1. Selection of cement
The mineral composition and grade of cement have a direct influence on the strength of concrete. As we all know, the mineral composition of Portland cement is mainly composed of C3S. C2S。 C3A。 C4AF。 To prepare high-strength concrete, cement with low C3A content and high C .. sulfur content should be selected, which has low hydration heat and good strength development in the later stage.
High-strength concrete generally adopts high-standard ordinary portland cement, and the specification stipulates that the dosage of cement is not more than 550kg/m3, and the total amount of cement and mineral admixture is not more than 600kg/m3. The test shows that when it exceeds 500kg/m3, the influence of cement dosage on the strength growth of concrete is not obvious, and the utilization coefficient of cement decreases. At this time, increasing the dosage of cement will not only increase the cost, but also have adverse effects. The optimum cement dosage varies with the dispersion of additives and the effect of reducing water.
Cement grade has a great influence on the strength of concrete. When the strength grade does not exceed C60, ordinary cement No.425 can be used, and cement No.525 or higher grade should be used for concrete higher than C60. The reason why cement with lower grade can be used to prepare concrete with higher strength is that fine standard sand and high water-cement ratio are used to test cement grade, and superplasticizer is added to concrete, so that cement can be fully diffused and hydrated.
2. Selection of coarse aggregate and fine aggregate
Gravel or pebbles with a particle size greater than 5 mm should be used for high-strength concrete. From the perspective of rock types, granite, feldspar and basalt are the best, followed by gneiss, quartzite and limestone. The strength of rock is expressed by ultimate compressive strength and crushing value (only crushing value is measured for pebbles). The ratio of rock compressive strength to concrete strength should not be lower than 1.5, and the crushing value index should be lower than 10% ~ 15%.
Other physical properties of aggregate also affect concrete. If the water absorption rate is low, there are few pores and defects, the concrete has high strength, strong frost resistance and small shrinkage. Aggregate should also have the thermal expansion coefficient and elastic modulus similar to that of cement stone, otherwise it will easily produce micro-cracks, increase the permeability of concrete and reduce the durability; If the coarse aggregate is too hard, when the concrete is subjected to temperature and humidity changes, it is easy to cause volume changes, and when the interface stress between cement stone and aggregate is large, it is easy to form cracks.
Because of the high strength of cement stone in high-strength concrete, the bonding force (bonding strength) between cement stone and aggregate is also strong. In the failure section of concrete, the proportion of coarse aggregate is large, so the strength of coarse aggregate often becomes an important constraint factor of high-strength concrete. In addition, the micro-cracks and weak areas that may exist in the parent rock will be destroyed along the cracks during crushing, so the small crushed stones formed after crushing often obtain higher strength than the large samples of the parent rock. This is one of the reasons why high-strength concrete should use small-size aggregate.
Macroscopically, concrete consists of coarse and fine aggregate and cement stone. Microscopically, the cement paste at the interface between cement paste and aggregate is also very different from the cement paste matrix in the system. The interface area forms a higher water-cement ratio and more pores, which are generally weaker than the other two phases of concrete and are the weakest part of the system. If the bonding strength of the interface region is low, it will be destroyed along the interface region.
Many studies have pointed out that coarse aggregate with small particle size should be used in high strength concrete. Due to the relative increase of the surface area of coarse aggregate with small particle size, the stress per unit surface area in the interface area between cement paste and aggregate will be greatly reduced, which will reduce the requirements for bonding strength in the interface area, so it is possible to obtain an interface area with higher bonding strength and bear higher external load. On the contrary, coarse aggregate has a large particle size and a small surface area bonded with cement stone, which is easy to form a weak interface area and weaken the strength of concrete. In ordinary low-grade concrete, the requirement for the bond strength of the interface area is not very high, which is easy to meet the requirements. If conditions permit, aggregate with larger particle size should be used as much as possible to save cement. But it is different for high-strength concrete with higher strength grade. If coarse aggregate with small particle size is not used to increase the surface area, it is impossible to prepare high strength concrete. Abroad, it is considered that the maximum particle size of aggregate should be 10 ~ 15 mm, and in order to save cement, it is generally appropriate not to exceed 20 ~ 25 mm
The influence of fine aggregate on high strength concrete is less than that of coarse aggregate, but it cannot be ignored. Clean medium and coarse sand with fineness modulus of 2. 6 ~ 3.2 coarse sand with fineness modulus of 3.0 is the best. Where is a number greater than Smm and less than 0. 3 15mm should be less. Because fine aggregate has a larger surface area than coarse aggregate, the ratio of fine aggregate to coarse aggregate (sand ratio) has a great influence on cement consumption and concrete strength. The best sand ratio can obtain the best strength. Due to the large amount of cement, proper reduction of sand ratio will not affect workability.
