Concrete is generally composed of cement, sand, stone and water. In order to improve certain properties of concrete, an appropriate amount of admixtures and admixtures are often added. Therefore, concrete is mainly composed of six major components: ① cement, ② water, ③ coarse aggregate (mainly stone), ④ fine aggregate (mainly sand), ⑤ mineral admixture (mainly fly ash or other Admixtures), ⑥ additives (such as expansion agent, water reducer, retarder, etc.).
In concrete, sand and stone act as the skeleton, which is called aggregate or aggregate; cement and water form cement slurry, which wraps on the surface of the aggregate and fills its voids. Before the concrete hardens, the cement slurry, admixtures and admixtures play a lubricating role, giving the mixture a certain fluidity and facilitating construction operations. After the cement paste hardens, the sand and stone aggregates are cemented into a solid whole. Sand and stone generally do not participate in the chemical reaction between cement and water, and their main functions are to save cement, bear loads and limit the shrinkage of hardened cement. Admixtures and admixtures not only improve the performance of concrete, but also save cement.
Influence Factors of Six Major Components on Concrete Quality
1. Cement
The selection of cement materials and grades affect the strength of concrete and the heat of hydration of concrete solidification, etc. The quality of related finished products plays a major role in the quality of finished concrete.
2. Water
The PH value of water, water quality, sulfate and other contents affect the strength and quality of concrete.
3. Coarse aggregate (mainly stone)
The strength and material of stones affect the strength of concrete and the quality of finished concrete.
4. Fine aggregate (mainly sand)
The mud content of the sand, the material of the sand body, and the content of harmful substances in the sand affect the strength and setting time of the concrete to varying degrees.
5. Mineral admixtures (mainly fly ash or other admixtures)
Different admixtures affect factors such as workability, strength curve, and appearance of concrete products.
6. Admixtures (such as expansion agent, water reducer, retarder, etc.)
The type and amount of admixture affect factors such as the setting time, strength, and physical properties of concrete.
The technical requirements of the six major components:
1. Cement
The choice of cement strength grade should be compatible with the design strength grade of concrete. Generally, the strength grade of cement is 1.5 to 2.0 times that of concrete, and 0.9 to 1.5 times for high-strength concrete. When using low-strength grade cement to prepare high-strength grade concrete, the amount of cement will be too large, which is not economical, and will also affect other technical properties of concrete. When using high-strength cement to prepare low-strength concrete, the amount of cement will be too small, which will affect the workability and compactness, resulting in poor durability of the concrete. Therefore, a certain amount of mixed materials should be added when it is necessary to do so.
2. Fine aggregate
Aggregate with a particle size below 4.75mm is called fine aggregate, which refers to sand in ordinary concrete. Sand can be divided into natural sand and artificial sand. Natural sand includes river sand, lake sand, mountain sand and desalinated sea sand (chloride ion content is not more than 0.06%); artificial sand is a general term for machine-made sand and mixed sand after soil removal treatment. Because the river sand is clean and meets the requirements of relevant standards, it is the most commonly used in the preparation of concrete. The technical requirements of fine aggregate for concrete are as follows:
Particle gradation and degree of thickness:
The particle gradation of sand refers to the ratio of the particles of different sizes in the sand to match each other. When the size of the particles is well matched, the gaps between the sand particles are the least. The thickness of sand refers to the overall thickness of sand particles of different particle sizes mixed together, usually divided into coarse sand, medium sand and fine sand. Under the same quality conditions, fine sand has a larger total surface area, while coarse sand has a smaller total surface area. In concrete, the surface of the sand needs to be covered with cement paste, and the gaps between the sand grains need to be filled with cement paste. In order to achieve the purpose of saving cement and improving the strength, the total surface area of the sand and the voids between the sand grains should be minimized, that is, the selected grade It is better to mix with good coarse sand or medium sand.
The particle gradation and thickness of sand are usually determined by sieve analysis. According to the cumulative sieve residue of the 0.63mm sieve hole, the sand is divided into three gradation zones: I, II, and III. The gradation area is used to represent the particle gradation of the sand, and the fineness modulus is used to represent the thickness of the sand. The larger the fineness modulus, the coarser the sand. According to the fineness modulus, sand can be divided into three grades: coarse, medium and fine.
When choosing sand for concrete, the particle size distribution and thickness of the sand should be considered at the same time. Zone II sand should be preferred when preparing concrete. When sand in zone I is used, the sand rate should be increased and sufficient cement dosage should be maintained to meet the workability requirements of concrete; when sand in zone III is used, the sand rate should be appropriately reduced to ensure the strength of concrete. For pumped concrete, medium sand should be selected, and the particles smaller than 0.315mm in the sand should not be less than 15%.
Harmful impurities and alkaline activity:
Concrete sand requires cleanliness and less harmful impurities. The mud, silt, mica, organic matter, sulfide, sulfate, etc. contained in the sand will have an adverse effect on the performance of the concrete. They are harmful impurities, and their content needs to be controlled not to exceed the relevant specifications. The sand used in the concrete of important projects should also be tested for alkali activity to determine its applicability.
