By Kayla Hanson, P.E.
An emerging classification of concrete, called ultra high performance concrete, is changing the way precasters can diversify and expand their product lines. The material allows construction of high strength, low profile and sustainable designs with limitless architectural and structural possibilities.
Kelly Henry, an architecture project manager at Lafarge North America, said the innovative material can help rethink the way concrete is used in building design. She said recently a designer inquired about using UHPC tubes as a capsulated “people mover,” similar to the system used at drive-through bank teller windows, but sized for humans.
“We are approached with many design ideas on a daily basis,” she said. “UHPC is truly a new building material that architects and engineers are using to push the limits of their design every day.”
UHPC is capable of reaching unconfined compressive strengths over 20,000 psi and flexural strengths up to 7,000 psi, with some mixes exposed to specialized curing methods capable of compressive strengths approaching 120,000 psi.1 By comparison, conventional precast concrete generally reaches compressive strengths between 4,000 and 6,000 psi at 28 days, and flexural strengths of about 10% to 20% of the compressive strength. The extreme strengths achieved by UHPC are made possible by reducing porosity and increasing homogeneity within the concrete mixture. However, to achieve this, mix designs must be meticulously controlled and placed within stringent tolerances and all constituent materials must meet strict quality and dimension standards.
Raw materials that help put the U in UHPC
UHPC mix designs differ significantly from those of conventional concrete or even high-performance concrete. The basic composition of a typical UHPC mix, by descending percentage of the mix by weight, includes fine sand, cement, pozzolanic supplementary cementitious materials, ground quartz, fibers, water, plasticizing or water reducing admixtures, and accelerator.
Among other products, the chemical reaction between cement and water produces calcium silicate hydrate and calcium hydroxide. CSH is a stable gel that serves as the main binder in concrete, while CH is an undesirable byproduct of the reaction that dissolves in water and has the potential to weaken the concrete matrix. The introduction of pozzolanic SCMs, such as silica fume, into the concrete mix allows the byproduct CH to react with the SCMs. The result is the production of more CSH and a reduction in the amount of CH. This reduces the volume of the interfacial transition zone between the matrix and aggregates, and strengthens and densifies the concrete.
Silica fume is commonly used in quantities ranging from 5% to 10% of the cementitious materials by mass, but can be used in amounts approaching 30%. Condensed silica fume has a surface area of about 97,765 ft2/lb, where, in comparison, the surface area of Type I range from about 1,466 ft2/lb to 1,955 ft2/lb and Type II from 2,444 to 2,933 ft2/lb, respectively. Due to its high surface area and the subsequent increased water demand, as the silica fume content of a mix increases, the concrete becomes increasingly sticky and less workable, and slump flow values steadily decrease.
SCMs like silica fume can have average particle diameters as small as 3.94-6 in. – about one hundredth that of cement – which allows the SCM particles to fill the interstitial voids between cement grains and ground quartz particles and ultimately improve the bond between the aggregate and hydrated cement. This improved particle packing contributes to the higher density of UHPC microstructures, and the resulting disconnection of pore spaces decreases penetrability.
Role of fibers in UHPC
The ductile behavior exhibited by UHPC members is due to the use of reinforcing fibers, typically less than 1.18e-2 in. in diameter, and elimination of passive reinforcing bars. Fibers are typically the final component added during mixing, and when used properly, are uniformly distributed throughout the batch. Structural UHPC requires steel fibers, while architectural applications require organic plastic (e.g. polyvinyl alcohol), fibers.
Fibers are capable of inhibiting the development of both plastic shrinkage cracks and service cracks. As excessive forces are applied to a member and cracks begin to develop, the even distribution of fibers throughout ensures they will be present at the site of fatigue. Once cracks form, the tensile forces applied transfer to the fibers, which can have tensile strengths in excess of 245,000 psi. The fibers bridging the cracks lend their strength to the member, allowing it to remain ductile, withstand increasing stresses, and impede crack propagation. Unlike UHPC, conventional concrete does not possess the same degree of ductile behavior before cracking, and are not as apt to support increasing loads after reaching their yield strength.
