UHPC Recipe & Resources
What is UHPC?
Ultra-high performance concrete is an innovative concrete technology that is rapidly gaining momentum throughout the world. With a compressive strength of at least 150 MPa (21.7 ksi), the component materials self-consolidate to yield extremely high durability and mechanical properties. UHPC is reinforced by steel fibers, which enhance the material’s tensile strength and energy dissipation capacity.
Does open recipe UHPC work as well as commercial UHPC?
Our team spent countless hours researching the recipe for UHPC so that it could be distributed widely and freely. Research has shown that there are no additional benefits gained from purchasing proprietary UHPC–it can be made from simple, off-the-shelf components and doesn’t require specialized curing or placement. Open recipe UHPC performs similarly but is 75% less expensive than commercial UHPC mixtures.
Like its commercial counterpart, generic UHPC can achieve exceptional short term and long term properties.
- Compressive strength in the 170-200 MPa (25-30 ksi) range
- Direct tensile cracking strength of 7-12 MPa (1-1.7 ksi)
- Direct tensile peak strength of 8-24 MPa (1.2-3.5 ksi)
- Negligible exposed surface mass loss (g/cm²) after more than 100 freeze thaw cycles
- Negligible passed charge (coulombs) in a rapid chloride penetration test
- Extremely small autogenous shrinkage
UHPC derives its unique properties from its high packing density, which is achieved by carefully controlling the size and distribution of the constituent particles, and incorporating steel fibers. The uniformity of the matrix results in a discontinuous pore structure, which prevents water from entering the material, leading to its exceptional durability properties.
Figure above: comparison between regular concrete and UHPC. Note the uniform nature of UHPC.
Watch a video of compression test below
Nonproprietary UHPC is made of ordinary Portland Type I cement, ground granulated blast furnace slag (GGBS) cement (slag cement), silica fume, two types of silica sand and short steel fibers. To ensure workability, a superplasticizer is used. Optimum packing density of the particles is achieved as discussed in “How is High Packing Density Achieved?” The mix proportions by weight of a generic UHPC mix are shown in the table. Note that the weight of Portland Cement + Ground Granulated Blast Slag Cement = 1.0 and that all other components are scaled to this.
Table: Mixed proportions by weight of cement (OPC+GGBS = 1.0)
White Portland Type I cement was used in the early development of generic UHPC due to its low tricalcium aluminate (C3A) and the high combination of di- and tricalcium silicate (C2S+C3S) resulting in exceptional performance in the fresh and hardened states. However, white cement is expensive (at $275 per ton [2019 price]). Research has shown that ordinary Portland cement Type I, which is much cheaper (at $150 per ton [2019 price]), can be successfully used. In general, the selected cement must have a tricalcium aluminate (C3A) content lower than 8% and a relatively low Blaine fineness to reduce water demand during the hydration. Many suppliers in the US can meet this requirement.
Silica fume is a by-product of the production of silicon alloys. The superfine spherical particles and pozzolanic reactivity densify the microstructure and significantly improve the compressive strength of UHPC. The median particle size is in the range of 0.1 to 10 microns. A lower carbon content is preferred because that decreases the water demand while promoting high flowability. Eliminating the coarse aggregate promotes high compressive strength. Instead of coarse aggregate, two types of quartz silica sand are used, with grain sizes of 70–200 μm and 400–800 μm. These grain sizes are optimized to enhance packing density.
Unlike regular concrete, UHPC uses a lot of cement, which increases costs and has environmental and ecological burden. It also has a negative impact on the hydration heat, which can lead to shrinkage problems. Therefore, slag cement (GGBS) is commonly added to make the mixes more environmentally friendly since GGBS is a byproduct of the steel making industry. GGBS is a beneficial mineral mixture for concrete because of its pozzolanic property and is known to positively affect the durability of concrete materials.
