In concrete technology, there is a highly remarkable type of concrete named Ultra-High-Performance Concrete (UHPC). Just like Normal Strength Concrete (NSC), UHPC is composed of the same standard materials like cement, aggregate, mineral admixture, chemical admixture, and water. The UHPC or Reactive Powder Concrete (RPC) [1], is an elastic material reaching extraordinary strength. The material can reach compressive strengths not less than 150 MPa and tensile strengths of not less than 7 MPa [2-4].

In concrete technology, the development of various types of concrete is considered to be the concrete evolution. Since the early 100, the construction material Normal Strength Concrete (NSC) has been around and responsible for countless civil engineering structures like The Pantheon (Italy, 125), Pont Camille de Hogues (France, 1900), and Hoover Dam (USA, 1935). Around 1970, NSC developed to High Strength Concrete (HSC) a type of concrete which exhibits high compressive strengths (55 MPa or more) [5]. The technological revolution of concrete can be placed around 1980, where HSC has contributed to engineering structures like Water Tower Place (USA, 1970), Two Union Square (USA, 1988), and Petronas Twin Tower (Malaysia, 1997). Around the year 1997, the introduction of Ultra-Hugh-Performance Concrete (UHPC) introduced strengths (compressive) well over 200 MPa along with multiple performance capabilities for concrete. UHPC has been used for Sherbrooke Bridge (Canada, 1997), Shepherds Creek Bridge (Australia, 2005), UHPC girder bridge (USA, 2008), Haneda Airport slabs (Japan, 2010), and Spun concrete columns (Germany, 2012) [6].

UHPC is divided into four essential components; (1) powders and fine materials, which can be Ordinary Portland Cement (OPC), quartz sand, or fine silica sand, (2) superplasticizers (also known as High-Range Water Reducers, HRWR), mostly polycarboxylate ether-based, (3) fibers, usually steel fibers, and (4) of course water [7]. This unique composite material is a combination of superior properties and designs flexibility offering answers with benefits such as reduced construction time, improved aesthetics, excellent durability, and corrosion, abrasion, and impact protection [8-10]. All these lead to reduced maintenance and a longer lifespan for the civil engineering structure. Working with UHPC leads to a reduction in materials (e.g., cross-section, material thickness) and very elastic behavior due to the addition of fibers. In short, some characteristics of UHPC are a low water-to-cementitious ratio (w/c), a high binder content (w/b), and an optimum packing density [2, 11].

For UHPC to be more durable and sustainable, the cement is often wholly or partially replaced by a large quantity of supplementary cementitious materials (SCMs). In order to speed up the chemical reaction with the cement high-temperature curing or pressure curing (autoclave curing) is applied, which considerably results in improved properties of UHPC concrete. Properties include reduced porosity, enhanced strength, improved matrix and aggregate/ fiber bond [12, 13]. Compressive strength of 200 MPa can be reached with eight hours of pressure curing for 3 or 4 percent fiber reinforcement UHPC [13].

There is a distinguished relationship between the micro-scale properties of UHPC and the engineering characteristics of UHPC. The key factor in producing UHPC is the improvement of the micro and macro properties of the material mixture. This enables for a denser packing of the particles and mechanical homogeneity leading to a much denser type of concrete. This ultimately results in a type of concrete with enhanced and improved mechanical and durable properties like high compressive strengths (>150 MPa) and abrasion-resistant, corrosion-resistant, etc. With its improved micro-scale properties, the silica fume (SF) pozzolanic reaction tends to speed up.

Another key point is the investigation of the microstructure and the mechanical properties of UHPC. A method used is by exploring the isothermal mechanism and thermal gravimetric analysis [14]. The investigated activation methods, namely chemical and thermal results in improved microstructural properties. By an accelerated hydration process and increased C-S-H amount formation a porosity decrease is noticeable [15].

The engineering characteristics of UHPC are characterized as having exceptional and enhanced performance capabilities for concrete. The addition of fiber as additional reinforcement in the concrete mixture results in improved mechanical features as strength (compressive, tensile, and flexural), durability features, ductility, and resistance [16].

