Shrinkage reduction measures of high strength self-compacting concrete?

As one of the most widely used building materials, it is of great significance to improve the quality and broaden the performance of concrete to ensure the durability and safety of concrete structures such as buildings, roads, bridges and water conservancy projects. Complexity and high-rise are the main trends in the development of concrete engineering in recent years, but they are faced with increasingly complex construction techniques and concrete reinforcement difficulties, and high-strength self-compacting concrete can effectively solve the above problems because it does not need vibrating molding. Based on the above background, this paper mainly analyzes the composition characteristics and mechanical properties of high-strength self-compacting concrete, and expounds the effects of shrinkage reducing agent and super absorbent resin on the strength and volume stability of concrete respectively, for the reference of relevant practitioners.

1 Introduction

At present, most super-high-rise buildings in China use reinforced concrete composite structures as external frames, and these structures need to be poured with concrete inside and outside the steel structure. The poor vibrating condition of concrete pouring makes the core temperature of mass high-strength self-compacting concrete increase, the hydration and hardening speed increase, and the self-shrinkage and temperature shrinkage increase, which increases the possibility of concrete structure void or crack. Therefore, in order to reduce the shrinkage of high-strength self-compacting concrete, measures such as adding shrinkage reducing agent can be adopted, which can effectively reduce the risk of structural cracking and help ensure the compressive strength and volume stability of high-strength self-compacting concrete.

2 raw materials for high-strength self-compacting concrete

(1) cement. Cement is the foundation of concrete construction, and its quality, variety and color have great influence on the appearance quality of self-compacting concrete. The same project should always use the same manufacturer, the same variety and the same strength grade of cement, and the color of cement should not change obviously with age. (2) Mineral admixture. Mineral admixture can partially replace cement and enhance the workability and durability of concrete, but its variety, impurity quality and uniformity will also affect the appearance quality of self-compacting concrete. Usually, the preparation technology of composite cementitious materials is mainly used in engineering. By adding composite mineral admixture of fly ash and slag, the workability of concrete is improved, slump loss is reduced, and the color and durability of self-compacting concrete are adjusted. (3) Coarse aggregate and fine aggregate. When preparing self-compacting concrete, medium sand or coarse medium sand is generally preferred for fine aggregate, and granite and limestone are selected for coarse aggregate. Select fine aggregate with fineness modulus of 2.6~3.0, and the particle content passing through the sieve hole with nominal diameter of 3 15 should be ≥15%; Coarse aggregate silt content should be ≤ 65438 0.0%, silt content should be ≤0.5%, and needle particle content should be ≤ 5.0%; In the mix design of self-compacting concrete, the maximum particle size of coarse aggregate is below 25mm; (4) admixture. There are many kinds of additives for preparing self-compacting concrete, which have great influence on the performance of concrete. For example, polycarboxylate-based high-performance water reducer has the advantages of high water reducing rate, low dosage, rapid strength growth, low slump loss and low drilling stagnation resistance, which can make up for the shortcoming of large autogenous shrinkage of concrete to some extent.

3 preparation of high strength self-compacting concrete

3. 1 principle of self-compacting concrete preparation

Self-compacting concrete has good fluidity, spanning ability and anti-segregation ability, and its properties are good fluidity, strong ability to span steel bars, no bleeding, no segregation and uniform material distribution. According to the rheological theory, the rheological equation of self-compacting concrete is: τ=τ0+ξγ, where τ-stands for shear stress; τ0- represents yield shear stress; η-represents plastic viscosity; γ-indicates the shear speed. Under the action of external force, when τ=τ0, concrete flows. Under the same external force, the smaller η is, the greater the velocity is. The preparation principle of self-compacting concrete is: using new concrete admixture and a large number of fine active mineral admixtures, through the design and selection of coarse and fine aggregate, admixture, cementing material and other raw materials, the τ0 of concrete is reduced to a suitable range, and at the same time, it has enough η, so that the aggregate is suspended in the cement paste without bleeding or segregation, so that it has high fluidity and does not need vibration, and can freely flow and fully fill the space in the model, forming a uniform and compact structure and solving the vibration problem.

3.2 Workability and mechanical properties of self-compacting concrete

Good workability is the key characteristic of self-compacting concrete mixture. The molding characteristic of self-compacting concrete is that it can flow freely and fill the model evenly and densely even under the condition of dense steel bars, only relying on the self-weight of concrete mixture and without vibration. Filling, crossing and segregation resistance are the workability of self-compacting concrete, and compressive strength, tensile strength, flexural strength, elastic modulus and stress-strain relationship are the basic mechanical properties of self-compacting concrete. Theoretically, the amount of coarse aggregate in concrete mixture is directly proportional to the elastic modulus, but it can be seen from the research of relevant personnel that the elastic modulus of low-strength self-compacting concrete is higher than that of ordinary concrete with the same strength. When the content of fly ash is small, the elastic modulus of self-compacting concrete is slightly smaller than that of ordinary concrete. When the content of fly ash is large, its elastic modulus is higher than that of ordinary concrete, which may be related to the changes of self-compacting concrete after hardening.

