Apron details in collapsible loess regions: cast-in-place concrete, bedding widened 500 mm, expansion joints at 6-10 m and flexible sealing (GB 50025-2018 clause 5.3.4 / GB 50037-2013 clause 6.0.20 item 4)

Topic Positioning

The special apron details for buildings on collapsible loess (cast-in-place concrete, widened bedding, closer joints, flexible sealing) and how they differ from ordinary regions.

Core Conclusions

GB 50025-2018 Standard for Building in Collapsible Loess Regions, clause 5.3.4, original:

Aprons shall be cast in place with concrete and shall comply: 1 a 150 mm lime-soil bedding or 300 mm soil bedding shall sit below, projecting 500 mm beyond the apron or the outer face of the external wall foundation; 2 expansion joints preferably every 6-10 m. The apron-to-wall junction and the apron joints shall be sealed with flexible waterproof material; open drains along the outer edge are unsuitable.

Commentary: after the foundation is built the flank is usually narrow and backfilled poorly; to stop roof and surface water entering the ground sideways, widening the apron and its bedding helps by covering the foundation flank fill. Ordinary regions space apron joints 6-12 m; most loess regions see wide day-night swings and cold, so aprons crack by frost and become seepage paths - hence the closer spacing.

GB 50037-2013 Code for Design of Building Floors, clause 6.0.20 item 4, same basis, refined:

4 In collapsible loess regions aprons shall be cast-in-place concrete on a 150 mm 3:7 lime-soil or 300 mm compacted plain-soil bedding; the bedding edge shall project 500 mm beyond the apron and beyond the outer face of the wall foundation. The apron fall shall not be under 5%, with expansion joints preferably every 6-10 m. The junction to the wall shall be jointed, its width and that of the apron joints preferably 20 mm, filled with flexible sealant. The apron width shall comply with GB 50025. Open rain drains along the outer edge are unsuitable.

Key points:

• Cast-in-place concrete mandatory.
• Bedding: 150 mm of 3:7 lime-soil or 300 mm compacted plain soil.
• Bedding edge projects 500 mm beyond both apron and wall foundation, covering the flank fill.
• Apron fall not under 5% (stricter than the 3-5% of ordinary regions).
• Joints every 6-10 m (closer than the 6-12 m of ordinary regions - wide temperature swings, cold, frost cracking).
• Junction and joint widths preferably 20 mm, flexibly sealed.
• No open (rain) drains along the outer edge.
• Width per GB 50025.

Applicability Boundaries

• Collapsible loess regions only; ordinary regions take 3-5% falls and 6-12 m joints.
• Concealed (planted) aprons on loess sites still meet the cast-in-place, widened-bedding and sealing requirements.
• The point of the joints and seals is to stop surface water entering the ground through them - the core control on loess sites; "an intact-looking apron" is no substitute for seal quality.
• The clause fixes no width; width follows GB 50025 with the site's collapse grade.

Further Reading

• GB 50025-2018, clause 5.3.4 and commentary (section 5.3, design)
• GB 50037-2013, clause 6.0.20 (item 4 and commentary)
• Construction sets width and bedding with the site's collapse grade and ground treatment