Keywords

1 Introduction

With the rapid development of domestic infrastructure construction, and the demand for concrete materials is becoming more urgent. Ultra High Performance Concrete (UHPC) is a kind of concrete with large amount of cementing material, low water-binder ratio and large shrinkage characteristics, which usually requires the use of high-quality raw materials and the incorporation of appropriate admixtures to ensure its workability and mechanical properties [1].

There are many researches on UHPC at present, such as Men et al. conducted an experiment based on the effects of slag powder, silica fume and steel fiber on the mechanical properties of UHPC, which results showed that the steel fiber content had the greatest impact on the mechanical properties of UHPC [2]. Wang et al. studied the content of superplasticizer in UHPC based on the closest packing theory, his research showed that when the content of superplasticizer was 3%, the obtained UHPC had the highest strength, and the UHPC prepared with the content of superplasticizer was 4% had the best working performance [3]. Although UHPC has excellent mechanical properties, but its low water-binder ratio, large amount of cementing material and lack of coarse aggregate increase the viscosity and decrease the fluidity of UHPC [4], resulting in increased difficulty in the pumping and construction process of ultra-high performance concrete mixes [5].

Traditional UHPC preparation processes often require particularly high-quality raw materials such as high-quality aggregate, special cement, copper-plated fine steel fibers, etc. [6]. To achieve high strength, special curing and protection measures such as high-temperature steam curing are often required [7].

This topic aims to prepare low-energy-consuming ecological E-UHPC by optimizing the proportion and gradation of aggregate, cementing material, and admixture, using ordinary sand as aggregate, industrial tailings as admixture, and organic dispersible fiber instead of steel fiber, under conventional technology (i.e., without special mixing equipment, steam curing, etc.). A set of basic mix design methods for ecological ultra-high performance concrete (E-UHPC) and the proposed E-UHPC basic material ratio are proposed.

2 Methods and Material

2.1 Test Material and Scheme Design

The test materials were made up by P.O 52.5 cement, river sand, and A and B polycarboxylic acid superplasticizers with content of 15% and 8%, respectively, and the water-binder ratio is 0.19. The particle size of river sand is 2.5–4.75 mm, 0.45–0.9 mm, 0.22–0.45 mm, 0.125–0.22 mm, respectively. The test scheme of different superplasticizer content and sand particle size is shown in Table 1.

Table 1. Test scheme
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2.2 Test Methods

Fluidity Test. The fluidity test was carried out in accordance with GB/T 50448-2015 ‘‘Technical code for application of cementitious grout’’. The tests is shown in Fig. 1 and Fig. 2.

Fig. 1.
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Variation of fluidity with sand particle size

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Fig. 2.
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Flow state diagram raw materials and fluidity for water reducer test

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Flexural Strength Test. The test of mechanical properties of E-UHPC mainly includes flexural strength and compressive strength, which is in accordance with GB/T 17671-2021 ‘‘Test method of cement mortar strength (ISO method)’’. The flexural strength places one side of the specimen on the support column of the testing machine, and the axis of the specimen is perpendicular to the supporting cylinder, and the load is uniformly applied vertically on the opposite side of the prism until it is broken through the loading rate of 50 N/s. The flexural strength is calculated according to Eq. (1)

$$R_{f} = \frac{{1.5F_{f} L}}{{b^{3} }}$$
(1)

In the Eq. (1), Rf represents the flexural strength, MPa; and Ff represents the load applied to the middle of the prism when its broken, N; and L represents the distance between the supporting columns, mm; and b is the side length of a prismatic square section, mm.

Compressive Strength Test. After the flexural strength test, the two halves were taken out and the compressive strength test was carried out. In the whole loading process, the rate of 2400 N/s is uniformly loaded until it is destroyed. The compressive strength is calculated according to Eq. (2)

$$R_{c} = \frac{{F_{c} }}{A}$$
(2)

In the Eq. (2), Rc represents the compressive strength, MPa; and Fc represents the maximum load when the material is failure, N; and A is compression area, mm2.

3 Results

3.1 Analysis on the Fluidity of E-UHPC

The fluidity of E-UHPC under different particle sizes is shown in Fig. 3. Figure 3(a) shows the fluidity of E-UHPC with 15% of superplasticizer. It can be seen from the Fig. 3a that the fluidity value shows an exponential growth trend with the particle size. When the particle size is less than 1 mm, the fluidity increases linearly with the increase of the particle size. And when the particle size is larger than 1mm, the increasing trend of fluidity is slow. Figure 3(b) shows the fluidity of E-UHPC with 8% of superplasticizer. The development trend of the fluidity value of E-UHPC with 8% of superplasticizer is consistent with that of with 15%. Which can be expressed by Eq. (3).

$$f = ae^{k/b} + c$$
(3)

In the Eq. (3), f represents the fluidity value, mm, k represents the sand particle size, and a, b, and c are the fitting parameters, as shown in Table 2.