3.3 Proportion calculation of self-compacting concrete

① Powder system: high-performance water reducer+water+fine aggregate+coarse aggregate+powder, in which powder is divided into three types: powder mixed with cement only, powder mixed with cement and an additive, powder mixed with cement and two additives; ② tackifier system: high performance water reducer+water+fine aggregate+coarse aggregate+cement+tackifier; ③ Combination system: high performance water reducer+water+fine aggregate+coarse aggregate+cement+tackifier+powder.

3.4 Self-compacting concrete mixture ratio optimization

Scholars have conducted in-depth research on self-compacting concrete, focusing on the analysis of formula ratio, and compared and analyzed the strength, corrosion resistance and compression resistance of concrete made according to different formulas, so as to get the best ratio scheme. The test results show that the working performance of concrete slag will be significantly improved with the continuous increase of its output, and the general trend is to increase. For example, compared with adding 10% ground slag, adding 20% ground slag improves the strength. Therefore, replacing 20% ground slag with the same amount of cement can effectively change the viscosity of concrete, avoid segregation and other problems, and the fluidity will not decrease, so as to improve the working performance and improve the strength of self-compacting concrete.

Production, transportation and construction of high-strength self-compacting concrete

4. 1 output

(1) Sufficient raw materials are prepared and the supply is sufficient to ensure the consistency of varieties, supply and quality; (2) Check the measurement system before delivery to ensure that the measurement accuracy meets the specifications; (3) The laboratory is responsible for determining and adjusting the mixture ratio. Other departments are not allowed to adjust the mixture ratio at will, and operators should do a good job of testing and rechecking regularly; (4) Each vehicle's concrete mixture can leave the factory only after passing the performance inspection, and full-time personnel shall be arranged to strengthen the detection of concrete slump; Increase the detection frequency of aggregate moisture content in the production process. When the moisture content changes greatly, the laboratory personnel will adjust the mixture ratio; (5) The mixing time of concrete mixture is 50s, and the slump of concrete is 240mm±30mm；; When the slump is less than 220mm, the original admixture can be adjusted appropriately, and the adjusted admixture content should be determined through experiments. After adding additives, the mixer truck should rotate at a high speed 10 minutes before discharging, and it will be considered as qualified after adjustment.

4.2 transportation

(1) According to engineering quantity, construction speed, real-time traffic environment, transportation distance, etc. , reasonable arrangement of concrete production and transportation speed, to ensure continuous pumping and pouring in the construction site; (2) The mixer truck should rotate at a certain speed during transportation. After arriving at the construction site, the mixing drum should rotate at least 1min at medium and high speed before discharging, so that the loaded concrete can be evenly mixed, and then discharged; (3) Pump truck drivers and mixer truck drivers shall not make any adjustments to the concrete delivered to the site, and the remaining concrete shall not be reused for self-compacting concrete, otherwise it will produce color difference.

4.3 architecture

(1) pouring: concrete must be poured continuously, and construction joints must be left at the open joints to avoid the impact of cold joints on the visual quality of concrete; Grasp the vibrating time of concrete, the concrete surface is smooth, the cement slurry is uniform, and there is no obvious sinking and a large number of bubbles rising; In order to reduce the air bubbles on the concrete surface, the secondary vibrating technology is adopted, namely, after the concrete is put into the formwork 1, before the second layer of concrete is poured, and generally after 0.5h, the top layer is vibrated. (2) Maintenance: When the strength of the concrete specimen under the same conditions reaches 80% of the standard strength and is not less than 14d, the formwork shall be removed to ensure that the angle will not fall off when the formwork is removed. Maintenance should be carried out in time after formwork removal, because the surface humidity of concrete after formwork removal is low, which is easy to cause sand to reduce the surface hardness, and there will be color difference and shrinkage cracks. The combination of plastic film covering and water spraying maintenance is often used in engineering. Water should be sprayed before formwork removal and during maintenance to keep the concrete surface moist.

5 conclusion

To sum up, in the process of self-compacting concrete mixing, the construction personnel should reasonably adjust the mixture ratio of different materials according to the concrete performance and the actual needs of engineering construction. In the process of mixing, the specific properties of concrete can be changed by changing the types and contents of additives, thus improving the strength and stability of concrete. In addition, there are many aspects of self-compacting concrete that need to be further understood and studied, such as the changes of mechanical properties and durability of self-compacting concrete after adding a large number of additives and admixtures, and their changing laws.

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