Professor Lu Xinying · UHPC Technical FAQ 100 Questions

Consider cost-effectiveness: Prioritize metallic fibers (steel fibers), followed by non-metallic fibers. When mixing different fibers, conduct careful experiments before making reasonable selections.

Specific recommendations: High-strength carbon steel fibers (tensile strength >2800MPa, diameter 0.2mm, length 13mm, aspect ratio ~60) are the first choice; carbon fibers, PVA, PP fibers, and glass fibers should be used with caution; mixed metallic and non-metallic fibers are not recommended.

For toughening requirements: Import PVA fibers are currently recommended.

For fire resistance requirements: PP fibers can be used. Organic fibers, glass fibers, or other mineral fibers are currently recommended only for non-load-bearing or decorative components.

This involves three core issues:

① Fracture mechanics: Original crack size and defect density

② Homogeneity: Large particle spatial distribution

③ Mass transfer: Defect size, connectivity, and defect density

Unless the component cross-section is sufficiently large, aggregates >5mm are generally not recommended (unless truly necessary, such as for shrinkage reduction, temperature lowering, cost reduction, etc.).

Size control recommendation: Control according to 1/30 of the minimum cross-section dimension. For example: if the minimum cross-section is 30cm, coarse aggregate not exceeding 10mm can be used.

Exception: For reinforced structures with surface protection (such as anti-corrosion coating or stainless steel/FRP sleeve), 1/5 control can be relaxed—that is, 5cm reinforcement layer can use aggregates below 10mm.

Most particle packing calculation software treats all particulate materials as inert spherical particles, and correction for chemical effects is troublesome and not easily implemented.

Recommended approach: Limit material dosage ranges based on hydration characteristics—cement 600±200Kg/m³, silica fume 200±50Kg/m³—then calculate.

Material selection recommendations:

• Heat-treated (Steam Curing/autoclave) UHPC: Consider using partial blast furnace slag

• Non-heat-treated: Do not use blast furnace slag for now

• Steel slag powder: Currently prohibited

• Fly ash: Grade I and II qualified products are recommended

General principle: Minimize mineral raw materials and their dosages that increase shrinkage and viscosity.

The software treats all particulate materials as inert spherical particles without considering the hydration characteristics of each material, resulting in unrealistic calculation results.

Solution: First set constraints based on the aforementioned dosage ranges (cement 600±200Kg/m³, silica fume 200±50Kg/m³), then calculate the mix proportion. Do not let the software freely iterate to zero.

Conventional mixers: Fibers should be added after the cement matrix is mixed

High-efficiency convective mixers: Order doesn't matter

Key principle: Ensure steel fibers are evenly dispersed in the cement matrix without clumping. Without high-efficiency mixers, fibers should not be mixed with powder materials. It is recommended to use professional fiber dispersion equipment.

Nano-silica has two preparation methods: wet and dry, with different fineness and activity, and dosage is generally minimal.

Recommendation: Remove nano-silica (unless you want to research it or publish papers).

Professor Huang Zhengyu's additional note: The fineness and activity of nano-silica vary greatly with preparation process, and special caution is needed when dosage is minimal.

Absolutely yes. This is one direction of UHPC industrialization. The key to pre-mixed UHPC is maintaining high-quality powder superplasticizer (currently, domestic market powder superplasticizers have obvious air-entraining issues, and imported products are restricted without supply).

Cement type selection is important; efflorescence may sometimes occur.

Solution: If efflorescence is possible, surface pretreatment can be performed. Reference existing surface protection technologies. If steel fibers are used in UHPC decorative panels, they may rust easily. Consider using stainless steel fibers or other non-metallic fibers as alternatives.

No need to worry much when fiber content is low; for high content (above 5%), consider gap-graded design and special design is needed. A reasonable description requires further research. Professor Huang Zhengyu suggests that this issue may be encountered when using dense packing design, and gap-graded design can be considered.

Based on current domestic production levels:

Standard cubic Compressive Strength: 150~200MPa

Tensile strength: 7~12MPa

Permeability: Two orders of magnitude lower than C80

These indicators will change with future technological development.

UHPC mix proportion design involves a 9-equation system with multiple constraints including Compressive Strength, tensile strength, slump Flow Spread, etc. There is no unique solution. It requires particle packing theory and iterative trial mixing:

① Requirements for particle gradation of the particle packing body (e.g., meeting a certain continuous gradation curve);

② Test particle gradation curves of various particulate materials;

③ Use particle gradation analysis software (such as Elkem Emma) to calculate dosages of various particulate materials;

④ Comprehensively consider superplasticizer and water content, design trial mix proportions;

⑤ Conduct trial mixing and test performance;

⑥ Readjust based on results, iterate until ideal mix proportion is obtained.

