.Analysis of cracking causes of plastering gypsum layer

1. Reason analysis of plastering gypsum raw materials

a) Unqualified building plaster

Building gypsum contains high content of dihydrate gypsum, which leads to faster bonding of plastering gypsum. In order to make plastering gypsum have a proper opening time, more retarder should be added to make the situation worse; soluble anhydrous gypsum in building gypsum AIII High content, AIII expansion is stronger than β-hemihydrate gypsum in the later stage, and the volume change of plastering gypsum is uneven during the curing process, causing expansive cracking; the content of curable β-hemihydrate gypsum in building gypsum is low, and even the total amount of calcium sulfate is low ;Building gypsum is derived from chemical gypsum, the fineness is small, and there are many powders above 400 mesh; the particle size of building gypsum is single and there is no gradation.

b) Substandard additives

It is not within the most active pH range of the retarder; the gel efficiency of the retarder is low, the amount of use is large, the strength of the plastering gypsum is greatly reduced, the interval between the initial setting time and the final setting time is long; the water retention rate of cellulose ether is low , water loss is fast; cellulose ether dissolves slowly, not suitable for mechanical spraying construction.

Solution:

a) Select qualified and stable building gypsum, the initial setting time is more than 3min, and the flexural strength is more than 3MPa.

b) Choose cellulose ether with small particle size and excellent water retention capacity.

c) Select a retarder that has little effect on the setting of the plastering gypsum.

2. Reason analysis of construction personnel

a) The project contractor recruits operators without construction experience and does not carry out systematic induction training. The construction workers have not mastered the basic characteristics and construction essentials of plastering gypsum, and cannot operate according to the construction regulations.

b) The technical management and quality management of the engineering contracting unit are weak, there are no management personnel at the construction site, and the non-compliant operations of the workers cannot be corrected in time;

c) Most of the existing plastering and gypsum plastering works are in the form of cleaning work, focusing on quantity and ignoring quality.

Solution:

a) Plastering project contractors strengthen on-the-job training and conduct technical disclosure before construction.

b) Strengthen construction site management.

3. Reason analysis of plastering plaster

a) The final strength of plastering gypsum is low and cannot resist the shrinkage stress caused by water loss; the low strength of plastering gypsum is due to unqualified raw materials or unreasonable formula.

b) The sagging resistance of the plastering gypsum is unqualified, and the plastering gypsum accumulates at the bottom, and the thickness is large, causing transverse cracks.

c) The mixing time of the plastering gypsum mortar is short, resulting in uneven mixing of the mortar, low strength, shrinkage and uneven expansion of the plastering gypsum layer

d) The plastering gypsum mortar that has been initially set can be used again after adding water.

Solution:

a) Use qualified plastering gypsum, which meets the requirements of GB/T28627-2012.

b) Use matching mixing equipment to ensure that plastering gypsum and water are evenly mixed.

c) It is forbidden to add water to the mortar that has been initially set, and then use it again

4. Cause analysis of base material

a) At present, new wall materials are used in the masonry of prefabricated buildings, and their drying shrinkage coefficient is relatively large. When the age of the blocks is insufficient, or the moisture content of the blocks is too high, etc., after a period of drying, cracks will appear on the wall due to water loss and shrinkage, and the plastering layer will also crack.

b) The junction between the frame structure concrete member and the wall material is where two different materials meet, and their linear expansion coefficients are different. When the temperature changes, the deformation of the two materials is not synchronized, and separate cracks will appear. Common wall columns Vertical cracks between the beams and horizontal cracks at the bottom of the beam.

c) Use aluminum formwork to pour concrete on site. The surface of the concrete is smooth and poorly bonded to the plastering plaster layer. The plastering plaster layer is easily detached from the base layer, resulting in cracks.

d) The base material and the plastering gypsum have a large difference in strength grade, and under the joint action of drying shrinkage and temperature change, the expansion and contraction are inconsistent, especially when the base-level light wall material has low density and low strength, the plastering gypsum layer often produces ice. Stretch cracking, even a large area of hollowing. e) The base layer has a high water absorption rate and a fast water absorption speed.

Solution:

a) The freshly plastered concrete base should be dry for 10 days in summer and more than 20 days in winter under the condition of good ventilation. The surface is smooth and the base absorbs water quickly. Interface agent should be applied;

b) Reinforcing materials such as grid cloth are used at the junction of walls of different materials

c) Lightweight wall materials should be fully maintained.

5. Reason analysis of construction process

a) The base layer is too dry without proper wetting or application of interface agent. The plastering gypsum is in contact with the base layer, the moisture in the plastering gypsum is quickly absorbed, the water is lost, and the volume of the plastering gypsum layer shrinks, causing cracks, affecting the increase in strength and reducing the bonding force.

b) The construction quality of the base is poor, and the local plastering gypsum layer is too thick. If plastering plaster is applied at one time, the mortar will fall and form horizontal cracks.

c) Hydroelectric slotting has not been properly handled. Hydropower slots are not filled with caulking gypsum or fine stone concrete with expansion agent, resulting in shrinkage cracking, which leads to cracking of the plastering gypsum layer.

d) There is no special treatment for the punching ribs, and the plastering gypsum layer constructed in a large area cracks at the punching ribs.

Solution:

a) Use high-quality interface agent to treat the base layer with low strength and fast water absorption.

b) The thickness of the plastering gypsum layer is relatively large, exceeding 50mm, and it must be scraped in stages.

c) Execute the construction process and strengthen the quality management of the construction site.

6. Cause analysis of construction environment

a) The weather is dry and hot.

b) High wind speed

c) At the turn of spring and summer, the temperature is high and the humidity is low.

Solution:

a) Construction is not allowed when there is a strong wind of level five or above, and construction is not allowed when the ambient temperature is higher than 40 ℃.

b) At the turn of spring and summer, adjust the production formula of plastering gypsum.