High-Performance Concrete (HPC) or High Strength Concrete (HSC) is a concrete mixture having more strength and durability compared to Normal Strength Concrete (NSC). The material reaches compressive strengths 55 MPa and more after 28 days of curing and classes as a material requiring special design requirements for the production and testing [17]. In civil engineering, the application of HPC reaches various areas which are not limited to bridges (e.g. long-span bridges), high-rise buildings, and highway pavements.

Structural design guidelines and standards have been written in countries of almost every continent, ranging from Europa (Germany [18]) to Asia (China [3]). The article presents a review of Ultra-High-Performance Concrete (UHPC) and is focused on (1) the principles, (2) supplementary cementitious materials including fibers, and (3) the mechanical and durable properties of UHPC.

There are some basic principles for UHPC as previously identified by several researchers [9, 19-21]. This composite material is based on the following characteristics: (1) a high binder content, reduction of (w/b), (2) reduced porosity, (3) optimized packing density (a close packing of the raw materials), (4) reduced water, < 0.25 (w/c), (5) accelerates early and drying shrinkage due to (temperature, steam or pressure) curing thus improving material properties, and (6) improved ductility due to fibers [2, 4, 12, 13, 18, 22, 23].

UHPC uses a high binder content compared to NSC giving it a reduced (w/b) around 900-1100 kgm^-3. By adding Silica Fume (SF) to the binder mix helps fill the voids between the various particle sizes (sand, aggregates) while also improving the workability.

In examining the porosity reduction, the compressive strength (f_c) and the porosity have a certain correlation [24], [25] as shown in Eq (1-4). The four models help in forecasting the materials effected through the porosity. Reducing the pores leads to increased compression strength and improved toughness of cement-based materials [26].

Equation of Balshin

(1)

Equation of Hasselmann

(2)

According to the research group, Chen et al. [27]. the predictions with the exponential equation (Ryshkevitch) and the logarithmic equation (Schiller) are almost identical except at 0 percent and 100 percent porosity.

Equation of Ryshkevitch

(3)

Equation of Shiller

(4)

In the equations above f_c,0, f_c, P₀, P, A, B, and D are defined as [4]:

-f_{c,0 is} the strength when the porosity is nil,

-f_{c is} the strength when the porosity is P,

-P₀ is the porosity when the strength is nil,

-P is the porosity,

-n is a to be determined coefficient,

-A, B, and D are the empirical constants [4].

By effectively using the admixture ingredients results in a broader concrete technology process. Normally the concrete strength is controlled by the water/binder ratio (w/b) [18], which is based on the mass. With the introduction of the water/ultrafine particles ratio (w/F_v) (Eq. 5), (Fehling et al., 2014), the combinations of ultrafine particles were effectively used for controlling the strength. This process effectively increases the strength and is based on the volume. The approach also establishes improved UHPC mix designs (extended k+ value, optimized Walz curve) [18].

(5)

where w is the volume-based water, F_v is the ultrafine particle, and Σ is the sum

The elimination of coarse aggregate plays a vital role in the composition of UHPC. According to Ma et al. [28], there is a clear difference in the portion of concrete, the time for mixing, and in the autogenous shrinkage. Comparing two UHPC types consisting of the powders Ordinary Portland Cement (OPC), white Silica Fume (SF), and Quartz Powder (QP). To improve the packing density of the powder mixture Quartz Powder (QP) was used as a micro filler. The difference in coarse aggregate size was around 625-670%. Both UHPC compositions had the same sand and superplasticizer (SP), which was polycarboxylate ether (PCE). The PCE ensures a high fluid ability as self-compacting concrete for both UHPC compositions giving it better flowability. Due to the PCE’s chemical structure, it allows water to be reduced up to 40% which enables good particle dispersion with a reasonably small amount (0.15–0.3% by cement weight) [29]. In Fig. (1) the aggregate volume fractions (sand and coarse) for NSC and UHPC is shown.

