how to readonly true textbox inside gridview using jquery in asp.net

<asp:GridView ID="gvFeeStructure" CssClass="mGrid" runat="server"
AutoGenerateColumns="False" onrowdatabound="gvFeeStructure_RowDataBound" Width="450px" >
<Columns>
<asp:TemplateField HeaderText="Ins. No">
<ItemTemplate>
<asp:Label ID="lblInsNo" runat="server" Text='<%# Eval("insNo") %>'></asp:Label>
</ItemTemplate>

</asp:TemplateField>
<asp:TemplateField HeaderText="Ins.&nbsp;Date">

<ItemTemplate>
<asp:Label Id="lblInsDate" runat="server" Text='<%# Eval("insPaidDate","{0:dd-MMM-yyyy}") %>' ></asp:Label>
<asp:TextBox ID="txtInsDate" runat="server" Text='<%# Eval("insPaidDate","{0:dd-MMM-yyyy}") %>'
Width="80px" CssClass="fundtext readonly" ></asp:TextBox>
<asp:ImageButton ID="imgbtnDatePicker" CssClass="datepickericon" AlternateText="Select Date" ToolTip="Select Date" runat="server" ImageUrl="~/Images/calendar_icon_small.png" />
<asp:CalendarExtender ID="calendarAdmissionDate" runat="server" CssClass="calendarstyle" DefaultView="Months" Format="dd-MMM-yyyy" TargetControlID="txtInsDate" PopupButtonID="imgbtnDatePicker">
</asp:CalendarExtender>

<asp:RequiredFieldValidator ID="reqInsDate" ValidationGroup="date" CssClass="error" ControlToValidate="txtInsDate" runat="server">*</asp:RequiredFieldValidator>
<%-- <asp:CompareValidator id="cvDate" runat="server" cultureinvariantvalues="true" display="none" enableclientscript="true"
ControlToValidate="txtInsDate" ErrorMessage="installment date must be greater than current date" type="Date" setfocusonerror="true" Operator="GreaterThanEqual" text="installment date must be greater than current date" />--%>
<asp:HiddenField ID="hfId" runat="server" Value='<%# Eval("srno") %>' />
</ItemTemplate>
<ItemStyle Width="150px" Font-Bold="True" ForeColor="Black" />
</asp:TemplateField>
<asp:TemplateField HeaderText="Ins.&nbsp;Amount">
<ItemTemplate>
<asp:Label Id="lblInsAmount" runat="server" Text='<%# Eval("InsAmount") %>' ></asp:Label>
</ItemTemplate>
</asp:TemplateField>

<asp:TemplateField HeaderText="Paid&nbsp;Amount">
<ItemTemplate>
<asp:Label Id="lblInspaidAmount" runat="server" Text='<%# Eval("insPaidAmount") %>' ></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Balance">
<ItemTemplate>
<asp:Label Id="lblBalance" runat="server" Text='<%# Eval("balance") %>' ></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
<EmptyDataTemplate>
Sorry no record exist.
</EmptyDataTemplate>
</asp:GridView>

<script src="http://code.jquery.com/jquery-1.8.2.min.js"></script>
<script type="text/javascript">
$(document).ready(function () {
$('.readonly').change(function () {
$(this).attr('readonly', true);
});

});



</script>