Silence IS an admission in Civil cases as well as criminal matters. It depends on the circumstances surrounding the silence when silence is followed in response to an accusation. Silence can be an admission by conduct: i.e. silence, flight etc.
A defendant can assert his 5th amendment rights in a civil case just as he can in a criminal case. He does not admit liability or negligence by doing so.
The difference between taking the 5th in a civil case and a criminal case is that the jury in a civil case may infer negligence from the defendant's silence. The jury in a criminal case may not consider the defendant's silence for any reason and may not infer guilt.