Galaxy clusters are extremely useful tracers of structure formation. I show the use of clusters to test the cosmological model in two different tests using a unique spectroscopic data set with more than about 300 bound galaxies per cluster for about 8 (at the 0.2 <~ z <~ 0.5 redshift range) of the 20 X-ray selected CLASH clusters. First, I will provide a new observational test for a key prediction of the LCDM cosmological model: the contributions of mergers with different halo-to-main-cluster mass ratios to cluster-sized halo growth. For mass ratios between ~0.2 and ~0.7, we found a ~1 sigma agreement with LCDM expectations based on the Millennium simulations I and II. Second, I estimate the clusters concentration-mass relation, which is another key test for the cosmological model. Finally, I compare our results for the relation to the theoretical expected ones.