Cracking of Precast Slabs in Rural Houses

Cracking of precast slabs (cast-in-place floor slabs) in rural houses is primarily caused by construction details. Common causes and corresponding measures are as follows:

1. Unsealed Formwork Joints

If the gaps between formwork panels are not sealed with tape or specialized sealing materials, concrete slurry can leak out during pouring, resulting in honeycombing, pitting, and holes on the floor slab surface, reducing local strength. Over time, under stress, these areas can develop through-cracks. Measures: Before construction, strictly inspect formwork joints and seal all gaps with waterproof tape or sealing strips. Conduct a water spray test before pouring to ensure no leakage.

2. Improper Rebar Configuration

1. Non-compliant Rebar Specifications or Spacing: Floor slabs with a span of ≤5 meters should use national standard deformed steel bars with a diameter of 8-10 mm, arranged in a double-layer, bidirectional pattern with a spacing of 15-20 cm. If the rebar diameter is too thin, spacing is too wide, or single-layer arrangement is used, the flexural capacity is insufficient, leading to transverse cracks.
2. Missing Concrete Cover: If the lower-layer rebar is not supported by cement spacers and the upper-layer rebar is deformed by trampling, the rebar may be exposed or displaced. Insufficient concrete cover thickness accelerates rebar corrosion, causing cracks along the rebar. Measures: Arrange rebar strictly according to design drawings, use specialized spacers to control cover thickness, and avoid trampling on the upper-layer rebar during pouring.

3. Missing Radial Rebar

The corners of floor slabs (e.g., room corners) are areas of stress concentration. If radial rebar (≥7 pieces, length ≥1.5 meters) is not installed, the concrete is prone to 45-degree diagonal cracks due to shrinkage stress. Measures: Install radial rebar at the corners and tie it firmly to the main rebar to enhance local crack resistance.

4. Concrete Construction Issues

1. Uncontrolled Mix Proportions: If self-mixed concrete has an excessively high water-cement ratio, too much fine sand, or if water is added arbitrarily during construction, the concrete shrinkage rate increases, raising the risk of dry shrinkage cracks. Measures: Use commercial concrete or strictly follow mix proportions for stirring. Prohibit adding water during pouring.
2. Improper Formwork Handling: If the formwork is not cleaned properly or not pre-wetted, the dry formwork absorbs moisture from the concrete, leading to insufficient surface strength and cracking. Measures: Clean the formwork and apply a release agent before pouring. Fully wet the formwork until no standing water remains.
3. Insufficient Curing: If the concrete is not promptly covered for moisture retention (e.g., with straw mats or plastic film) after final setting or if the curing period is less than 7 days, dry shrinkage cracks worsen. Measures: Begin curing within 12 hours after pouring and keep the surface moist until the design strength is achieved.

5. Foundation and Backfill Issues

If the foundation is not excavated to hard soil, the ground beam is not adequately cured, or the backfill is not compacted in layers, uneven settlement can occur, causing cracks in both walls and floor slabs. Measures: Conduct a geological survey before construction to ensure uniform foundation bearing capacity. Use layered compaction for backfill (each layer ≤30 cm thick) and maintain a curing period of at least 14 days for the ground beam.

6. Concentrated Pre-embedded Pipelines

Crossed or stacked electrical and water pipelines weaken the concrete cross-section strength, especially in densely piped areas, leading to cracks. Measures: Distribute pipelines evenly and avoid three-dimensional crossing. Install anti-crack steel wire mesh (mesh size ≤100 mm × 100 mm) in areas with concentrated pipelines to enhance local tensile strength.

Summary: By strictly supervising key aspects such as formwork sealing, double-layer bidirectional rebar arrangement, radial rebar installation, concrete mix proportions, and curing, the risk of cracking in cast-in-place floor slabs can be significantly reduced. If cracks have already appeared, targeted repair measures should be taken based on crack width (≤0.3 mm can be sealed; >0.3 mm requires pressure grouting) and cause.