Synergistic effect between aggregate and fibre

The order of reinforcement on flexural properties is as follows: 30 mm medium hook-ended fibre > 60 mm long 5D fibre > 13 mm short straight fibre for UHPFRC with the Dmax of 8 mm, while 60 mm long 5D fibre > 30 mm medium hook-ended fibre > 13 mm short straight fibre for UHPFRC with the Dmax of 25 mm. To reveal the interaction between aggregates and steel fibres, the relative size effect (ratio of fibre length to maximum aggregate size, L/Dmax) on the fibre utilization factors (η_σ1, η_σp, η_Tf and ϕ) are illustrated in Figure 5.11.

Figure 5.11: Correlation between L/Dmax and fibre utilization.

The lowest utilization efficiency is obtained with the short steel fibres, which is in line with the results observed in the UHPFRC incorporating coarse aggregates [125]. Firstly, the straight fibres have weaker bond compared to other fibres with anchoring effect at ends.

Additionally, the short fibres cannot completely overlay aggregates with a large size, thus

No fibre Short straight Medium hook-ended Long 5D

0.9

Reinforcing factor, p Reinforcing factor, 1

20

providing a limited fibre-bridging interlock stress. Furthermore, if the size of coarse aggregate is too large compared to the fibre length, the fibre distribution in matrix can be significantly disturbed, as shown in Figure 5.12. The non-random orientation of steel fibres adversely affects the reinforcement efficiency and decreases the compactness.

Figure 5.12: Interaction between aggregates and steel fibres on granular skeleton [214].

Preferential synergistic effects are observed between aggregates with the maximum size of 8 mm and 30 mm medium hook-ended fibres (A8F30), aggregates with the maximum size of 25 mm and 60 mm long 5D fibres (A25F60), considering all fibre utilization factors (η_σ1, η_σp, η_Tf and ϕ) in Figure 5.11. Thus, a longer fibre is not always better, and an appropriate length of steel fibre is needed to match the size of coarse aggregate. On the one hand, longer fibres are beneficial to overlay coarse aggregates, enhance the interlock between fibres and coarse aggregates and then improve the flexural performance [213,218]. On the other hand, limiting the particle size to half the fibre length is recommended from the workability point of view [214], which can also decrease the probability of the ‘fibre balling’ phenomenon.

Han et al. indicated that the rational range of the ratio of steel fibre length to coarse aggregate maximum size for steel fibre reinforced concrete is 1.25–3 by considering the reinforcements on splitting tensile strength and flexural properties [215]. Rui et al. summarized that the fibre length is mostly about 2-4 times of the maximum aggregate size in normal concrete, at least not shorter than the aggregate size [221]. Based on the results acquired here, the length of steel fibre (L) is recommended to be between 2 and 5 times the maximum size of aggregate (Dmax) for UHPFRC systems, as illustrated in Figure 5.11.

5.4 Conclusions

This chapter studies the synergistic effect between steel fibres and aggregates on the mechanical properties of UHPFRC. The aggregate size effect, steel fibre type effect, and their interaction are analysed. The key conclusions can be summarized:

 Coarse basalt aggregates up to 25 mm can be successfully introduced in UHPFRC with a significantly lower cement consumption, designed by using a particle packing model, with limited influence on compressive and tensile splitting strengths. However, the negative influence of larger basalt size is more obvious on flexural toughness. The maximum size of coarse aggregate is suggested to be no more than 25 mm.

 The 13 mm short straight steel fibres show a good reinforcement on compressive strength due to the more homogenous distribution in UHPFRC matrix. While, the 30 mm medium

hook-ended or 60 mm long 5D fibres are more efficient in reinforcing tensile and flexural properties.

 Based on the analysis of fibre reinforcing factors, the flexural toughness is more sensitive to the steel fibres, followed by the flexural peak strength, while the first crack strength is mainly controlled by the UHPFRC matrix.

 The deflection or strain hardening behaviour can be acquired by utilizing 2 vol.% of 30 mm medium hook-ended or 60 mm long 5D steel fibres, while the 13 mm short straight fibres only trigger a very limited strain hardening behaviour.

 A preferential synergistic effect is observed between the coarser aggregates and the longer steel fibres. The length of steel fibre is suggested between 2 and 5 times the maximum size of aggregate. The order of reinforcement on flexural properties is: 30 mm hook-ended fibre > 60 mm 5D fibre > 13 straight fibre for UHPFRC with the Dmax of 8 mm, while 60 mm 5D fibre > 30 mm hook-ended fibre > 13 straight fibre for UHPFRC with a Dmax of 25 mm.

Chapter 6

6 Functionally graded UHPC beams

In this chapter, functionally graded ultra-high performance cementitious composite beams are developed by applying the composite concepts of Ultra-high Performance Concrete (UHPC), Two-stage Concrete (TSC) and Slurry-infiltrated Fibrous Concrete (SIFCON). The functionally graded composite beam (FGCB) is fabricated with a bottom layer of SIFCON and top layer of TSC, and the two layers are synchronously cast by using UHPC slurry. The novel concept of FGCB is proposed towards more economical and high performance structural systems, namely an excellent flexural bearing capacity and impact resistance, low cement consumption and high steel fibre utilization efficiency. The fresh and hardened properties of UHPC slurry, flexural properties of FGCB are measured (the impact resistance will be analysed in following chapter). The results reveal that the designed FGCB has superior flexural properties and energy absorption, without showing any interfacial bond problems. The fibre utilization efficiency of the designed FGCB is very high compared to the traditional UHPFRC and SIFCON beams. The 30 mm medium hook-ended steel fibres show the best utilization efficiency compared to the 13 mm short straight and 60 mm long 5D steel fibres, and 3 vol.% medium fibres are optimum to design FGCB.

