Upto now, the knowledge building influence of the fundamental communicative functions during an on-line collaborative learning (OLCL) session, i.e. argumentative, responsive, elicitative, informative and imperative have been mainly based on results from qualitative studies, results that could have been strengthened by quantitative approaches. Starting from a literature review, we formulate a dual quantitative model of an on-line collaboration knowledge building (OCKB) that described these communicative functions, and aim to validate this model in a computer science topic related OCKB with a total of n=44 participants. Corpuses are collected for manual dialog act coding and communicative function variable calculations. A regression analysis failed to provide for the hypothesized model on seven of the eight tests on the basis of quantitative data. Findings suggest the imperative communication function best explain the assessment results statistically alone and in some significance in combination with some of the other communicative functions.