his calculations, then raised his head and replied, "Director Qin, if the coefficient is 3." The formulation strength is 27 MPa, while the coefficient of 1.645 is 21.6 MPa. The difference between the two is 5.4 megapascals, which is more than a strength grade. Does this mean that if the strength guarantee rate is too high, the less economical the concrete mix ratio will be? ”

Qin Haijuan nodded and smiled: "Yes, Xiao Li, you can draw inferences, this is very good." We who do technical work just have to think more and think about why. To know what it is, but also to know why. Okay, let's get down to business, after the formulation strength is calculated, it's time to move on to the next calculation. In the second step, we need to calculate the water-cement ratio according to the Paul Mi formula. Let's take a look at how it should be calculated. ”

Li Liangfeng looked at it. In fact, the calculation of the water-cement ratio is to use several values, one is the preparation strength calculated in the first step, the second is the measured strength of cement, and the third is the selection of regression coefficient. In addition, in order to ensure the durability of concrete. The water-cement ratio shall not be greater than the maximum water-cement ratio specified in the table, if the calculated water-cement ratio is greater than the specified maximum water-cement ratio, the specified maximum water-cement ratio shall be taken.

However, the measured strength of cement is not yet available, and the choice of regression coefficient does not know how to choose, Li Liangfeng looked at Qin Haijuan with some confusion, and whispered: "Director Qin, we don't have this measured strength of cement yet!" I have just made the strength of the mortar less than two days, and I still don't understand this regression coefficient, how to choose it! ”

Qin Haijuan nodded and said, "Well, you have just come into contact with mix ratio design now, and it is understandable that you are not clear." The measured strength of cement, if not, you can choose to multiply the mark by a coefficient, generally choose 1.15, for example, our 42.5 Portland cement, use 42.5 times the surplus coefficient of 1.15 to get the strength of cement. ”

After a pause, Qin Haijuan said: "The choice of regression coefficient is mainly related to the variety of coarse aggregate used in concrete. For example, the regression coefficient of broken limestone and pebbles fished out of the river is different. This is because gravel has a rough surface and many edges and corners, so it has strong adhesion and grip strength, while pebbles, because the surface is smooth and the shape is close to round, so the adhesion is poor. In this way, in the case of the same water-cement ratio, the concrete strength of the pebble preparation will be lower, and this regression coefficient is also calculated by the expert group that compiles the concrete mix design regulations, and the number of thousands of sets of tests is calculated for us to choose when designing the mix ratio. ”

Li Liangfeng suddenly realized: "Oh, so that's the case! I didn't expect that there are so many principles in the concrete mix design, and I really underestimated it before. I thought that concrete was just cement, river sand and gravel, and mixed with water in a certain proportion? Experienced builders can estimate how much cement they use, but now it seems that they are still not very reliable with experience points! ”

Li Ruoyu listened to it for a long time, and then he laughed and said, "That's right, Xiao Li!" There's still so much to learn! Concrete is a very deep science, if you want to study it thoroughly, it will take decades of hard work. And with the development of science and technology, concrete, an ancient building material, will continue to get new development with the progress of the times, we can not be limited to the concrete we see now, we must take a long-term view. Of course, the most important thing for you now is to lay a good foundation, combine theory and practice, and truly master the application technology of concrete. “

Li Liangfeng said excitedly: "Well, Mr. Li, I will definitely study hard." (To be continued.) )