3.3. Bond–Slip Curves
Based on Equation (1) and
SF, the bond–slip curves of the specimens were plotted and are shown in
Figure 8.
Scholars have observed that there are four characteristic points on the bond–slip curves of rebar and NC, namely internal cracking, splitting, limit and residual, which were used to classify five stages [
23] in this study. According to the bond–slip curves and the failure patterns of the SFRC specimens, the bond–slip curves of SFRC at different high temperatures were divided into five stages by considering the effects of steel fibers and the temperature. A typical bond–slip curve of SRFC is shown in
Figure 9.
τu is the ultimate bond stress and
su is the free-end slip corresponding to the ultimate bond strength in
Figure 9.
- (1)
Initial micro-slide phase (OA)
The concrete around the rebar near the loaded end of the specimen is elastically deformed at the beginning of central pull-out test. A small relative slip between rebar and concrete appears. At this stage, the bond between rebar and concrete is dominated by the chemical bond force and supplemented by the mechanical bite force caused by the small slip. The bond–slip curve at this stage is essentially linear. Steel fibers do not play a role in preventing cracking because the concrete around the rebar has not yet cracked.
- (2)
Slip phase (AB)
With the increase in pullout force, the chemical bond force disappears. The oblique squeezing force of the rebar rib on the concrete increases and its radial component causes the concrete around the rebar to produce radial cracks and relative slip from the loaded end. The bond force is mainly provided by the mechanical bite force of the rebar rib on the concrete and the frictional resistance between the surface of the rebar and the concrete. At this stage, the steel fibers, which are in the same direction as the main tensile stress, start to play a crack-arresting role. The development of cracks is thus greatly restricted. Therefore, the bond–slip curve enters a nonlinear growth phase.
- (3)
Splitting failure phase (BC)
As the oblique squeezing force increases, the ultimate tensile strength of concrete at the periphery of the rebar is reached. Cracks in the concrete appear along the direction perpendicular to the rebar ribs. The crack-bridging effect of steel fibers sharply increases, which effectively delays the development of cracks. There is a trend of slowly rising until the bond–slip curve reaches the peak point. As shown in
Figure 8, the rising trend of curves becomes flatter with the increase in
Vf before the peak point. The bond–slip curve reaches the peak of the curve at point C. Then, the concrete surface starts to produce obvious splitting cracks.
The temperature has a significant effect at this stage. The ultimate bond strength of SFRC specimens decreases significantly as the temperature rises. The bond–slip curve near the peak point flattens out with the increase in temperature at the same Vf. The incorporation of steel fibers greatly improves the ductility of the specimen at high temperature.
- (4)
Stress drop phase (CD)
When the ultimate bond strength is reached, the concrete in contact with the rebar rib is crushed and broken. The bond stress is provided by mechanical bite and frictional resistance. The contact surface between the rebar rib and the surrounding concrete gradually smooths out. The roughness of the contact surface decreases as the slip increases, which causes a reduction in mechanical bite force.
As a result, after the peak point, the bond stress between rebar and concrete gradually deteriorates with the increase in slippage. The bridging effect of the steel fibers diminishes with the increase in slippage. Cracks in the concrete around the rebar gradually become deeper and wider and rapidly extend to the boundaries of the concrete.
- (5)
Residual pull-out phase (after the D point)
At this stage, the bond–slip curve tends to flatten out. The specimen fails as the rebar is pulled out of the concrete. Because frictional resistance, aggregate bite and steel fiber remain between rebar and concrete, the bond–slip curves of SFRC specimens appear as longer horizontal segments. This indicates that the residual bond stress is maintained at high temperatures.
3.4. Bond Strength
The experimental results of pull-out tests are shown in
Table 5.
To precisely understand the effect of
Vf and high temperature on the bond strength, the steel fiber reinforcement factor (
βV) and temperature reduction factor (
βT) were introduced, as shown in Equations (2) and (3).
where
τuV is the ultimate bond stress when steel fiber is mixed with
Vf (
Vf = 0.5%, 1% and 1.5%);
τu0 is the ultimate bond stress of the NC specimen.
τuT is the ultimate bond stress when the temperature of the bond zone is
T (
T = 200, 400 and 600 °C);
τu20 is the ultimate bond stress at room temperature (20 °C).
