Alonso set the fastest time on Day 3 at the Circuit de Catalunya, completing 97 laps.
"Today we concentrated on a comparison of the compounds, starting with the Soft and moving on to the Hard in the afternoon," said the Spaniard.
"The track was colder than yesterday, especially in the morning, which made it harder to adapt the car and find the right grip level. We tried various aerodynamic configurations and for the first time, we made some set-up changes to get a good understanding of tyre behaviour and degradation."
"With every change we made, the car reacted as I expected and it was useful to try out the different settings, especially to understand which direction we need to go in next week, when there will be a greater emphasis on performance."
Alonso also acknowledges the importance of Pirelli's tyres.
"Once again this year, the tyres will be one of the crucial aspects to manage: they are softer and quicker, but also less consistent and they degrade rapidly with every lap. We will have a lot of work to do to understand how best to balance the car, but that applies to all the teams."
Felipe Massa returns to the helm of the F138 for the final day of running on Friday.