Application and Common Problems of Ready-Mixed Plastering Mortar in Construction Engineering

Since the promotion and application of ready-mixed mortar in construction engineering, significant progress has been made. Ready-mixed mortar is gradually being accepted and adopted by the construction industry in our city. However, some quality issues and disputes have also emerged in the engineering application of ready-mixed mortar, mainly concentrated in plastering mortar, especially on concrete shear walls, where the most problems occur. As a new material, ready-mixed mortar inevitably requires a process for people to become familiar with and master it. This paper discusses the common problems in the construction of ready-mixed plastering mortar to seek solutions and promote its further application.

I. Changes in Construction Requirements Brought by Ready-Mixed Plastering Mortar

(1) Higher Requirements for Plasterers' Ability to Adapt to Changes in Mortar Performance

When using on-site mixed mortar, if workers found the workability unsatisfactory, they could adjust the mortar mix or enhance mixing to meet their habits. For example, if the mortar was too sticky, they could add more plasticizer or extend mixing time; if it lacked cohesion and sagged excessively, adding more cement would solve the problem. After long-term use of on-site mixed mortar, workers have developed proficient handling techniques and construction habits, adjusting mortar performance on-site to suit their preferences.

The advantage of this method is the quick, simple, and targeted adjustment of workability. However, the major drawback is the arbitrary modification of the mortar mix, leading to large fluctuations in quality, which may fail to meet engineering requirements and fail to effectively guarantee project quality.

With the adoption of ready-mixed mortar, the mortar is factory-produced, and its performance cannot be altered on-site by adjusting the mix. This requires plasterers to possess higher skills, adapt their working methods to the mortar's characteristics, and strictly follow construction specifications to ensure quality.

(2) New Changes in Plastering Technology and Interface Treatment Due to Increased Mortar Strength

The use of lower-strength mortar can improve plastering efficiency and reduce hollowing and cracking. Since relevant standards do not specify strength requirements for plastering mortar, low-strength mortar was often mixed on-site. A considerable number of projects used plastering mortar that failed to meet strength requirements, resulting in a powdery, easily removable plaster layer that could not satisfy decoration standards. This problem was particularly evident in winter due to slow strength gain, leading to numerous customer complaints.

Ready-mixed plastering mortar specifies mortar strength, resulting in higher supplied strength. With increased strength, the stiffness of the plaster layer rises, increasing the tensile stress exerted on the interface treatment layer. This necessitates a corresponding improvement in the quality of base surface treatment and strict adherence to plastering specifications to ensure construction quality.

II. Analysis of Common Problems and Causes in Ready-Mixed Plastering Mortar Construction

(1) Cracking and Hollowing Due to Uneven Base Wall Without Leveling

If the thickness of the mortar layer applied in one pass is uneven, the thicker areas dry slower while the thinner areas dry faster. As the mortar dries, it hardens and shrinks. Areas that dry faster harden and shrink earlier, while slower-drying areas harden and shrink later, creating internal stress within the mortar layer. When the thickness deviation is significant, this inconsistent deformation can cause structural damage, resulting in hollowing or cracking.

Localized cracking often occurs where the layer is locally thick and mostly thin, especially with a steep thickness gradient. Hollowing, where the mortar layer detaches from the base, frequently happens at the edges of door/window openings or wall corners where the layer is locally thin, the majority is thick, and the bond strength between the mortar and substrate is weak.

Therefore, if the plastering base is uneven, it must be leveled first. After leveling, allow it to cure for 5 to 7 days before applying the plastering mortar.

(2) Cracking Caused by Sagging of the Mortar Layer

If the plaster layer remains fluid and does not stiffen for an extended period, it may sag or slide under its own weight, leading to cracking. This mainly occurs when the base surface has low water absorption, the single plaster thickness is excessive, or the mortar is exposed to rain before setting. It is particularly common in winter with low temperatures.

To prevent this, strictly follow technical specifications: limit the single plaster thickness, ensure sufficient time intervals between layers in multi-layer plastering, avoid plastering on a wet base surface, appropriately reduce the water content in the mortar, protect fresh plaster from rain, and enhance ventilation after plastering in cold weather.

