$datS =  get-date -Format "yyyyMMdd"
$datF =  get-date -Format "yyyyMMddHHmm"

$backupDir = "C:\temp\$($datS)"

if(-not $(Test-Path $backupDir)) {
    mkdir $backupDir
}

$waUrl = "http://contoso"

$sites = Get-SPSite -Limit ALL -WebApplication $waUrl

foreach($site in $sites){
    $urlStr = $site.RootWeb.ServerRelativeUrl.Replace("/", "_")
    $fname = "$($backupDir)\$($datF)$($urlStr).bak"

    backup-spsite -Identity $site.Url -Path $fname -Verbose -NoSiteLock -Confirm:$false
}