Ultra-high Performance Concrete Mixtures Research Articles

The Influence of Materials on the Mechanical Properties of Ultra-High-Performance Concrete (UHPC): A Literature Review.

Ultra-high-performance concrete (UHPC) is a cementitious composite combining high-strength concrete matrix and fiber reinforcement. Standing out for its excellent mechanical properties and durability, this material has been widely recognized as a viable choice for highly complex engineering projects. This paper proposes (i) the review of the influence exerted by the constituent materials on the mechanical properties of compressive strength, flexural tensile strength, and elastic modulus of UHPC and (ii) the determination of optimal quantities of the constituent materials based on simplified statistical analyses of the developed database. The data search was restricted to papers that produced UHPC with straight steel fibers at a content of 2% by volume. UHPC mixture models were proposed based on graphical analyses of the relationship of constituent materials versus mechanical properties, aiming to optimize the material's performance for each mechanical property. The results proved to be in accordance with the specifications present in the literature, characterized by high cement consumption, significant presence of fine materials, and low water-to-binder ratio. The divergences identified between the mixtures reflect how the constituent materials uniquely impact each mechanical property of the concrete. In general, fine materials were shown to play a significant role in increasing the compressive strength and flexural tensile strength of UHPC, while water and superplasticizers stood out for their influence on the material's workability.

Effect of nano SiO2 on rheology, nucleation seeding, hydration mechanism, mechanical properties and microstructure amelioration of ultra-high-performance concrete

The use of Nano SiO2 products will likely change cement, concrete construction sector. Their stability, low toxicity, and ability to functionalise with chemicals and polymers have drawn interest. When mixed with hybrid nanomaterials, nanostructured silica with unique geometric properties can improve biomaterial conjugates. Few scientists have studied the effects of Nano SiO2 on cement-based composites microstructural characterisation and mechanical behaviour. In this study four combinations were made by adding 0%, 1%, 2%, and 3% Nano SiO2 by cement weight. The tested Ultra-High-Performance Concrete (UHPC) mixes with various mechanical and microstructural characterisation techniques utilised. The finding suggests that the UHPC mixture with 2% Nano SiO2 has better mechanical and microstructural capabilities than the others. The study found that UHPC samples with more Nano SiO2 had higher mechanical strength than the reference sample. The study found that adding UHPC 2% Nano SiO2 to the mix increased compressive, split tensile, and flexural strength by 56%, 37%, and 28%, respectively. In UPV testing, this UHPC NS-2% mix had the greatest average velocity 4883 m/s and 4951 m/s for 28 and 90 days, suggesting a stronger concrete matrix. In optimum UHPC mix due to nucleation effect of Nano SiO2 and hydration of cement generates to more Calcium Silicate hydrate gel formation and less ca/si ratio is 0.54. It will lead to produce the dense UHPC with lowest porosity. The environmental and economic benefits of UHPC with Nano SiO2 make it a suitable building material for various civil engineering applications.

Properties of Ultra-High Performance Concrete incorporating iron tailings powder and iron tailings sand

Iron tailings (IT) is a kind of solid waste, which is usually divided into iron tailings powder (ITP) and iron tailings sand (ITS). It has been reported that IT has potentials to be used in ultra high performance concrete (UHPC). However, the recovery rate of IT is relatively low, and rare research mentioned the co-utilization of ITP and ITS to get full-scale use of IT. Therefore, in this study, ITP and ITS with different replacement ratio were used to replace cement and quartz sand, respectively, in UHPC. The effects of ITP and ITS substitution on the mechanical properties, microstructure, hydration products, durability and environmental evaluation of UHPC mixtures were studied. The optimal ITP replacement ratio is 20% that 20% ITP-contained UHPC mixtures had better mechanical properties. Further increasing the ITS replacement ratio, the mechanical properties initially increased and then suffered a reduction. The UHPC with 20% ITP and 50% ITS had the maximum compressive strength (145.6 MPa) and flexural toughness (110.99 N·m). Appropriate addition of IT could improve the microstructures of UHPC mixtures by its filler effect and pozzolanic potential. The average pore diameter was reduced from 97.00 nm to 27.04 nm, and the ratio of harmful pores (＞ 100 nm) was reduced from 85.7% to 60.9% when ITP and ITS were added. In the durability tests, the addition of IT has less effect on the electric flux passed. Owing to the pores refining effect of IT, the drying shrinkage of UHPC mixtures was reduced. According to the environmental evaluation, the amount of leached heavy metal ions can be neglected. The IT contained UHPC had relatively low green degree and ITS/ITP surplus in 1 m3 UHPC production which implies its good environmental performance. Overall, ITP and ITS can be co-used in developing environmentally friendly UHPC to minimize the environmental burdens.

