Wordpress Post thumbnail URL

<div class="row">
          <ul>
            <?php

$args = array( 'post_type' => 'infrastructure');
$loop = new WP_Query( $args );
while ( $loop->have_posts() ) : $loop->the_post();
$postid = get_the_ID();
$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'thumbnail_size' );
$url = $thumb['0'];
?>
            <li class="col-lg-4 col-md-4 col-xs-6 col-sm-4">
              <div class="image-block image_size_230 portfolio-hover"> <?php echo the_post_thumbnail();?>
                <div class="pcontent">
                  <div class="hover-content">
                    <div class="view-large"> <a href="<?php echo $url; ?>" class="swipebox" title="<?php the_title();?>"><i class="fa fa-search-plus"></i> </a></div>
                  </div>
                </div>
              </div>
            </li>
            <?php endwhile;?>
          </ul>
        </div>