Summary

 Quaternary blends with cement-slag-silica-limestone have considerable advantage of environmental sustainability for UHPC pastes compared to the pure Portland cement, with maximum improvements of 59% CO2 emission reduction and 130% sustainability efficiency based on strength.

 The designed quaternary binders in general slightly accelerate the hydration process and dilute the heat flow and total heat, but significantly improves the hydration degree and efficiency of cement in UHPC pastes. Furthermore, the pore structures of UHPC pastes with quaternary binders are densified compared to the mixture with pure Portland cement.

 Limestone powder contributes to better environmental sustainability, spread flow and wet packing density, but causes enlarged total free shrinkage and diminished strength of UHPC pastes due to dilution effect, while application of silica powder is an effective counter measure to overcome those disadvantages due to nucleation, pozzolanic and filling effects.

 Slag cement possessing a relatively lower Ca/Si ratio is preferred to a lower amount but finer silica in the presence of limestone powder to achieve enhanced hardened properties (3% nano silica for the quaternary binders), compared to the Portland cement with a higher Ca/Si that needs more silica even with coarser particle size (5% micro silica for the ternary binders).

 Positive synergies in term of strength, fibre-to-matrix bond and total free shrinkage can be observed in UHPC pastes with quaternary binders (cement-slag-silica-limestone) compared to binary (cement-slag) and ternary (cement-silica-limestone) ones. It demonstrates the reasonability of quaternary blends for developing sustainable UHPC system instead of binary or ternary ones.

3.4 Conclusions

This chapter aims to optimize the binder by mineral admixtures addition with low cement clinker consumption for UHPC system, towards environmentally sustainable and cost-efficient purposes. First, the roles and optimum content of limestone powder in eco-friendly and low-cost UHPC are investigated. Then, the sustainable quaternary binder cement-slag-limestone-micro/nano silica are developed.

 Limestone powder shows a positive mineral plasticization effect that should be considered in designing UHPC. The degree of secondary pozzolanic hydration is more

intensive than C3S/C2S hydration, which can enhance the later-age strength development potential.

 The optimum content of limestone powder appears to be 50 vol.% of the total powder content in UHPC, and contribute to a higher strength, denser pore structure, diminished total free shrinkage and higher sustainability efficiency.

 Quaternary blends with cement-slag-limestone-silica in UHPC pastes have considerable advantage of reducing embedded CO2 emission and improving sustainability efficiency.

 Positive synergies in term of strength, fibre-to-matrix bond and total free shrinkage are observed in UHPC pastes with quaternary binders compared to binary and ternary ones.

Chapter 4

4 Introduction of coarse aggregate in UHPC system

Currently, most UHPC mixtures are designed without coarse aggregates to ensure the homogeneity. This chapter attempts to introduce coarse aggregates into the UHPC system, in order to reduce the powder content and costs, improve the volume stability and penetration impact resistance, etc. Firstly, UHPC applying coarse basalt aggregates with a maximum particle size Dmax of 16 mm are designed by using a particle packing model and considering optimal powder proportion. The basalt aggregate size effect, powder content effect and fibre reinforcing effect are analysed and discussed. The coarse basalt aggregates have limited reducing effect on the mechanical strength of UHPC. The optimal powder content of about 800 kg/m³ and 700 kg/m³ is found for UHPC when the Dmax is 8 mm and 16 mm, respectively.

Furthermore, a distribution modulus q of 0.19 for the modified Andreasen and Andersen packing model is recommended for designing UHPC with coarse aggregates. Secondly, a novel concept of two-stage UHPC (TS-UHPC) is proposed towards maximum volume of coarse aggregate utilization and ultra-low binder consumption. Results show that TS-UHPC has a low binder amount (e.g. 364 kg/m³) and high binder efficiency (e.g. 0.417 MPa·m³/kg), possessing an excellent compressive strength of up to 151.8 MPa at 91 days. New formulas are proposed to describe correlation between compressive and splitting tensile strength of TS-UHPC, and to predict the strength of TS-UHPC by grout strength.

This chapter is partially published elsewhere:

P.P. Li, Q.L. Yu, H.J.H. Brouwers. Effect of coarse basalt aggregates on the properties of Ultra-high Performance Concrete (UHPC). Construction and Building Materials. 170 (2018) 649-659.