This chapter is partially published elsewhere:

P.P. Li, M.J.C. Sluijsmans, H.J.H. Brouwers, Qingliang Yu. Functionally graded ultra-high performance cementitious composite with enhanced properties. Composites Part B:

Engineering. (2020) 107680.

6.1 Introduction

Concrete is one of the most widely used construction building materials in civil engineering.

The brittle behaviour subjected to tensile or flexural loading is an adverse property, which causes negative influences, e.g. abrupt failure without warning, reduced service life due to crack formation and propagation. To overcome this shortcoming, fibre reinforced concrete was proposed by adding discrete steel fibres into the plain concrete matrix [222,223]. In the 1990s, UHPFRC was invented and further extended to the concept of fibre reinforced concrete, which is characterized by high dosages of steel fibres, large amounts of reactive powders without any coarse aggregate, and a very low water content [1,2,28]. Although UHPFRC already possesses excellent microstructure, strength, durability, ductility and impact resistance [10,224,225], its tensile and flexural strengths are still relatively low, especially compared to the compressive performance [6,79,226,227]. In addition, the high content of steel fibres and reactive powders in UHPFRC have adverse impacts, causing economic and environmental problems [27,228]. Thus, how to develop more eco-friendly UHPFRC materials and components is of great interest for both researchers and engineers.

The aggregate-to-powder ratio is a key factor to determine the powder consumption and control the cost of UHPC. Chapter 4 revealed that incorporating an appropriate amount of coarse aggregates with proper sizes can significantly reduce the powder content of UHPC, still possessing a comparable mechanical strength [125]. However, the coarse aggregates usually occupy a limited volume by normal mixing methods, less than 40% of total UHPC matrix. To further enlarge the volume of coarse aggregates and diminish the powder content, we applied the two-stage concrete (TSC) concept in UHPC system, i.e. we first place coarse aggregates in mould and subsequently inject ultra-high performance slurry into the voids by gravity pressure. The designed two-stage UHPC can significantly enhance the utilization potential of coarse aggregates, up to 55% - 60%, which consequently greatly decreases the powder demand and creates great economic benefits.

The high strength steel fibre is another key factor to remarkably address the brittle behaviour of UHPC, however it is much more expensive than the other ingredients. Thus, it is of great significance to improve the fibre utilization efficiency of ultra-high performance fibre reinforced concrete (UHPFRC). Meng et al. [229] studied the rheology to control fibre dispersion uniformity, which improved the flexural performance of UHPFRC. Yoo et al.

[213] suggested to use long steel fibres to enhance flexural properties. Controlling fibre orientation [24,25] and using hybridization [26] are also efficient measures to increase the utilization efficiency, both static and dynamic. Another solution to efficiently utilize steel fibres is to position more steel fibres into the tensile zones instead of compressive areas, due to the more remarkable reinforcement of steel fibres on tensile behaviour rather than compressive behaviour. According to this design concept, multiple layered (or functionally graded) concrete composites have been developed with good flexural performance, fracture energy, penetration impact resistance, as well as economical benefit [211,230–232].

However, the functionally graded concrete composites have potential interfacial bond problems, namely weak bond or even delamination in the case of casting the top layer on the hardened bottom layer [233], or wavy layers and uneven thicknesses in the case of casting

the top layer onto the bottom layer, that is still not hardened due to gravity force from the top layer [230]. Furthermore, sometimes high dosages of steel fibres are needed to achieve stronger and energy absorptive UHPFRC beams, especially for protective structures subjected to impact and blast loadings. It is rather difficult or even impossible to add a high volume fraction of steel fibres in the bottom (tensile) layer because of the workability reduction and ‘balling’ phenomenon [7]. Thus, it is reasonable to use Slurry-infiltrated Fibre Concrete (SIFCON) in the tensile layer, which can easily achieve a fibre volume fraction up to 10 vol.% [234].

To develop a superior cementitious composite beam subjected to flexural and impact loadings, we propose a novel functionally graded composite beam (FGCB) concept by applying the combined concepts of UHPC, TSC and SIFCON. The bottom layer consists of SIFCON to withstand high tensile stress, while the top layer is designed by two-stage UHPC to achieve an excellent compressive strength with very low cement consumption. The UHPC slurry is injected into the voids of steel fibres (bottom layer) and coarse aggregates (top layer) simultaneously to acquire superior interfacial bond. The 13 mm straight, 30 mm hook-ended and 60 mm 5D steel fibres are investigated with volume fraction from 0 to 3 vol.%, in order to find an optimal type and content of steel fibres on the flexural properties. Furthermore, the superior performance, low cement consumption and high fibre utilization of FGCB are revealed by comparing them with conventional UHPFRC and SIFCON beams.

6.2 Materials and experiments

6.2.1 Materials

The UHPC slurry is composed of Portland cement CEM I 52.5 R (PC), micro-silica (mS), limestone powder (LP), fine sand (S), tap water (W) and PCE-type superplasticizer (SP3 from Chapter 2 is used). The physical and chemical properties of those raw materials can be found in Section 3.2.1, 3.3.1, 4.2.1 and 4.3.1. The coarse basalt aggregate (BA) with particle sizes between 16 and 25 mm is selected by considering its high inherent strength and passing ability of slurry based on our preliminary tests. Three types of steel fibres (SF) are used to investigate the type and dosage effect on the performance of FGCB, described in Section 5.2.1.