Tip: First achieve good particle continuous gradation and dense packing, then consider chemical hardening of binder and admixture adjustment.

Recommended to use Emma software from Elkem (earlier version Lisa also works). This software is also applicable to Self-Leveling concrete and refractory ramming material mix design. Of course, calculation software can also be developed independently based on particle gradation calculation methods.

For now, don't consider the differences in testing methods; they can be used directly for calculation. Because the calculation results are only for reference! Compared to mechanical multi-factor orthogonal experimental design, it just looks more scientific.

① Cement particle gradation can use literature data from the same manufacturer and brand if testing conditions are not available;

② Silica fume particle gradation is often inaccurately measured; use manufacturer or literature data directly;

③ If the particle gradation curve of micro-nano powders is inaccurately measured, it may cause difficulties in calculation and trial mixing;

④ For quartz powder and quartz sand, select at least 3 or more particle gradation materials from fine to coarse.

Adjustment of the finest and coarsest materials can be relatively rough, but the middle particle gradation materials must be taken very seriously!

Obviously not! Physical and chemical properties and geometric characteristics of all particulate materials must also be considered. For example: Are the particles hydration-active or inert? Are the particles approximately spherical or angular? Various factors must be considered. Calculation + sufficient experiments, combined with experience in HPC and other cement-based composite materials, will definitely find at least one relatively ideal mix proportion.

① Focus on high-strength carbon steel fibers, such as tensile strength >2800MPa, diameter 0.2mm, length about 13mm (aspect ratio usually around 60);

② Carbon fibers, PVA, PP fibers, glass fibers, etc. are not necessary to use; mixed metallic and non-metallic fibers should not be considered for now;

③ Focus on particulate materials when designing UHPC mix proportions; fiber calculation is not needed.

① Design trial mixes according to elastic limit tensile strength (first-crack strength) requirements to produce UHPC matrix material close to that strength value;

② Trial mix with 2.0% by volume steel fiber, and adjust steel fiber content by testing direct tensile performance after standard Steam Curing.

For UHPC with strain-hardening requirements, matrix tensile strength should be at least 5MPa, and steel fiber volume content above 2.0%.

If you don't have a high-efficiency (cyclone or vortex) mixer, fibers should not be mixed with powder materials; mix the powder to ideal flow state first, then add fibers gradually while continuing to mix until uniform. If professional fiber dispersion equipment can be used to help, it will save a lot of effort!

① Try a more efficient superplasticizer. Water-reducing rate should preferably be above 38%, and air-entraining content should be as low as possible; start with liquid agents, not powder agents;

② If the superplasticizer is not the problem, then the particle gradation is poor, or some unnecessary particulate materials are used. Components should be readjusted and trial mixing conducted again.

Most UHPC is self-compacting or Self-Leveling, requiring no vibration. In actual construction, slopes or areas with construction angle requirements are often encountered, such as in traffic pavement projects. In this case, Self-Leveling UHPC should not be used, and appropriate slump should be controlled. For UHPC with slump between 120~180mm, flat plate vibrators can still be used; laboratory specimens can also be vibrated on a vibrating table, but vibration time should be shortened appropriately to avoid severe fiber segregation.

Both abrasion resistance and fire resistance of cement-based materials require special design and have no fixed relationship with Compressive Strength or density.

Fire resistance improvement: Add PP fibers, change fiber content, add coarse aggregate, or use aluminate cement

Abrasion resistance improvement: Add wear-resistant agents or aggregates on the surface, consider fiber type/content or surface reinforcement

UHPC's abrasion and fire resistance can be designed to be excellent, but achieving order-of-magnitude improvement compared to C80 is not easy.

The elastic modulus of a material essentially depends on the main bond types in the material. UHPC is still a cement-based material, and the bond types have not fundamentally changed. Significant improvement is not possible.

Editor's supplement: Adding high-strength, high-hardness aggregates to UHPC can improve the elastic modulus to 60-70GPa.

Strain value recommendations:

Important conclusion: Only by making good UHPC matrix material can you make good UHPC—the matrix contribution accounts for 70~80% of the elastic region.

Main conversion coefficients:

Practical reference: When UT07 requires ftu≥7.7MPa, Flexural Strength ≥20MPa, cylindrical splitting strength ≥15MPa, and small 8-shaped specimen tensile strength ≥12MPa can be required.

⚠️ Prohibition: GB/T 50081 UHPC Flexural Strength must NEVER be converted to T/CBMF37 tensile strength!

Like all cement-based materials, not resistant to acid corrosion (including sulfate bacteria attack, although much better than OPC).