Robustness:
The firmness of sand refers to the ability of sand to resist cracking under the action of climate, environmental changes or other physical factors. The solidity of the sand is tested with sodium sulfate solution, and the mass loss of the sample after 5 cycles should meet the requirements of the relevant standards.
3. Coarse aggregate
Aggregate with a particle size greater than 5 mm is called coarse aggregate. Coarse aggregates commonly used in ordinary concrete are gravel and pebbles. Coarse aggregates obtained by crushing and screening natural rocks or pebbles are called crushed stones or crushed pebbles. Coarse aggregates formed by rocks due to natural conditions are called pebbles. The technical requirements of coarse aggregate for concrete are as follows:
Particle gradation and maximum particle size
There are two kinds of particle gradation of crushed stone or pebble for ordinary concrete: continuous particle size and single particle size. Among them, single-grain aggregates are generally used to combine into continuous-grain grades with required gradation, and it can also be mixed with continuous-grain crushed stones or pebbles to improve their gradation. If resources are limited and single-grain aggregates must be used, measures should be taken to avoid segregation of concrete.
The upper limit of the nominal particle size in the coarse aggregate is called the maximum particle size. When the particle size of the aggregate increases, its specific surface area decreases, and the amount of cement in the concrete also decreases. Therefore, under the premise of meeting the technical requirements, the maximum particle size of the coarse aggregate should be selected as large as possible. In reinforced concrete structural engineering, the maximum particle size of coarse aggregate shall not exceed 1/4 of the minimum size of the structural section, and shall not be greater than 3/4 of the minimum clear distance between steel bars. For concrete solid slabs, aggregates with a maximum particle size of 1/3 of the slab thickness are allowed, but the maximum particle size shall not exceed 40mm. For concrete that is pumped, the maximum particle size of gravel should not be greater than 1/3 of the diameter of the conveying pipe, and the maximum particle diameter of pebbles should not be greater than 1/2.5 of the diameter of the conveying pipe.
strength and sturdiness
The strength of crushed stone or pebble can be expressed by rock compressive strength and crush index. When the concrete strength grade is C60 and above, the rock compressive strength test should be carried out. The ratio of the compressive strength of the rock used to make the coarse aggregate to the strength grade of the concrete should not be less than 1.50. For regular production quality control, the crushing index value can be used to test.
Coarse aggregate used in concrete with antifreeze requirements requires determination of its firmness. That is to say, test with sodium sulfate solution, and the mass loss of the sample after 5 cycles should meet the requirements of the relevant standards.
Harmful impurities and needles, flake particles
Mud, silt, fine chips, sulfate, sulfide and organic matter contained in coarse aggregate are harmful substances, and their content should meet the requirements of relevant standards. In addition, it is strictly forbidden to mix calcined dolomite or limestone blocks into the coarse aggregate.
Crushed stones or pebbles used in concrete for important projects should also be tested for alkali activity to determine their applicability. Too many needles and flaky particles in the coarse aggregate will make the workability and strength of the concrete worse, so the content of needles and flaky particles in the coarse aggregate should meet the requirements of the relevant standards.
4. water
The water quality inspection items for concrete mixing water include pH value, insoluble matter, soluble matter, C1-, SO₂²-, alkali content (inspection when alkali active aggregate is used). The tested water samples should also be compared with the drinking water samples for cement setting time and cement mortar strength. In addition, concrete mixing water should not float obvious grease and foam, and should not have obvious color and peculiar smell; concrete enterprise equipment washing water should not be used for prestressed concrete, decorative concrete, aerated concrete and concrete exposed to corrosive environments. Not for use in concrete using alkali-active or potentially alkali-active aggregates. Untreated seawater is strictly prohibited for use in reinforced and prestressed concrete. In the absence of water sources, seawater can be used for plain concrete, but it is not suitable for decorative concrete.
The water quality inspection items of concrete maintenance water include pH value, C1-, SO₂²-, alkali content (inspection when alkali-activated aggregate is used), and the insoluble and soluble matter, cement setting time and cement mortar strength may not be inspected.
5. Additives
Admixtures are substances that are added before or during concrete mixing, and the amount is generally not more than 5% of the cement mass (except in special cases), and can improve the performance of concrete as required. The application of various concrete admixtures improves the performance of freshly mixed hardened concrete, promotes the development of new technologies for concrete, promotes more applications of industrial by-products in cementitious material systems, and also helps save resources and protect the environment. It has gradually become an indispensable material for high-quality concrete.
6. Mineral admixtures
In order to improve the performance of concrete, save cement, and adjust the strength level of concrete, natural or artificial mineral materials added during concrete mixing are collectively referred to as concrete admixtures. Concrete admixtures are divided into active mineral admixtures and inactive mineral admixtures. Inactive mineral admixtures basically do not react with cement components, such as ground quartz sand, limestone, hard slag and other materials. The active mineral admixture itself does not harden or hardens very slowly, but it can react with Ca(OH)2 generated by cement hydration to form hydration products with gelling ability, such as fly ash and granulated blast furnace slag powder , silica fume, zeolite powder, etc.
