system.xml fields are not saving in magento

Hi today i'm working on system.xml file. it's not saving correctly.

system.xml

<custom_field translate="label">
     <label>Add Class and it's Params</label>
     <frontend_model>hello/adminhtml_form_field_addclass</frontend_model>
     <backend_model>hello/system_config_backend_addclass</backend_model>
     <sort_order>6</sort_order>
     <show_in_default>1</show_in_default>
     <show_in_website>0</show_in_website>
     <show_in_store>0</show_in_store>
</custom_field>

Test\Block\Adminhtml\Form\Field\AddClass.php

<?php

class Test_Hello_Block_Adminhtml_Form_Field_AddClass extends Mage_Adminhtml_Block_System_Config_Form_Field_Array_Abstract
{

    protected function _prepareToRender()
    {
        $this->addColumn('add_param', array(
            'label' => Mage::helper('hello')->__('Add Parameter'),
        ));
        $this->addColumn('add_param_class', array(
            'label' => Mage::helper('hello')->__('Add Parameter Class'),
            'style' => 'width:100px',
        ));
        $this->addColumn('add_param_class_field', array(
            'label' => Mage::helper('hello')->__('Add Parameter Class Fields'),
            'style' => 'width:100px',
        ));
        $this->_addAfter = false;
        $this->_addButtonLabel = Mage::helper('hello')->__('Add Class');
    }

}

Test\Model\System\Config\Backend\AddClass.php

<?php


/**
 * Backend for serialized array data
 *
 */
class Test_Hello_Model_System_Config_Backend_AddClass extends Mage_Core_Model_Config_Data
{
    /**
     * Process data after load
     */
    protected function _afterLoad()
    {
        $value = $this->getValue();
        Mage::log($value,null,'AddClass');
        //$this->setValue($value);
    }

    /**
     * Prepare data before save
     */
    protected function _beforeSave()
    {
        $value = $this->getValue();
        //$this->setValue($value);
        Mage::log($value,null,'AddClass');

    }
}

it's saving core_config_data table like below formate.

path = customtab/custom_attributes/custom_field

value = Array,Array,Array,

Logs are created like below formate

2015-08-10T10:42:55+00:00 DEBUG (7): Array,
2015-08-10T10:58:26+00:00 DEBUG (7): Array
(
    [_1439204268814_814] => Array
        (
            [add_param] => acceptNewClass
            [add_param_class] => AcceptNewClassFields
            [add_param_class_field] => ID,PrimariContact
        )

    [_1439204284271_271] => Array
        (
            [add_param] => TokenID
            [add_param_class] => 
            [add_param_class_field] => 
        )

    [_1439204298185_185] => Array
        (
            [add_param] => memberID
            [add_param_class] => 
            [add_param_class_field] => 
        )

    [__empty] => 
)

how to save & retrieve this?

Any help on this

Thanks.