The type of curing regime plays a vital role in increasing the strength of UHPC (Table 1) as presented by previous researchers [2, 12, 13, 30-33]. As studied by Alsalman et al. [2] the compressive strength showed the highest with curing at 60 °C (two days) followed by 90 °C (three days). The effect of pressure curing (autoclave curing) [30] (0.5 MPa, 1.0 MPa, and 1.5 MPa) and the period (6 h, 8 h, 10 h, and 12 h) showed a maximum increase of 37.5% and 30.3% of the strength. Both the curing regime and the use of mineral admixtures increase the mechanical and environmental performance of UHPC [34].

Fig. (1). Aggregate volume fractions (sand and coarse) for NSC and UHPC.

The use of fibers has a positive effect on UHPC, mainly on the ductility and the impact resistance. Several factors affect the mechanical performance of UHPC by adding fibers. Table 5 presents the influence of fibers on the strength performance of UHPC. The most common type of fibers used is steel fibers having a tensile strength of 2500 MPa or beyond [64]. Adding a volume fiber ratio between 0-5 percent has the most positive effect for increased strength. The testing results of cubical and cylindrical specimens increased by 4% and 8% [34] with an addition of 3 percent fibers as presented by the research group Alsalman et al. [2]. The research group Pyo et al. [83] achieved compressive strengths of 150 MPa by adding between 0.5-2 percent amounts of steel fiber volume.

Another factor is the addition of two or three different types of fibers taking into account the fiber length [4]. The initial placement of the fibers parallel to the longitudinal direction instead of transversely gave a 61% larger flexural strengths of UHPC and a 5.5% larger initial cracking of UHPC. The use of steel fiber also increases the compressive and splitting tensile strengths, along with enhancing the compression tension ratio [84].

By adding steel fibers, the compressive strength improves working as an enhancing material to UHPC [45] (28 days curing). The fibers (Figs. 2 and 3). can be divided into (1) deformed fibers, mostly hooked or twisted, (2) straight or smooth fibers, and (3) the length of the fibers short or long. It is worth mentioning that steel fibers have a high resistance in an alkaline environment, shows high strength, and a high elastic modulus [85].

In recent years, there have been three generations of mix design methods developed for UHPC, namely (1) the Linear Packing Density Model (LPDM), (2) the Solid Suspension Model (SSM), and (3) the Compressible Packing Model (CPM) [86, 87]. Most researchers have developed the UHPC mix designs based on the CPM concept (empirical concept) [20, 88-90]. However, UHPC also contains small particles (on a microscopic level) in order to improve the packing density. Research has discovered that there are surface forces (electrical charges, steric- and van der Waals forces), which influence the interaction of these small particles [4, 91]. Therefore, for microscopic particles, using the CPM method to predict the particle packing can considerably result in differences compared to the experimental results. The mixtures containing fine particles cannot be used to predict the behavior by CPM [92] accurately. Since the UHPC mixture is made of various components of non-identical classes, the CPM mix design method only contemplates packing the packing of monosized classes [91].

A typical UHPC mixture proportion is given in Table 6 where the cement is taking around 28.5% only of the mixture.

With the particle packing model of Andreasen and Andersen [93], the allocation of the UHPC grain size can be established. The particles allocation is calculated according to Eq (6), where coarse mixtures are realized when 0.5>q and small particles are realized when q <0.25 [85, 93].

(6)

In Eq (6) P(D_i) is the fragment of the concrete parts less than the specified diameter D_i; D_max is the maximum particle diameter; q is the distribution modulus, which relies on the kind of concrete.

Fig. (2). Different types of steel fibers.

A revised form of the Andreasen and Andersen model (Eq. 6) is shown in Eq. (7), which introduces a minimum particle size (D_min). Enhancing concrete mix designs for NSC and lightweight concrete has been well accepted by this equation as well as UHPC [85, 90].

