100 Questions about Ready-Mix Concrete (1-20)

What is ready-mixed concrete?

Answer: According to the requirements of the purchaser, components such as cement, water, sand, gravel, admixtures, and mineral admixtures are mixed in a certain proportion at the mixing plant, metered, mixed, and then sold. The concrete mixture is transported by a mixing truck and delivered to the purchaser's delivery location within the specified time.

What are considered general-purpose products?

Answer: Ready-mixed concrete with a strength grade no greater than C60, slump no greater than 180mm, maximum aggregate size between 20～40mm, and no other requirements.

What are considered special products?

Answer: Ready-mixed concrete where any index exceeds the specified range for general-purpose products or has special requirements. For example, fine aggregate concrete, pile foundation concrete, waterproof concrete, and antifreeze concrete.

What is the delivery location?

Answer: The location for the handover of concrete as determined in the contract between the supplier and the purchaser, which is the construction site.

What is factory inspection?

Answer: Inspection of the quality of ready-mixed concrete before it leaves the factory. This includes sampling and testing the slump, making strength test specimens, visual inspection of slump and workability, etc.

What is delivery inspection?

Answer: Inspection of ready-mixed concrete at the delivery location by the supplier, purchaser, and supervisor. This includes testing the slump, making test specimens, visual inspection of slump and workability, etc.

What is cement? How many common types of cement are there?

Answer: Any water-hardening cementitious material made by finely grinding silicate cement clinker, a reasonable proportion of admixture, and an appropriate amount of gypsum is called cement. Common cements include: Portland cement, ordinary Portland cement, and composite cement. Portland cement is further divided into P·Ⅰ and P·Ⅱ types, with strength grades of 52.5 and 62.5. Ordinary Portland cement is divided into ordinary type and early strength type, with codes P·O, and strength grades of 42.5, 42.5R, 52.5, 52.5R. Composite cement has the code P·C, with strength grades of 32.5, 32.5R, 42.5, 42.5R.

What are the items for cement re-inspection?

Answer: Setting time, soundness, strength, fineness (specific surface area).

What is aggregate? How many types of aggregate are there?

Answer: The material that acts as a skeleton in concrete is called aggregate (also called aggregate). Gravel is called coarse aggregate, and sand is called fine aggregate. Gravel is divided into pebbles and crushed stone, and is classified by stone type into limestone crushed stone, granite crushed stone, etc. Sand can be divided into river sand, sea sand, mountain sand, and artificial manufactured sand, etc.

How many types of sand are there according to its fineness modulus?

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.

Why is medium sand preferred for pumped concrete?

Answer: Because the grading of medium sand is reasonable, the content of particles below 0.3mm can reach about 15%. These particles adhere to the pipe wall in the mortar of the concrete inside the pump pipe, resulting in very little frictional resistance, which is beneficial for pumping. Coarse sand has high frictional resistance, which is not conducive to pumping. Fine sand has a large surface area, and more cement is used under the same strength, increasing the probability of cracking.

How many types of aggregate gradations are there?

Answer: Single-sized and continuous-sized. Single-sized includes: 10～20, 16～31.5, 20～40. Continuous-sized includes: 5～10, 5～16, 5～25, 5～31.5, 5～40.

Why is continuous-sized aggregate preferred for pumped concrete?

Answer: Because, for the same maximum aggregate size, the void ratio of single-sized aggregate is larger than that of continuous-sized aggregate, and the large voids need to be filled with mortar. Under the same conditions, the surface area of aggregate per cubic meter of concrete is larger when using single-sized aggregate than when using continuous-sized aggregate, and the corresponding amount of cement required is also larger. Furthermore, concrete made with continuous-sized aggregate has better fluidity than that made with single-sized aggregate, which is beneficial for pumping construction. Therefore, continuous-sized aggregate should be preferred.

What are the main quality requirements for good aggregate?

Answer: The main requirements are:

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

(2) There are no yellow skins (surface mud), mud eggs, large mud lumps, or large aggregate particles;

(3) The content of needle-shaped and flaky particles should be as low as possible (the content of needle-shaped and flaky particles in aggregate for pumped concrete should not exceed 10%);

(4) The content of shale should be as low as possible (because shale has low strength and increases the content of flaky particles).

What are the quality requirements for good sand?

Answer: The requirements are:

(1) The particle size distribution meets the requirements for medium sand;

(2) The particle size is medium sand, with a low content of particles larger than 10mm, not exceeding 10% at most;

(3) No mud eggs, large mud lumps, and the mud content does not exceed 3%;

(4) No roots, grass roots, mud, plastic bags, or other debris.

What is fly ash?

Answer: Fly ash is the fine powder collected from the flue gas in the pulverized coal boiler of a power plant by dust removal equipment.

Into how many grades is fly ash classified according to its technical requirements? Where are they respectively applicable?

Answer: It is divided into three grades: Grade I, Grade II, and Grade III; Grade I is applicable to C40 and above concrete, Grade II is applicable to C15~C40 concrete, and Grade III is applicable to cement admixtures.

Why is fly ash added to pumped concrete?

Answer: Because the microstructure of fly ash is spherical particles, and its fineness is finer than cement. It also has a certain activity. Adding it to cement can replace part of the cement, increase the fluidity of the concrete mixture, greatly improve the pumpability of pumped concrete, and also improve the later strength and durability of the concrete.

What items should be inspected for fly ash upon arrival at the factory?

Answer: The fineness and water demand ratio of fly ash should be inspected. The key item is the water demand ratio. If the fineness slightly exceeds the standard, and the water demand ratio meets the requirements of Grade II, the ash can be used; otherwise, it cannot be used.

What are concrete admixtures?

Answer: Additives used to improve the properties of concrete are called concrete admixtures.