Conceptual design and performance evaluation of two-stage ultra-low binder ultra-high performance concrete

Abstract

This study proposes a novel concept of two-stage ultra-high performance concrete (TS-UHPC), towards ultra-low binder consumption. The effects of grout and coarse aggregate are investigated and their compatibility is evaluated. Results show that TS-UHPC has a low binder amount (down to 364 kg/m³) and high binder efficiency (up to 0.417 MPa·m³/kg), possessing excellent compressive strength of up to 151.8 MPa at 91 days. Microstructural analysis reveals that grout with a sand-to-powder ratio of 1.0 shows a higher hydration degree, denser structure, and increased later strength. Coarser basalt aggregate tends to slightly lower compressive and splitting tensile strength, 14% and 12% reduction with the maximum size from 8 mm to 25 mm, respectively. The TS-UHPC has an excellent interfacial transition zone that induces a water-permeable porosity of 0.91%–1.32%. New formulas are proposed to describe correlation between compressive and splitting tensile strength of TS-UHPC, and to predict strength of TS-UHPC by grout.

Introduction

Ultra-high performance concrete (UHPC) is an advanced building material with wide application potentials, attributed to its superior workability, mechanical property, impact resistance and durability [[1], [2], [3], [4]]. One of the key mix design principles is to eliminate coarse aggregate and utilize large amount binder to increase homogeneity of UHPC and overcome inherent weakness of interfacial transition zone (ITZ) [[5], [6], [7]], usually more than 900 kg/m³ binders are consumed [8]. It causes some disadvantages, such as poor economic benefit and sustainability issue, large autogenous shrinkage, and thermal induced cracks in mass concrete.

Introducing appropriate coarse aggregates into UHPC is a promising attempt to solve the above mentioned problems. The utilization of coarse aggregates in conventional concrete has apparent advantages, such as reduced autogenous shrinkage [2], improved elastic modulus and workability [9], enhanced stress-strain behaviour of confined concrete [10], strengthened impact resistance under high velocity projectile [11]. In recent years, researchers have attempted to incorporate coarse aggregates into UHPC and acquired good results. Our previous research showed that coarse basalt aggregates have negligible negative effect on the mechanical strength, with a significantly reduced powder amount of UHPC [12], and improved low-velocity impact resistance [13]. Pyo et al. [14,15] produced UHPC incorporating coarser aggregates with maximum particle size of 5.2 mm, and revealed excellent abrasion resistance, strain hardening behaviour and a limited decrease in tensile strength. Liu et al. [16] designed UHPC combined with coarse aggregates (5–20 mm) and fibres, and found that coarse aggregates could be successfully introduced into the system of UHPC without impairing tensile properties at a favourable replacement level (25% by the volume of UHPC matrix). Wu et al. [17,18] investigated the projectile impact resistance of basalt or corundum aggregated UHPC, proved that enlarging the size of the coarse aggregates could help to reduce the penetration depth, impact crater area and volume. However, those attempts only use limited volume replacement levels (e.g. 25% by the volume of UHPC matrix in Ref. [17]) and maximum particle sizes (e.g. 5.2 mm in Ref. [15]) of coarse aggregates, and the powder contents are still quite large (e.g. 770–1100 kg/m³ in Ref. [18]). Besides, coarse aggregates with low density and strength are not compatible with the relatively high strength of UHPC matrix. While high strength coarse aggregates usually have dense structure and high density, which more easily causes segregation problem in UHPC system. Hence, how to further increase the volume and size of coarse aggregate and reduce the binder consumption in UHPC system is still an issue and potential research subject.

Two-stage (preplaced aggregate) concrete (TSC) is an effective way to extend the utilization of coarse aggregates, which is produced by first preplacing aggregates in a formwork and subsequently injecting grout [19,20]. High volumes of large size aggregates can be easily used, due to its fabrication methodology without any segregation concerns [20]. A higher volume (e.g. 53%–59% in Ref. [19, 21]) means a much lower binder consumption. A larger maximum particle size of the aggregates (e.g. 40 mm in Ref. [22]) indicates a better resistance against bullet or projectile impact [18]. TSC has already been successfully used in applications including underwater concrete construction, massive concrete structure, casting concrete in areas with narrowly spaced reinforcement, concrete repair, heavyweight concrete and low-shrinkage concrete. Nevertheless, the strength of current TSC is relatively low, usually ranging between 10 MPa to 60 MPa [[19], [20], [21], [22], [23], [24], [25]], which is probably attributed to low intrinsic strength of coarse aggregates, relatively low strength of the grout, weak homogeneity, stress concentration at the contact points between aggregates, inherent weakness between coarse aggregate and paste matrix. To sum up, UHPC and TSC have some complementary characteristics on coarse aggregate utilization, binder consumption and mechanical properties. Hence, there is potential to design a novel building material to make full use of advantages of TSC and UHPC and overcome their individual shortages.

This paper aims to develop two-stage ultra-high performance concrete (TS-UHPC) as a novel building material, including fabrication methodology, excellent mechanical properties, high volume coarse aggregate and very low binder consumption, possessing widely potential application, e.g. impact resistant, underwater, massive, repaired, heavyweight, low-shrinkage and narrowly spaced reinforced concrete. The properties of grouts are assessed by fresh behaviour, hydration kinetics, pore structure and compressive strength. The mechanical behaviour of TS-UHPC is evaluated in terms of compressive and splitting tensile strength, as well as binder efficiency. The compatibility between grout and aggregate is analysed by assessing the interfacial transition zone (ITZ). New models are proposed and validated to correlate the compressive strength and tensile splitting strength of TS-UHPC, and compressive strength of TS-UHPC and grout. The proposed TS-UHPC concept further contributes to sustainability development of advanced concrete materials and the proposed models can be applied to predict the materials property.