WordPress: Bootstrap grid

<?php

                $columns_num = 2;
                echo '<div class="row">';

                while (have_posts()) : the_post();
                    echo '<div class="col-sm-12 col-md-6 col-lg-' . 12 / $columns_num . ' posts">';
                    echo '  <div class="thumb-wrapper">';
                    echo '    <div class="data">'.get_the_time('d/m/Y').'</div>';
                    echo '    <a href="'.get_permalink().'">' . get_the_post_thumbnail( get_the_ID(), 'slides-size', array( 'class' => 'post-thumb', 'title' => get_the_title(get_the_ID()) ) ) . '</a>';
                    echo '  </div>';
                    echo '  <div class="excerpt-wrapper">';
                    echo '    <h2><a href="'.get_permalink().'">' . get_the_title() . '</a></h2>';
                    echo      get_the_excerpt();
                    echo '  </div>';
                    echo '</div>';

                endwhile;

                echo '</div>';
            ?>