3. Mixed with high-quality mineral admixture
Mineral admixtures such as fly ash, ground slag, silicon powder and zeolite powder can improve the workability of concrete and increase the cohesiveness of mixed materials. Due to the change of cohesiveness, the water migration channel is cut off, which can reduce bleeding and segregation. Bleeding and segregation are the main reasons for the microstructure defects of concrete, especially the interface structure between cement paste and aggregate. Therefore, adding mineral admixture to high strength concrete is not only to save cement, but also to improve the microstructure and properties of concrete.
Slag powder, fly ash and silica powder are all pozzolanic materials, which will slowly hydrate in the presence of water and lime to form cementitious materials, increase strength and fill the pores in concrete. The active components are mainly amorphous silicon dioxide and aluminum. o .。 Si0. And Ca (OH) in cement. Chemical action occurs, and their reaction formula is as follows:
xCa(OH)2 +SiOz+mH20 -，xCa0。 Silica titanium water
When fly ash is not added, not only the interface area between aggregate and cement paste will form a high water-cement ratio area, but also Ca (OH) will be generated. The crystal is large, oriented, porous and plate-like with weak structure. After adding fly ash, Ca (OH) can be reduced. The amount of orange in the interface area; If finer silicon powder or zeolite powder is added, it will react more with Ca (OH). Reaction. The generated C-S-H gel has an amorphous and compact structure, which obviously improves the microstructure of the two-phase interface area and can improve the strength, impermeability and durability of Kunming clay.
Silica fume contains more than 90% silica. 5% ~ 10%. Concrete mixed with silica fume has good workability and stability. Zeolite is a porous crystalline mineral composed of silica tetrahedron. After adding zeolite powder to concrete, the hydration can be uniform and the reaction can be sufficient, thus improving the compactness and strength. If the mineral admixture with low activity is added, it will be evenly distributed in the cement slurry and become the deposition core of a large number of hydrates, and the cement slurry will become evenly distributed microporous amorphous hydrates, reducing the uneven structure of crystal blocks, which is also very beneficial to strength and impermeability.
4. Mix with superplasticizer
Because high-strength concrete must adopt high cement content and low water-cement ratio, Kunming concrete will inevitably have high viscosity and poor rheology. After adding superplasticizer, it has a strong dispersing and lubricating effect on cement, which can greatly reduce the water consumption and make it possible to reduce the water-cement ratio to the maximum extent. Therefore, superplasticizer naturally becomes one of the essential components of high strength concrete.
Water cement ratio is the main factor to control the strength of cement paste and concrete. The test shows that when the water-cement ratio is below 0.35, the performance of the interface area is obviously improved, and the compactness and strength are obviously improved. The theoretical bound water required for the hydration of portland cement is about 28%, and the chemical bound water measured directly is about 25%. When the actual water-cement ratio is lower than this value, the hydration of cement is incomplete, and the compactness of cement stone can not reach enough. Considering the mixing, pouring and curing conditions, the water-cement ratio of high-strength concrete with water reducing agent should not exceed 0.35, and the strength should not exceed 0.25 ~ 0.30 when it is above 80MPa.
5. Strengthen construction control and early maintenance.
According to the characteristics of high-strength concrete with high cement content and hydration block, the following measures should be taken in construction:
(l) step-by-step feeding and mixing are adopted. That is, cement, admixture, sand and water are mixed for about 40s, and then stones and water are added.
The water reducer stirs Imin out of the machine. This can give full play to the role of water reducer, maintain good castability and improve strength. In hot conditions, various materials should be kept as low as possible.
(2) mixer truck transportation. From stirring to pouring, it can be completed within a limited time according to the temperature.
(3) adopt high-frequency vibration. Even if fluid concrete is used, it should be vibrated to improve the compactness.
(4) Strengthen early maintenance. Because hydration is fast, water should be sprayed at an early stage to ensure that hydration is not short of water, so as not to affect the strength development.
(5) Minimize slump. When conditions permit, adding as little water as possible and adopting small slump are still important measures to improve strength.