Ativar no Apache :

  # a2enmod proxy

No VirtualHost:

     ProxyRequests Off
     ProxyPass /sistema http://localhost:8081/sistema
     ProxyPassReverse /sistema http://localhost:8081/sistema
 
     <Location "/sistema">
         Order allow,deny
         Allow from all
     </Location>

Acrescentar no arquivo /etc/tomcat6/server.xml

     <Connector port="8081" protocol="HTTP/1.1" 
              connectionTimeout="20000" 
              URIEncoding="UTF-8"
              redirectPort="8443"
              proxyName="server.tic.ufrj.br"
              proxyPort="80" />