# Fatigue in Concrete under Low-Cycle Tensile Loading Using a Pressure-Tension Apparatus
## 3 Experimental Results and Discussion
### Tensile Strengths (Cyclic Loads)  
Twelve wet and four dry concrete cylinders (a total of sixteen specimens) have been tested under a cyclic loading condition. Since the tensile strength of the specimen can be varied substantially for the wet specimens as shown in Table 2, finding the fatigue threshold associated with a low-cycle loading protocol can be quite challenging. The results are shown in tabular format for these specimens.  
Table 3 shows the cyclic test results for two wet specimens (No. 7 and No. 8). Each one failed during the cyclic loading and under a tensile stress that was below the maximum applied tensile stress in previous cycle(s). Both specimens have not been loaded prior to the cyclic loads. It is worth mentioning that the stress at failure is also less than the average tensile strength of 4.45 MPa (as indicated in Table 2).  
Specimens No. 9 to No. 14 have been subjected to 100 cycles of tensile loads with a maximum tensile stress of 2.23 MPa to 3.01 MPa. Then, they have been loaded up to the failure point and the tensile strengths have been recorded to see the effect of the cyclic loading on the final tensile strength. The results are tabulated in Table 4. It seems that the tensile strength of concrete will not be affected if the maximum tensile stress in the cyclic loading is below a threshold (i.e., low-cycle  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Specimen No.**} & \multirow{2}{*}{**Condition**} & **Ultimate Tensile Strength (MPa)** & **Average (MPa)** & **Standard** & **Coefficient of** **Deviation (MPa)** & **Variation (\%)** \\ \hline
1 & Dry & 2.90 & 3.02 & 0.16 & 5.39 \\
2 & Dry & 3.13 & & & \\ \hline
3 & Wet & 3.83 & & & \\
4 & Wet & 4.33 & & & \\
5 & Wet & 4.08 & & & \\
6 & Wet & 5.55 & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Tensile strength of concrete exposed to ambient conditions for two years.  
Figure 3: Broken specimen under tension using pressure-tension apparatus: (**a**) Elevation view; (**b**) Cross-sectional view.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specimen No.**} & \multirow{2}{*}{**Condition**} & **Failed during the Load Cycle No.** & **Max. Tensile Stress Applied** & **Stress at** **in Previous Cycles (MPa)** & **Failure (MPa)** \\ \hline
7 & Wet & 2 & 3.43 & 3.23 \\
8 & Wet & 12 & 3.13 & 3.03 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Cyclic load results for specimens 7 and 8.  
loading). This threshold can be more than 62% of the tensile strength (based on the limited number of test results presented in Table 4) but more test results are required for verification. The difference between these specimens and those reported in Table 3 is that they have been subjected to a cyclic loading for 100 cycles and then tested up to the failure point. The specimens in Table 3 failed during the cyclic loading.  
Specimens No. 15 to No. 18 have been subjected to a cyclic loading with a maximum of 20 cycles. The cyclic loading started at a low level (about 50% of the average tensile strength of the wet specimens as indicated in Table 2) and the maximum load in the next 20 cycles has been increased gradually (by about 5% of the average tensile strength of the wet specimens as indicated in Table 2). The purpose of this test method was to prove that there is a threshold for the cyclic loading at which the fatigue failure will happen at a low-cycle loading. Test results for these specimens are shown in Table 5.  
Table 6 shows the test results for specimens No. 19 to No. 22. They have been tested in a dry condition. The same procedure explained for testing specimens No. 15 to No. 18 has been followed for these tests. It is clear that the condition of the concrete cylinders (i.e., wet versus dry) plays an important role in the tensile capacity of the concrete in cyclic loading (Tables 3-6) and non-cyclic loading (Table 2). As an example, the cyclic loading on specimen No. 19 is shown in Figure 4, in which the specimen failed during the sixth cycle.