(3) Cracking Due to Rapid Water Loss of the Mortar Layer

During stiffening, drying, setting, and hardening, the mortar layer undergoes shrinkage. Shrinkage during stiffening and drying is called early-age shrinkage, while further shrinkage after setting due to continued hydration is called later-age shrinkage. Early-age shrinkage is significantly greater than later-age shrinkage, typically around 1%.

The shrinkage of masonry walls is generally within 0.3%, mainly concentrated in mortar joints, while concrete walls shrink by approximately 0.05%. The large difference in shrinkage between the plaster layer and the base material, combined with the much higher stiffness of the base, easily causes cracking in the plaster layer—one of the main reasons why complete avoidance of cracking is difficult. The faster the mortar layer loses water and dries, the more severe the shrinkage and the higher the risk of cracking.

This frequently occurs in summer with high temperatures or strong winds. To prevent rapid water loss cracking, adjust summer construction hours, protect the finished plaster layer from direct sunlight, and implement proper curing measures.

(4) Cracking Due to Different Water Absorption Rates of the Base Material

Variations in water absorption rates of the base material cause differences in the stiffening, hardening, and shrinkage rates of the mortar layer, making cracks prone to appear at the junctions of different base materials. For example, at the interface between masonry walls and concrete beams/columns, concrete has a much lower water absorption rate than shale sintered bricks or aerated concrete blocks, making these areas particularly susceptible to cracking.

To avoid this, the main construction measure is to overlay a layer of steel wire mesh or fiberglass mesh across the junction of different substrates. Apply a thin layer of mortar first, then proceed with the main plastering.

(5) Cracking Due to Structural Changes in the Plastering Base

Depending on restraint conditions, mortar shrinkage is divided into free shrinkage and restrained shrinkage. Free shrinkage is greater than restrained shrinkage; under sufficient restraint, a mortar element may not shrink at all. This is why adding fibers to mortar or embedding steel wire mesh in the layer can reduce or prevent cracking.

For the plaster layer on a base surface, the inner surface bonded to the base experiences restrained shrinkage. As it attempts to shrink, the base pulls against it, resulting in less shrinkage than the outer surface exposed to air. Structural changes in the base lead to inconsistent restraints on mortar deformation, causing differential shrinkage across the plaster layer.

For instance, the edges of door/window openings are under three-dimensional restraint, while the central area is under four-dimensional restraint; the same applies to wall and beam/column corners. Due to the differential deformation at these locations, cracking is common. Therefore, an additional layer of steel wire mesh or fiberglass mesh should be installed around door/window openings to prevent cracking.

(6) Hollowing and Delamination Due to Insufficient Strength of the Interface Treatment Layer

When the base surface is smooth and the plastering mortar has difficulty bonding, interface treatment (e.g., roughening) is required to ensure a firm bond. This layer is known as the interface treatment layer.

After the mortar sets, continued hydration and hardening in the later stage are accompanied by shrinkage. This later-age shrinkage creates tensile stress in the interface treatment layer. The thicker the plaster layer, the higher the mortar strength, and the finer the sand, the greater the tensile stress exerted on the interface layer.

If the bond strength between the interface treatment layer and the base is insufficient, or the interface layer itself is weak, the later-age shrinkage of the plaster layer can cause the interface layer to debond from the base or be torn by the plaster, resulting in hollowing and delamination. This problem often manifests without initial issues but becomes increasingly severe after about half a month as time progresses.

To prevent this, first, improve the bond strength between the interface treatment layer and the base, as well as the strength of the interface layer itself, to ensure a firm bond. Second, avoid excessive single-pass thickness; prohibit one-time plastering to the full thickness, and maintain proper time intervals between layers. For plastering thickness exceeding 20mm, a multi-layer plastering process must be adopted.

Ready-mixed mortar is a new material that requires a process of continuous familiarization and mastery. Its application imposes stricter requirements on the quality of concrete interface treatment, the thickness of each plastering layer, and the adoption of a multi-layer process for over-thick plastering. Many current construction problems can be avoided by strictly following the existing mortar construction procedures.

Common Problems in the Construction of Ready-Mixed Plaster Mortar and Corresponding Process Measures