Larger autogenous shrinkage: Early-age cracking sensitivity is theoretically higher (actually not necessarily), requiring control through reasonable mix proportion and curing.

Durability assessment principle: Look at whether water, water vapor, oxygen, and other harmful substances can flow freely in cracks—if yes, local deterioration cannot be ignored; if no, you can sleep soundly.

Not necessarily. Larger specimen cross-section requires larger load cell range, and measurement accuracy may not be guaranteed. Additionally, larger size may increase size effect and defect influence, and dispersion may sometimes be greater. It is more appropriate if both the typical cross-section of actual components and testing equipment accuracy can be considered.

It's hard to say. Some think force control is more suitable for the elastic region and displacement control is more suitable for post-cracking; others think either force control or displacement control can be used uniformly. If actual component working conditions can be referenced, appropriate loading methods can be specified; when it's difficult to specify, one loading method can be chosen for comparison.

Yes. UHPC's first-crack strength is usually close to the matrix tensile strength, typically slightly higher by 10~20%. First-crack strain is also close to the matrix—for example, if the matrix is at 150~160 microstrain, fiber-reinforced may be at 180~200 microstrain. Therefore, 70~80% of UHPC's elastic region contribution comes from the matrix; so, only by making good UHPC matrix material can you make good UHPC!

Post-cracking deformation performance depends on fiber performance and fiber-matrix interface bond strength. When fiber tensile strength and deformation capacity are strong, if the bond strength with the matrix is higher, apparent strain hardening is easier to achieve; otherwise, it is more difficult. Generally, higher matrix strength means higher density, and higher fiber-matrix bond strength. Higher matrix strength also contributes relatively more to UHPC post-cracking deformation performance.

For compressive or tensile first-crack strain, 200 (150~200) microstrain can usually be taken; peak compressive strain can be 2000 (1500~2000) microstrain, and ultimate compressive strain can be 4000 microstrain; for apparent strain-hardening UHPC, peak tensile strain should be no less than 1500 microstrain, and ultimate strain should be no less than 1500~2000 microstrain.

All methods have limited effects:

• Shrinkage-reducing agent: Conventional method, limited effect

• Micro-expansion agent: Conventional method, limited effect

• Micro-nano material pretreatment: Zeolite, molecular sieve pre-saturation technology, still not ideal

• Adding coarse aggregate: Provided it does not significantly reduce impermeability and mechanical properties

Recommendation: Before mature effective methods are available, minimize on-site casting and adopt more factory Steam Curing precast methods.

Supplement: UHPC has physical shrinkage, which is a unique problem of bodies with large amounts of fine particles, different from traditional OPC and HPC. Measurement results vary greatly (600~900 to several thousand microstrain), because of differences in reference length measurement start time, sealing conditions, and measurement techniques. The bellows method is recommended, and measurement start time should be controlled within 30 minutes after water addition and mixing.

In addition to scientific mix proportion, timely covering and surface spray curing are necessary measures. It is recommended to use heat curing treatment when possible instead of natural curing.

Special attention in northwest windy areas: After casting one layer, the surface skins over severely. Do not directly cast the second layer on top (this will create cold joints). Use spray curing (not water curing) and timely covering. It is recommended that relevant engineering technical personnel understand the spray curing methods for silica fume HPC in Northern Europe.

Early curing key points: Consider internal wet curing and timely surface covering and spray curing. Some suggest using surface curing agents or surface moisturizing agents.

Because UHPC has low water-to-binder ratio, sulfoaluminate cement or similar expansion agents do not easily form ettringite under normal conditions, so shrinkage reduction effect is often poor. High-alumina cement is generally not considered for shrinkage reduction under normal circumstances.

Professor Lian Huizhen's note: Aluminate cement itself will have strength regression due to crystal transformation at above room temperature; sulfoaluminate cement will also have volume regression due to ettringite dehydration under dry conditions.

Many have tried using Ca/Mg expansion agents for UHPC shrinkage reduction with some effect, but not yet ideal, requiring further research.

Shrinkage-reducing agents have some effect on UHPC, but expectations should not be too high. Professor Huang Zhengyu points out: Shrinkage-reducing agents have some effect on UHPC shrinkage reduction, but all methods are limited and need to be combined with other measures for comprehensive control.

Main purpose: Eliminate adverse effects of later-age hydration of UHPC, such as Shrinkage & Cracking, Creep, etc.; also can improve UHPC strength and density.

Sufficient Steam Curing (wet-heat curing, heat curing, heat treatment) allows cementitious materials that can hydrate at room temperature to complete hydration early, reducing later-age time-dependent changes.

