Design provisions for expansion strengthening bands (definition and three forms, 2000 mm width, 30-60 m spacing, casting timing, extra reinforcement and expansive agent dosage) and their differences from conventional post-cast strips in mechanism, width and casting time

Topic Positioning

Design provisions for expansion strengthening bands - definition and forms, width and spacing, casting timing, extra reinforcement and expansive agent dosage - and their differences from conventional post-cast strips in mechanism, structural dimensions and closing time.

Core Conclusions

1. Definition and three forms (JGJ/T 178-2009 clause 2.0.7, terminology): "expansion strengthening band (expansive strengthening band): a technical measure that reduces or eliminates post-cast strips and expansion joints and lengthens continuous casting runs by casting shrinkage-compensating concrete at the post-cast strip positions preset in the structure; it divides into continuous, intermittent and post-cast forms. The continuous form: the band concrete and the adjacent concrete on both sides are cast together; the intermittent form: the band concrete is cast together with the adjacent concrete on one side while the other side is a construction joint; the post-cast form is cast the same way as a conventional post-cast strip."

2. Setting and width (clause 4.0.3 of the same regulation): the post-cast strips of mass, large-area and super-long concrete structures may be replaced by expansion strengthening bands, complying: 1 the band may take the continuous, intermittent or post-cast form; 2 the band may follow the setting principles of conventional post-cast strips; 3 the band width is preferably 2000 mm, with dense-mesh steel (plate) mesh on both sides separating the concrete inside from outside; 4 a non-settlement band may be cast after the shrinkage-compensating concrete on both sides has cured 28 d, and the band in mass concrete is cast only when the centre temperature of the concrete on both sides has fallen to ambient.

3. Spacing and form choice (clause 4.0.4): the casting method and structural form of shrinkage-compensating concrete follow table 4.0.4 by structural length - "the spacing of expansion strengthening bands is preferably 30-60 m. Strongly restrained slab structures preferably use post-cast bands and cast in stages"; table 4.0.4 embodies "weak restraint, short total length and thin members allow longer continuous casting segments, and vice versa". Where a band replaces a post-cast strip, the continuous form is preferred, the intermittent or post-cast forms also acceptable, the casting complying with JGJ/T 178 (JGJ/T 492-2023 clause 6.5.6).

4. Differences from conventional post-cast strips.

• Mechanism: crack control in reinforced concrete offers "resist" and "release" - setting an expansion strengthening band is "resist", a post-cast strip or a post-cast-form band is "release"; underground structures and thinner members favour "resist", superstructures and thick members suit combining "release". The band is in substance a widened, strengthened post-cast strip - wider than a post-cast strip, otherwise similar in detail (thicker slabs generally need no waterstop).
• Casting (closing) time: with ordinary concrete the codes differ - GB 50108-2008 requires the strip at 42 d after the flanking concrete; high-rise post-cast strips at 14 d after the roof slab concrete; the commentary to GB 50010-2002 deems two months appropriate. With shrinkage-compensating concrete the drying shrinkage is compensated and, per abundant practice, the strip may be cast at 28 d; in mass concrete the band is cast only when the centre temperature of the flanking concrete falls to ambient. Walls generally use the post-cast form of band, backfilled and cast 7-14 d after the flanking concrete.

5. Materials and reinforcement.

• GB 50945-2013 clause 5.2.11 (seamless design of super-long concrete structures): with shrinkage-compensating concrete and expansion strengthening bands, "the expansive agent dosage inside the band is 14%-15% (expansion about 4x10-4 to 6x10-4), outside the band 8%-12% (about 2x10-4 to 3x10-4), replacing cement equally. The bars run through the band with extra reinforcement of area half the main reinforcement, and the concrete strength is one grade higher." The agent may be UEA or another type.
• GB 50108-2008 clause 5.2.3: "post-cast strips shall be cast with shrinkage-compensating concrete whose impermeability and compressive strength grades are not lower than the flanking concrete." The mix design of shrinkage-compensating concrete shall further meet the impermeability and strength grades of waterproof concrete.
• DB11 T 1832.3-2021 clause 27.1.1: post-cast strip concrete may be shrinkage-compensating, its strength grade per the design; absent design requirements preferably one grade above the flanking concrete; the mix shall be trialled, testing slump and casting strength, free-expansion and restrained-expansion cubes.
• GB 50666-2011 clause 8.3.10 (treatment before recasting post-cast strips and the one-grade-higher requirement) is on related cards.

