The quality of plastering construction has always been an important concern for project delivery and house inspection by owners. In particular, the hollowing and cracking of the plaster layer ranks first among the owners’ quality complaints. How to control the risk of hollowing and cracking of the plaster layer is an important task for our engineers.

Ⅰ. Causes of hollowing

1. Inadequate control of raw materials

There is too much stone powder in the machine-made sand in the mortar, which is too sticky and inconvenient to operate. After the plastering is completed, it is easy to cause cracking and hollowing of the wall; for the convenience of operation, the on-site workers privately add admixtures such as mortar king, mortar essence and even washing powder, resulting in insufficient strength and hollowing.

2. The base layer is not properly treated and not watered

There is no special person to supervise and accept the base layer treatment, and the key parts and nodes are not properly treated. Enter the next process with “disease”. There is too much floating slurry on the concrete surface and the interface is not flushed. The wall surface was not watered and moistened 1 day in advance before slurrying, and the base wall was not watered and moistened in advance (2 days, once in the morning and evening) before the wall was plastered.

3. No mesh treatment

Wire mesh was not hung at the junction of different structures and the repair of pipe grooves.
The wire mesh was not hung at the groove of the wire pipe.
Wire mesh was not hung at the junction of different materials.

4. The slurry was not standardized and the maintenance was not in place.

The slurry distribution was uneven and missed. The slurry area did not meet the requirements and the effect was not up to standard. The slurry was not maintained in time after the slurry was applied, resulting in the slurry strength not meeting the standard.

5. The fiberglass mesh was not set as required.

The paving rate of the fiberglass mesh on the plastered wall was less than 100%. And it was not set on the upper part of the plastered surface layer, which did not prevent cracking and hollowing.

6. Maintenance

The maintenance cycle was insufficient, the number of maintenance times per day was too few, and the water loss was too fast, causing hollowing and cracking.

Ⅱ. Preventive measures for hollowing and cracking

(1) Raw material control

Strictly control the quality of mortar entering the site, and conduct on-site inspection and testing in units of vehicles. Mortar with problems must be withdrawn; the stone powder in the machine-made sand in the mortar should be controlled at about 10%, which is conducive to workability and viscosity, and convenient for workers to operate. During the construction process, a dedicated person controls the mortar. Once it is found that workers privately add additives such as mortar king, mortar essence or laundry detergent, they will be severely dealt with.

(2) Process control

1. Base treatment

Screw holes, channel steel holes, and wall connection holes are sealed as required; structural defects are repaired and handled in time.

2. Mesh hanging measures

The junctions and slots of different materials are sealed and meshed. The gap between the flue and the wall is filled fully and densely, and then the wire mesh is fully hung. The overlap width of the glass fiber mesh/wire mesh and the base is ≥150mm on each side. Flat and warp-free, the overlap width between the meshes is ≥100mm.

3. Requirements for slurry shooting

▶ Homemade wire mesh shooting: Cut the 15mm×15mm×1mm wire mesh into a size of about 200mm×2400mm, then fold it into 200mm×300mm, and finally use a DN25 galvanized pipe as a handle to fix the folded wire mesh to the end of the galvanized pipe. After the wire mesh shooting is completed, the slurry shooting effect test is carried out on the wall. After meeting the requirements, the slurry shooting operation is started.
▶ Before slurry shooting, the wall surface should be sprinkled with water one day in advance. The wet standard is preferably 8~10mm infiltrated into the masonry.
▶ Use construction glue, cement and an appropriate amount of fine sand to prepare a slurry in a certain proportion for slurry shooting. Control the slurry consistency, the slurry point has no dripping, the slurry is evenly distributed, the burrs are 3-5mm, and the coverage area reaches more than 95% of the entire wall.
▶ Timely maintenance after final setting, the strength should be light and continuous, and there is a tingling feeling when the hand is placed on it, which can improve the adhesion of the base and effectively reduce hollowing.

4. Anti-cracking of the surface layer

Anti-cracking fiberglass mesh is added to the plaster surface layer and set on the upper part of the surface layer. The surface layer should not be exposed when the slurry is raised and the surface is closed. Do not rub it hard (otherwise, the layer will be delaminated and hollow); when the surface layer is constructed from bottom to top, the upper end should not be closed too hastily. Wait for all the gas behind the surface layer to be discharged (otherwise, bubbles will form and hollows will form).

5. Timely maintenance

After the plastering is completed, arrange personnel to carry out watering and maintenance in time according to the weather conditions. Watering the wall should be done without leaving dead corners. Let the plastering surface fully absorb moisture. Generally, it is carried out after 12 hours at room temperature, no less than 3 times a day (appropriately increase the frequency in summer). The maintenance period is not less than 7 days.
Plaster hollowing and cracking not only affect the quality of use and appearance. Repair is also labor-intensive and time-consuming, requiring a lot of manpower and material resources. Strengthening early planning and process control is the key to solving the problem.