A polycarboxylate-based high range water reducers (HRWR), also known as superplasticizer, is used in UHPC mixture designs. Also, steel fibers with high strength (>2000MPa [>290 ksi] yield strength) are employed. Steel fibers are generally 0.2 – 0.3 mm in diameter and 13 mm – 25 mm in length.
About High Packing Denisty
How Is High Packing Density Acheived?
Packing theory is the basic method used for developing dense concrete using different sized particles.
Proper application of packing theory can control the fresh and hardened properties of concrete because the improved particulate packing leads to more usable water as a lubricant. The Andreasen and
Andersen (A&A) model is commonly used to design UHPC with various solid constituents and high
fluidity. According to A&A theory, optimal packaging can be achieved when the cumulative particle size
distribution (PSD) obeys the following equation:
where, P(D) is the fraction that can pass a sieve with opening D; Dmax) is the maximum particle size of
the mix. The distribution modulus q has a value between 0 and 1. The Andreasen and Andersen model
doesn’t contain the minimum particle size. To account for that, a modified version of A&A model
suggested by Funk and Dinger is commonly used
where, Dmin accounts for the minimum particle size in the mix. Andreasen and Andersen found that
optimum packing is obtained when q = 0.37. However, for mixtures with a high amount of powders
(<250 μm), a smaller q value is recommended in the range of 0.22∼0.25.
Figure above: Particle gradation in UHPC matches the ‘ideal’ one
What is Strain Hardening?
UHPC has a higher tensile strength than conventional concrete, it also exhibits strain hardening response after initial cracking when properly reinforced with steel fibers. The typical stress strain
curve for UHPC is shown in Figure. In the initial stage, the material’s tensile behavior is elastic, which continues up until the first cracking strength (σcc). Following this, the material then exhibits
strain hardening up until its peak point (σpc). The strain hardening behavior of segment II is typically characterized by multiple cracks development in the gauge length of the specimen.
Figure above: Typical tensile strain response in UHPC
Applications of UPHC
UHPC is used anywhere regular concrete is used. Due to its extreme durability (an order of magnitude greater of that of regular concrete), it is especially effective in harsh environments. The very high strength of UHPC allows regular concrete components to be redesigned, resulting in large weight savings (up to two thirds [67%] reduction in weight). Redesigning structures to take advantage of the unique properties of UHPC usually allows the end structure to be significantly more economical over its lifespan than if it were built with regular concrete.
UHPC can be used in a broad range of products. It is particularly well suited for precast components and systems, such as Precast Bridge Elements and Systems (PBES) and Accelerated Bridge Construction (ABC).
- Bridges: Bridge decks, joints between bridge decks, bridge girders, foundation piles
- Buildings and Garages: Lightweight girders (prestressed and precast), lightweight slabs
(prestressed and precast)
- Lightweight, thin-walled concrete pipes
- Architectural facades
- Outdoor furniture
UHPC used for the closure joints between precast bridge decks
Bond Behavior of Steel Rebars
UHPC has exceptional bond with steel rebars, which allows for an extremely small embedment length.
To get a sense for this excellent performance, bridge closure pours can be reduced in size from 500 mm (20 in) to just 100 mm (4 in). Based on research in, Alkaysi and El-Tawil, an assumed bond stress, τbond, equal to 1.1√f’c (MPa), can be used for estimating the required embedment length in UHPC.
Failure modes observed during pullout of steel rebars embedded in UHPC
Fiber Pullout: An Indicator of Fiber Effectiveness
The bond-slip relationship between fibers and the surrounding matrix directly influences the mechanical properties of the UHPC composite. Bond-slip response is activated when fibers bridge cracks that are trying to open further. The resistance they offer against cracking opening promotes beneficial multiple cracking and strain hardening tensile behavior. However, the fiber-matrix bond characteristics are carefully tailored to the strength of the matrix and the characteristics of the fiber to achieve an optimal UHPC response.
