High-performance concrete is a new type of high-tech concrete made by modern concrete technology on the basis of greatly improving the performance of ordinary concrete. Below, I will share with you the construction technology and method of high performance concrete preparation. Welcome to read and browse.
1 Performance characteristics of high performance concrete
Durable concrete must be able to resist weathering, chemical erosion, abrasion and other damage processes, that is to say, high-performance concrete should not only have high strength, but also have high stiffness, small volume change, virtually impermeable to water, difficult to penetrate chloride ions, high elastic modulus, small shrinkage and creep, and small thermal strain. Therefore, high performance concrete should be different from ordinary concrete in composition and structure, and should first have the following structural characteristics [1]:
1) porosity is very low, and there are basically no macropores larger than 100nm;
2) The content of Ca(OH)2 in hydrate decreases, while the content of C-S-H and Aft increases;
3) There are many unhydrated particles, and the centrality effects of unhydrated particles and fine mineral admixtures at all levels increase, and the centrality network skeleton is strengthened;
4) The interfacial transition layer has a small thickness and low porosity, so the amount of Ca(OH)2 decreases, the degree of orientation decreases, and the grain size of hydration products decreases, which is closer to the distribution of hydration products in cement paste, so it is strengthened.
Secondly, the preparation of high-performance concrete is characterized by low water-binder ratio, adding superplasticizer and fine mineral admixture, so from the perspective of composition and proportion, high-performance concrete should also have the following characteristics:
1) water cement ratio (W/C)? 0.38
According to r? Ch proposed the phase diagram [2], when the water-cement ratio >: 0.38, after the cement is completely hydrated, the cement paste contains cement gel, gel water, capillary water and voids. Capillary water can diffuse and permeate in concrete, that is, W/C >; 0.38, capillary exists in concrete, which reduces impermeability and durability. Therefore, when preparing high performance concrete, the water cement ratio should not be greater than 0.38.
2) Superplasticizer is a necessary material to reduce the water-cement ratio in concrete, and it is also an indispensable part of high-performance concrete. In order to make concrete have good working performance, superplasticizer should not only have high water reduction rate, but also have the function of effectively controlling slump loss.
3) Mineral admixture is one of the functional components of high performance concrete. Under the same water-binder ratio, it can fill the gaps of cement and improve the fluidity and hardened strength. More importantly, it can improve the interface structure between cement stone and aggregate in concrete and improve the strength, impermeability and durability of concrete.
4) When high performance concrete has frost resistance or other requirements, air entraining agent and other related additives, such as rust inhibitor, should be added.
Therefore, when preparing high-strength and high-performance concrete, the variety and quality of raw materials (such as the maximum particle size and variety of cementing materials, admixtures, admixtures, aggregates and coarse aggregates, etc.) should be determined. ) According to the actual durability, fluidity and strength requirements of engineering and environment, select reasonable process parameters and determine the concrete mixture ratio. In addition, the construction of high performance concrete needs strict quality control.
2 composition of high performance concrete
Like NC, HPC must use cement, aggregate and water, as well as additives and fine mineral admixtures. However, due to the requirement of high performance, HPC requires higher material quality, and the quantity and proportion of its constituent materials are obviously different from those of NC. In order to obtain high-strength and high-fluidity high-performance concrete, besides cement, water and aggregate, the following technical measures must be taken:
1. Add highly active mineral admixtures, such as silica fume, ground slag micropowder, superfine fly ash, natural zeolite powder, etc. Make full use of the interstitial action of superfine powder to form a meso-dense system, improve the pore structure of concrete, improve the compactness of concrete, improve the interface structure of concrete and enhance the interfacial bonding strength.
2. Adding high-quality superplasticizer such as retarding superplasticizer can not only ensure sufficient fluidity of concrete, but also effectively control slump loss of concrete.
3 mix design of high performance concrete
3. 1 high performance concrete preparation objectives and influencing factors
1) Durability: As mentioned above, the goal of HPC programming is mainly durability. Low permeability is the first line of defense for concrete, because most physical or chemical corrosion that leads to concrete deterioration is caused by harmful media soaking in water. The main factors affecting the permeability of concrete are the uniformity and stability of the mixture, as well as the compactness of hardened concrete, the formation of central network, interface structure, dimensional stability and the quality of raw materials used.
