The core of mix design for ultra-high performance concrete lies in ultra-low water cement ratio, high dosage of cementitious materials, and fiber reinforcement. It is necessary to choose a UHPC planetary mixer for strong mixing and achieve the goal of high strength and toughness through multi-scale collaborative control. When designing, it is not enough to simply apply ordinary concrete formulas, but to conduct refined trial mixes based on target strength, workability, and durability.
How to choose materials
Cement should be selected for strength: Portland cement or ordinary Portland cement with a strength grade of 52.5 is usually used. For concrete above C80, it is recommended to use grade 52.5, and the amount of cement used should be strictly controlled within a reasonable range.
Admixtures are key: Silica fume is a "sharp tool" to improve strength, with a dosage generally ranging from 5% to 10% of the cement content, which can significantly fill the gaps. Fly ash and slag powder can improve workability and reduce costs, with fly ash content commonly ranging from 20% to 30%.
The aggregate should be clean and well graded: medium sand should be used for fine aggregate, with a fineness modulus of 2.6-3.0 and a low mud content. The maximum particle size of coarse aggregate usually does not exceed 20mm, and its strength needs to be greater than 1.2 times the design strength of concrete.
Fiber cannot be reduced: Steel fiber is standard for UHPC, and the recommended volume ratio is between 0.5% and 1.5%. Excessive fiber can easily clump and affect strength. There are also studies attempting to blend polyethylene fibers to enhance flexural performance.
How to set key parameters
The water cement ratio is extremely low: this is the core that determines the strength, and the water cement ratio of UHPC is usually between 0.18 and 0.25, much lower than that of ordinary high-performance concrete below 0.4.
High total amount of cementitious materials: The total amount of cementitious materials used is generally between 550 and 800 kg/m ³, which needs to be adjusted according to the strength target, but attention should be paid to controlling the hydration heat.
The sand ratio needs to be determined through testing: UHPC commonly uses fine sand or quartz sand, and the sand ratio has a significant impact on fluidity. Generally, the optimal value is determined through trial mixing.
Admixtures should be efficient: Polycarboxylate based high-efficiency water reducing agents should have a water reduction rate greater than 20%, and the dosage is usually between 1.5% and 2%. The optimal dosage should be found through experiments. Official website
How to make it more stable
Calculate first and then try: After initially determining the water cement ratio and the amount of cementitious material, it is necessary to conduct trial mixing adjustments, as the Baum formula is no longer applicable to concrete above C60.
• Staged mixing: It is recommended to dry mix the sand and gravel for 1 minute first, then add the cementitious material and stir by suitable UHPC mixer, and finally add fibers and remaining water to ensure even fiber distribution.
• Proper maintenance: Demoulding should be done 24 hours after pouring, and curing should be carried out for 28 days in a room temperature environment of around 20 ℃. Early humidification and cooling can reduce the risk of hydration heat cracking.
Multi objective optimization: Modern design tends to comprehensively consider strength, durability, workability, and economy, and can use computer simulation to assist in determining the mix proportion. Only by selecting a reasonable mix proportion and using a UHPC mixer with excellent mixing effect can high-performance concrete with excellent performance be prepared.
