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Key points for construction of UHPC ultra high performance concrete
Ultra High Performance Concrete (UHPC), also known as Ultra-High Performance Concrete (RPC), is the most innovative cement-based engineering material in the past three decades, achieving the performance of engineering materials. Big leap. “Ultra-high performance concrete” consists of two aspects of “ultra-high” – with high durability and high mechanical properties (compression, tensile and high toughness). Ultra-high performance concrete, its ultra-high performance solves almost all problems compared to ordinary concrete!
UHPC concrete raw materials should be accurately weighed according to the construction mix ratio requirements. The allowable deviation of weighing should meet the following requirements (by weight): gelatinous materials (cement, admixture, etc.) ±1%; admixture ±1%; Aggregate ± 2%; mixing water ± 1%. The horizontal axis, planetary or counter-current forced mixer should be used to mix the concrete, and the electronic measuring system is used to measure the raw materials. Stirring time should not be less than 2min, and should not exceed 3min. When mixing concrete in hot season or cold season, effective measures must be taken to control the temperature of raw materials to ensure that the mold temperature of concrete meets the requirements.
Effective measures should be taken to ensure that UHPC concrete maintains uniformity during transportation and that various performance indicators do not fluctuate significantly. Insulation measures should be taken for transportation equipment to prevent local concrete from rising in temperature (summer) or frozen (winter). Appropriate measures should be taken to prevent moisture from entering the shipping container or evaporating.
(1) Before the concrete is put into the mold, special equipment shall be used to measure the working performance of concrete temperature, slump, gas content, water-to-binder ratio and bleeding rate; only the concrete whose performance meets the design or mix ratio requirements can be used. Casting into the mold. The mold temperature of concrete should generally be controlled at 5 ~ 30 ° C
(2) The free fall height when concrete is poured shall not exceed 2m. When it is greater than 2m, the chute, stringer, funnel and other appliances shall be used to assist the concrete transportation to ensure that the concrete does not have delamination.
(3) The pouring of concrete shall be carried out by means of layered continuous displacement. The gap time shall not exceed 90 minutes, and the construction joint shall not be left at will.
(4) The temperature difference between newly poured concrete and adjacent hardened concrete or geotechnical medium shall not exceed 15 °C.
The vibrating equipment such as a plug-in vibrating rod, an attached flat vibrator, and a surface flat vibrator can be used to vibrate the concrete. Avoid collisions with formwork, steel bars and embedded parts when vibrating. When inserting vibrator to vibrate UHPC concrete, it is advisable to use vertical point vibration to vibrate. The vibration time of each point is based on surface slurry or no large bubbles. Generally, it should not exceed 30s to avoid over-vibration. If you need to change the horizontal position of the vibrating rod in the concrete mixture, first pull the vibrating rod vertically and slowly, then move the vibrating rod to a new position. Do not place the vibrating rod in the mixture. The inner flat drag.
UHPC concrete curing has two purposes: one is to create conditions that enable the cement to be fully hydrated, to accelerate the hardening of the concrete; the other is to prevent the concrete from forming beyond the normal range due to natural factors such as sun, wind, dryness and cold. Shrinkage, cracks and damage. The standard curing conditions for concrete are temperature (20 ± 3) ° C, relative humidity maintained above 90%, time 28d. In actual engineering, standard maintenance conditions are generally not guaranteed, and only measures can be taken to achieve the best possible curing effect under economic and practical conditions. The early strength of high-performance concrete increases rapidly, generally reaching 60% of the design strength in 3 days and 80% of the design strength in 7 days. Therefore, early curing of concrete is particularly important. Usually, after the concrete is poured, the mold-maintenance is mainly used, and the watering and maintenance is supplemented to keep the concrete surface moist. The curing time is no less than 14 days. Concrete conservation can be divided into two categories: natural conservation and heat conservation.
Sixth, quality acceptance
In addition to strict quality inspection of raw materials before construction, in the concrete construction process, the following indicators of concrete should be inspected and controlled: concrete mixture: water-to-binder ratio, slump, gas content, mold temperature, bleeding rate, uniform Qualitative. Hardened concrete: compressive strength of standard curing test pieces, compressive strength and impermeability of the same condition curing test piece.
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