Concrete block wall and Portland cement-based plaster or stucco are very complementary since they are both made from the same primary ingredients. Therefore, if properly done, a concrete block wall can be a great base framework for the stucco.
A concrete block must have an open or rough texture to apply stucco. This opening or coarseness allows the stucco to interlock with the exterior of the concrete block providing a mechanical key bond.
A mechanical bond refers to the entanglement in space among multiple component parts. It means that there can be no separation without distorting or breaking chemical bonds between atoms.
Some concrete blocks have a smooth surface texture. This type of concrete block has an inadequate mechanical bond for the stucco to clutch. But if the suction bond can be developed, it can still be considered a great base framework for stucco installation.
A suction bond refers to the permanent bond created by the absorption of another agent’s element. For example, if you are laying bricks and those bricks, due to their porous characteristic, absorb water quickly. It is this suction that absorbs the mortar into a brick. Therefore, creating a permanent bond between them. Likewise, in the case of stucco and concrete block wall, suction bond refers to the absorption of water and Portland cement from the mortar into the surface of the block. This occurrence caused a bonding effect at the interface of the stucco and the block wall.
In a block wall that is to receive the stucco, the intersections are cut flush with the block. In order to prevent the reduction of mechanical keying needed for a good bond, sponging or floating of mortar joints is considered a prohibited practice. As defined in ACI 530 Chapter 3, the walls must be aligned properly to ward off large variations in stucco thickness and plane tolerances. The should be tidy and free from any form oil, efflorescence, mortar drippings, loose block, roof tar, etc. The wall must be checked for suitability before plastering. You should address cracks or voids in the mortar joints before applying the stucco.
Before applying the scratch coat, determine the bond sufficiency. You should not process if the bond is questionable. You may consider the other options available:
- Dash‐Bond Coat – this provides a mechanical bond for succeeding plaster. It is composed of a thick slurry of Portland cement and fine sands with enough water.
- Bonding Agents – since there are several variations of this material in the market, it is important to identify the right bonding agent to be used. Some are integrally combined into the stucco mortar while some are surface applied to the block prior to the application of the scratch coat.
- Metal Plaster Base Assembly – for this application, it is recommended to have a paper backed, self‐furring metal lath properly anchored to the wall. The reason for this is for the paper is to ensure that there is a uniform un-bonded condition and that the stucco is not sticking to the block which causes cracking.
One of the greatest benefits of using block for a base is that it just needs to be connected at the control joints in the base wall with the correctly bonded stucco.
If a metal plaster base is applied over a block wall, the same thing must be applied with frame construction.
Differences in the bases will cause the stucco to crack if walls where block construction adjoins another type of construction. As a precaution, it is recommended to have the control joint at the juncture of the block and frame walls. In this way, the crack will be hidden and pre-aligned.
The stucco mix refers to the properties of the aggregates and materials available in each location. The most important characteristic of a stucco mix is its utility. Adhesiveness is important in order for it to stick on the trowel from the hawk to the wall. Plasticity is also important to make troweling easy. But above all, the mix should bond properly with the block.
A minimum of five minutes of mixing time after adding the final ingredient is sufficient to achieve a proper bond.
Coats and Thickness
For block wall construction, provisions for either a two‐coat or three‐coat application was contained in ASTM C 926, “Installation of Portland Cement-Based Plaster”.
To ensure mechanical key and section bond, stucco should be applied with adequate pressure to push the material into the surface of the block. It’s not enough to just spread it on the wall. Also, in order to achieve the preferred thickness of the coat, sufficient substance should be applied. For the scratch coat, sufficient thickness is very important because it is prone to bond failure and “dry out”.
Curing is important in any stucco work. There are no definite codes and standards regarding proper curing. Generally, there must be “sufficient moisture and time between coats”. In most cases, moist curing like a fog mist applied several times a day for the first two or three days should be sufficient. It is not allowed for the stucco to dry out during this stage.