Figure above: (a) Pullout test setup; (b) Pullout performance of Hiper fiber and smooth fiber
How to make UHPC
Generic UHPC is made of the following ingredients:
- Ordinary Portland Type I cement
- Ground Granulated Blast Furnace Slag (GGBS) cement (slag cement)
- Silica fume
- Graded silica sand
- Steel fibers
- High Range Water Reducer (HRWR)
The mix proportions of a generic UHPC mix per cubic yard are shown in the table. Four mixes are listed with different amounts of High Range Water Reducers (HRWR).
Table: Mix proportions by weight of cement (OPC+GGBS = 1.0). Weights listed in pounds.
1Mixes A, B, C and D have HRWR dosages of 1.5%, 2%, 2.5% and 3%, respectively
2Grain sizes: 80-200 microns
3Grain sizes 400-800 microns
4Polycarboxylate ether-based high range water reducer
5High range water reducer dosage rates can be adjusted to meet the paste flowability requirements, Dosages range vary with the type of silica fume and range from 1.5% to 3.0% by weight of the cement.
6The steel fibers are 2% by volume.
Figure above: Color of silica fume with different carbon content.
Figure above: Silica sands used for the preparation of UHPC
Conventional concrete is generally easy to mix using commonly available mixers and can be conveniently adapted to most construction conditions. However, mixing UHPC requires equipment that provides more energy and shear than regular concrete due to the low water content and high powder content. In general, the expected performance (including fresh and hard-solid properties) of the selected mixture cannot be achieved when low-mix energy mixers are used to mix UHPC. Moreover, use of a low-energy mixer will also increase the turnover time of the mixture, causing the temperature of the mix to rise, which is detrimental to the UHPC mixing process (high temperature delays mix turnover).
UHPC can be mixed in plants that have large capacity pan mixers with multiple mixing paddles. The paddles should have bottom and side scrapers to ensure a good mix result. The mixing process is as follows:
- The silica sand and silica fume are first dry-mixed for about five minutes. Cement and GGBS are added to the mixture and dry-mixed for another five minutes.
- Water and HRWR are first mixed together and then added gradually to the dry material. Premixing the HRWR and water aids in more uniformly distributing the HRWR during mixing. The UHPC mixture shows appropriate workability (turn over) approximately five to seven minutes after the addition of water and HRWR.
- Once an adequate mixture consistency is achieved, the steel fibers are added into the mixer and allowed to mix until the fibers are dispersed
WARNING: Dry mixing silica fume, fine sand and cement can cause dust which when inhaled can lead to silicosis. Appropriate protection equipment must be worn at all times during mixing. A N95 mask and full Tyvek suit are recommended.
Once mixing has concluded, a spread test should be carried out to ensure that the material has mixed appropriately. The spread should be between 175 mm (7 in) and 300 mm (12 in).
Figure above: Precasting plant mixer with multiple paddles. Note the bottom scrapers. The mixer also has side scrapers (not in view)
See video below showing how to mix generic UHPC
UHPC can be successfully mixed in the field using commonly available mortar mixers. One mixer that has worked well is the Mortarman 750 model by Imer. Onsite mixing has special challenges that go beyond plant mixing. We have devised various means to alleviate those challenges.
Field mixers have limited power. UHPC’s viscosity increases dramatically at turnover and the mixer’s engine can labor noticeably at higher mix loads and even stall. To address this limitation, a revised mix procedure can be used that has been shown through research to yield similar fresh and hardened properties as obtained from the original mix protocol. The new mixing process is shown schematically below and is as follows:
- Dry mix cement, GGBS, silica fume, and a portion of the silica sands for 5 minutes.
- Add water and superplasticizer till turnover and formation of thick slurry.
- Incorporate remaining silica sands gradually and mix another 5 minutes.
- Add fibers and continue to mix until fluidity is optimized (between 5 and 8 minutes).
Hot Weather Mixing: Mixing and casting UHPC on a warm day leads to two complications:
- A reduction in the spread (flowability) because the excessive temperature compromises the
effectiveness of the HRWR, and
- The potential for evaporation of water during mixing and placement.