6. Other post-cast strip details for reference. SJG 19-2019 clause 8.3.4: strips sit where force and deformation are small, position and spacing fixed by the structural design, width preferably 700-1000 mm; cast with shrinkage-compensating concrete of strength and impermeability grades not below the flanks, bars continuous, a delayed-swell water-swellable rubber waterstop strip at mid-depth on both sides with embedded grouting pipes preferably provided, the enhancement layer preferably 2 mm flexible waterproofing lapping 300 mm beyond the strip each side. JGJ 3-2010 clause 12.2.3: construction post-cast strips through roof, floor and wall slabs may be set every 30-40 m, closing preferably 45 d or more later. DB T 29-278-2020 clause 5.9.4 (top-down underground works): strips or strengthening bands of super-long, super-wide structures are preferably cast when the flanking concrete reaches 90 d (design requirements prevailing); cast with shrinkage-compensating concrete placed at low temperature, impermeability and compressive grades not below the flanks, strength preferably one grade higher; quick-closure expanded-metal mesh may serve as the strip formwork. Full post-cast strip details per GB 50108.

Applicability Boundaries

• This card states design and construction provisions, not pricing bases; pricing (item application and measurement) is on related cards.
• The expansive agent dosage and the "half the main reinforcement" extra bars in GB 50945-2013 clause 5.2.11 belong to that code's optical-fibre plant chapter; other works follow the design documents - no unconditional application.
• Closing (casting) times of 14 / 28 / 42 / 45 / 90 d belong to different codes and methods: 28 d presupposes shrinkage-compensating concrete, 90 d suits top-down underground works; choose by work type and the current local standard.
• Local standards (SJG 19-2019 Shenzhen, DB11 T 1832.3-2021 Beijing, DB T 29-278-2020 Tianjin) apply only in their regions or methods; key parameters follow the design documents and current local standards.
• The number and shape of expansion strengthening bands are set case by case by the design and construction engineers to suit the works.

Further Reading

• JGJ/T 178-2009 Technical Regulation for Application of Shrinkage-Compensating Concrete, clause 2.0.7 (terminology), 4.0.3 (setting, width, casting timing), 4.0.4 (table 4.0.4 casting method and form, 30-60 m spacing)
• JGJ/T 492-2023 Standard for Seamless Construction of Super-Long Concrete Structures, clause 6.5.6 (bands replacing post-cast strips)
• GB 50108-2008 Technical Code for Waterproofing of Underground Works, clause 5.2.3 (strip concrete, impermeability and strength grades) and the post-cast strip detail chapter
• GB 50945-2013 Technical Code for Optical Fibre Plant Engineering, clause 5.2.11 (seamless design: agent dosage, extra reinforcement, structural steel, insulation)
• DB11 T 1832.3-2021 Construction Technology Code for Building Works, Part 3: concrete structures, clause 27.1.1
• SJG 19-2019 Technical Standard for Waterproofing of Building Works in Shenzhen, clause 8.3.4 (strip waterproofing detail, figure 8.3.4)
• DB T 29-278-2020 Technical Regulation for Top-Down Underground Works in Tianjin, clause 5.9.4 (figure 5.9.4, added bars at strips)
• JGJ 3-2010 Technical Specification for Concrete Structures of Tall Buildings, clause 12.2.3 (strip spacing 30-40 m, closing preferably 45 d or more later)