100 Questions about Ready-Mix Concrete (21-40)

What is the difference between water reducer and pumping agent?

Answer: Water reducer is an admixture that reduces the amount of water used while maintaining the workability of the concrete mixture, thereby increasing the concrete strength; while the main raw material of the pumping agent is water reducer, with a small amount of plasticizing and slump retention components added to facilitate the pumping construction of concrete.

What are the different types of concrete admixtures?

Answer: There are water reducers, pumping agents, retarding agents, accelerating agents, early strength agents, antifreeze agents, antifreeze pumping agents, expanding agents, and waterproof agents, etc.

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

Answer: The following must be inspected: water reduction rate, setting time, 1h slump change over time, compressive strength ratio. Routine testing can use the theoretical mix ratio test for C30 production. The flowability of neat cement paste cannot be used to replace the concrete mix ratio test (the results of neat cement paste and mix ratio test are too different, which is not conducive to production guidance).

What is the detection status of a good pumping agent?

Answer: When a good pumping agent is tested for mix ratio, the water reduction rate is not less than 20%, the workability of the concrete mixture is good, the mortar wraps the stones easily, and the slump loss over time is within 50mm. Conversely: if some stones are not coated with mortar, the fluidity is poor, and the slump loss over time is >80mm, it indicates that the quality of this batch of pumping agent is poor.

What types of water should be used for concrete mixing?

Fresh water, tap water, groundwater, drinking water, recycled water, and vehicle washing water in the mixing plant (partially used) should be used. Industrial wastewater, domestic wastewater, and seawater must not be used.

What is the meaning of the word 砼 (tónɡ)? How is it pronounced?

It means artificially made stone, representing concrete. The dictionary pronunciation is tóng, and it can also be read as hùn níng tu.

What is the function of the expanding agent?

Answer: When the expanding agent is added to the cement, it reacts chemically with the cement hydrate to generate ettringite and other substances, which increases the density of the concrete and produces micro-expansion in volume, reducing the shrinkage of the concrete and playing a role in crack resistance and seepage prevention.

How many grades are concrete divided into according to its strength? How is it divided?

Answer: According to the strength of concrete, it is divided into 13 grades: C10, C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C70, and C80. Its grade is based on the strength of the standard concrete test block after standard curing for 28 days, and each grade is divided every 5MPa or 10MPa. C is the first capital letter of the English word for concrete, which represents concrete, and the number in the corner indicates the strength grade.

What are the different types of concrete according to its performance?

Answer: It is divided into ordinary concrete, high-strength concrete (greater than C50), waterproof concrete, pumping concrete, antifreeze concrete, frost-resistant concrete, mass concrete, self-consolidating concrete (self-leveling concrete), dry-hard concrete, and high-performance concrete.

What unit does MPa represent, and how is it converted?

Answer: MPa is a unit of strength: 1MPa=106Pa, 1Pa=1N/m2, 1MPa=1N/mm2. 1MPa≈10kg/cm2.

Which properties of concrete mixture are mainly tested?

Answer: The main tests are the workability, slump, and air content of the mixture. Workability is comprehensively represented by cohesion, water retention, and fluidity;

(1) Cohesion—Observed with the eyes, when doing the slump test, the specimen collapses around, indicating good cohesion; if the specimen collapses to one side or some stones leak out, and the surface is not coated with mortar, it indicates poor cohesion.

(2) Water retention—After the concrete mixture is filled into the slump cone and compacted, there is little or no thin slurry at the bottom of the cone, indicating good water retention; if the concrete mixture is poured onto an iron plate, and water or cement slurry is separated on the surface, it indicates poor water retention (i.e., segregation phenomenon).

(3) Fluidity—If the mixture flows around immediately after being piled up, it indicates good fluidity; otherwise, it indicates poor fluidity.

What is the slump of concrete?

Answer: The slump cone is placed on an iron plate, filled with concrete mixture and compacted, then the slump cone is vertically lifted, and the concrete specimen collapses downwards. The distance between the top of the cone and the highest point of the specimen is the slump. The slump of ordinary concrete is 30～90mm. The slump of pumping concrete is 100～230 mm; the higher the pumping height, the greater the slump, and vice versa.

What is the spread of concrete?

Answer: When the slump of concrete is greater than 220mm, a steel ruler is used to measure the maximum and minimum diameters of the concrete mixture after spreading, and the average value is the spread. The greater the spread, the better the flowability. The spread of self-leveling concrete should be measured.

What does segregation of concrete mixture look like?

Answer: If cement slurry flows out from the edge of the pile of concrete mixture. After a few minutes, the sand and gravel sink, forming a grabbing phenomenon, which is the segregation phenomenon of the concrete mixture. Segregation is a manifestation of poor workability of concrete, which is easy to cause pipe blockage during pumping and will reduce the strength of concrete. The common cause of segregation is excessive amount of pumping agent, and the amount can be reduced by 0.2% and retested.

Sometimes, some stones in the concrete mixture do not adhere to the mortar, or it looks like tofu residue and does not stick. What is the reason?

A: Some stones do not adhere to the mortar because of insufficient thickening agent in the admixture. The tofu-like state is due to the incompatibility between the admixture and some components in the cement.

36. What is a standard test block?

A: A test block with sides of 150mm×150mm×150mm is a standard test block. A test block with sides of 100mm×100mm×100mm is a non-standard test block, and its compressive strength result needs to be converted to the standard test block strength by multiplying by a conversion factor of 0.95.

37. How should the quality of concrete test molds be checked?

A: Use a vernier caliper to measure the length, width, and height of the test mold, with an allowable error ≤0.5mm. Use the back of the caliper and a 0.02-1mm stainless steel gauge to measure the flatness of the four inner surfaces, with an allowable error ≤0.05㎜ (the standard method is to gauge 100mm in length); when testing a 150mm×150mm×150mm test mold, it is best to gauge the entire length along the diagonal of the inner surface, and then use the gauge to measure the flatness, with an allowable error <0.13mm, this method is more accurate than the standard method; when the 150mm×150mm×150mm test mold passes the standard method test, the flatness of the side of the test block may not be qualified. If there is no gauge, a razor blade can also be used; as long as the blade cannot be inserted into the gap between the caliper and the inner surface of the test mold, it is considered qualified. Use a standard protractor to measure the angle between adjacent surfaces, with an allowable error ≤0.3°; or use a standard drawing triangle to clamp one side of two adjacent surfaces from the inside, and use a gauge to measure the gap between the triangle and the end of the other surface, with an allowable error <0.5mm (100mm×100mm×100mm), <0.8mm (150mm×150mm×150mm). Most of the 150mm×150mm×150mm cast iron test molds currently sold on the market have concave sides, while most of the 150mm×150mm×150mm plastic test molds have convex sides; therefore, carefully check the flatness of the sides before purchasing.

38. What constitutes standard curing?

A: Placing the formed concrete test blocks in an environment with a temperature of 20±2℃ and a humidity greater than 95% is considered standard curing.

39. What is curing under the same conditions?

A: Curing the formed concrete test blocks under the same conditions as the curing conditions used for the components at the construction site is called curing under the same conditions.

40. How many common curing ages are there for concrete?

A: 3d, 7d, 14d, 28d, 60d, 90d. However, the national standard for determining whether the concrete strength is qualified is based on the standard curing 28d strength of the standard test block; when constructing large-volume concrete, the 60d or 90d strength can be specified in the "Construction Organization Design" as the evaluation basis.