1. Introduction
To overcome the casting difficulty of concrete in structural members and joints with complex shapes and dense reinforcements, and to reduce the environmental pollution of vibration noise, a high-performance self-compacting steel fiber reinforced concrete (SFRC) becomes more and more applicable in engineering structures [
1,
2,
3,
4]. Self-compacting SFRC not only realizes the self-compacting performance with high passing and filling workability without segregation and bleeding, but also overcomes the brittleness of self-compacting concrete (SCC) by using steel fibers [
5,
6,
7].
Based on literatures reported, many previous works sacrificed the workability of SCC to inhibit the shrinkage by using the constant mix proportion of base materials without adjusting with the increase of volume fraction of steel fiber. Aslani and Nejadi [
8] reported that the drying shrinkage of SCC was reduced by 9.3% at 364 d by directly admixing hooked-end steel fiber (circular section,
lf = 60 mm,
df = 0.75 mm) with volume fraction
vf = 0.38%, but the diameter of J-ring flow was decreased by 11.5%. Corinaldesi and Moriconi [
9] reported that by admixing the hooked-end steel fiber (circular section,
lf = 30 mm,
df = 0.7 mm) with volume fraction
vf = 0.64%, the drying shrinkage of SCC decreased about 37.5%, while the time
T50 of slump-flow to diameter of 50 cm and the elapsed time to gain the final configuration increased respectively by 50% and 30%. Grabois et al. [
10] studied the self-compacting lightweight concrete which prepared with coarse and fine lightweight aggregates by adding hooked-end steel fiber (circular section,
lf = 35 mm,
df = 0.55 mm) with volume fraction
vf = 0.5%, and the drying shrinkage decreased about 7%, but the V-funnel flow time increased by 30 s. Meng and Khayat [
11] investigated the effect of hybrid fibers on properties of ultra high-performance concrete, of which the straight steel fiber (circular section,
lf = 13 mm,
df = 0.2 mm) and the hooked-end steel fiber (circular section,
lf = 30 mm,
df = 0.5 mm) were used. Results showed that the autogenous shrinkage reduced by 30% with the increase of
vf from 2% to 5%, but the V-funnel time increased by 61.5%. Bensaci et al. [
12] found that although the drying shrinkage of self-compacting SFRC reduced by 33% with the
vf increased from 0% to 1%, the workability became bad with an increase of the
T50 by 81.4%. Therefore, these studies did not take into account the shrinkage performance of hardened self-compacting SFRC with the premise of ensuring workability on a fresh mixture. This produces the hidden issue in casting and molding quality of self-compacting SFRC. One of the risks is the strength reduction of self-compacting SFRC. In the study of Bensaci et al. [
12], the cubic compressive strength decreased by 9.36% with the
vf having increased from 0% to 1.5%. Grabois et al. [
10] also found that the V-funnel flow time of self-compacting lightweight concrete increased 30 s by using steel fiber with a volume fraction of
vf = 0.5%, and the cylindrical compressive strength decreased by 14.3%. Khaloo et al. [
13] reported that with the increasing
vf from 0% to 2% of hooked-end steel fiber (rectangular section,
lf = 20.4 mm), the slump-flow reduced from 800 mm to 640 mm, while the cylindrical compressive strength reduced by 18.6%.