(7)

As studied by previous researchers a wide range of materials is compared. The raw materials for the UHPCs consisted of the following binder powder composition: Ordinary Portland Cement, the SCMs (Metakaolin (MK), Silica Fume (SF), Fly Ash (FA), Slag (Ground Granulated Blast furnace Slag (GGBS) and Copper Slag (CS)); the filler compositions variables were: quartz and sand; the superplasticizer, which was Polycarboxylate Ether (PCE) based enables to reduce the water content of the mixture and steel fibers.

The research of Vande Voort [94] highlighted the durability properties of UHPC (including fibers), HSC, and NSC. The results showed a significant improvement in the durability of UHPC compared to HSC and NSC. The indicators in Fig. (3) are the salt scaling mass lost, chloride ion diffusion coefficient, water absorption, carbonation depth, reinforcement corrosion ratio, and abrasion relative volume loss and are relative to NSC.

The information in Table 7 outlines the chemical properties of some Supplementary Cementitious Materials (SCMs) as researched by Wu et al. [13, 51, 95].

For the production of UHPC, it is important that the composition improves the material properties on a micro and macro scale. The mixture will guarantee mechanical consistency, maximum particle packing density and minimum size of flaws [19, 96, 23]. For that reason, the composition of UHPC should both emphasize on selecting suitable materials with physical and chemical characteristics as well as the different grain sizes. In Table 8 there are some commercially available UHPC mixtures presented [64, 97]. These cement-based composites are known as BSI, Ductal, and Dura and are further elaborated. As the table shows the composition of UHPC is based on powder cement, SF, and sand. As the cost for the production of UHPC still surpasses that of Normal Strength Concrete (NSC) it is impeccable to reduce the material cost of UHPC and still produce a high-quality product.

Because UHPC has a high autogenous shrinkage, the addition of SCMs in UHPC mixture has a positive effect on it. However, studies have shown that the proportions of the UHPC mixture were mostly assumed instead of designed, and the replacing PC and SF by SCMs were tested [91]. Moreover, it is noticeable that Fly Ash or Slag can replace Silica Fume, temperature curing accelerates the hydration process, and the mechanical properties can be enhanced by using straight long or deformed (hooked and twisted) long steel fibers [98].

Fig. (3). Length of steel fibers.

As UHPC is a relatively new construction material, there have not been available standards and recommendations for long. In 2014 Germany, Fehling et al. [18] published the English version of Beton-Kalender regarding UHPC guidelines standards, which highlighted the fundamentals, design, and some UHPC examples. With the German Committee for Structural Concrete (DAfStb – Deutscher Anschluss für Stahlbeton) working on UHPC guidelines focusing on the design, technology, and quality [104]. In France (2016) the technical guidelines and recommendations for UHPC were replaced for the French national standards for UHPC [105]. In previous years, the design and production of UHPC, as well as researchers, referred to the French Interim Recommendations [106] and the Japanese Recommendation [107] for guidance.

With a rise in popularity of UHPC several countries have started to work on their standards and guidelines like the Chinese standards [3]. On the American continent (Canada and the USA) an Emerging Technology Report (ETR) gives an overview of the Structural Design of Ultra-High Performance Concrete by the subcommittee ACI 239C [108] (in 2013). The report marks as a beginning for the understanding of design methodologies for UHPC and briefly introduces these concretes, their properties and design principles for their use. For several years the FHWA - Federal Highway Administration (USA) has published reports [109] regarding UHPC, which provides a framework for understanding, developing and using UHPC. In Fig. (4), a comparison of the first usage of UHPC divided by continents is presented in Fig. (5).

As most of the available literature review is well known among researches, there are still some major essentials in order to fully commercialize Ultra-High Performance Concrete (UHPC). Along with the materials unique and desirable characteristics suitable in a variety of constructions [110-112], challenges like (1) lack of sufficient design codes, guidelines and standards, (2) producing sustainable UHPC (less CO₂ emission), (3) requirement of high cement content.

(1) The concrete industry has always provided sufficient and adequate design codes, guidelines and standards for concrete, which are regularly updated when needed. These official documents are on a continental level as well as a country level (e.g. China, Germany, etc.). However, for UHPC proper documentation regarding the operations, maintenance, and inspection still are needed.