2) Strength: Due to the advantages of reducing the cross-section of columns and long-span bridges in high-rise buildings, reducing the self-weight of structures and expanding the use area, the strength of concrete should be improved as much as possible where the cross-section is allowed to be reduced. At present, the requirement of structural design for concrete is still based on compressive strength, so the visible design basis of concrete mixture ratio is compressive strength first. The main factors affecting strength and compactness are water-binder ratio and the amount of mineral admixture. At the same time, influenced by the interface, the particle size, sand ratio and slurry content of coarse aggregate will also affect the strength.
3) workability: the workability of HPC is very important, which is the key to ensure the quality of concrete pouring. High-performance concrete should have high fluidity and pumpability, and the mixture should have the characteristics of stable volume, no segregation and no bleeding. At the same time, in order to ensure the construction quality, we should consider reducing the loss of fluidity when batching. The main factors affecting the workability of high performance concrete mixture are the amount of cement slurry (involving water-binder ratio, the amount of cementitious materials and sand ratio, etc.). ), aggregate gradation, additive variety and dosage, etc.
3.2 Mix proportion law of high performance concrete
According to the characteristics of high performance concrete, the mix design should follow the following principles:
1) lime-water ratio rule: the strength of concrete is directly proportional to the strength of cement and the ratio of lime to water. Once the ash-water ratio is determined, it cannot be changed at will. Here? Gray? Including all cementing materials, also known as glue ratio.
2) Law of dense volume of concrete: The total volume of concrete in plastic state is the sum of dense volumes of water, cement (cementing material), sand and stone. This law is the basis of calculating concrete mix proportion.
3) Minimum unit water consumption or minimum cementing material consumption rule: under the condition that the ratio of lime to water is fixed and the raw materials are fixed, the concrete with stable volume and economy can be obtained with the minimum water consumption (that is, the minimum amount of slurry) that meets the workability.
4) Minimum cement consumption rule: In order to reduce the temperature rise and improve the ability of concrete to resist environmental factors, the cement consumption in cementing materials should be reduced as much as possible on the premise of meeting the early strength requirements of concrete.
3.3 Selection of Mix Proportion Parameters of High Performance Concrete
The mix parameters of high performance concrete mainly include water-binder ratio, slurry-aggregate ratio, sand ratio and water reducing agent content.
1) water-binder ratio: Water-binder ratio (or water-binder ratio) has a great influence not only on the strength of concrete, but also on the impermeability and durability of concrete. The water-binder ratio of cement stone is larger, the permeability increases and the durability decreases. Due to the requirement of durability, the Technical Specification for Mix Design of Ordinary Concrete stipulates the maximum water-cement ratio and the minimum cement dosage. For high performance concrete, in order to achieve low permeability and ensure the durability of concrete, its water cement ratio should not be greater than 0.35 ~ 0.40. The research shows that when portland cement is hydrated, the combined water accounts for about 22% of the cement weight, that is, under the conditions of cement and superplasticizer currently used, the optimal water-cement ratio is about 0.22 by adopting common mixing, pouring and curing technical measures. If the water-cement ratio is less than 0.22, the cement stone will not reach enough compactness, so the water-cement ratio range of high performance concrete should be 0.22 ~ 0.35. For example, the water-binder ratio of the wall in Shaanxi Zhongjia is 0.236, and C80 pump mixes secondary fly ash to transport fluid concrete [3].
2) Slurry-aggregate ratio: After the water-cement ratio is determined, the total amount of cementitious materials reflects the ratio of cement slurry to aggregate, that is, the slurry-aggregate ratio. Experiments show that the volume ratio of cement slurry/aggregate in high performance concrete is 35/65. In order to ensure the good fluidity of concrete, the total amount of cementitious materials is required to be large, but with the increase of the dosage of cementitious materials, the elastic modulus of concrete will decrease and the shrinkage of concrete will increase. According to experience, the total amount of cementitious materials for HPC should not exceed 550 kg/m3, and decrease with the decrease of concrete strength grade, but at least it should not be less than 300 kg/m3.
In addition, high performance concrete is easy to cause volume deformation and temperature cracks because of its low water-cement ratio, high cement content, high hydration heat and large temperature rise. Therefore, considering the technical performance and economy, it is necessary to add auxiliary cementing materials to reduce the temperature rise and drying shrinkage of concrete, improve the chemical corrosion resistance, increase the compactness and reduce the cost. Generally, cement is replaced by 10% ~ 30% auxiliary cementing material, which can be mixed with silica fume, slag and fly ash respectively, or mixed with silica fume and fly ash or mixture of silica fume and slag.
