This paper tackles the problem of autonomic admission control for web clusters. The main contribution of this work is the proposal of a new session admission algorithm that self-configures a dynamic constraint on the rate of incoming new sessions to guarantee the respect of Service Level Agreements (SLA). Unlike other approaches, our policy does not need any prior information on the incoming traffic, nor any assumption on the probability distribution of request inter-arrival or service time. Furthermore, it does not require any manual configuration or parameter tuning. We performed extensive simulations under a range of operating conditions and compared our algorithm to other previously proposed approaches. The simulations show that our policy rapidly adapts to the given traffic profile and improves service throughput while respecting the response time constraints imposed by the SLAs. It also improves service quality by reducing the oscillations of response time and number of active clients common to other policies.