Concrete, as an important material in the field of modern architecture, its fundamental knowledge is crucial for personnel engaged in related work. I believe that everyone has gained a simple understanding of concrete foundation knowledge from the previous article "Concrete Foundation Knowledge, Scratch from scratch!" In this article, we will continue to delve deeper into concrete foundation knowledge!

Why is medium sand preferred for pumping concrete?

Answer: Sand is divided into coarse sand (3.1-3.7), medium sand (2.3-3.0), and fine sand (1.6-2.2) according to its fineness modulus. The grading of medium sand is reasonable, and the particle content below 0.3mm can reach about 15%, which is beneficial for pumping. Coarse sand has high frictional resistance, which is not conducive to pumping. The surface area of fine sand is large, and the use of cement at the same strength increases the probability of cracking.

What are the main quality requirements for good stones?

Answer: There are mainly the following items:

(1) The particle size meets the requirements, and the particle size distribution is continuous;

(2) No yellow skin (sticky mud on the surface) mud eggs, large mud blocks, and large pebbles;

(3) The lower the content of needle and flake particles, the better (the needle and flake particle content of stones used for pumping concrete should not exceed 10%);

(4) The less shale content, the better (because shale has lower strength and increases the content of flaky particles)

Why should continuous graded stones be prioritized for pumping concrete?

Answer: Single particle size and continuous particle size. Single particle sizes include: 10-20, 16-31.5, 20-40. Continuous particle sizes include 5-10, 5-16, 5-25, 5-31.5, and 5-40. When the maximum particle size of the stone is the same, the porosity of a single particle size is greater than that of a continuous particle size, and the large porosity needs to be filled with mortar. The flowability of concrete mixed with continuous grade stones is better than that of concrete mixed with single grade stones, which is conducive to pumping construction. 

What are the inspection items for fly ash entering the factory??

Answer: Fly ash is a fine powder collected through dust removal equipment in the flue gas of the power plant's coal powder furnace, divided into three levels; Level I, II, III. The fineness and water demand ratio of fly ash should be tested, and the key item is the water demand ratio. If the fineness slightly exceeds the standard and the water demand ratio meets the level II requirement, this ash can be used, otherwise it cannot be used.

Which items should be inspected when pumping agents into the factory?

Answer: Must be tested: water reduction rate, setting time, 1-hour slump change over time, compressive strength ratio. Conventional testing can use theoretical mix ratio testing for C30 production, and net slurry flowability cannot replace concrete mix ratio testing (the results of net slurry and mix ratio testing differ too much, which is not conducive to guiding production). Good pumping agents usually have a reasonable water reduction rate, good fluidity of concrete mixtures, and good slump retention.

What are the main performance tests for concrete mixtures?

Answer: It mainly tests the workability, slump, and gas content of the mixture, and workability is represented by cohesion, water retention, flowability, and comprehensive properties;

(1) Cohesiveness. Observing with the eyes, when measuring the slump, if the specimen collapses to the surrounding area, it indicates good cohesion. If the specimen collapses to one side or some stones leak out, and the surface does not adhere to mortar, it indicates poor cohesion.

(2) Water retention - After the concrete mixture is compacted into a slump cylinder, if there is a slight slurry at the bottom of the cylinder or if there is no good water retention, if the concrete mixture is poured onto an iron plate and there is water or cement slurry on the surface, it indicates poor water retention (i.e. segregation phenomenon).

(3) Liquidity - When the mixture is piled up and immediately flows around, it indicates good fluidity, while vice versa indicates poor fluidity.

What does the segregation of concrete mixtures look like?

Answer: If it is found that cement slurry flows out from the edge of the concrete mixture pile. After a few minutes, the sand and gravel sink, forming a bottom grabbing phenomenon, which is the segregation phenomenon of the concrete mixture. The occurrence of segregation is a manifestation of poor workability of concrete, which can easily block pipes during pumping and reduce the strength of concrete. The common reason for the occurrence of segregation phenomenon is due to the excessive dosage of pumping agent. The dosage can be reduced by 0.2% and tried again, or concrete anti segregation agent can be used to improve the segregation state of concrete.