Recommendation: Steam Curing should be used whenever possible. There is no need to blindly pursue non-steam-cured technology. Non-steam-cured does not always mean advanced; adapt to materials and circumstances.

Recommended NEL method (Nernst-Einstein-Lab), based on the Nernst-Einstein equation for chloride ion diffusion coefficient laboratory testing.

Reason for selection:

• Deterioration of cement-based materials all involves water. When pore structure does not allow fluid flow, diffusion becomes the main mass transfer process

• Chloride ion is chosen because of its strong migration ability, low technical threshold for quantitative measurement, and only used as a ruler without requiring actual chloride environment

⚠️ Important warning: Any method using color development depth to calculate diffusion coefficient can be completely abandoned for UHPC!

Exemption condition: Permeability inspection can be exempted when strength ≥150MPa. Never use directly cast circular specimens for measurement.

Saturation judgment standard: Volt-ampere curve is approximately a straight line through the origin (definitely not when split to see color development with chloride ions everywhere).

Recommended paper-based strain gauge—convenient and reliable, easy to obtain first-crack time and strain.

Evaluation of other methods:

• Extensometer/displacement meter: Requires high precision, high sampling rate, very expensive

• Digital micrometer: Insufficient precision, prohibited

Advice: When publishing, all data can be published simultaneously. Never select only the best curve to publish, and never brag about only the best curve, or eventually the bluff will burst or you'll fall into a pit.

If required by structural design, it definitely should be done.

Load determination methods:

• Consult structural engineer

• Reference existing domestic and international relevant standards

• If no clear value, consider loading 40% first

For UHPC, 1.5% water absorption indicator can definitely be achieved. As for testing standards and CMA stamps, check which unit has the corresponding testing items and qualifications.

Reference: The national inspection center of China Building Materials Academy currently can conduct UHPC basic performance testing according to T/CBMF 37-2018 standard with CMA stamps.

Reason: This is the typical performance that distinguishes UHPC from OPC and HPC, and is the mechanical property most interesting to structural designers. Direct tensile test is the most direct and easiest.

Note: Specimen shapes and sizes vary by country—plate specimens, dumbbell (dog-bone) specimens, prisms; with/without notch; ground/unground surface laitance; grip fixed/pinned. Different testing methods cannot be directly compared. Size effect and testing technique differences must be considered.

Can you ask the structural design unit (or through the supervisor suggest the design unit) to provide the minimum value or range of relevant Flexural Strength requirements?

Absolutely. For material suppliers and users, this can be agreed upon through contracts. If only tensile performance is specified for the material or product, and the following conditions are met, both parties can still conduct inspection and entry acceptance based on Flexural Performance:

① The supplier has sufficient self-test data proving a stable correspondence between the UHPC material or product's tensile and Flexural Performance indicators;

② A third party recognized by both supply and demand parties proves through testing that a stable correspondence exists between the supplier's UHPC material or product's tensile and Flexural Performance indicators.

Reference GB/T50081 four-point Flexural Strength test, specify specimen size as 100×100×400mm, attach paper-based strain gauge at the mid-span bottom of the specimen to detect first-crack Flexural Strength, and set displacement meter at the mid-span position of the neutral axis to test Flexural Strength; through extensive testing and data statistics, find the correspondence or conversion relationship between first-crack Flexural Strength and Flexural Strength with the elastic limit tensile strength and peak tensile strength measured by T/CBMF37, and simultaneously determine the error range.

Note: Strain changes cannot be matched!

Don't use cubic splitting test. The Brazilian splitting test can be used—adopting GB/T50081 cylindrical splitting specimen Φ100×200mm, attach paper-based strain gauge or displacement meter at the center of the bottom circle to test the specimen's first-crack strength or splitting strength, then compare with the elastic limit tensile strength or peak tensile strength measured by T/CBMF37. Through extensive data statistics, see if a corresponding or conversion relationship can be established.

Note: The first-crack strength and splitting strength of fiber-reinforced UHPC cylindrical specimens are not easy to measure accurately; a suitable conversion relationship may be found between matrix strengths without fibers.

They can certainly be used as tensile specimens, and testing first-crack strength and tensile strength is technically feasible, but the strain data is not usable because the reference tensile length interval is unclear.

Additionally, small 8-shaped specimens have small size, and fiber distribution differs significantly from actual components. Therefore, strength values measured from them will be significantly higher than actual values.

Some foreign specimen preparation has this requirement, mainly considering the influence of fresh UHPC fluidity and fiber distribution and orientation. If special emphasis is placed on casting method and fiber orientation influence, French and Swiss practices can be referenced—large-size components can be cast according to actual casting process, then specimens can be cut from different positions considering different fiber orientations for performance testing. Regardless of the method, as long as enough test specimens are tested, typical representative statistical data for the material can always be found.