From
Figure 10 and
Table 5, it can be seen that both ultimate bond stress and peak slip increase as
Vf increases. As the temperature rises, the ultimate bond stress decreases and the peak slip increases.
The effect of
Vf on bond strength is shown in
Figure 11. With the increase in
Vf,
βV generally shows a linear growth trend.
βV is small at 20 °C and 200 °C, i.e., less than 1.5 in both cases. The slope of the
βV–
Vf curve is steeper between 400 and 600 °C, which indicates that the effect of
Vf on the improvement in bond strength is more obvious between 400 and 600 °C.
βV of the same Vf increases as the temperature rises. The main reason for this is that high temperatures significantly reduce the bond strength of NC.
The effect of temperature on bond strength is shown in
Figure 12. It can be seen that there is less
βT decay of NC and SFRC between 20 and 200 °C.
βT of NC shows large decay between 200 and 400 °C. However, the degree of
βT decay of SFRC is consistent with that for temperature between 20 and 200 °C. The slope of the
βT–
T curve for SFRC is greater between 400 and 600 °C than between 200 and 400 °C. This indicates that the bond strength between rebar and SFRC experiences greater decay between 400 and 600 °C.
At the same temperature, βT increases as Vf is increased. This indicates that the increase in Vf improves the bond strength between rebar and SFRC.
The reasons for the effect of Vf and temperature on bond strength can be explained as follows.
- (1)
The interaction of the steel fibers and the aggregate creates a rigid skeleton for bridging cracks [
17,
21,
22]. As a result, the bridging action not only retards the expansion of concrete cracks but also moderates the degree of stress concentration at crack tips. This allows the scale and number of crack sources within the concrete matrix to be effectively controlled.
- (2)
The difference in the thermal properties of rebar and SFRC at high temperatures destroys the bond behavior of rebar and SFRC. Firstly, there was a significant difference in the thermal properties of rebar and SFRC. During the heating process, the thermal conductivity of rebar was higher than that of SFRC. Therefore, the stress concentration generated by the temperature difference exists in the bond zone between rebar and SFRC. Secondly, while the thermal expansion of rebar and SFRC were of the same order of magnitude under temperature influence, the temperature expansion of rebar and SFRC above 400 °C varied considerably [
24]. The differences in the expansion and deformation of the rebar and SFRC aggravated the extension of cracks in the concrete between the ribs in the radial direction of the rebar. This led to a decrease in the bond strength between rebar and SFRC as the temperature rose.
- (3)
The continued high temperatures cause cumulative temperature damage to the matrix concrete, which results in the change in concrete strength properties [
25]. At temperatures between 20 and 200 °C, hairline cracks and voids form inside the specimen due to the evaporation of free water inside the concrete. The water and water vapor in the crevice are pressurized due to temperature rise, which creates tension on the surrounding concrete. Stress is concentrated at the seam tip after the specimen is loaded. Stress concentration contributes to crack expansion and a slow decrease in bond strength. The free water inside the specimen evaporates when the temperature is between 200 and 300 °C. Coarse aggregates and cement paste within the concrete have unequal temperature expansion coefficients and the temperature deformation difference causes cracks to form at the aggregate interface. This causes a slight reduction in the tensile strength of the concrete. These factors of the reduction in bond strength are noticeable. However, after 400 °C, the temperature deformation difference between the aggregate and cement paste continues to grow and interface cracks develop and spread. Cement hydration products, such as Ca(OH)
2, dehydrate and expand in volume, which causes the cracks to expand. When the temperature rises above 600 °C, the quartz component of the unhydrated particles and aggregates in the cement decomposes and crystallizes, which results in the development of large-scale expansion. As a result, cumulative damage at high temperatures appears as follows: cracks and voids are formed inside the concrete by water evaporation; the difference between the thermal properties of coarse aggregates and cement paste produces deformation gaps and internal stresses and interface cracks are formed; coarse aggregates are ruptured by thermal expansion. Eventually, these microcracks in turn reduce the tensile strength of concrete more severely than the compressive strength of concrete, leading to the different failure patterns of the NC specimens at different temperatures. However, steel fibers in the heated specimens are able to control these cracks. With the increase in
Vf, the ultimate bond strength of the SFRC specimens increases.