Replacing fly ash with a natural pozzolan in normal strength concrete and ultra-high performance concrete

Fly ash is a widely-used supplementary cementitious material (SCM) in concrete. Common reasons for using fly ash in normal strength concrete (NSC) are to reduce cement consumption and mitigate alkali-silica reaction. Additionally, fly ash is being used in ultra-high performance concrete (UHPC) to reduce use of silica fume which is more expensive than fly ash. Fly ash and silica fume are important SCMs in UHPC that provide a dense microstructure, improving UHPC strength and durability. However, fly ash is becoming less available because the energy industry has been investing in renewable energy production and removing coal burning power plants from operation. Therefore, there is a need to identify sustainable SCMs to replace fly ash in NSC and UHPC mixtures. Natural pozzolans are a diverse class of siliceous or siliceous and aluminous materials that can form compounds possessing cementitious properties in the presence of water. Since natural pozzolans are easily mined, they are usually considered to be more sustainable than Portland cement and nearly as sustainable as other SCMs such as fly ash and silica fume. This experimental study evaluated the effects of a natural pozzolan (pumicite) as a potential replacement for fly ash that can produce comparable rheological and mechanical properties in NSC and UHPC mixtures. Results indicated that workability of both NSC and UHPC mixtures containing pumicite was consistent enough to achieve the targeted workability in just a few trials. Additionally, NSC mixtures with 30% pumicite had comparable compressive strength compared to NSC mixtures with 30% fly ash. Modulus of rupture (MOR) for NSC mixtures containing 30% pumicite (greater than 4.10 MPa) exceeded the MOR for specimens containing 30% fly ash. For UHPC mixtures, results showed that up to 75% of the fly ash in the control mixture could be replaced with pumicite while still producing acceptable compressive and flexural strengths. Based on these results, it appears that pumicite can be a reliable alternative for fly ash in NSC and UHPC mixtures in terms of rheological and mechanical properties.

Effect of curing regimes on the rheological, mechanical, hydration and microstructure of ultra-high-performance concrete (UHPC) using indigenous resources: progress towards sustainable construction practices

ABSTRACT Ultra-High-Performance Concrete (UHPC) was renowned for its exceptional mechanical strength and durability. While contemporary UHPC is made using high-quality materials and equipment, its enormous use of natural resources in construction has had global ecological impacts. Employing materials that are accessible locally without specialized procedures. This study concentrates on UHPC mixtures, formulated with indigenous materials, aiming to develop an economical product without the need for high-speed mixers or special fibers. The mixtures are designed with optimal proportions of various supplementary cementitious materials (SCM’s). The study begins with an analysis of flow behaviour, followed by an evaluation of mechanical properties using non-destructive techniques (NDT) under different curing conditions. The UHPC mixtures demonstrate compressive strengths from 70–85 MPa to 110–144 MPa under standard and hot water curing. Relationships between destructive and NDT test outcomes are also determined. The hot water curing process leads to the formation of calcium-silicate-hydrate(C – S – H) gel, with calcium-hydroxide (CH) crystals observed within the amorphous C – S – H gel. The use of high-temperature curing improves mechanical characteristics by (35–40%) and densifies the microstructure. Consequently, producing UHPC using widely accessible materials without further processing and common technologies like conventional casting and standard curing would increase its cost-effectiveness and environmental sustainability.

