Set time interval between loops in tcpreplay

I need to replay packets at good speeds, but also I need to take a break about 20 seconds before the next round

tcpreplay -i eth1 --pktlen -p3200 -l10 Sample.pcap

simple little shell script:

#!/bin/bash
i=0
while [ $i -lt 10 ]; do
    tcpreplay -i eth1 --pktlen -p 3200 Sample.pcap
    i=$(($i + 1))
    sleep 20
done