P.P. Li, Q.L. Yu, H.J.H. Brouwers, W. Chen. Conceptual design and performance evaluation of two-stage ultra-low binder ultra-high performance concrete. Cement and Concrete Research. 125 (2019) 105858.

4.1 Introduction

To avoid the drawbacks of limited intrinsic strength of coarse aggregates, overcome the inherent weakness between coarse aggregates and paste matrix, increase the homogeneity and eliminate stress concentration at the contact points between those aggregates, most UHPCs are designed by using only fine aggregates or refined aggregates [1,79,156].

However, concrete containing appropriate type and content of coarse aggregates can possess certain advantages. Rozalija and Darwin [157] reported that high-strength concrete containing basalt aggregates yields higher mechanical properties than high-strength concrete containing limestone, which is attributed by the intrinsic strength of the rock. Ma et al. [158]

reported that coarse aggregates can improve the elastic modulus and alter the workability of UHPC more easily, as well as reduce the cost. Some researchers presented that an addition of coarse aggregates does not reduce or even exhibits a slightly higher compressive strength [159,160]. With the utilization of coarse aggregates, the autogenous shrinkage was reduced by approximately 40% [79]. Peng et al. [161] suggested to use coarse basalt aggregates to improve the penetration impact resistance. Tai et al. [162] presented that at higher loading rates (impact loading), the cracks form quickly and can propagate through the aggregates, consequently increasing the impact resistance. Both the disadvantages and advantages are very considerable for concrete incorporating coarse aggregates. To utilize coarse aggregates in UHPC, these contradictions should be well balanced. Hence, it is of importance to study the aggregate size effect in UHPC. In this study, basalt aggregates are used to match the high strength of paste matrix of UHPC.

Currently, most UHPCs are designed with a high content of powder, which leads to poor economic benefit and low efficiency [102]. On the other hand, a relatively high content of powder is needed to fill the voids between the aggregates to reduce the contact stress concentration and obtain a homogenous stress distribution through the matrix [163].

Normally, the powder volume fraction in UHPC containing coarse aggregates is lower than that without coarse aggregates [158]. Some investigations show that the compressive strength of self-compacting concrete (SCC) increases noticeably with the increase of powder content, especially at lower water-to-cement ratios [164]. But Domone [165] pointed out that there was no discernible trend of variation on the mechanical properties of SCC when increasing the powder contents. Therefore, it is necessary to further understand the powder content effect and find an optimal amount for UHPC when coarse basalt aggregates are utilized. Steel fibre is a critical ingredient because of its considerable reinforcement on mechanical properties, especially for tensile strength, ductility and energy dissipation. The interaction effect between coarse aggregates and steel fibres is also researched on the UHPCs with different powder contents in the present study, as well as the discussion of fibre length for UHPC with coarser aggregates.

However, some attempts only used limited volume replacement levels (e.g. 25% by the volume of UHPC matrix [166]) and maximum particle sizes (e.g. 5.2 mm [167]) of coarse aggregates, and the powder contents are still quite large (e.g. 770 - 1100 kg/m³ [168]).

Besides, coarse aggregates with low density and strength is 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 the UHPC system. Hence, how to further increase the volume and size of coarse aggregates and reduce the binder consumption in UHPC systems 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 [169,170]. High volumes of large size aggregates can be easily used, due to its fabrication methodology without any segregation concerns [170]. A higher volume (e.g. 53% - 59% [169,171]) means a much lower binder consumption. A larger maximum particle size of the aggregates (e.g. 40 mm [172]) indicates a better resistance against bullet or projectile impact [168]. 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 [169–175], 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 aggregates 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 shortcomings.

The objective of this chapter is to investigate the effect of coarse aggregates and the consequent alteration of powder content on the properties of UHPC. The basalt aggregate size effect on mechanical strength is measured and analysed. The powder content effect on compactness and strength of UHPC with coarse basalt aggregates is analysed and discussed, and the optimum powder content and corresponding value of distribution modulus q are suggested. Furthermore, two-stage UHPC as a novel building material is developed, including fabrication methodology, excellent mechanical properties, high volume coarse aggregates 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 compatibility between grout and aggregates 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.