3) Sand ratio: Under the condition of a certain amount of cement slurry, the influence of fine aggregate on concrete mixture ratio is more significant than that of coarse aggregate. Under a certain weight, the surface area of fine aggregate is much larger than that of coarse aggregate, and all aggregates need to be wrapped with cemented slurry, so the size of sand particle size and sand ratio directly affect the demand for slurry. High-performance concrete has a large amount of cementitious materials. If the fine aggregate is less and the coarse aggregate is larger, the amount of mortar can be reduced, which is more economical and can obtain higher strength. Therefore, under the condition that the workability can meet the construction requirements, a smaller suitable sand ratio can be selected. P.K.Mehta thinks that [2] high performance concrete with good dimensional stability can be prepared by using suitable coarse aggregate and the volume ratio of cement slurry to aggregate is 35/65. When the maximum particle size of coarse aggregate is 12 ~ 19 mm, the recommended sand ratio is 36% ~ 39%. Through the statistics of the mix proportion of typical foreign projects and laboratories, it is found that the sand ratio of high performance concrete with 28d compressive strength of 60 ~ 120 is mostly in the range of 34% ~ 44%. When the strength is between 80 ~ 100, the sand ratio is mainly between 38% and 42%. And with the increase of concrete strength, the sand ratio tends to decrease. For example, the sand ratio of Inner Mongolia aviation coal rock is 0.36, and C80 high performance concrete is prepared by adding 650m2/kg superfine slag [4].
4) dosage of water reducing agent: it is very difficult to prepare concrete with high construction performance under the above-mentioned low water cement ratio, so high-efficiency water reducing agent must be applied. Superplasticizer has a strong dispersing effect, and the water reduction rate can be as high as 30% or more, especially when the cement dosage is large or the cement particles are fine. Generally, the dosage of water reducer is about 1.0% of the cement quality, or 0.8% ~ 1.0% of water reducer and 0.2% of calcium lignosulfonate are added to properly control the slump loss of concrete. When the dosage of cementing material is large, it is necessary to increase the dosage of water reducer.
Construction and quality control of high performance concrete
If the material selection and mix design are correct, the durability of high performance concrete depends largely on the construction quality, and the manufacture and construction of concrete determine the performance of concrete. Correct feeding sequence, sufficient and even mixing, no segregation of concrete mixture during transportation, loading and unloading, vibrating compaction and sufficient curing are important factors to ensure the quality of high performance concrete.
High-performance concrete can be constructed with ordinary concrete construction equipment, but its construction quality control should be stricter, the measurement error of ingredients should be within the allowable range, the inspection times of raw material quality change should be increased, and the concrete mixing should be thorough and uniform to ensure the good construction performance of fresh concrete. One aspect of high performance concrete is that it should have proper workability to ensure satisfactory pouring quality. In order to facilitate pouring, HPC usually needs a large slump, such as 10 ~ 20 cm. However, due to the large amount of HPC cementing material and low water cement ratio, the concrete mixture is sticky and slump loss is fast. Excessive slump loss is not conducive to concrete pouring, compaction and homogenization, and affects the overall quality of the structure. Therefore, in the process of high performance concrete construction, in addition to requiring high efficiency water reducer to have good slump loss control performance, special attention should also be paid to the construction organization arrangement to minimize the slump loss of concrete.
In addition, due to the high hydration temperature rise of high-strength and high-performance concrete, it reaches the highest temperature in about 24 ~ 48h after pouring according to different concrete components and environmental conditions. Therefore, it is not appropriate to dismantle the formwork prematurely in HPC construction, and it is not appropriate to dismantle the formwork immediately after formwork removal. It should be protected continuously for several hours to avoid cold shock. At the same time, correct plastering and water conservation are important steps to obtain impervious surface [5]. For high-performance concrete with low water-cement ratio, it is necessary not only to keep the internal moisture from evaporating, but also to pay attention to replenishing water from the external environment, and to carry out external moisture maintenance to ensure the full hydration of concrete and improve the comprehensive performance of concrete.
References:
[1], even. High performance concrete [M]. Beijing: China Railway Press, 1999.
[2] edited by Feng Naiqian. High performance concrete [M]. Beijing: China Building Industry Press, 1996
Zhong Jiaqiang, Liu Kai, et al. Experimental study on C80 pumping Ⅱ fly ash in high fluidity concrete. Proceedings of the Third National Conference on Commercial Concrete in 2006.
Hangzhou, Zhao Gentian,. Study on the preparation of C80 concrete with superfine slag powder. Concrete, 2007
[5] Zhang Chuancang, Yang Limin and others. Engineering practice of mass concrete temperature measurement technology. Concrete, 2007, (4)
;