Considering temperature deformation, and such testing processes as strain gauge attachment and displacement meter installation, it is best to cool to room temperature and test under unified conditions. If only lateral comparison, or wanting to quickly know if a certain characteristic value meets the specified requirement, such as first-crack strength or tensile strength, testing immediately after Steam Curing under specified same conditions is also acceptable. However, test results at different temperatures cannot be directly compared or equated.

Neither can be said to be better! If machining quality is poor, causing invisible defects in the tensile zone, negative deviation may occur; the directly cast forming surface is usually the weak zone, and direct tension mostly cracks from there. Given the influence of the casting forming surface, direct tensile test results of cast specimens are usually lower than machined specimens, more conservative and safer.

This is determined based on extensive finite element calculations and actual tests, also considering testing convenience and operability. The real knowledge lies in the design of specimen variable cross-section and direct tensile zone, focusing on eliminating local stress concentration.

It is convenient, but the influence of fiber orientation sometimes cannot be ignored!

Deterioration of cement-based materials (including carbonation) all involves water. When pore structure does not allow fluid flow, diffusion becomes the main mass transfer process.

Why choose chloride ion: Strong migration ability, low technical threshold for quantitative measurement, only used as a ruler without requiring actual chloride environment

International practices: France uses porosity, air permeability, chloride ion diffusion coefficient under electric migration; Switzerland uses surface water absorption

⚠️ Important warning: Any method using color development depth to calculate diffusion coefficient is completely unsuitable for UHPC. Even the NEL method cannot fully apply to current UHPC, but it is a relatively good and reliable testing method.

NEL = Nernst-Einstein-Lab, referring to the chloride ion diffusion coefficient laboratory testing method based on the Nernst-Einstein equation.

Corresponding is the NEF method: Nernst-Einstein-Field method (chloride ion diffusion coefficient field testing method based on the Nernst-Einstein equation), awaiting development.

It must be done when required by design.

Exemption condition: When both UHPC matrix Compressive Strength and fiber-reinforced UHPC Compressive Strength can reach 150MPa or above (fiber reinforcement may sometimes reduce strength if not done properly), permeability testing can be exempted.

Absolutely not! Because the surface skin effect cannot be ignored. Testing must be done according to specified requirements.

The volt-ampere curve obtained during testing being closer to a straight line through the origin indicates more accurate and reasonable test results; the closer parallel specimen test results are to each other, the better the testing quality.

Redo if conditions permit. If re-testing is not possible, depending on the data distribution of parallel test results, take the median or maximum value as the test value for that material. If strictly required, take the maximum value.

Simply moisten the copper electrode surface or specimen test area with saturated salt solution.

No. Because the initial conditions for the second measurement are already different from the first measurement.

If encountering random events such as power outage or equipment problems, salt-saturated specimens can be naturally soaked in salt solution for 72h or one week or more before retesting; if it's a batch of specimens, retest under the same conditions in the same batch, and use data from the same batch only. Do not mix data from different batches.

No. Unless you just want to see what changes occur.

Currently, there is no way to know in advance whether the specimen is fully salt-saturated; this can only be known during or after measurement.

What does fully salt-saturated mean? As long as the volt-ampere curve obtained during testing is approximately a straight line through the origin, it is considered fully saturated. It is not like some people imagine—splitting the specimen with a press, spraying color-developing agent, and seeing chloride ions throughout the entire surface. This needs to be explained by material defect density and continuous channel model.

First, for non-steam-cured cast-in-place UHPC: Use finishing process similar to high-strength silica fume HPC. After finishing, immediately cover the surface (such as plastic film), or cover while finishing, to prevent moisture loss; can also spray high-efficiency curing agent while finishing; even if curing agent is sprayed, timely covering is still needed.

⚠️ Special attention: Do not perform premature water spraying or spray curing on UHPC surface. Water spraying or spray curing can be done 1 day after casting.

Second, for cast-in-place UHPC intended for Steam Curing: First cure within 1 day using the above method; then Steam Curing after 1 day. Generally reference "standard Steam Curing process"—90°C, 48h. Pay attention to monitoring temperature and humidity at different positions to ensure uniformity as much as possible.

Third, for factory precast Steam Curing or autoclave UHPC components: Reference the above methods or make appropriate adjustments.

For UHPC constructed at room temperature, demolding can be done after 1 day of normal curing. For UHPC cast under other environmental conditions, adjustments can be made according to actual situations.

For structures needing demolition and reconstruction: Demolish promptly. Later demolition cost will be higher.