Rapid chloride penetration test: An evaluation of corrosion resistance in ultra-high performance concrete

Six Ultra-High Performance Concrete (UHPC) mixtures were tested to determine their chloride ion ingress potential using the rapid chloride penetration test (RCPT). The UHPC mixtures showed low to negligible chloride ion penetrability, depending on their water to cementitious materials (w/cm) ratio and curing regimen. Electrical resistivity and conductivity of the mixtures were determined based on the recorded electric current during the RCPT test. The electrical resistivity varied from 49 to 1419 Ω-meter, while conductivity ranged from 0.0007 to 0.0013 milli-Siemens/meter, indicating a moderate to very low expected corrosion rate for the mixtures. The chloride diffusion coefficient was calculated using the Nernst-Plank Equation, making corrections for Joule effect and temperature adjustment due to the temperature increase of the electrode solution during the RCPT test. The chloride ion diffusion coefficients of the UHPC mixtures ranged from 0.53–3.01 × 10−15 m2/s. These results suggest that UHPC mixtures with a w/cm ratio as high as 0.30 can exhibit excellent corrosion resistance.

Utilization of solid waste from tunnel excavation as manufactured sand with different lithology and pre-washing process for preparation of eco-friendly ultra-high performance concretes: Properties and microstructural analysis

Although manufactured sand is becoming increasingly popular to prepare the cleaner and sustainable construction production in face of the depletion of natural river sand, it needs to be further promoted in some advanced concretes like ultra-high performance concrete (UHPC). Different lithological manufactured sands with the same particle gradation were used as fine aggregates to prepare UHPC in this study. Meanwhile, the UHPC mixtures prepared with washed or unwashed sands were also compared. The following aspects were evaluated: flowability, rheology, setting time, mechanical strength, volume stability, thermogravimetric behavior, pore distribution, microstructure characteristics, environmental and economical benefits. Results indicated that sand washing process greatly lowered the apparent viscosity and yield stress, and improved the flowability of UHPC. Increments of 16.7 %, 19.6 % and 15.7 % in flowability were observed for washed granite, tuff and limestone sand UHPC. Pre-washing process significantly delayed the setting times. Tuff sand UHPC demonstrated the optimal mechanical properties (compressive, flexural and tensile strengths), with a compressive strength exceeding 150 MPa at 28 days. In most cases, sand washing process had a slightly beneficial effect on mechanical strengths. The use of washed sand reduced the autogenous shrinkage of UHPC by 5.1%–10.6 %. Tuff sand UHPC possessed the largest cumulative pore volume, while limestone one had the fewest macropores (>100 nm). The cumulative pore volume and total porosity decreased with the addition of washed sands. Compared with traditional UHPC, the utilization of manufactured sand with different lithology and pre-washing process in UHPC could not only illustrate good workability and mechanical strengths, it also demonstrated lower carbon emissions and production costs. The prepared eco-friendly UHPC in this study effectively utilized the solid waste and lessened the environmental burden, which proposed an emerging approach for the application of manufactured sands in green advanced concretes in future work.

Influence of vibrating compaction time on the strength and microstructure of ultra-high performance concrete

During the forming process of ultra-high performance concrete (UHPC), the duration of vibrating compaction plays a pivotal role in influencing its performance. Our study delved into the impact of various factors on the macroscopic mechanical properties, considering the influence of vibrating compaction time on the pore size distribution in the matrix, fibre distribution characteristics, and fibre–matrix interface. We employed SPSS software to analyse the correlation between different factors and UHPC strength. Based on our experimental findings, the influence of vibrating compaction time on the flexural and compressive strengths of UHPC varied within the 0–15 s vibration range. Notably, vibration time had a significant effect on the distribution of pores and fibres within the UHPC matrix. The alterations in pore size distribution and fibre distribution induced by vibrating compaction time were primarily responsible for its impact on UHPC strength. The study provides a solid theoretical foundation for optimising the vibrating compaction time in UHPC mixtures.

