Prefabricated concrete components may develop cracks, but the severity and causes vary depending on the specific area. Understanding the different reasons behind these cracks can help avoid unnecessary sealing in prefabricated projects.
Cracks Caused by Production
During the manufacturing process at the prefabrication plant, cracks may form due to issues such as template leakage, improper vibration, leakage, or inadequate quality control. Common defects after production include missing or uneven edges and corners, warping, uneven surfaces, protruding ribs, rough textures, peeling, sanding, contamination, honeycombing, and holes.
Although quality inspection systems are in place, components that don’t meet standards may still develop cracks later due to environmental factors or other external issues after leaving the factory.
Large-span prefabricated panels are particularly vulnerable during transportation and lifting. Compression between panels or excessive deflection during handling can cause cracks that may extend throughout the panel, leading to significant damage. Therefore, special care is required when producing and transporting large-span components, which is a critical consideration for construction teams during material customization.
Cracks Caused by Installation
Cracks may also appear during installation, with causes and solutions varying depending on the location. Below is a detailed analysis of cracks in floor slabs, roofs, and walls.
Floor Area
There are multiple reasons for cracks in floor areas. Insufficient or uneven grouting before installing floor slabs, or failing to lay mortar at the slab supports, reduces the constraint force and causes instability. This can lead to diagonal shaking or twisting during construction, accelerating longitudinal cracking of the slabs.
Additionally, failing to adjust joints after slab installation can result in joints that are too narrow or blind, which diminishes their ability to transmit forces effectively. Floating ash on slab edges can weaken the bond between the concrete and slab, reducing adhesion strength.
Roof Panel Area
1. Direct Causes
- Poor installation and improper overlapping of roof tiles on reinforced concrete lightweight roofs.
- Loose assembly or mortar filling between joints of hollow slabs and large roof panels.
- Displacement of roof panels due to various factors, leading to cracks.
- Poor extension or adhesion of waterproof coatings and joint materials, causing tearing or detachment.
- Construction defects such as bubbles, delamination, or damage to waterproof coatings and weak bonding of joint materials.
- Aging and cracking of waterproof coatings and joint materials.
2. Indirect Causes
- Lack of appropriate measures for concealed pipe laying in board joints. Insufficient joint width and uneven conduit thickness can cause the conduit to become stuck, resulting in ineffective jointing.
- Following up sealing with subsequent processes too quickly without proper technical intervals. Filling joints immediately after floor slab installation and then adding upper structures can cause slab deflection and deformation before the joint concrete gains sufficient strength.
- Excessive localized loading during construction. Overconcentration of materials on floor slabs beyond their bearing capacity can cause excessive deformation and hidden dangers in slab joints.
Wall Area
- Neglecting proper curing during joint filling. If the joints are not moistened beforehand and the mortar dries too quickly, the joint strength will be compromised, weakening force transmission.
- Improper removal of top supports on cantilever beams. Formwork should only be removed once the concrete has reached full design strength.
- Poor bonding between surface layers and floor slabs, resulting in low stiffness of prefabricated slabs.
Cracks Caused by Material Issues
Variations in cementitious materials and fine aggregates used in prefabricated components can increase cracking risks. Due to the scarcity of natural sand, alternatives like fine sand, mountain sand, and manufactured sand are increasingly used. However, these materials have stricter property requirements, and concrete produced with them is more prone to cracking.
Recommended Solutions
- Enhance quality inspection of floor slabs: Ensure all slabs have component qualification certificates before arriving on site. Conduct spot checks for deviations, surface quality, and concrete density. Remove slabs with significant thickness differences or visible cracks. Prestressed slabs should never be installed with joints.
- Ensure grout quality: When installing threshold concrete floor slabs, grout should be applied evenly, with a typical thickness of 10–15 mm. Supports must be flat, and slab bottom heights consistent. Ideally, slabs should be lifted into place in one go. If not, the floor must be leveled after installation, and grout issues addressed after joint adjustments.
- Proper gap filling: Joints in prefabricated floor slabs should only be filled after the previous floor’s main structure is complete. This prevents deformation and cracking caused by material transport and stacking. Roof panel joints should also be allowed to gain sufficient strength before constructing upper structures.
- Preparation before jointing: Clean joints thoroughly, removing debris such as wood and brick slag. Rinse board joints with clean water, and use hard, non-deformable materials as bottom molds.
- Ensure joint filling and concrete curing quality: Properly cure the concrete to enhance strength and durability.
- Strictly control construction load on floors: Distribute materials evenly in batches to avoid excessive load concentration. When load reduction is not possible, add temporary supports to prefabricated panels.
Article source: Construction Technology, China Construction Technology Information Platform
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