You have to try like this.
string pDetails = string.Empty;
foreach (Control c in gpPaymntdetl.Controls)
{
if (c.GetType().ToString().Equals("Sys.Windows.Forms.RadioButton"))
{
if(((RadioButton)c).Checked == true)
{
pDetails += Convert.ToInt16(((RadioButton)c).Name);
break;
}
}
}
objBll.BltPayDetails = pDetails.ToString();