Aggregate is the largest raw material in the concrete system, and has always been considered to play a role in filling and framing, reducing the instability of the slurry and improving the stability of the concrete. The aggregate is rich in sources, the price is lower than the cementing material, and has been considered as a filler, making many people form the idea of “heavy rubber, light aggregate”. Under the domination of this idea, it is generally believed that as long as the quality of rubber material is controlled well, and even the quality of cement is controlled well, concrete can be done well, and then the control of aggregate performance is ignored, resulting in large fluctuations in concrete quality.

 

According to the formation of the aggregate can be divided into natural aggregate and artificial aggregate, natural aggregate is mainly river sand, river egg (gravel) stone, in addition to mountain sand, mountain gravel and Gobi sand and sea sand, sea gravel; Artificial aggregate is mainly rock broken by mechanical gravel, artificial sand, in addition to industrial waste broken into recycled aggregate, as well as artificial baked ceramics and so on. According to the particle size, aggregate can be divided into coarse aggregate and fine aggregate. The particle size below 4.7mm is fine aggregate, that is, sand; The particle size above 4.75mm is coarse aggregate, that is, stone.

 

In order to standardize the technical index requirements of concrete aggregates, relevant state departments have formulated relevant technical specifications. It is generally believed that achieving the technical indicators specified in codes and standards is usually easy to prepare concrete that meets the quality requirements. However, it needs to be clear that some indicators specified in the specifications and standards do not play a decisive role in the quality of concrete, in other words, some technical indicators of aggregate do not meet the requirements of the specifications and standards, but also to prepare concrete that meets the requirements. For example, the grading of aggregate, voidage, needle sheet content, fineness modulus, etc., only has an impact on the amount of cement (cementing material) of concrete, and has a greater impact on the economy of concrete. Although some indicators affect the quality of concrete, such as weak particle content, crushing index, mica content, etc., they can be used differently according to different strength levels, that is, concrete with high quality requirements cannot be used, and concrete with low quality requirements may be used, which is ultimately determined by concrete tests. Harmful components such as mud content, organic content, sulfide content, salt content exceed the provisions of the aggregate, should be strictly controlled, can not be used in concrete without treatment.

 

Concrete aggregate should first be selected according to the concrete technical indicators, choose the aggregate that can meet the quality of concrete, and then consider the local (or nearby) materials to reduce costs as much as possible. In order to ensure the quality requirements of concrete, aggregate is generally required: (1) hard texture, low crushing value, strength should be greater than 1.5 to 2 times of concrete; (2) The chemical properties are stable, and should not have bad chemical effects in concrete; (3) Control the content of mud and other harmful substances to reduce the adverse impact on concrete; (4) Good grain shape, excellent grading and low voidage.

 

(1) Fineness modulus and workability of concrete

Sand fineness modulus is a technical index reflecting the thickness of fine aggregate, but can not reflect the gradation of fine aggregate. Generally speaking, the fineness modulus of the sand decreases, the fine particles of the aggregate increase, the total surface area increases, the remaining lubricating slurry after wrapping the sand surface decreases, and the slump of the concrete decreases. The fineness modulus of sand increases, the fine particles decrease, the water retention of concrete mix is poor, the water is easy to bleach, the segregation, the cohesion is poor, and the mix appears rough. On the contrary, the fineness modulus is reduced, the concrete has good water retention, almost no water secretion, good workability, and it is not easy to separate, but when the mix ratio and other raw materials are unchanged, the water consumption will increase. In addition, with the increase of slump and water consumption, the volume of mortar increases slightly, and the amount of coarse aggregate decreases slightly. When slump decreases, the volume of mortar decreases slightly, and the amount of coarse aggregate increases slightly. When the requirements of coarse aggregate type, maximum particle size, gradation, sand thickness and slump are determined, the practical results show that the bulk volume of coarse aggregate changes very little, except a few, not more than 3%, mostly not more than 1.5%.

 

(2) Adjustment of fineness modulus and concrete mix ratio

 

In the concrete production process, when the sand fineness modulus is obviously reduced, the concrete slump is reduced, and it is even difficult to construct. On the contrary, when the sand fineness modulus suddenly increases, the concrete slump will increase, but it is easy to leak, segreate, and even cannot be tamped (insufficient mortar). The reason is that when the sand becomes coarse, the content of coarse particles increases, and when the void of coarse aggregate is filled, the sand of larger particles “squeeges” the void between the coarse aggregate, which increases the void ratio of coarse aggregate, and requires more mortar filling. When the slurry is insufficient, the concrete appears dry and has poor water retention. In contrast, fine sand particles are small, easier to fill the gaps of coarse aggregate, and the “extrusion” phenomenon is reduced, that is, with less sand, mortar can fill the gaps of coarse aggregate. Therefore, when the thickness of sand changes, it has a great impact on the sand rate and slurry amount of the concrete mix. In production practice, attention should be paid to adjusting the sand rate and slurry volume according to the change of sand, but attention should be paid to keeping the water-binder ratio unchanged.

 

(3) Fineness modulus and concrete strength

In the concrete production process, when the sand suddenly becomes fine, the concrete slump becomes smaller, and the operator will often increase the water consumption to maintain the predetermined concrete slump. The increase of water consumption directly changes the original water-binder ratio and decreases the strength of concrete. This may also be one of the reasons why many people have the illusion that the strength of fine sand concrete is lower than that of coarse sand concrete. In fact, as long as the water-cement ratio is maintained, and there is a certain fluidity (that is, there is enough cement slurry to fill the sand void and wrap the sand surface), the thickness of the sand will not affect the strength of the concrete.

 

(4) Aggregate voidage and concrete performance

The size of the voidage of the aggregate mainly affects the size of the aggregate and slurry volume in the concrete, the smaller the voidage of the aggregate, the larger the aggregate volume in the concrete, the smaller the slurry volume, the lower the cost of concrete. Conversely, the higher the aggregate voidage, the higher the cost of concrete. Therefore, when determining the concrete composition, the voidage of aggregate should be reduced as much as possible. Generally speaking, the larger the particle size of coarse aggregate, the smaller the voidage of aggregate and the smaller the surface area. On the contrary, the larger the porosity of aggregate, the larger the surface area. The larger the coarse aggregate particle size, the continuous gradation, the lower the voidage, the smaller the surface area, the less the volume of mortar filling the aggregate void and wrapping the aggregate surface area in the concrete, so the smaller the amount of sand (sand rate), the more conducive to improving the volume stability of the concrete and preventing cracks.

 

(5) Coarse aggregate grade concrete performance

Gradation is the coordination of the graded aggregates, also known as particle composition. The more reasonable the aggregate grading, the smaller the voidage and surface area, and the smaller the volume of concrete cement paste. The selection of the best graded aggregate is very beneficial to reduce the concrete cost, due to the small water consumption and cement dosage, the concrete volume stability is good, not easy to crack.


Post time: Oct-06-2023