For structures needing repair: Promptly roughen or cut, then repair promptly; repair materials can be the same as original construction materials. When repairing with the same UHPC material, no other bonding agents are needed.

Cutting tools: When conventional cutting tools are not suitable, consider using hand-held high-pressure water jet for cutting.

If core strength is seriously insufficient, demolish, blast, or reinforce as appropriate.

If core strength is about 20% less than design strength, consider: Can the age be extended before testing? Or can heat curing be applied before testing?

⚠️ In any case, final evaluation should be done by a professional structural engineer.

For UHPC without abrasion, high-temperature action, or acid corrosion, generally no special routine maintenance is needed.

Special requirement treatments:

• High appearance cleanliness requirement → Perform surface hydrophobic or anti-fouling treatment in advance

• High color requirement → Apply UV-resistant or color-fixing agents

• Reflectivity requirement → Acidification, surface roughening, or matte treatment

• Requirement for NOx and other toxic gas decomposition → Incorporate titania powder and other degradation agents, or zeolite powder and other adsorbents in advance

• Surface contamination → High-pressure water, sandblasting, shotblasting, or grinding, then appropriate surface treatment

• Abrasion action → Conduct routine inspection, replace when critical abrasion degree is reached

• High-temperature action → Pay attention to bursting and spalling conditions, replace promptly if serious damage occurs

• Acidic gas or acid corrosion environment → Perform hydrophobic, film-covering, or corrosion-resistant surface treatment before use; conduct routine observation during use

Non-copper-plated exposed steel fiber ends undergo ordinary electrochemical corrosion; copper-plated exposed steel fiber ends undergo galvanic corrosion.

If it looks unsightly: Lightly grind the UHPC surface, then perform hydrophobic treatment, such as brushing silane or other organosilicon materials, or other organic coatings (transparent or opaque).

If it doesn't look unsightly: No treatment needed, because corrosion depth is usually very shallow, even after several years.

If such conditions are not allowed: New structures can use stainless steel fibers or other non-metallic fibers; if not replacing steel fibers, find ways to prevent exposure, or perform timely surface protection treatment after demolding.

Pores in high-quality UHPC are mainly nano (nm) level and below, with extremely poor connectivity. Water, water vapor, and oxygen are difficult to transmit over long distances. Therefore, steel fibers or rebars buried a few millimeters deep inside UHPC are difficult to corrode.

Additionally, because UHPC has very low water-to-binder ratio (usually <0.18), the matrix material has larger ohmic drop, which avoids macro-cell corrosion.

Conclusion: Even with higher salt content in UHPC, following the above principles, there is no need to worry about steel fiber or rebar corrosion. This is what distinguishes UHPC from OPC and HPC.

It can all be attributed to insufficient free water or reaction water in UHPC. This applies to frost resistance, carbonation, alkali-aggregate reaction, sulfate attack, etc.

It can also be attributed to harmful substances being difficult to transmit over long distances within it.

First, look at the degree of cracking and crack width and depth.

For cracks below 0.2mm as permitted for traditional OPC/HPC, there may be no need to worry too much in UHPC; however, for wider cracks, especially cracks penetrating to the rebar surface, if water and oxygen can transmit through them, rebar corrosion at that location cannot be ignored.

Core principle: Look at whether water, water vapor, oxygen, and other harmful substances can flow freely in cracks—if yes, local deterioration cannot be ignored; if no, there is no need to worry.

Surface abrasion resistance: Like traditional concrete, wear-resistant agents or aggregates can be added to UHPC surface. Simply increasing fiber dosage does not necessarily improve surface abrasion resistance.

Impact abrasion: May need to be treated differently—wear-resistant coarse aggregates can be considered for surface abrasion resistance; for impact abrasion, fiber type, content, or surface reinforcement may also need consideration; even surface impact buffer protection layers can be considered.

Using PP fibers, changing fiber content, adding coarse aggregates, etc. can sometimes improve UHPC's fire resistance; if conditions permit, using aluminate cement or designing according to refractory ramming materials may achieve better results.

Absolutely yes! If power facilities (such as electric poles, tower foundations, power station foundations, etc.), transportation facilities (such as roads, bridges) in these areas are replaced or surface-reinforced, many technical problems that have long troubled the power and transportation systems can be completely solved.

If existing concrete structures (such as docks, bridges, etc.) or steel structures on salt flats are all given a UHPC outer coat, many traditional deterioration problems will be solved.

Yes. If possible, consider first using the same UHPC as the base for surface overlay; of course, any other overlay material you want to use can also be used.

Of course yes. UHPC is a very good inorganic adhesive or structural reinforcement adhesive, and also an excellent anchoring or grouting material.

