# Pullout Response of Ultra-High-Performance Concrete with Twisted Steel Fibers
## 3 Experimental and Verification
### Concrete Samples with a Single Twisted Steel Fiber  
The twisted steel fibers with rectangular section were used in the experiment, as illustrated in Figure 3. The fiber length \(l\), equivalent diameter \(D_{\mathrm{e}}\), fiber pitch \(l_{0}\), tensile strength \(\sigma_{p}\) and elastic modulus \(E\) are 45 mm, 0.5 mm, 0.53 \(\times\) 0.37 mm\({}^{2}\), 0.1961 mm\({}^{2}\), 10 mm, 2500 MPa and 206 GPa, respectively. Only rectangular-section twisted steel fiber in high-performance fiber concrete was studied in this paper as the twisted steel fibers with triangular section cannot be purchased in the Chinese market.  
There are three fiber embedment lengths of 15, 20 and 25 mm and six replicate specimens (T15/20/25-1, -2, -3, -4, -5, -6) were prepared for each fiber embedment length to obtain the representative testing values. The minimum fiber embedment length of 15 mm is 50% higher than the fiber pitch value of 10 mm; while the maximum fiber embedment length of 25 mm is 2.5 times fiber pitch and approximately half of fiber length. This arrangement ensures the whole pullout force curve can be completely obtained.  
Eighteen fiber reinforced specimens in total were prepared in the following process: First, a twisted fiber with a hook anchor was fixed in the middle of a steel mold and the concrete mixture prepared in above Section Concrete samples was poured into the mold for compaction. After 24 h, the first half specimen was demolded. The second half specimen with a certain embedment length of fiber (i.e., 15, 20 and 25 mm) was then cast and carefully demolded after 24 h. The whole sample was then conditioned for 28 days. As the concrete strength during the sample preparation is low, cares should be taken to protect the fiber-matrix interface from being damaged during the operation. To ensure no rotation of sample during the pullout process, both sides of sample were restrained using a fixture. The single fiber pullout experiment was then carried out according to the testing standard [21]. The loading actuator controls the displacement values, which are used as the fiber end slip. Note that the maximum pullout force value of approximately 250 N is so small that the fiber deformation can be negligible. Displacement control at a low pull velocity of 0.01 mm/s was used in our quasi-static loading experiment.  
The concrete matrix of samples before and after the pullout test was scanned and high-resolution images were obtained by using the Nikon XTH320 \(\upmu\)XCT facility at Zhejiang University. The scanning of the specimen before pullout test was to observe the fiber-matrix interface; while the scanning of the matrix after pullout was to observe the embedded channel of the twisted steel fiber.  
A series of two-dimensional images were first obtained and then three-dimensional images were reconstructed by using VGStudio MAX 3.0 software [22]. The images were then analyzed and compared (before pullout vs. after pullout) to observe the damage situation of the fiber-matrix interface and understand the pullout mechanism of twisted steel fiber. The following scanning parameters were used: exposure time (500 ms), voltage (200 kV), current (100 \(\upmu\)A) and scanning resolution (approximately 30 \(\upmu\)m).