Fig. 3.
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Variation of fluidity with sand particle size

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Table 2. Parameter of the exponential function
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The fluidity of E-UHPC with two types of superplasticizer content at different grading is shown in Fig. 4. As can be seen from the Fig. that the greater dosage of superplasticizer, the smaller the fluidity. The fluidity of cement mortar with different grading is the same, which will decreases with the decrease of sand particle size. The main reason is that the smaller the particle size of sand particles, the larger the specific surface area, and the amount of cement paste required to wrap the surface of sand particles increases. Therefore, under the same content of water and superplasticizer, the fluidity of cement mortar decreases significantly. The fluidity of cement mortar with 8% superplasticizer is 17%–21% higher than that with 15%, hence the effect of 8% superplasticizer content on the flow performance of cement mortar is more significant.

Fig. 4.
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Comparison of fluidity of E-UHPC with 15% and 8% superplasticizer

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3.2 Analysis on the Flexural Strength of E-UHPC

The 28-day flexural strength E-UHPC under different particle sizes is shown in Fig. 5. The flexural strength of E-UHPC corresponding to 8% and 15% water-reducing agent is basically the same, which is increased first and then decreased with the increase of particle size. When the particle size is at range of 0.125–0.9 mm, the flexural strength increases linearly with the change of particle size.

When the sand particle size is at range of 0.125–0.22 mm, the flexural strength of E-UHPC corresponding to two types of superplasticizers is at a small value. The main reason is that the smaller the sand particle size is, the larger the surface area is, and the bonding property between sand particles is weakened, which affects the mechanical properties of E-UHPC to a certain extent. When the sand particle size is at range of 0.45–0.9 mm, the flexural strength corresponding to the two types of superplasticizers reaches the maximum, and the flexural strength corresponding to the 8% superplasticizers is about 22% of the 15% superplasticizers Dosage. With the particle size increasing to 2.5 mm–4.75 mm, the flexural strength corresponding to the two content superplasticizers showed a linear decrease trend, mainly because the sand surface area decreased due to the doubling of the sand particle size.

Fig. 5.
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Variation of flexural strength with sand particle size

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Comparison of 28-day flexural strength of E-UHPC with 15% and 8% superplasticizer is shown in Fig. 6. As can be seen from the Fig. That the flexural strength of E-UHPC corresponding to the 8% superplasticizers is larger than that to 15% superplasticizers at every partical size, which is about 14–22%.

Fig. 6.
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Comparison of 28-day flexural strength of E-UHPC with 15% and 8% superplasticizer

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Fig. 7.
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Variation of compressive strength with sand particle size

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3.3 Analysis on the Compressive Strength of E-UHPC

After the flexural strength test, the two halves were taken out and the compressive strength test was carried out. The 28-day compressive strength of E-UHPC under different particle sizes is shown in Fig. 7. The results in the Fig. show that the compressive strength of E-UHPC corresponding to 15% superplasticizer increases rapidly at first and then steadily with the increase of particle size, and reaches the maximum when the particle size is at range of 2.5 mm–4.75 mm. The compressive strength of E-UHPC corresponding to 8% superplasticizer increases rapidly with the increase of particle size at first and then remains basically unchanged, and reaches the maximum when the particle size is at range of 0.22–0.45 mm.

By comparing the 28-day compressive strength of E-UHPC with two types of superplasticizer content, it can be found that the 28-day compressive strength of E-UHPC with 8% superplasticizer content under the same particle size is about 14% to 28% larger than that with 15%.

4 Conclusions

By comparing the fluidity, flexural strength and compressive strength of E-UHPC with two different contents of water reducing agent and four kinds of sand particle sizes, The fluidity of E-UHPC with the superplasticizer content of 8% is 17%–21% higher than that with the superplasticizer content of 15%, which shows a better peaceability. The 28-day flexural strength of E-UHPC with two types of superplasticizer content increases first and then decreases with the increase of particle size, while the 28-day compressive strength increases rapidly first and then steadily. The 28-day flexural strength and 28-day compressive strength of flexural strength with superplasticizer content of 8% are obviously higher than that of 15%. Therefore, E-UHPC with superplasticizer content of 8% has better working and mechanical properties.