Typical applications:

• Offshore wind turbine tower construction—bonding different components

• Offshore drilling platform repair and reinforcement

• Bonding steel plate reinforcement for existing structures

• Grouting materials for precast building sleeve connections (recommend using fiber-free UHPC matrix material)

• Wet joints and expansion joints of precast bridges and piers

For cracks at mm level and below, mature organic crack grouting or sealing materials are recommended; for larger cracks at several cm level, UHPC is more suitable.

Low-viscosity UHPC matrix material without fibers should be applicable, though cost will be somewhat high. However, after appropriate composition adjustment, the grouting material and prestressing system can be well integrated with the concrete structure.

For large anchor boxes, UHPC with coarse aggregate can be considered for anchoring or reinforcement.

Building sector: Complex building nodes, shear/earthquake-resistant structures, precast stairs, door/window frames, integrated kitchen and bathroom, garage floors

Public facilities: Venues, commemorative buildings, ancient building restoration

Outdoor applications: LOGOs, sculptures, tables and chairs, benches, fences, flower walls, artistic parking lots, tree surrounds

Cultural and creative sector: Furniture, lamps, speakers and other decorations; molded replicas of wood carvings, brick carvings, stone carvings; replication of ancient cultural relics

Important infrastructure in the South China Sea is very suitable for UHPC, especially military facilities!

Core advantage: UHPC is an excellent impact-resistant and penetration-resistant material

Typical military facilities: Ammunition depots, aircraft hangars, ship sheds, other weapons and equipment storage

Extremely suitable for nuclear industry engineering!

• Nuclear power plant facility construction

• Storage and solidification of low and medium-level radioactive spent fuel

• Can be used for high-level radioactive waste disposal after special technical measures

Other suitable fields: Space infrastructure, toxic or harmful environmental protection engineering, harbor engineering, underground pipe galleries, PCCP, tunnels, subway/urban rail/high-speed rail, highway transportation

For large structures, this is somewhat difficult. Simply considering conventional reinforced concrete blasting probably won't work. Specialized cutting equipment or processes, such as water jet cutting, may need to be combined.

Axial compressive strength fcp / Cubic Compressive Strength fcu = 0.82~0.90, recommended 0.86

Specimen specifications: 100×100×300mm (axial compression) vs 100×100×100mm (cubic)

Four-point Flexural Strength fb / Direct tensile strength ftu = 2.2~2.8, recommended 2.5

Specimen specifications: 100×100×400mm

Practical requirement: UT07 requires ftu≥7.7MPa → recommend fb≥20MPa

Cubic splitting fct / Direct tensile ftu = 1.9~2.5, recommended 2.2 → recommend fct≥17MPa

Cylindrical splitting fct / Direct tensile ftu = 1.6~2.2, recommended 2.0 → recommend fct≥15MPa

Specimen specifications: Cubic 100×100×100mm; Cylindrical Φ100×200mm

Small 8-shaped ftu-8 / Standard specimen ftu = 1.0~1.5, recommended 1.5

Practical requirement: UT07 requires ftu≥7.7MPa → recommend ftu-8≥12MPa

Note: Small 8-shaped specimens have small size, and fiber distribution differs significantly from actual components. Strength values measured from them will be significantly higher than actual values.

✅ Recommended: Ordinary carbon steel rebar (fully meets requirements)

❌ Use with caution/not necessary:

• Negative-size high-strength rebar (brittle)

• Stainless steel rebar/clad stainless steel (rebar corrosion not a concern)

• Epoxy-coated rebar (not necessary)

• Hot-dip galvanized rebar (not necessary)

⚠️ Special note: Plain round rebar is sometimes superior to ribbed rebar—UHPC's high shrinkage and high viscosity generate large local constraint stresses on ribbed rebar, which may cause adverse effects.

Steam Cured UHPC: Prestress can be applied after Steam Curing ends (generally 4th day after casting); some apply during Steam Curing

Non-steam-cured UHPC: Apply prestress after material strength reaches specified strength (e.g., when design is 150MPa, can apply at 100MPa)

⚠️ Key note: UHPC's early shrinkage is largest within 48~72h, and elastic modulus and strength change very quickly. It is usually required to avoid the rapid change period and apply prestress 2 or 3 days after casting. It is not recommended to apply prestress at 1 day after casting.

Applied prestress / UHPC Compressive Strength at prestress application = 30~50%, recommended 30~40%

Example: UHPC Compressive Strength at prestress application is 100MPa → prestress controlled at 30~50MPa

Recommendation: Apply prestress after reaching 60~70% of design strength. In most cases, applied prestress is within the elastic deformation region of UHPC.