In order to improve the workability of self-compacting SFRC, the mix proportion should be adjusted. El-Dieb and Reda [
14] studied the effect of fiber factor (the product of the aspect ratio with the volume fraction) and cementitious material content on properties of fresh mixture. Results indicated that with the maximum fiber factor of 50, 90, and 100, the cement content of mixtures should increase to 350 kg/m
3, 400 kg/m
3 and 500 kg/m
3, respectively. By keeping a constant thickness of mortar wrapped on the fibers and coarse-aggregates, Khayat et al. [
15,
16] produced the self-compacting SFRC with high filling and passing ability and sufficient stability, of which the cementitious materials was 475 kg/m
3, and the dosage of coarse-aggregate was reduced 8.8% with the
vf = 0.5%. Ding et al. [
17,
18] proposed a method to get a result of indirect reduction of the coarse-aggregate content by regarding steel fibers as coarse-aggregates, of which the binders’ content and sand ratio were increased with the volume fraction of steel fiber. However, with the premise of ensuring the workability of a fresh mixture by adjusting mix proportion, the hardened self-compacting SFRC faces a problem of great shrinkage compared with SCC due to the content of cementitious materials increased with the volume fraction of steel fiber [
16,
17,
18,
19,
20]. An experimental study exhibited that to keep the diameter of slump-flow at 600 mm for the self-compacting SFRC with the
vf of hooked-end steel fiber (circular section,
lf = 30 mm,
df = 0.5 mm) increased from 0% to 1.4%, the binders content was increased by 16.3% and the dosage of coarse-aggregate was reduced by 35%. In this condition, the autogenous and drying shrinkages respectively increased by 19.8% and 53% [
21].
In general, the contradictory changes of workability and shrinkage of self-compacting SFRC are difficult to harmonize by only adjusting the contents of base materials of SCC with steel fibers. The greater shrinkage of self-compacting SFRC is needed to be controlled by a means of adding an expansive agent [
22,
23]. Su et al. [
24] reported that the drying shrinkage of ultra high-performance concrete was reduced 29.5% by using a calcium–magnesium composite expansive agent. Choi et al. [
25] found that the shrinkage of alkali activated material mortar was reduced by 23.1% due to admixing calcium-sulfoaluminate expansive agent. However, this also raises another problem of the reduction of compressive strength of SCC. He et al. [
26] pointed out that when the dosage of calcium-aluminate expansive agent was less than 9%, the cubic compressive strength of SCC decreased by 9.6%. Li et al. [
27] found that the cubic compressive strength of SCC decreased by 11.5% by adding a calcium-sulfoaluminate and calcium-oxide composite expansive agent. This mainly comes down to the transversal deformation with the unconfined expansion of SCC specimens. Therefore, the presence of steel fibers in SCC could confine the expansion [
28,
29], the expansion rate abated 81.5% of self-compacting SFRC with steel fiber (circular section,
lf = 35 mm,
df = 0.55 mm) of
vf = 0.75% compared to that of SCC under the standard curing condition at (20 ± 2) °C temperature and
RH ≥ 95%, while a slight decrease of the expansion rate at about 0.07% of self-compacting SFRC with the
vf = 0.25–0.75% was produced under the sealed cure and top-surface exposure curing conditions. uAfroughsabet et al. [
30] presented that the conventional SFRC with K-type expansive cement was improved by 28.4% of cylindrical compressive strength and 39.2% of splitting tensile strength with the
vf = 1.0% hooked-end steel fibers, while the expansion was fully cancelled due to the inhibition effect of fibers. Geng et al. [
31] also reported that to ensure the workability of a fresh mixture by increasing 7% binder and reducing 8.8% coarse-aggregate, the drying shrinkage of conventional SFRC with the
vf = 0.8% increased by 22.2%, which could not be removed by adding expansive agent. This means that a coordination exists between the dosage of expansive agent and the content of steel fiber to get the balance of expansion and shrinkage for self-compacting SFRC.
Based on the above analyses, the workability, shrinkage, and strength of self-compacting SFRC are complexly affected by the proportion of base materials of SCC, the content of steel fiber, and the addition of expansive agent. For engineering application, the study should be done on the premise of a rational workability of self-compacting SFRC with different volume fraction of steel fiber, and then the effect of considered parameters on basic mechanical properties and shrinkage of self-compacting SFRC should be verified to satisfy the design requirements. According to this technical route, this paper investigates the workability, mechanical properties, and shrinkage of self-compacting SFRC with calcium-sulfoaluminate expansive agent. The optimal dosage of the expansive agent was firstly determined, and the volume fraction of steel fiber was selected as the main parameter. The mix proportion of self-compacting SFRC was designed with the absolute volume method, of which steel fibers are considered as the distributed coarse aggregates. Results are discussed combined with the influencing mechanisms of studied properties.