① Select various raw materials such as cement and aggregates carefully;
② It is necessary to use high-efficiency water reducing agents to reduce water consumption and water cement ratio;
③ High quality mineral admixtures should be used to improve the microstructure of cement paste and interface zone, enhance density and bonding strength;
④ Careful selection of mix proportions is necessary to determine a reasonable sand ratio and water cement ratio, in order to reduce cement usage and improve concrete strength; ⑤ Strictly control the construction quality and do a good job in early maintenance. Only by comprehensively adopting the above technical measures can effective preparation of high-strength concrete be implemented.Use UHPC mixer to make the materials mixed homogeneously
Selection the UHPC mixer
High uniformity mixing effect
Planetary operation+high-speed auxiliary mixing make UHPC mixing more ideal.
Complex mixing curve, no dead corners, full coverage in 5 seconds.
It can evenly distribute fibers within the cement-based material in a short period of time, solving the problems of clumping and pushing during the mixing process, with a mixing uniformity of 100%.
Advanced and flexible design without leaking slurry
Upward drive, no slurry leakage during mixing.
The discharge door can be opened 1-3 times to meet the personalized needs of users.
The UHPC mixer is designed with a compact structure, easy maintenance, and high reliability.
2. Selection of cement
The mineral composition and grade of cement have a direct impact on the strength of concrete. As we all know, the mineral composition of Portland cement is mainly composed of C3S, C2S, C3A, C4AF. It is advisable to use cement with low C3A content and high C.S. content for the preparation of high-strength concrete. This type of cement has lower hydration heat and better strength development in the later stage.
High strength concrete generally uses high standard ordinary Portland cement, and the specification stipulates that the cement dosage should not exceed 550kg/m3, and the total amount of cement and mineral admixtures should not exceed 600kg/m3. Experiments have shown that when the cement dosage exceeds 500kg/m3, the effect of cement dosage on the strength growth of concrete is no longer significant, and the utilization coefficient of cement decreases. At this point, increasing the amount of cement not only increases costs, but also has adverse effects. The optimal amount of cement varies with the dispersion and water reducing effect of the admixture.
The cement grade has a significant impact on the strength of concrete. When the strength grade does not exceed C60, ordinary grade 425 cement can be used. For concrete higher than C60, grade 525 or higher cement should be used. The reason why lower grade cement can be used to prepare higher strength concrete is because the detection of cement grade uses finer standard sand and a larger water cement ratio. In addition, high-efficiency water reducing agents are added to the concrete, allowing the cement to fully diffuse and hydrate.
3. Selection of coarse and fine aggregates
High strength concrete should use crushed stones or pebbles with a particle size greater than 5mm. In terms of rock types, granite, feldspar, basalt, etc. are the best, followed by gneiss, quartzite, limestone, etc. The strength of rocks is expressed in terms of ultimate compressive strength and crushing value (only crushing value is measured for pebbles). The ratio of compressive strength of rocks to the strength of concrete should not be less than 1.5, and the crushing value index should be below 10% to 15%.
The other physical properties of aggregates also have an impact on concrete. If the water absorption rate is low and there are few pores and defects, the concrete will have high strength, strong frost resistance, and low shrinkage rate; Aggregates should also have a thermal expansion coefficient and elastic modulus similar to that of cement stone, otherwise they are prone to micro cracks, increase the permeability of concrete, and reduce durability; If the coarse aggregate is too hard, it is easy to cause volume changes when the concrete is subjected to temperature and humidity changes. When the stress at the interface between cement stone and aggregate is high, it is easy to form upward cracking failure.
Due to the high strength of cement stone in high-strength concrete, the bonding strength between cement stone and aggregate is also strong. In the failure section of concrete, the proportion of coarse aggregate being damaged is relatively high, so the strength of coarse aggregate often becomes an important limiting factor for high-strength concrete. In addition, the weak areas of micro cracks that may exist in the parent rock will break along the cracks during fragmentation, so the small crushed stones formed after fragmentation often have higher strength than larger samples of the parent rock. This is also one of the reasons why high-strength concrete should use small-sized aggregates.
From a macro perspective, concrete is composed of a two-phase system of coarse and fine aggregates and cement stone. From a microscopic perspective, there are significant differences between the cement stone in the cement aggregate interface zone and the cement matrix in the system. The interface zone has a higher water cement ratio and more pores, generally weaker than the other two phases of concrete, making it the weakest part of the system. If the bonding strength in the interface area is low, it will be destroyed along the interface area.
