ACC block stands for autoclaved cellular concrete block. The ACC block is a valuable addition to the masonry units accessible to masonry contractors.

 Advantages of cellular concrete block masonry over brick masonry are its

• lightweight design,
• minimal mortar use, and
• faster working speeds

Many contractors are unfamiliar with the usage of autoclaved cellular concrete blocks. This procedure will assist them in understanding the important constructional needs and specifications for this form of masonry.

Use of Mortar in Masonry

Mortar shall not be spread so much ahead of the actual laying of the units that it tends to stiffen and loose its plasticity, thereby resulting in poor bond.

For most of the work the joints, both horizontal and vertical, shall be 10 mm thick.

The mortar joints shall be struck off flush with wall surface. Mortar is compressed with a rounded or U-shaped tool (jointer) when it starts to stiffen.

This compaction is important, since mortar, while hardening, tends to shrink slightly and thus pull away from the edges of the block. The mortar shall be pressed against the units with a jointing tool after the mortar has stiffened. This result in intimate contact between the mortar and the masonry unit and obtain a water-tight joint.

 

Procedure for Laying AAC Block Masonry

A. Laying The First Course

The first course of cellular concrete block masonry shall be laid with greater care, making sure that it is properly

• aligned,
• levelled, and
• plumbed,

as this may assist the mason in laying succeeding courses to obtain a straight and truly vertical wall.

The first layer of cellular concrete block masonry on plinth should preferably have groove/offset outside so that rainwater coming down the wall falls out.

B. Preparatory work before laying the first course

1. Before laying the first course, the alignment of the wall shall be marked on the damp-proof course.
2. The blocks for this course shall first be laid without mortar along a string stretched between properly located corners of the wall. This helps to determine the correct position of the blocks. It also includes those of the cross walls jointing it and adjust their spacing.
3. When the blocks are set in proper position, the two corner blocks shall be removed, a mortar bed spread, and these blocks laid back in place truly level and plumb.
4. The string shall then be stretched tightly along the faces of two corner blocks and the faces of the intermediate ones adjusted to coincide with the line.
5. Thereafter, each block shall be removed and relaid over a bed of mortar.

After every three or four blocks have been laid, their correct alignment, level and verticality shall be carefully checked.

6. The construction of walls may be started either at the corners first or started from one end proceeding in the other direction.
7. If the corners of the wall are built first, they shall be built four or five courses higher than the center of the wall.
8. As each course is laid at the corner, it shall be checked for alignment and level and for being plumb.
9. Each block shall be carefully checked with a level or straight-edge to make certain that the faces of the blocks are all in the same plane. This precaution is necessary to ensure truly straight and vertical walls.

The use of a storey pole which is simply a board with markings 200 mm apart, provides an accurate method of finding the top of the masonry for each course.

10. Each course, in building the corners, shall be stepped back by a half-block. The horizontal spacing of the block shall be checked by placing a mason’s level diagonally across the corners of the blocks.
11. When filling in the wall between the corners, a mason’s line shall be stretched from corner to corner for each course and the top outside edge of each block shall be laid to this line.

The manner of handling or gripping the block shall be such as to position the block properly with minimum adjustment.

C. Working with mortar

The following things to be considered while working with mortar

• To assure satisfactory bond, mortar shall not be spread too far ahead of actual laying of the block, or it will stiffen and loose its plasticity.
• As each block is laid, excess mortar extruding from the joints shall be cut off with the trowel and thrown back on the mortar board to be reworked into the fresh mortar.
• If the work is progressing rapidly, the extruded mortar cut from the joints may be applied to the vertical face shells of the blocks just laid.
• If there be any delay long enough for the mortar to stiffen on the block, the mortar shall be removed to the mortar board and reworked.
• Dead mortar that has been picked up from the scaffold or from the floor shall not be used.

D. Closure Block

The closure block is nothing, but the last or final block placed in any course.

12. When installing the closure block, all edges of the opening and all four edges of the closure block shall be buttered with mortar.

The closure block shall be carefully lowered into place. If any mortar falls leaving an open joint, the closure block shall be removed, fresh mortar