Performance and life cycle of ultra-high performance concrete mixes containing oil refinery waste catalyst and steel fibre recovered from scrap tyre

An economic solution to recycle spent equilibrium catalyst (Ecat) disposed of by petrochemical manufacturers and steel fibre recovered from scrap tyre would be their use in the production of structural concrete. In this study, the performance and life cycle of ultra-high performance concrete (UHPC) mixes containing these waste materials was examined. They were made using a total binder content of 1160 kg/m3 and also contained fly ash and silica fume in addition to Ecat and cement. Their average compressive strength at the end of 28 days was found to be above 150 MPa. The replacement of cement with Ecat reduced the CO2 emission and material cost of recycled steel fibre (RSF) reinforced UHPC samples by 15% and 6%, but there is no significant change in their compressive, flexural and split tensile strengths at the end of 28 days. Only 2% of the elastic modulus of these samples increased as a result of the addition of Ecat by weight of the binder. Ecat, an aluminosilicate material, is generally classified as a pozzolanic material, but its addition together with fly ash and silica fume in the UHPC mixture did not affect the strength gain properties. Based on our experimental findings, the use of waste materials including Ecat and RSF in the preparation of precast UHPC elements for explosion and earthquake resistant structures is recommended.

Freeze-Thaw resistance and mechanical properties of UHPC reinforced with a lower amount of hybrid fibers

Sustainable development is becoming critical in many industrial fields, including building materials. The rapid development of the construction industry in Kazakhstan requires construction materials having high-quality, low maintenance, and good durability. The main durability issue of construction material associated with Kazakhstan’s severe climate is the volume change of water in concrete caused by the freezing and thawing (F-T) cycles, which threatens the material’s strength and microstructure. Therefore, ultra-high-performance concrete (UHPC), a relatively new material with high compressive and tensile strength and improved microstructural properties, is suggested. Nevertheless, a limited number of researches have been done regarding the F-T resistance of UHPC. This research focuses on the effect of combining two types of fibers (steel and polypropylene (PP) fibers) on the compressive, flexural, and direct tensile strength and F-T resistance of UHPC. Test results indicated higher dosages of PP fiber in UHPC increased strength before exposure to F-T cycles. Adding PP fiber to UHPC led to more prolonged strains for each UHPC mixture. However, adding more steel fibers was beneficial for UHPC exposed to F-T cycles. Although mixtures with high steel fiber dosage presented a more brittle type of failure and lower strain values than mixtures with higher PP fiber dosage, the compressive, flexural, and tensile strength of UHPC mixtures containing higher steel fiber dosages increased after 90F-T cycles. Relative dynamic modulus of elasticity (RDME), which is a common indicator of F-T resistance of concrete, tends to increase for all tested mixtures, with the highest values belonging to ones with higher steel fiber dosage. Hence, it is suggested that UHPC with higher steel fiber dosage is more beneficial in terms of the F-T resistance of UHPC.

Materials, Structure, and construction of a low-shrinkage UHPC overlay on concrete bridge deck

As ultra-high performance concrete (UHPC) has superior mechanical and durability properties, it is a promising solution to apply a thin layer of UHPC on top of normal concrete bridge decks to extend the service life and enhance the structure integrity. However, the large autogenous shrinkage characteristics of UHPC is still an issue, and there is a lack of research focus on the structural design and stress analysis of the concrete bridge deck and UHPC overlay system. Thus, this study first designs and evaluates a new low-shrinkage UHPC mixture internally cured with a high-strength, high-abrasion resistant, and porous aggregate (calcined bauxite, CB), and the optical mixture design is validated by testing the fresh, mechanical, autogenous shrinkage, abrasion, and impact properties of the UHPCs. Second, the outdoor UHPC overlay-NC composite slab experiments and the numerical modeling are carried out to analyze the shrinkage performance of the UHPC, debonding potential of the overlay layer, stress and strains development in the system. Then, the significant influencing factors and the key measures to assure the UHPC-NC interface bonding properties are obtained, including proposing a grouped L-shape steel rebar arrangement for strengthening the interface. Finally, the applicability of the new low-shrinkage UHPC mixture as overlay materials on a real concrete bridge deck is demonstrated. Based on these, the engineering design scheme and the construction recommendations of an environmentally-cured low-shrinkage UHPC overlay on concrete bridge deck is given. The utilization of UHPC overlay on concrete bridge decks offers a novel paving system that substantially enhances strength, abrasion resistance, and durability.

