Work sequence, coat-thickness limits for trowel-pressed layers and covered moist-curing requirements of high-ductility concrete (HDC) overlay strengthening (DB62/T 3159-2019, DBJ 61/T 112-2021, T/CECS 0001-2023)

Topic Positioning

This card consolidates the workmanship controls of high-ductility concrete (HDC) overlay strengthening: sequences for masonry walls and concrete members, interface treatment and concealed acceptance, per-coat thickness limits, pressing intervals, overlay thickness provisions and moist-curing rules - for method statements, technical briefings and pricing listings.

Core Conclusions

1. Work sequences.

• Masonry (DB62/T 3159-2019 annex C.7.1 item 1, original): "HDC strengthening of masonry structures preferably proceeds: strip the original wall plaster -> chase or groove the joints -> (fix the rebars) -> blow off the dust -> wet the wall -> trowel-press the HDC -> cover and moist-cure -> decorate."
  • Where the original face is badly alkali-attacked, first remove the loose parts and face with HDC; loosened pointing mortar is picked out; cracked walls are repaired per GB 50702 (Code for Design of Strengthening Masonry Structures); damaged or loose bricks are replaced or locally repaired.
• Concrete members (annex D.6.1 item 1, original): "HDC strengthening of concrete members preferably proceeds: clean and scabble the strengthened face -> fix the new rebars (planted bars included) -> interface treatment -> trowel-press the HDC -> cover and moist-cure -> finish."
  • Finishes or oil on the original surface are cleaned off first and loosened concrete picked out; cracked members are repaired per GB 50367 (Code for Design of Strengthening Concrete Structures) before strengthening.
  • Before pressing, the interface treatment and the grade and fixing quality of the new rebars (planted bars included) pass concealed-works acceptance; the HDC is pressed or cast before the interface agent (adhesive) dries.

2. Coat thicknesses and intervals.

• Masonry walls (annex C.7.1 items 4 and 5): wet the wall before pressing and press in coats once wet without free water, each coat preferably within 15 mm; save the last finish pass that needs levelling, earlier passes keep a rough face - not trowel-smoothed. Where one pass falls short of the design thickness, the next pass follows as soon as the previous begins initial hardening, the interval preferably not over 4 h.
• Concrete members (annex D.6.1 item 6): pressing in coats, each pass preferably within 10 mm; the overlay thickness per the design; where one pass falls short, the next follows after initial hardening of the previous, and the face of all but the last pass is roughened (scratched), not smoothed.
• DBJ 61/T 112-2021 clause 6.2.6: overlays may be sprayed or hand-pressed in coats, keeping the fibres evenly dispersed, working continuously around corners - no cold joints at corners; sprayed faces are levelled before initial set and cured; hand pressing works in coats within 15 mm each. Thin overlays go by hand; large areas or thick overlays gain output by spraying.
• T/CECS 0001-2023 clause 8.4.5, original: "a single HDC pressing must not exceed 15 mm; overlays over 15 mm are preferably pressed in coats, the interval between coats not exceeding 4 h."

3. Covered moist curing. After the overlay, covered moist curing starts within 12 h and after final set and lasts not less than 7 d; in summer shield from sun, in winter insulate against frost (annex C.7.1 item 6; annex D.6.1 item 7).

4. Companion overlay-thickness provisions.

• Overlays generally run 10-25 mm; over 30 mm, add a reinforcement mesh within the overlay forming a reinforced HDC overlay, the vertical bars reliably connected or anchored at the wall foot; strengthening masonry uses Class I HDC with the original bedding mortar not below M0.4; existing finishes at the strengthened zones are stripped clean first (annex C.1.2).
• Shear and seismic strengthening: preferably double-sided, overlay not under 10 mm and preferably not under 15 mm; single-sided overlays not under 15 mm. Ground-floor overlays outdoors preferably descend 200 mm below ground or reach the ground ring-beam top; reinforced ground-floor overlays preferably thicken below grade and descend 500 mm (annex C.6.3).

Applicability Boundaries

• Applies to the construction, briefing and acceptance control of HDC overlay / reinforced-overlay strengthening of masonry and concrete structures.
• The standards word the coat limits slightly differently (masonry per coat 15 mm, concrete per pass 10 mm, single pass 15 mm) - follow the design documents and the current local standard; "no cold joints at corners" is universal.
• Curing is by covering and moisture; the method, temperature control and striking conditions follow the design and season; the thickness limits cap single coats, the overlay design thickness setting the number of passes.

Further Reading

• DB62/T 3159-2019 Technical Standard for Application of High-Ductility Concrete, annex C (C.1.2 wall provisions, C.6.3 shear and seismic strengthening, C.7.1 masonry construction) and annex D (D.6.1 concrete member construction).
• DBJ 61/T 112-2021 Technical Regulation for Application of High-Ductility Concrete, clause 6.2.6 (construction).
• T/CECS 0001-2023 Technical Regulation for Strengthening Masonry Structures with High-Ductility Concrete, clause 8.4.5 (construction).
• GB 50702 Code for Design of Strengthening Masonry Structures; GB 50367 Code for Design of Strengthening Concrete Structures (crack repair basis).