Water reducers in UHPC
UHPC mixes boast water-to-cementitious materials ratios (w/cm) ranging from 0.20 to 0.26, depending on the application, where conventional concrete is typically designed with a w/cm between 0.32 and 0.48. The extreme water restriction in UHPC allows highly flowable mixes to be placed with ease and formed into complex figures due to the use of plasticizing or water – reducing chemical admixtures. These admixtures are surfactants that, when introduced to a concrete mix, attach to the cement particles and cause them to repel one another. This results in a more homogeneous suspension and reduces the probability of change to cement particles, in turn giving the fresh concrete the characteristic flowability and workability essential to UHPC.
The use of water reducing chemical admixtures in UHPC also contributes to the densification of the mix. When used in combination, SCMs and water reducers drastically decrease the total porosity of the concrete matrix, in addition to significantly lowering the percentage of capillary pores and the average pore radius, as seen in Table 1. Depending on the mix design, slump flow values for UHPC mixes can range from 26 in. to upwards of 30 in., whereas conventional self-consolidating concrete can display spreads between 18 in. and 32 in.
Coarse aggregates are omitted from UHPC and the fine aggregates (i.e. fine sand) typically have a maximum diameter of 1.97e-2 in. UHPC often relies on portland cement with low tricalcium aluminate (C3A) contents to decrease water demand while improving the properties of the mix. Additionally, accelerators are able to shorten set time or increase the rate of early strength gain (strength accelerators) during the curing process.
Benefits of UHPC
UHPC mix designs systematically improve the hardened characteristics of conventional concrete, resulting in the development of extreme strength, improved performance and enhanced service life. In addition to its strength and ductility, UHPC is highly resistant to acid chloride and sulfate penetration, which is attributed to the increased density and decreased porosity of UHPC in comparison to conventional concrete. UHPC also demonstrates extreme resistance to freeze-thaw cycles, abrasion and other aggressive environments.
UHPC has an approximate weight of 156 lb/ft3, where conventional prestressed concrete and normal strength reinforced concrete have weights of about 150 lb/ft3.
Ease of manufacturing and placement
UHPC can be placed using traditional concrete equipment by pouring, injection or extrusion.
The technical properties and resulting mechanical behavior of UHPC enable its use in unique structural and architectural settings.
Along with the lack of passive reinforcement, UHPC’s ability to self-consolidate allows for production of complex shapes and lattices, thin cross sections, and excessive void rates. Additionally, UHPC has enabled architects to combine the numerous functions of a building envelope – external wall system, weatherproofing, insulation, solar gain moderation, support for joints, etc. – into a single element.
Henry said the most innovative UHPC architectural project to date is the Museum of Civilizations from Europe and the Mediterranean or MuCEM in Marseille, France (see photos) because of the different ways UHPC was used in the post-tensioning columns, the perforated façade and the footbridge.
She said the inherent strength and durability of UHPC also facilitates construction of infrastructural necessities including bridges, dams, utility and energy structures.
“Under the structural category, we (Lafarge) have many projects that are utilizing UHPC to create stronger, more durable bridges and the Pulaski Skyway stretching from New Jersey to New York,” she said. “Due to the size of this bridge, they are utilizing a technique to pump the UHPC into the joints of the new bridge deck, which will give a new life span from the durability afforded by UHPC.”
Also due to its strength and excellent impact and blast resistance, security and barrier systems are becoming increasingly reliant on UHPC.
“The type of applications for UHPC are growing every year,” she said. “We try to keep our minds open to all potential applications.”
UHPC is an emerging industry and continuously evolving technology. Its inherent strength and durability make it a reliable option for engineers; an economical choice for governments, owners and developers; and an opportunity for architects to add intrigue to traditional construction and infrastructure.
Kayla Hanson, P.E. is a technical services engineer with NPCA.
1 Strengths of this magnitude are the result of lab curing conditions. In this case, the specimen was heat treated at 480 degrees Fahrenheit. Employing conventional curing methods in a plant setting, can result in compressive strengths around 40,000 psi.
Design and Control of Concrete Mixtures, Steven H. Kosmatka and Michelle Wilson
Ultra High Preformance Concrete: Preceedings of the Second International Symposium on Ultra High Performance Concrete, Ekkehard Fehling, Michael Schmidt, S. Stürwald
Ultra High Precast Concrete (UHPC), NPCA’s White Paper