Cementless ultra-high performance concrete (UHPC) using CaO-activated GGBFS and calcium formate as an accelerator

This study proposes an innovative approach to develop cementless ultra-high performance concrete (UHPC) by activating ground granulated blast furnace slag (GGBFS) without an alkaline-activating solution. To develop cementless UHPC, calcium oxide (CaO) (5% by weight) was used to activate GGBFS, and calcium formate (CF) was used as an accelerator at concentrations ranging from 0 to 6 wt%. The results showed that CF significantly improved the mechanical properties until 5 wt% and all the samples, including CF, exhibited a compressive strength of over 150 MPa. Uniaxial tensile tests revealed strain-hardening behavior, similar to typical cement-based UHPC. The addition of CF led to an increase in hydration products and pore size refinement, resulting in improved mechanical properties. The life cycle assessment results indicated that the developed UHPC mixture had nearly 70% lower CO2 emission than cement-based UHPC. The experimental results highlight the potential of this method for sustainable UHPC development.

Influence of 3D-printed reinforcement on the mechanical and fracture characteristics of ultra high performance concrete

The current research makes an attempt to examine the possibility of using the 3D printing technique for the reinforcement of Ultra High-Performance Concrete (UHPC). In this regard, the mechanical properties and fracture mechanism of UHPC containing 3D-printed shell reinforcements made from Polylactic Acid (PLA) materials have been investigated. At first, the 3D-printed shell reinforcements were plotted with the aid of 3D Max and Rhino 6 software. Then, they were fabricated through Fused Deposition Modeling (FDM) method and positioned into the cubic, cylindrical and prismatic molds. At the next step, the prepared UHPC mixture was poured into the molds and the samples were cured for 28 days. Finally, the compressive, tensile and flexural strength tests were carried out on the samples. The results indicated that the compressive, tensile, and flexural strengths of reinforced samples were less than the unreinforced ones, respectively. Although the inclusion of 3D-printed reinforcement decreased the mechanical properties of the UHPC samples, it was able to change the fracture mechanism of concrete from brittle to ductile.

Development and Performance Evaluation of UHPC and HPC Using Eco-Friendly Additions as Substitute Cementitious Materials with Low Cost

Ultra-high-performance concrete (UHPC) and high-performance concrete (HPC) are widely used in construction engineering applications. The quality and economy of this type of concrete are the main challenges in real construction systems due to their expensive cost. In the present investigation, the performances of UHPC and HPC were improved using eco-friendly additives from natural sources or industrial wastes. Accordingly, different kinds of concrete mixtures were prepared with the addition of various eco-friendly materials, such as metakaolin (10, 15, and 20%), silica fume (2.5, 5, 10, and 15%), cement kiln dust (CKD) (0, 5, and 10%), and 1 vol.% of steel and polypropylene fibers. All of these materials were subjected to efficient treatment and purification processes. The results indicated that the prepared UHPC was characterized by high compression and flexural strengths. The prepared UHPC (sample CR-2) with metakaolin (10%), CKD (10%), and 1 vol.% of steel fibers provided the highest compressive strength of 135 MPa at 28 days. Moreover, the results showed that reducing the cement amounts to 750, 500, and 250 kg/m3 provided concrete with efficient structural requirements and specifications and can be characterized as UHPC and HPC. Also, the mixture (sample CM15) with a metakaolin addition of 15%, CKD of 100 kg/m3, and 1 vol.% of steel fibers showed the highest flexural strength of 19.14 MPa at 28 d. Moreover, the highest splitting tensile strength of the prepared UHPC cylinders was 9.6 MPa at 28 d for the MSS1000 sample, which consisted of 15% metakaolin, a cement content of 1000 kg/m3, silica fume of 10%, and steel fibers of 1% vol. The prepared UHPC mixtures will reduce the amount of consumed cement and the production cost, with a high performance in comparison to classical concrete. Finally, it was clear that the prepared UHPC and HPC concrete with green additions can serve efficiently in specific construction applications, with high performance, economic feasibility, and safe environmental impacts.

