The calculation logic and engineering-value considerations for the anchorage depth of chemically planted rebar. There are three core conclusions: First, the anchorage depth must be determined by taking the larger value between the formula l_s = 0.2·α_spt·d·(f_y/f_bd) and the minimum constructive depth, and must not be back-calculated from short-term pull-out test values. Second, the final design value l_d must be obtained by superimposing multiple correction factors including bearing capacity, displacement ductility, and splitting, with particularly significant correction magnitudes in seismic zones and for cantilever members. Third, load-bearing structures and seismic zones must use Grade A adhesive; Grade B adhesive is limited to non-structural members only.

I. Code Basis and Scope of Application

Rebar-planting design involves multiple current codes and specifications; execution should follow the current version in force at the project location and the requirements of the competent authority:

Technical Specification for Concrete Structure Rebar Planting Systems, T/CECS 1847-2025

Technical Specification for Post-Installed Fastenings in Concrete Structures, JGJ 145-2013

Design Code for Strengthening of Concrete Structures, GB 50367-2013

General Code for Assessment and Strengthening of Existing Buildings, GB 55021-2021

Among them, Clause 7.2.1 of JGJ 145-2013 imposes a mandatory provision on the use of rebar planting as the main load-bearing connection for cantilever members and other principal force connections. It is the higher-level constraint that determines 'whether rebar planting may be used' and cannot be circumvented merely by increasing the anchorage depth.

II. How to Calculate the Anchorage Depth

2.1 Basic Anchorage Depth Formula

The basic anchorage depth of planted rebar is calculated as follows:

l_s = 0.2 · α_spt · d · (f_y / f_bd)

The meaning of each symbol is as follows:

l_s: basic anchorage depth of planted rebar (mm).

α_spt: concrete anti-splitting calculation coefficient, dimensionless, determined by the rebar spacing and edge-distance conditions.

d: nominal diameter of the rebar (mm).

f_y: design value of the rebar tensile strength (N/mm2).

f_bd: design value of the bond strength between the structural adhesive and concrete (N/mm2).

The formula shows that the anchorage depth is directly proportional to the rebar diameter and rebar strength and inversely proportional to the adhesive bond strength. For the same ribbed rebar, HRB500 (f_y = 435 N/mm2) requires about 1.25 times the anchorage depth of HRB400 (f_y = 360 N/mm2); HPB300 plain round bars, with no ribs and poorer bond performance, require the anchorage depth to be increased to 1.3-1.5 times that of ribbed rebar, with a 180° hook at the end and a straight extension after the bend of not less than 3d.

2.2 Minimum Constructive Anchorage Depth

The basic anchorage depth obtained from the formula must also be taken as the larger value against the following constructive minimums; both must be satisfied simultaneously to qualify:

Tensile rebar: max{0.3l_s, 10d, 100 mm} - take the largest of the three.

Compressed rebar: max{0.6l_s, 10d, 100 mm} - take the largest of the three.

Tie rebar: max{5d, 60 mm}, per T/CECS 1847-2025.

Cantilever members: multiply the corresponding above values by a correction factor of 1.5.

It must be specially noted that cantilever members must not use rebar planting as the main load-bearing connection. If it is truly necessary, in addition to the 1.5 factor, a special demonstration is required, and additional measures must be specified in the design documents.

2.3 Quick-Reference Comparison by Rebar Grade

The table below gives the basic anchorage depth (mm) by rebar grade under the formula for Grade A adhesive under normal environment, applicable to ribbed rebar. The values are for quick on-site estimation only; formal design shall be based on calculation.

In the original draft, the large HRB400 table broken down by spacing and edge distance (three groups: s1≥5d/c≥2.5d, s1≥6d/c≥3d, s1≥7d/c≥3.5d) - their differences are all essentially reflected in the α_spt value and have been merged into Section 2.4; the list is not repeated.

2.4 Correction Factors and Design Anchorage Depth

The basic anchorage depth must be multiplied by the various correction factors to obtain the final design anchorage depth:

l_d = ψ_N × ψ_ae × l_s

The values and applicable conditions of the three factors are as follows:

Bearing-capacity correction factor ψ_N: 1.0 for normal loading; 1.1-1.15 for general members; 1.15-1.27 for important members; 1.5 for cantilever members; an additional 1.1 when rebar diameter exceeds 25 mm.

Displacement-ductility correction factor ψ_ae: for concrete strength not higher than C30, take 1.10 for 6-degree seismic zones and 7-degree zones on Site Classes I and II, and 1.25 for 7-degree zones on Site Classes III and IV and 8-degree zones; for concrete strength higher than C30 and non-seismic zones, take 1.00.

Splitting-influence correction factor α_spt: 1.0 when s1≥7d and c≥3.5d; 1.1 when s1≥6d and c≥3d; 1.25 when s1≥5d and c≥2.5d.

By superimposing the above factors, the comprehensive correction magnitude for common working conditions can be estimated as follows, for quickly judging the required depth order of magnitude:

Non-seismic, general members: 1.1-1.15 (most common).

7-degree seismic, general members (≤ C30, Site Classes I, II): 1.21-1.27.

8-degree seismic, general members (≤ C30): 1.38-1.59 - note the relatively large depth.

Non-seismic, cantilever members: 1.65-1.9 (already includes the 1.5 cantilever factor).

8-degree seismic, cantilever members (≤ C30): 2.06-2.38 - rebar planting is not recommended.

III. Adhesive Selection: Grade A vs. Grade B

Rebar planting in load-bearing structures shall use Grade A adhesive and shall not use Grade B; Grade B adhesive may only be used for anchoring non-structural members. The main differences between the two types are as follows:

Because Grade B adhesive has lower bond strength, under the same conditions its anchorage depth needs to be about 15%-20% greater than Grade A. Taking HRB400, C30 concrete, s1≥6d, c≥3d as an example: under Grade A adhesive, the l_s for Φ12 rebar is about 240 mm; after switching to Grade B adhesive, the estimate is about 280-290 mm.