It's hard to say, because it is actually not easy to measure accurately. Different studies have widely varying conclusions (600~900 to several thousand microstrain).

Measurement difficulties: Reference length starting point, measuring apparatus (contact vs non-contact optical), sealing conditions

Recommended method: Bellows method, with measurement start time controlled within 30 minutes after water addition and mixing. When conditions permit, non-contact optical measurement can be used, possibly enabling measurement immediately after casting.

Test conclusions: Traditional expansion agents are difficult to achieve ideal early shrinkage reduction, with varying opinions.

Methods tried with limited effects:

• Early strength agent + expansion agent dual control (difficult to achieve)

• Adjusting fiber type and content (only achieves microscopic stress homogenization, not directly comparable)

• Nano material/polymer technology (may be promising, not yet breakthrough)

Core viewpoint: Some practices and experience from HPC can still be referenced, but newer and better approaches need to be developed

Recommended directions:

• Fine mix proportion regulation ✅

• Simply reducing cement content ❌ (under low water-to-binder ratio, large amount of cement remains unhydrated, actual effect does not meet expectation)

• Large amount of inert particles replacing ❌ (still believe shrinkage comes from hydratable cementitious materials)

• Adding large aggregate ⚠️ (has engineering value, but no advancement)

⭐ Recommended "aggregate external addition method": Mix UHPC separately, then externally add a certain volume of coarse aggregate as needed. Steps: Purchase UHPC premix with stable quality → add water and mix uniformly → externally add locally suitable coarse aggregate → fully mix and cast self-compacting or lightly vibrate.

First: Select sufficiently Steam Cured UHPC, and tension (or release, for pretensioned reinforcement) after Steam Curing

Second: For non-steam-cured UHPC, under normal construction conditions, avoid the rapid change period of 48~72h after casting, and tension on the 4th day

Third: Prestress tension level should not exceed 40% of material strength at tensioning

This requires more effort in material mix proportion design and structural local design. Construction processes, including the influence of specific construction procedures, must also be considered.

From a microscopic perspective, the constraining effect of fibers and their interface with UHPC matrix can also be fully considered to explore UHPC early tensile Creep control issues.

Deflection or stress loss of actual structures can only be understood and controlled through continuous observation (or real-time monitoring).

At the design stage, it depends on what Creep model or Creep coefficient you use for estimation. For any structure with long service life, estimation will not be accurate, but can be used for reference and scheme comparison.

Although long-term Creep of sufficiently heat-cured UHPC is not a concern; however, considering the material composition and stress non-uniformity of large-span or large-section components, Creep issues remain an important problem that cannot be ignored.

Very many!

• UHPC can incorporate more industrial solid waste (nano-level to mm-level)

• Sea sand, mountain sand, or desert sand (with low mud content) may also be widely used

• Special polymer materials integrated with UHPC will have great development prospects

• UHPC is expected to enter energy industry and waste treatment, such as battery waste solidification and treatment

• UHPC shines in solidification and isolation of toxic and harmful substances

Of course, very suitable! Using UHPC's electrical, magnetic, or mechanical properties, it can be made into intelligent components or structures for different purposes! It can even directly make sensors.

UHPC is currently used in the power industry, such as UHPC power poles, photovoltaic battery brackets, insulating foundations. In the future, it may also be used for manufacturing high-voltage insulators!

Have you heard of "cement" resistance? UHPC-like materials can be used to make different resistors, or insulating or conductive substrates. They can also be made into capacitors or other special semiconductors.

Very ideal for ecological materials. Through clever structural design, it can be used for different marine, lake, and sewage biological or microbial habitat or living environment construction, stable and durable!

Currently, it certainly cannot be directly used as active biological material, but drawing on UHPC preparation technology, different biological materials may be made in the future.

Considering high temperature, high humidity, high corrosion, and rockburst issues, UHPC is currently the most suitable material for this engineering, including bridges and tunnels!

If traditional OPC or HPC are still heavily used, it not only wastes manpower and material resources, but is also absolutely unworthy of the country and future generations!

⚠️ Note: For severe geological disasters (such as intense crustal movement), UHPC is also not effective!

Like the Sichuan-Tibet high-speed railway, UHPC may be the most suitable structural material for this water conservancy project, and can be given priority.

Uncertain. Currently, UHPC can be used in environments from -80°C to 300°C; improved UHPC can resist strong neutron or gamma radiation.

But whether it can be used in vacuum or strong particle irradiation space requires consultation with aerospace experts!

Of course yes! UHPC is resistant to seawater corrosion, durable, and high-strength, making it an ideal material for ocean ranch facilities. It can make artificial fish reefs, aquaculture cage fixing structures, docks, and trestles.