To address the former, it is recommended that about 40% of the mix water should be replaced with ice. Substantially hotter days will require greater ice quantities, which can be ascertained by trial and error. The objective is to cool the mix to less than 85 °F to ensure effectiveness of the superplasticizer. The latter issue can only be resolved by speeding up the mixing and placing processes.
WARNING: Dry mixing silica fume, fine sand and cement can cause dust which when inhaled can lead to silicosis. Appropriate protection equipment must be worn at all times during mixing. A N95 mask and full Tyvek suit are recommended
Figure above: Mortarman 750 mixer by Imer suitable for mixing UHPC onsite
Figure above: Field mixing process
Figure above: Revised mixing process to reduce burden on field mixer.
The spread test for freshly mixed UHPC is determined by testing the spread value in accordance with ASTM C1437. After mixing the paste, the fresh mix is placed into a spread cone (see figure). Special care should be taken to keep the spread cone and the base plate at the same humidity level prior to testing. Due to the inherent high flowability of the paste, there is no need to compact the UHPC in the mold and no vibration is required. The spread cone is filled up to the rim and then lifted at a fixed speed. The leftover material sticking to the wall of the cone is scraped off and the material on the base plate is left to spread. After 2 min ± 5 sec has elapsed, the diameter of the spread UHPC paste is measured along two perpendicular directions and the average diameter is calculated and recorded as the spread value.
The spread should be between 175 mm (7 in) and 300 mm (12 in). Spread values outside this range indicate that the mix should be rejected
Figure above: (a) mold used for spread test; (b) spread test results
See below for a video showing how to conduct the spread test
Since UHPC contains a small amount of mixed water, surface drying may affect its hardening properties, so it is important to cover the surface of the specimen with plastic sheets as quickly as possible after pouring to prevent moisture loss. Onsite or laboratory manufactured specimen should be cured according to the ASTM C31/C31M and ASTM C192/C192M, respectively.
If needed, the specimen can be heat treatment for 48 hours after demolding to achieve the desired strength at an early stage. The curing conditions are a temperature of 90 °C (195 °F), and relative humidity of 95%
Approximate market prices in 2019 for the components of generic UHPC are shown in the Table below. The total cost per cubic yard is about 70% less than the cost of commercial products, which is in excess of $2,500 per cubic yard (see El-Tawil et al. 2018). By using river sand, the cost can be further reduced to about one fifth (~20%) of commercial products. The cost of UPHC is driven by the price of steel fibers.
Note: Shown steel fiber cost is for imported fibers. Fibers that comply with the Buy America Act are more expensive.
Reducing Cost By Using River Sand
It is entirely feasible to replace silica sand, which is expensive, with natural (river) sand. River sand costs about 1/15 of the cost of silica sand, reducing the cost of the UHPC matrix significantly. Our research has shown that completely replacing silica sand with river sand can result in a viable UHPC. As shown in the figures below, the reason is that the matrix still achieves good packing density (the gradation matches the ideal packing curve). After 48 hours of heat treatment after demolding, the 7-day compressive strength of our specimens was 158.6 MPa (23.0 ksi).
Figure above: Gradations of UHPC mix with natural sand instead of silica sand.
Figure above: Spread from mix with natural sand meets specifications
Higher packing density, and therefore superior properties, can be achieved by replacing a portion of the silica sand with river sand. The results of our experiments show that superior performance can be achieved if just 10% of the total sand content (the rest being natural sand) is fine silica sand where the compressive strength and tensile strength of UHPC are 169.7 MPa (24.6 ksi) and 11.3 MPa (1.6 ksi), respectively
HiPer Fiber, LLC was founded in March 2019. Its founding team includes structural, materials and mechanical engineers with more than three decades of combined experience in mixing, casting, and testing open recipe Ultra High Performance Concrete (UHPC) for structural and non-structural applications.