How To Use Extra Fine Sand To Prepare Concrete

Ultra-fine sand concrete refers to concrete prepared by using sand with a fineness modulus of 0.70 to 1.50 as fine aggregate. Generally, natural medium-coarse sand with low mud content and less harmful impurities is used to prepare concrete. In recent years, with the continuous development and construction of civil engineering in my country, high-quality natural medium-coarse sand resources have gradually decreased, and medium-coarse sand resources have even been exhausted in some areas; at the same time, my country’s Chongqing, Ningxia, Inner Mongolia, Xinjiang, Guangxi, Yunnan and other places Extra-fine sand is rich in resources and convenient for mining and transportation.

Ultra-fine sand has the characteristics of small average particle size, poor particle gradation, small fineness modulus, large mud content, high porosity, and large specific surface area. If the design is unreasonable, it will lead to high viscosity of the mixed concrete and pumping Difficult, large shrinkage and easy to cause deformation cracks, poor wear resistance of concrete, etc. Therefore, to reasonably determine the mix ratio of concrete based on the characteristics of ultra-fine sand is an important technical basis to ensure the quality of ultra-fine sand concrete engineering and the long-term and durability of concrete structures.

1. Superfine sand concrete preparation method

When preparing ultra-fine sand concrete, the fineness modulus of the fine aggregate must be greater than 0.7 and must not contain a large amount of mud sand. The preparation of ultra-fine sand concrete can refer to or follow the following basic principles.

(1) Appropriately reduce the sand rate

Sand rate affects the workability of concrete. When the sand rate reaches the best effect, it will decrease as its fineness modulus decreases. As the sand rate increases, the use of coarse aggregate will be relatively reduced. In order to further ensure the original requirements of the workability and durability of the mixture, the amount of cement must be increased as a last resort. After the amount of cement increases, As a result, the viscosity of the mixture will increase, the fluidity will slow down, the flow rate will decrease, and the shrinkage of the concrete will increase, which greatly increases the probability of cracking. Compared with medium sand, the fineness modulus of ultra-fine sand is very low. Controlling the sand ratio at a low level can effectively suppress some problems caused by ultra-fine sand and improve the performance of concrete. When the fineness modulus of sand is 1.4, it is 7% to 13% lower than that of medium sand with a sand ratio of 2.6.

(2) Increase the amount of cementitious material appropriately

It is very important to control the amount of cement and the total amount of cementitious materials. It is meaningful for concrete performance strength and control of shrinkage and cracking caused by temperature rise of hydration heat. In addition, it also saves cement and reduces economic costs. The shortcomings of ultra-fine sand are its larger specific surface area and higher mud content. Compared with the concrete prepared with medium sand, the amount of cement and cementitious materials are higher. The amount of cement is generally increased by 20kg/m³.

(3) Minimize the concrete slump

Due to the large specific surface area and mud content of ultra-fine sand, the amount of water used to wrap the sand will increase significantly, which will inevitably affect the strength of the concrete; on the other hand, the concrete is prone to bleeding and sand inversion after vibrating. Phenomenon, affecting the quality of concrete construction. In addition, if the concrete fluidity is large, the content of the coarse aggregate of the concrete is relatively small, and the sand ratio follows the principle of low sand ratio. In order to ensure the original workability and strength of the concrete, more cement and cement will be added. The increase of materials, cement and cementitious materials will cause shrinkage and dry shrinkage, and the probability of early cracking will be greatly increased. Therefore, if you want to formulate high-fluidity concrete, it is best to choose sand with larger particle size and less mud content.

(4) Reasonable use of mineral admixtures and additives

The addition of mineral admixtures is conducive to the workability and durability of concrete, and can save cement, reduce production costs, and improve the workability of the mixture. Although the rational use of mineral admixtures and high-efficiency admixtures is an important principle for preparing concrete with other fine aggregates, as well as an important principle for preparing high-performance concrete, but due to the inherent characteristics of ultra-fine sand, mineral admixtures and high-efficiency additives The effect of the agent on improving and enhancing the comprehensive technical performance of ultra-fine sand concrete is more prominent.

2. Factors affecting the performance of ultra-fine sand concrete

(1) Factors affecting the strength of ultra-fine sand pumped concrete

⑴ The primary and secondary order that affects the strength of ultra-fine sand pumped concrete is water-binder ratio> fly ash content> water reducing agent content> sand ratio> coarse aggregate gradation. The effective way to use ultra-fine sand to prepare pumped concrete is to mix fly ash and water-reducing agent to minimize the water-binder ratio. Within the research range selected by the test, the sand ratio and coarse aggregate gradation have an effect on the strength of concrete. The impact is not significant, and its reasonable value range can be determined according to other technical indicators such as concrete pumpability.

⑵ The strength of pumped concrete with ultra-fine sand increases first and then decreases with the increase of sand ratio; the greater the water-binder ratio and the amount of fly ash, the lower the strength of the concrete; when the water reducing agent increases from small, the strength of concrete Increase first and then decrease.

⑶ There are significant interaction effects between sand ratio and water-binder ratio, sand ratio and fly ash content, water reducing agent content and fly ash content, and the difference in impact on concrete strength is very obvious.

(2) Factors affecting plastic shrinkage of pumped concrete with ultra-fine sand

(1) Use fly ash and water reducing agent to significantly reduce the crack width, length and area of ultra-fine sand pumped concrete.

(2) The smaller the sand fineness modulus, the more plastic cracks appear in concrete, especially when the fineness modulus is less than 1.3.

(3) When the sand ratio increases from small, the concrete crack area first decreases and then increases. The sand ratio cannot be too large or too small, and there is an optimal sand ratio.

(4) The plastic shrinkage of concrete increases with the increase of water-binder ratio. During construction, the bleeding on the surface of the concrete should be removed in time to prevent plastic cracks from appearing at the bleeding pool due to the increase of local water-binder ratio.

(5) The primary and secondary sequence of environmental factors affecting the plastic shrinkage of concrete is surface temperature>surface wind speed>ambient temperature>air relative humidity>concrete pouring temperature. Concrete pouring temperature can not be used as a key measure to prevent plastic shrinkage and cracking.

(3) Factors affecting the drying shrinkage of ultra-fine sand pumped concrete

(1) The drying shrinkage of concrete in the early stage increases with the increase of the water-binder ratio, and in the later stage it increases with the decrease of the water-binder ratio.

(2) Adding as many large-sized stones as possible to the ultra-fine sand pumped concrete is beneficial to the crack resistance of the concrete. The late drying shrinkage of reinforced ultra-fine sand pumped concrete will tend to slow down as the maximum size of coarse aggregate increases.

(3) Mixing fly ash and expansion agent can effectively reduce the drying shrinkage of concrete. Compared with non-fly ash concrete, the free drying shrinkage rate of concrete with 90d age is reduced by 10%-20%, and the restrained shrinkage is reduced by 15%-25%. The expansion agent reduces the free drying shrinkage by 15%-20%. The restraint shrinks 10% to 25%.

(4) The early-strength agent will increase the drying shrinkage of concrete, and the drying shrinkage of concrete will increase with the increase of the early-strength agent content during the entire test age. Adding 2% to 4% early strength agent at 90d age can increase the free drying shrinkage rate by 5% to 12% and the restrained shrinkage rate by 13% to 24%.