Li Ruoyu said with a smile: "Li Liangfeng, let's talk about it again, how to evaluate the workability of this plate of concrete!" ”

Li Liangfeng knew that it was time to test again, so he replied: "Usually the fluidity of the concrete mixture is determined, supplemented by intuitive experience to evaluate the cohesion and water retention, to determine the workability. The slump determination of concrete mixture is used to judge the fluidity of concrete mixture, which is mainly suitable for the determination of the consistency of plastic and fluidity concrete mixture with a slump value of 10~220 mm, and the maximum particle size of aggregate should not be greater than 40 mm. The internal dimensions of the slump cylinder are 200 plus or minus 2 mm at the bottom; Top diameter 100 plus or minus 2 mm; height 300 plus or minus 2 mm; The thickness of the cylinder wall is not less than 1.5 mm, the diameter of the ramming rod is 16 mm, the steel rod is 600 mm long, the end is rounded, and the slump cylinder is lifted vertically and smoothly. The lifting process of the slump cylinder should be completed within 5 to 10 seconds. The whole process from the beginning of the charge to the lifting of the slump cylinder should be carried out uninterrupted and should be completed within 150 seconds; After lifting the slump cylinder, the height difference between the height of the measuring cylinder and the highest point of the concrete test body after the collapse is the slump value of the concrete mixture. After the slump cylinder is lifted, if the concrete collapses or one side is sheared, it should be re-sampled and measured. If the above phenomenon still occurs in the second test, it means that the workability of the concrete is not good! ”

Li Liangfeng paused, and then said: "Judging from the collapse of our C15 concrete, the workability is good. Then observe the cohesion and water retention by gently tapping the side of the collapsed concrete cone with a tamper, at this time, if the cone gradually sinks or remains in its original state, it means that the cohesion is good; If the cone collapses, partially cracks, or segregation, it indicates that the viscosity is not good. The water retention is evaluated by the precipitation of slurry in the concrete mixture, and if more slurry is precipitated from the bottom after the slump cylinder is lifted, and the aggregate is exposed due to the loss of slurry, it indicates that its water retention performance is not good. If there is no slurry or only a small amount of slurry precipitates from the bottom after the slump cylinder is lifted, it means that its water retention performance is good. ”

Li Ruoyu smiled: "Okay, you can measure the slump." Let's check the cohesion and water retention of concrete. ”

Li Liangfeng nodded, and began to measure the slump of concrete with a ruler. After measurement, the concrete slump was 140 mm, which met the design requirements.

Li Ruoyu nodded and said, "Well, that's good." The requirements of the design have been met, Director Qin, you can record it. The slump of the concrete mixture is measured in millimeters, and the measurement is accurate to 1 millimeter, and the result is expressed to about 5 millimeters. Good cohesion and water retention. ”

Qin Haijuan began to make records on the trial matching record. Li Ruoyu said again: "Li Liangfeng, what tests should be done next?" ”

Li Liangfeng thought for a while and said: "Well, we also need to do the concrete apparent density test, the concrete slump time loss test, the concrete setting time test, and the concrete compressive strength specimen molding. ”

Li Ruoyu said with a smile: "The answer is correct, but the concrete setting time test is more troublesome according to the standard method, and the number of concrete we try is not enough, so we can only use the simple soil method." That's to press with your fingers, and I'll teach you a way, which is that your fingers can press down with a little force. It means that the initial setting time has been reached, and if you have to press hard to only have one fingerprint, it means that the final setting time has been reached. However, under normal circumstances, the difference between the initial setting time and the final setting time will not exceed 3 hours, so you should pay attention. Under normal circumstances, we need about six or seven hours for the initial setting time of concrete and ten hours for the final setting time. ”

The next step is the apparent density test, which is also very important, because the apparent density of the concrete is assumed when the mix is designed. However, the apparent density of the actual concrete may have a large error. For example, due to the difference in the air content of concrete, this error may be large.

Li Ruoyu knows that there have been many concrete quality accidents. It is caused by the excessive air content of concrete. However, in addition to the standard air content test method to test the air content of concrete, the apparent density test can also be used to judge whether the air content of concrete exceeds the standard. Because as long as the material is the same, if the air content is too high, the apparent density of the concrete will definitely drop sharply. Even when the apparent density reaches 2350 kilograms per cubic meter when designed, because the gas content exceeds the standard, the apparent density may drop to 2100 or even lower, so that the concrete strength will also drop significantly, and the compressive strength will not meet the standard, causing concrete quality accidents.

Li Liangfeng picked up a five-liter volumetric cylinder, washed it with water, then shook it off, weighed it on the scale, and then began to load the concrete.

After testing, the apparent density of concrete reached 2350 kilograms per cubic meter, which basically met the design index, and the difference between the assumed apparent density was within one percent, so that the final concrete mix ratio did not need to be checked.

Then there is the loss of concrete slump over time, which is generally measured after one hour or two hours, so that compared with the concrete slump in the initial state, generally speaking, the slump after one hour is 30 mm smaller than the initial state, even if it is acceptable. If the loss is too great, then in the actual production process, there will be a situation where the on-site construction requirements cannot be met.

Because the concrete from the factory to the construction site, it takes a certain amount of transportation time, sometimes the distance is too far or because of road congestion, it may take two hours to reach the construction site, and it may also be because there is too much backlog of vehicles, and the guide zhì waits for a long time at the site, so that many situations from the concrete mixer, to the delivery to the working face, it takes one to two hours, of course, no matter how long, it is difficult for the concrete to maintain the required slump.

At this time, some special technical measures need to be adopted to meet the requirements. For example, if the admixture is added on site to adjust, the slump of the concrete will increase when it leaves the factory, but this needs to be tested to increase the amount of rubber to meet the strength requirements.

Li Liangfeng put a dozen liters of concrete into an iron bucket and covered it with a lid, which he had to wait for an hour before testing. Now you need to form, and then do the next C20 concrete mix test, which can also save time, if you want to wait an hour to measure the slump loss, and then do the next test, then you can't do a few trials a day.

Fortunately, at the end of this day, all the match matches were relatively successful, and all indicators were within the expected range. (To be continued.) )