Modeling mixing kinetics for large-scale production of Ultra-High-Performance Concrete: effects of temperature, volume, and mixing method

The mixing of ultra-high-performance concrete (UHPC) featuring low water-to-binder ratios involves different evolution of microstructures and mixing torques from conventional concrete. This paper investigates the mixing kinetics of UHPC in the mixing process, presents a mixing kinetics model to predict the mixing torque at an arbitrary time instant, and develops a multi-batching method to reduce the mixing torque for large-volume production of UHPC. The presented mixing kinetics model considers the effects of mixing temperature, mixing volume, and mixing methods. The mixing kinetics model is calibrated using experimental data, and the calibrated model shows high prediction accuracy. The multi-batching method enables large-volume mixing of UHPC by reducing the mixing torque while retaining desired flowability and hardened properties of UHPC. Specifically, when the number of sub-batches is two, the peak mixing torque of the multi-batching method was approximately reduced to half of the peak mixing torque of the mono-batching method. Besides, the differences in workability, compressive strength, and autogenous shrinkage by using the multi-batching method and mono-batching method are within 5%. Finally, the reliability and repeatability of the presented mixing kinetics model are verified through the validation tests with different UHPC mixture designs and mixing methods. This study will advance understandings of the mixing kinetics for UHPC and promote large-volume UHPC production.

Influence of red mud on the engineering and microstructural properties of sustainable ultra-high performance concrete

This paper introduces a new application of industrial solid waste red mud to use as a cementitious material in the development of ultra-high performance concrete (UHPC) performances. In addition, utilization of red mud can reduce the environmental impact of red mud landfill. For gaining this aims, UHPC specimens were prepared with different contents of red mud and analyzed the effects of different RM contents on the fluidity, setting time, mechanical properties, durability, drying shrinkage and microstructure of the red mud-based UHPC. The results indicated that fluidity was reduced to 310 mm at 20% red mud level, but it still belongs to enormous fluidity, which is convenient for construction. In addition, it is worth noting that using red mud instead of cement can significantly shorten the setting time, providing a new method for applying the UHPC rapid-repairing process. Red mud-based UHPC mortar prepared with appropriate red mud instead of cement exhibits higher mechanical properties, better durability and reduced drying shrinkage of UHPC during curing. After curing for 28d, the highest compressive strength of UHPC samples containing 20% red mud was 129.5 MPa, the compressive strength of the UHPC samples' sulfate resistance was 136.6 MPa, and the drying shrinkage value was around 414 µm. Moreover, observed information from scanning electron microscopy (SEM) noted that red mud can fill the micro-cracks between particles of cementitious materials, make the structure more compact and form new hydrates. Finally, the two characteristics of low cost and energy consumption further promote the sustainable application of red mud-based UHPC. Therefore, replacing OPC with 20% RM is meaningful to produce a sustainable UHPC mixture and field applications.

Ispitivanje primjene otpadnih mineralnih dodataka u betonu ultravisokih svojstava

This study examined the effects of various waste materials on the properties of ultra-high-performance concrete (UHPC), along with their designs and ideal mix ratios. Several UHPC mixtures have been developed using the Taguchi L16 method, which generally forms part of the experimental programs for evaluating the properties of UHPCs. Furthermore, the proportions of the component materials were chosen based on approximate ranges found in literature. The samples were cured under two different regimes: standard immersion water curing (SC) and hot water immersion curing (HC). The properties of both hardened and fresh concretes were assessed. A flow test was conducted on the fresh concrete to determine the workability, and a standard test was conducted to assess the density. To investigate a hardened concrete sample, the compressive and flexural strengths were examined and density, absorption, and void tests were conducted. The results obtained from the Taguchi approach for the compressive strength at 28 days were found to be 20 % for SF, 0 % for FA, and 0 % for GBFS; the flexural strength was 10 % for SF, 0 % for FA, and 0 % for GBFS. The compressive strength was 147.07 MPa with SC and 150.13 MPa with HC and flexural strength was 26.88 MPa with SC and 27.31 MPa with HC (as conducted at 28 days in a mixture of 10 % SF and 10 % GBFS).