Many studies suggest that high-strength concrete should use coarse aggregates with small particle sizes. Due to the relatively increased surface area of small-sized coarse aggregates, the stress per unit surface area in the cement aggregate interface zone will be greatly reduced, reducing the requirement for bonding strength in the interface zone. Only in this way can a higher bonding strength interface zone be obtained, which can withstand higher external loads. On the contrary, coarse aggregate has a larger particle size, a smaller surface area for bonding with cement stone, and is prone to forming weaker interfacial zones, which weakens the strength of concrete. In ordinary medium and low-grade concrete, the bonding strength requirement in the interface zone is not very high, and it is relatively easy to meet the requirements, if conditions permit Require the use of larger particle size aggregates as much as possible to save cement. But for high-strength concrete with higher strength grades, it is different. If small particle size coarse aggregates are not used to increase the surface area, it is impossible to prepare high-strength concrete. Abroad, it is believed that the maximum particle size of aggregate should be 10-15mm, and to save cement, it is generally not more than 20-25mm.
The influence of fine aggregates on high-strength concrete is relatively small compared to coarse aggregates, but it cannot be ignored. Clean materials with a fineness modulus of 2 should be selected Medium coarse sand with a fineness modulus of 3.0 is the best, ranging from 6 to 3.2. The quantity greater than Smm and less than 0.315mm should be reduced. Due to the larger surface area of fine aggregates compared to coarse aggregates, the ratio of coarse and fine aggregates (sand ratio) has a significant impact on the amount of cement used and the strength of concrete. The optimal sand ratio can achieve the best strength, and reducing the sand ratio appropriately will not affect the workability of high-strength concrete due to the high amount of cement used.
4. Mixing high-quality mineral admixtures
Mineral admixtures such as fly ash, ground slag, silica powder, zeolite powder, etc. can improve the workability of concrete and increase the cohesiveness of the mixture. Due to changes in cohesiveness, the water migration pathway is cut off, which can reduce bleeding and segregation. Bleeding and segregation are the main causes of microstructural defects in concrete, especially in the interface zone between cement and aggregate. Therefore, adding mineral admixtures to high-strength concrete is not only to save cement, but mainly to improve the microstructure and performance of the concrete.
Silica fume contains over 90% Si0. The dosage is 5% to 10%. Concrete mixed with silica fume has good workability and stability. Zeolite is a porous crystalline mineral composed of silicon oxygen tetrahedra. When zeolite powder is added to concrete, it can promote uniform hydration and complete reaction, thereby improving compactness and strength. If low activity mineral admixtures are added and evenly distributed in the cement slurry, they become the core of a large amount of hydrate deposition, and the cement slurry becomes a uniformly distributed microporous amorphous hydrate, reducing the uneven structure of crystalline blocks, which is also very beneficial for strength and impermeability.
5. Adding high-efficiency water reducing agent
Due to the high cement content and low water cement ratio required for high-strength concrete, it inevitably leads to high viscosity and poor rheological properties of the concrete. After adding high-efficiency water reducing agents, it has a strong dispersing and lubricating effect on cement, which can significantly reduce water consumption and make it possible to minimize the water cement ratio. So, high-efficiency water reducing agents naturally become one of the necessary components of high-strength concrete.
The water cement ratio is the main factor controlling the strength of cement paste and concrete. Experiments have shown that when the water cement ratio is below 0.35, the properties of the interface zone are significantly improved, and the density and strength are significantly increased. The theoretical value of the bound water required for the hydration of Portland cement is about 28%, while the directly measured chemical bound water is about 25%. When the actual water cement ratio is lower than this value, the hydration of cement will be incomplete and the cement stone will not achieve sufficient compactness. Considering the mixing, pouring, and curing conditions, with the addition of high-efficiency water reducing agents, the water cement ratio of high-strength concrete should generally not exceed 0.35, and when the strength is above 80MPa, it should not exceed 0 25～0. 30.
6. Strengthen construction control and early maintenance
According to the characteristics of high cement content and hydration blocks in high-strength concrete, the following measures should be taken during construction:
(1) Adopt step-by-step feeding and mixing. Mix cement, admixtures, sand, and water for about 40 seconds by UHPC mixer first, then add stones and water reducer and stir for 1 minute before leaving the machine. This can fully utilize the effect of water reducing agents, maintain good pouring performance, and improve strength. Under hot conditions, various materials should be kept at a low temperature as much as possible.
(2) Transport by mixer truck. From mixing to pouring completion, it can be completed according to the temperature limit.
(3) High frequency vibration is used. Even when using fluid concrete, vibration should be carried out to improve compactness.
(4) Strengthen early maintenance. Due to rapid hydration, early watering and maintenance should be carried out to ensure that hydration does not lack water, so as not to affect strength development.
(5) Try to reduce the slump as much as possible. If conditions permit, adding as little water as possible and using a small slump is still an important measure to improve strength.
