## 2 Materials and Methods  
### Materials and Specim Preparation  
The mix proportions of substrate and repair materials are shown in Table 1. General use hydraulic cement and class F fly ash were used. Fine aggregate was natural sand and coarse aggregate was crushed rock with a maximum size of 10 mm. For the repair materials, beyond the plain control condition without fibers, three types of fibers with volume fractions V\({}_{\text{f}}\) = 0.5% and 1% were considered, namely: (i) 8 mm long Poly-Vinyl-Alcohol (PVA) fibers, (ii) 12 mm long PVA fibers (to compare the effects of different fiber lengths), and (iii) 13 mm long steel fibers (to compare fibers with similar lengths but very different materials, matrix bonding, and stiffness). As reported at the state of the art and discussed in the introduction, short fibers can be more effective than longer fibers with respect to bonding especially because their effectiveness depends upon the relative size of the fiber compared to the interface roughness. The technical information on the fibers is provided in Table 2.  
For the composite substrate-repair samples, concrete substrates were cast in pre-formed moulds designed to cast only half of the final specimen. Substrates were covered by plastic sheets immediately after casting, demoulded after 24 h, and eventually placed in cured at a standard temperature of 20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C and humidity 95% \(\pm\) 5%. After 28 day curing, the substrates were sandblasted. Sandblasting was done by a pressure washer at 4000 PSI (27.6 MPa) using a commercially available glass abrasive product containing less than 1% silica. The nozzle was kept at a distance of 10-20 cm from substrate surface for 1-2 minutes up to the point that the aggregate would be visible on the substrate surface. Afterwards, concrete substrates were put back into the moulds in optimum Saturated Surface Dry (SSD) condition [19] and the repair mortar was poured against the substrates. It is important to mention that, in this work and unlike in other studies, the mortar was cast parallel to the substrate. Although the mortar was well vibrated and compacted, this casting direction is expected to potentially penalize compaction at the Interfacial Transition Zone by the substrate and to affect fiber orientation near the bond plane (especially for steel fibers, which are stiffer). The repaired specimens were cured with the same procedure used for the substrates. When casting the substrate and repair components of the specimens for bond testing, monolithic specimens of the repair materials were cast in similar CDCB moulds and cured together with the bond samples, in order to characterize fracture properties of the repair materials as well. Three replicates were tested for each of the seven repair materials and each of the seven corresponding concrete-repair interfaces.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Layer** & **Cement** & **Fly Ash** & **Sand** & **10 mm** & **Water** & **Fiber Volume** \\  & & & & **Aggregate** & & **Fraction** \\ \hline
**Substrate** & 1 & 0.25 & 2 & 0.48 & 0.5 & — \\ \hline
**Repair** & 1 & 0.25 & 2 & — & 0.5 & 0\%, 0.5\% \& 1\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mix proportions of substrate and repair layers.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type of Fiber** & \begin{tabular}{c} **Diameter** \\ **[mm]** \\ \end{tabular} & \begin{tabular}{c} **Cut Length** \\ **[mm]** \\ \end{tabular} & \begin{tabular}{c} **Tensile Strength** \\ **[MPa]** \\ \end{tabular} & \
\begin{tabular}{c} **Young’s Modulus** \\ **(GPa)** \\ \end{tabular} &
\begin{tabular}{c} **Specific** \\ **Gravity** \\ \end{tabular} \\ \hline
8 mm PVA & 0.04 & 8 & 1600 & 40 & 1.3 \\ \hline
12 mm PVA & 0.10 & 12 & 1100 & 28 & 1.3 \\ \hline
13 mm Steel & 0.20 & 13 & 2750 & 210 & 7.85 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Technical information about Poly-Vinyl-Alcohol (PVA) and steel fibers adopted.