Mechanical and fracture characteristics of ultra-high performance concretes reinforced with hybridization of steel and glass fibers

Ultra-high performance concrete (UHPC) provides exceptional mechanical and durability properties, but it is highly prone to cracking. Despite short steel fibers have been introduced to UHPC mixtures to mitigate brittleness and improve ductility, its effect on the tensile strength and fracture toughness becomes rather limited. This experimental study addresses hybridization of micro steel and glass fibers, and long hooked steel fibers to improve mechanical and fracture properties of ultra-high-performance concretes (UHPCs). Totally, 22 concrete mixtures were cast using mono and binary combinations of micro steel fiber (MSF), long hooked steel fiber (HSF) and micro glass fiber (GF) at 0, 0.25, 0.50, 0.75, 1.0, 1.5, and 2% by total fiber volume. The UHPCs were tested for compression, splitting, flexure, load-deflection diagram, fracture toughness, and characteristic length. Combined use of fibers suggested the performance order of MSF + HSF, MSF + GF, and HSF + GF on the effects tested. Using hooked steel fibers (HSF) in the hybrid blends seems remarkably promising as it has provided much higher ductility and discernible strain hardening. Using mono-GF did not improve ductility, even showed brittle failure.

Predicting ultra-high-performance concrete compressive strength using gene expression programming method

There have been extensive experimental studies available on the composition and characteristics of Ultra-High-Performance concrete (UHPC). However, the relation between UHPC characteristics and mixture content, on the other hand, is extremely non-linear and challenging to distinguish utilizing typical statistical approaches. A comprehensive literature research was carried out for this aim to acquire experimental data on the compressive strength of UHPC. The dataset contains 810 experimental values of compressive strength and 15 most influential parameters that include cement, water, nano-silica, quartz powder, limestone powder, gravel, sand, slag, superplasticizer, fiber, temperature, age, fly ash, relative humidity, and silica fume, are considered as input. The suggested gene expression programming (GEP) model can estimate the compressive strength of UHPC by using simple mathematical formulations. There is no predetermined function to evaluate in the GEP technique, and it replicates or eliminates numerous combinations of factors to create the formulation that suits the experimental results. For verification and validation of model performance, various statistical measures, SHAP analysis, external validation checks, and comparing with the regression model, are applied. SHAP analysis provided that age, fiber, silica fume, superplasticizer, cement, sand, and water have a high influence on compressive strength while other input parameters have less influence on compressive strength. The model outcomes indicate the robustness and accuracy of the predictive potential of the proposed model. As a result, the GEP model can be used to give practical insights into the mixture design of UHPC for a variety of construction applications, resulting in better predictive capacity at a cheaper cost and in a considerably shorter period. Also, the present study findings can assist the design engineers and builders to understand the significance of each constituent in UHPC.

A Comprehensive Study on Non-Proprietary Ultra-High-Performance Concrete Containing Supplementary Cementitious Materials

Ultra-high performance concrete (UHPC) is a novel cement-based material with exceptional mechanical and durability properties. Silica fume, the primary supplementary cementitious material (SCM) in UHPC, is expensive in North America, so it is often substituted with inexpensive class F fly ash. However, future availability of fly ash is uncertain as the energy industry moves toward renewable energy, which creates an urgent need to find cost-effective and environmentally friendly alternatives to fly ash. This study investigated replacing cement, fly ash, and silica fume in UHPC mixtures with ground granulated blast-furnace slag (GGBFS), metakaolin, and a natural pozzolan (pumicite). To identify acceptable UHPC mixtures (28-day compressive strength greater than 120 MPa), workability, compression, and flexural tests were conducted on all mixtures. Then, durability properties including shrinkage, frost resistance, and chloride ion permeability (rapid chloride permeability and surface resistivity tests) were evaluated for the acceptable UHPC mixtures. Results showed that 75, 100, and 40% of fly ash in the control mixture could be replaced with pumicite, metakaolin, and GGBFS, respectively, while still producing acceptable strengths. Flexural strengths were greater than 14.20 MPa for all mixtures. For durability, UHPC mixtures had shrinkage strains no greater than 406 μstrain, durability factors of at least 105, and "very low" susceptibility to chloride ion penetration, indicating that these SCMs are suitable candidates to completely replace fly ash and partially replace silica fume in non-proprietary UHPC.