1. How to finish and cure UHPC?

 

First, for cast-in-situ UHPC without steam curing, the finishing process similar to high-strength silica fume HPC can be adopted. After finishing, the surface shall be covered immediately (e.g. plastic film) or covered while finishing to prevent water loss; Of course, you can also spray high-efficiency curing agent while finishing; Even if the curing agent is sprayed, it shall be covered in time; Premature sprinkling or spray maintenance should not be performed on the surface of UHPC. Sprinkling or spray maintenance can be carried out after pouring 1day.

 

 Second, for the cast-in-situ UHPC to be steam cured, the above methods can also be used for curing within 1day; Steam curing shall be carried out after 1day; Generally, refer to "standard steam curing process", i.e. 90 ° C, 48h; However, pay attention to monitor the temperature and humidity at different locations at any time to ensure uniformity everywhere as much as possible.

 

 Third, for factory prefabricated steam curing or autoclaved UHPC components, the above methods can be referred to or adjusted appropriately.

 

2. When will UHPC remove the formwork?

 

 For UHPC constructed under normal temperature, the formwork can be removed after 1day of normal curing. For pouring under other environmental conditions, it can be adjusted appropriately according to the actual situation.

 

3. What should be done if large-scale defects are found after UHPC formwork removal?

 

Those that need to be demolished and rebuilt should be demolished in time, otherwise the later demolition cost is high.

If repair is required, it shall be repaired in time after chiseling or cutting; The repair materials can be the same as the original construction materials. If the same UHPC material is used for repair, no other interface agent is required.

 When conventional cutting tools are not applicable, hand-held high-pressure water knife can be considered for cutting.

 

 4. What if the strength of cast-in-situ UHPC fails to meet the standard at 28day?

 

 If the coring strength is seriously insufficient, it should be dismantled, blasted and reinforced.

 

 If the coring strength is about 20% less than the design strength, we need to consider whether it can be measured after prolonging the age? Or after thermal curing? In either case, it shall be finally evaluated by professional structural engineers.

 

5. How to carry out daily maintenance of UHPC structure?

 

 For UHPC without wear, high temperature or acid corrosion, special daily maintenance is generally not required.

 

For UHPC with high requirements for appearance cleanliness, surface hydrophobic or antifouling treatment can be carried out in advance, and some surface synergists can be added during mixing in advance; For those with high color requirements, anti ultraviolet agent or color fixing agent can be coated appropriately; Acidification, surface roughening or matte treatment can be carried out if there are requirements for reflection; If there are requirements for the decomposition of toxic gases such as NOx, titanium dioxide can be added in advance for degradation, or some absorbent such as zeolite powder can be added for absorption. For UHPC whose surface has been polluted, high-pressure water or sand blasting, shot blasting or grinding can be used before appropriate surface treatment.

UHPC under the action of wear needs daily inspection and replacement after reaching the critical wear degree.

In case of high temperature, pay attention to the expansion, cracking and peeling of UHPC. In case of serious damage, replace it in time.

 For UHPC in acid gas or acid corrosion environment, hydrophobic, film coating or corrosion-resistant surface treatment can be carried out in advance before putting into use; Daily observation shall be carried out during use. Once the critical damage degree is reached, it shall be replaced in time.

 

 6.What if the exposed steel fiber end face of UHPC surface is corroded？

 

Common electrochemical corrosion occurs on the exposed steel fiber end face without copper plating; Galvanic corrosion occurs on the exposed steel fiber end face with copper plating.

 

If you feel it is inconvenient for your eyes, you can gently polish the UHPC surface and carry out hydrophobic treatment, such as brushing silane or other silicone materials, or other organic coatings (transparent or opaque).

 

If you don't think it's eye-catching, you don't need to deal with it, because its corrosion depth is usually very shallow, even after a few years.

If this is not allowed, stainless steel fiber or other non-metallic fiber can be selected for the new structure; When the steel fiber is not replaced, you can also find ways to prevent it from being exposed or conduct surface protection treatment in time after formwork removal.

 7. Why not worry about the corrosion of steel fibers or reinforcement in UHPC?

 

Because the pores in high-quality UHPC are mainly those below the nanometer (nm) level, its connectivity is very poor, and it is difficult for water, water vapor and oxygen to be transmitted for a long distance. Therefore, it is difficult for the internal steel fiber or reinforcement buried several mm deep in UHPC to be corroded again.

 

In addition, because the water binder ratio of UHPC is very low (usually < 0.18), the ohmic drop of the matrix material is large, which avoids the occurrence of macro cell corrosion.

 

Even if the salt content in UHPC is high, according to the principle just mentioned, there is no need to worry about the corrosion of steel fiber or reinforcement. This is also the difference between UHPC and OPC and HPC.

 

8. Why not worry about the frost resistance of UHPC or other traditional deterioration?

 

It can be attributed to that there is not enough free water or reaction water in UHPC. This is true for frost resistance, carbonization, alkali aggregate reaction, sulfate erosion, etc.

 It can also be attributed to the difficulty of long-distance effective transmission of harmful substances.

 

 9. Don't you have to worry about steel fiber or reinforcement corrosion in cracked UHPC?

 

First of all, it depends on the degree of crack and the width and depth of crack.

For cracks below 0.2mm allowed by traditional OPC / HPC, there may be no need to worry too much in UHPC; However, for wider cracks, especially those penetrating to the surface of reinforcement, if water and oxygen can be allowed to transfer in them, the reinforcement corrosion problem at this place can not be ignored; If it is subjected to dynamic load at the same time, it can not be underestimated, although the harm of reinforcement corrosion is much weaker than that in conventional concrete!

 In other words, we can master such a principle: it depends on whether water, water vapor, oxygen and other harmful substances can flow freely in the crack; If it can, the problem of local deterioration cannot be ignored; If not, there is no need to worry.

 

10. How to improve the anti-wear or impact wear performance of UHPC?

 

For the surface wear resistance, as with traditional concrete, adding wear-resistant agent or wear-resistant aggregate on the surface of UHPC can not necessarily improve its surface wear resistance by simply increasing the amount of fiber.

 

For impact wear resistance or impact wear, it may be treated differently. The former can consider using wear-resistant coarse aggregate, and the latter may also consider the type and amount of fiber or surface reinforcement; Even consider the use of surface anti-collision buffer protection layer.