Selection by location can be judged as follows: Grade A adhesive should be preferred for load-bearing or tension members such as longitudinal reinforcement of construction columns, lintel and ring-beam longitudinal reinforcement, and slab-bottom and beam-bottom strengthening bars; Grade B adhesive may be restrictively used for secondary structures and infill-wall tie bars where verified; Grade A adhesive must be used for important projects such as bridges and water conservancy; Grade B adhesive suffices for temporary facilities and non-structural members, and using Grade A there is uneconomical.

IV. Reference Anchorage Depth by Structural Location

The values below are common engineering reference values, given for HRB400 rebar, Grade A adhesive, C30 concrete, and non-seismic conditions. They are for quick on-site estimation only; formal design must be based on structural calculation. Among them, main tension members such as slab bottoms and beam bottoms must be further multiplied by the corresponding correction factors.

V. Additional Requirements for Seismic Fortification Zones

In seismic zones, the rebar-planting depth is usually 10%-25% greater than in non-seismic zones; the exact magnitude is determined by the seismic grade, and the following requirements must also be met:

Material: Grade A planting adhesive must be used; Grade B adhesive is strictly prohibited.

Concrete strength: the concrete strength grade in the anchorage zone shall not be lower than C20; not lower than C25 for important members.

Spacing and edge distance: in seismic zones, rebar-planting spacing and edge distance should be appropriately increased to avoid concrete splitting.

Failure mode: ensure the rebar-planting failure mode is bar yielding followed by pull-out, not concrete splitting or adhesive failure.

Grade correction: by seismic grade, multiply by 1.15 for Grades I and II, by 1.05 for Grade III, and by 1.0 for Grade IV.

VI. Pull-out Tests and Bearing-Capacity Inspection

6.1 Test Types and Acceptance Criteria

On-site inspection is divided into non-destructive and destructive types, with the following acceptance criteria:

Non-destructive inspection: the load value is 0.9 times the standard value of the rebar yield bearing capacity; the load drop within the 2-minute hold shall not exceed 5%, and the substrate shall have no cracks and the rebar no slip.

Destructive inspection: load until failure; the ultimate bearing capacity shall be no less than 1.2 times the design value.

Infill-wall tie bars: uniformly inspected at 6.0 kN, per GB 50203.

Taking HRB400 rebar as an example, the non-destructive inspection values and ultimate bearing-capacity reference magnitudes are: Φ12 ≈ 40.7 kN / 55 kN, Φ16 ≈ 72.4 kN / 98 kN, Φ20 ≈ 113.1 kN / 153 kN, Φ25 ≈ 176.7 kN / 239 kN. The complete comparison can be checked item by item based on the rebar cross-sectional area and yield strength.

6.2 Short-Term Pull-out Values Must Not Determine Anchorage Depth

Passing a short-term pull-out test does not equal long-term anchorage reliability. If the planting depth is insufficient during construction, merely improving grout fill, etc., the short-term pull-out test may still pass; but under the effect of time and repeated loads, the risk of anchorage failure will gradually emerge. This is precisely why the code emphasizes that 'anchorage depth shall not be determined from short-term pull-out test values' - the depth must be determined by formula and correction factors, and the pull-out test is only used to inspect construction quality.

VII. Construction Quality Control

7.1 Drilling

The hole diameter should be controlled at d+4-8 mm; the allowable depth deviation is 0-+20 mm (foundation), 0-+10 mm (superstructure members), and 0-+5 mm (connection nodes); verticality shall not exceed 50 mm/m (foundation) and 30 mm/m (superstructure members); position deviation shall not exceed 10 mm (foundation) and 5 mm (superstructure members).

7.2 Hole Cleaning

Hole cleaning is the key link in planting quality; incomplete cleaning can reduce the pull-out force by 30%-50%. The steps are: after drilling, first clean the hole wall with a brush, then blow out the dust with compressed air; repeat no fewer than 3 times. The acceptance standard is no dust, no standing water, and no loose concrete debris in the hole; grouting and planting should be carried out promptly after cleaning, with an interval preferably not exceeding 30 minutes, to prevent secondary contamination of the hole wall.

7.3 Grouting and Rebar Insertion

Grouting should start from the hole bottom, withdrawing the nozzle gradually outward as the adhesive is extruded, to ensure the hole is full; the grout volume should be about 2/3 of the hole depth. The rebar should be inserted slowly with rotation so that the adhesive evenly coats the rebar surface. The rebar must not be disturbed during the curing period after planting; the curing time shall follow the product manual. At normal temperature (25 ℃), the curing time shall be no less than 72 h; it should be extended accordingly in low-temperature environments. On-site anchorage bearing-capacity inspection may be carried out only after the adhesive has cured for a full 7 days.

VIII. Conclusion

Determining the rebar-planting anchorage depth is a three-step process of 'formula calculation - constructive maximum - factor correction,' and no step may be omitted:

Step 1: calculate the basic anchorage depth by l_s = 0.2·α_spt·d·(f_y/f_bd), and take the larger value against the constructive minimum corresponding to its force type.

Step 2: superimpose the bearing-capacity, displacement-ductility, and splitting-influence corrections by l_d = ψ_N × ψ_ae × l_s; seismic zones and cantilever members must be checked separately.

Step 3: review the anchorage depth by location and force characteristic, and implement the construction-quality control requirements for hole cleaning, grouting, curing, and pull-out inspection.

The quick-reference table and location reference values are for quick on-site estimation only; they shall not replace structural calculation, and still less shall anchorage depth be back-calculated from short-term pull-out test values.