(2) The need to produce more sustainable UHPC is gaining more attention among researchers and engineers alike. As many countries realize the negative impact that the concrete industry has in terms of energy and resource consumption, carbon consumption, and Greenhouse Gas (GHG) emissions. As known by researchers worldwide GHG emissions are linked with global climate change giving the latter much more needed attention.

(3) There is more research needed for UHPC in the concrete industry in terms of developing and producing more sustainable as well as economical UHPC. A proper balance between the benefits and the financial costs of UHPC should be accurately researched. This will enable to commercialize this high-performance type of concrete more, making it more attainable and favorable for future engineering structures.

Fig. (4). Comparison of the durability performance of UHPC, HSC, and NS (results provided by Vande Voort [90]).

Fig. (5). Comparison of the first usage of UHPC divided by continents.

This article reviews the application of various Supplementary Cementitious Materials (SCMs) and fibers in order to obtain the desired UHPC. The substitution of the SCMs showed enhanced mechanical and durable properties and reviewed several segments of UHPC, like (1) the (compressive, tensile, flexural) strength, (2) the flowability, and (3) the toughness. From the conducted analysis and comparison, the following conclusions are summarized:

1. The curing applied (high temperature and pressure) on UHPC resulted in improved durability properties. The use of high-temperature curing can decrease the drying shrinkage, while the use of pressure curing results in 30-38% growth of the compressive strength. Because of the porosity reduction, UHPC shows better water absorption. The result in Fig. (3) shows a better performance in the durability of UHPC compared to HSC and NSC.
2. Both the curing regime and the use of mineral admixtures in the form of SCMs increases the concretes mechanical and environmental achievement (200 MPa strength). In its solid phase, the effects of the type of curing environment, aggregate, mineral admixtures, the properties of the fiber, the size of the specimen, and strain-rate all influence the mechanical performance of UHPC. The use of chemical admixture, PCE, allows a water reduction of 40% resulting in a better flowability of UHPC.
3. The steel fiber is used as an improving material to UHPC and by adding steel fiber to the mixture the compressive strength (at 28 days) becomes larger. Adding 1% of fiber volume reaches compressive strengths well over 150 MPa. Fiber volume of 2% also allows a reduction of the autogenous shrinkage by 14-15%. Comparing UHPC to normal strength concrete (NSC) the compressive strength is higher by adding 4-8% of steel fibers, which acts as an enhancement material. The placement of the fibers also affects the performance of UHPC. Placing the fibers parallel to the longitudinal direction results in 61% larger flexural strengths.
4. Moreover, it is noticeable that (1) the SCM Fly Ash or Slag can replace Silica Fume; (2) curing with high temperatures speeds up the hydration process, leading to higher strengths by Fly Ash, Slag. Also (3) the mechanical properties of UHPC are enhanced using steel fibers, which are long deformed (hooked and twisted) or straight.
5. By replacing the Portland cement and the Silica Fume with the SCMs Metakaolin (flexural strength +2.6%), Fly Ash (200-250 MPa strength), and Ground Granulated Blast-Furnace Slag (250 MPa) result in the production of UHPC with similar or better mechanical and durability properties. Another positive effect is reducing UHPC’s autogenous shrinkage (40-60%). Using the SCM Metakaolin in the production of UHPC also gives a more aesthetically pleasing result with its white color.
6. Understanding the UHPC principles is one step closer in developing and producing the desired UHPC. As guidance, most studies have used the compressible packing model (CPM) to produce UHPC. This mix design method is not the most reliable, because the CPM method only considers monosized classes packing as proven UHPC mixtures consist of different size classes, which makes this method a debatable precision regarding the behavior of UHPC. Another point is the fact that UHPC contains fine particles, and the CPM concept does not take specific forces into account (electrical charges, steric- and van der Waals forces).
7. Because of the limited design codes available (only several countries have their standard and recommendation), the application of UHPC is not as fast as NSC. However, with a rise in popularity of UHPC several countries have started to work on their standards and guidelines.