Industry News

How to install anchor bolts and cast-in anchors that stay put: selection, installation and acceptance

2026-10-06

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why anchor bolts fail - base defects and load paths decide the outcome

Steel structures, tower-crane bases, large machine tools and wind-tower foundations all rely on anchor bolts or post-installed anchors to transfer equipment loads into the concrete base. Yet the most common failure mode on site is not bolt fracture but pull-out or edge splitting: concrete tensile strength is only about one-tenth of its compressive strength, and the design axial tensile value for C30 concrete is roughly 1.43 MPa. Once edge distance is insufficient or the hole wall is loose, a cone-shaped failure surface propagates from the hole bottom toward the free edge and the whole anchor group pulls out together with the concrete cone. Another hidden risk is the wrong load path: the repeated horizontal shear produced by equipment vibration, if resisted only by friction, will slip once preload relaxes. The correct approach is to let the anchor carry shear directly or to provide a shear key, so horizontal force enters the foundation rather than relying on friction. Temperature stress is also a factor - outdoor steelwork can see a 40 degC day-night differential, and with a linear expansion coefficient of 12 x 10^-6 /degC the steel moves relative to the concrete. If the anchor system has no compensation allowance, cyclic thermal expansion will consume preload and eventually cause loosening. Before installation, therefore, three things must be confirmed: concrete grade and age meet design values (normally C25 or above and 28 days), anchor edge distance and spacing satisfy code minima, and the load type - tension, shear, or combined - matches the anchor type.

Selection comparison: expansion anchors, chemical anchors and cast-in bolts

The three mainstream fixing methods each have boundary conditions. Mechanical expansion anchors work by a tapered stud spreading a sleeve to generate expansion force. The advantage is immediate load-bearing with no curing wait, suited to indoor static equipment and secondary members. The downside is tight hole-diameter tolerance, loss of expansion force in cracked concrete, and loosening under vibration. Per JGJ 145 (Technical Specification for Post-installed Fastenings in Concrete Structures), expansion anchors are not suitable for tension-zone cracked concrete or lifeline engineering. Chemical anchors (adhesive anchors) consist of a stud with an undercut or deformed profile plus epoxy or vinylester adhesive; after curing the adhesive bonds stud and hole wall into one body, suitable for cracked concrete, dynamic loads and critical connections. Curing time depends on adhesive and temperature: at 5 degC an epoxy may need more than 24 hours to reach design strength, so the schedule must allow for it. Cast-in anchors are embedded before the pour and become monolithic with the concrete, giving the highest capacity and best durability for heavy equipment bases and tall structures. The drawback is high positioning accuracy - if they are misplaced, correction is difficult. The selection decision can be simplified as: secondary static load, uncracked concrete -> expansion anchor; important structure, cracked concrete or dynamic load -> chemical anchor; heavy equipment, tall tower, long-term durability -> cast-in bolt. For material, outdoor and damp environments choose hot-dip galvanised or Dacromet finish; marine environments choose 316 stainless or duplex stainless. Strength grades per GB/T 3098.1 are normally 8.8 or 10.9; tall structures often use 10.9 to reduce anchor count and foundation size.

Installation and acceptance: drilling, hole cleaning, curing and preload control

The process chain can be summarised as mark-out, drill, clean, install, cure, preload, re-check. Mark-out: set out from design drawings; cast-in bolts need a template plate fixed to tolerance within plus/minus 2 mm; post-installed anchors must avoid reinforcement, so scan with a rebar locator before drilling and shift position if a main bar is hit. Drill: bit diameter is anchor size plus 2-4 mm, hole depth must reach design embedment (normally 8-12 times bolt diameter), and perpendicularity deviation must be within 2 degrees. For chemical anchors, hole-wall roughness affects bond strength, so use a diamond core bit rather than a percussion drill to avoid an overly smooth wall. Clean: this is the most neglected critical step; hole dust can reduce adhesive bond strength by more than 50 percent. The code requires blow-brush-blow - compressed air, nylon brush, repeat, then wipe with acetone or alcohol. Expansion anchors also need cleaning, otherwise debris compacts under the sleeve and expansion force is insufficient. Install and cure: for chemical anchors, inject adhesive from the bottom of the hole, filling at least two-thirds of depth; adhesive overflowing at the mouth as the stud is turned in is normal. Do not disturb the anchor during curing; in winter, heat curing is needed. Preload control: final preload for foundation bolts is normally designed at 0.7-0.8 times bolt yield strength; for an M24 grade 8.8 bolt this is about 180-220 kN. A torque wrench or hydraulic tensioner must be used, tightening in two or three symmetrical stages to avoid uneven base loading. Re-check after 24 hours, because concrete creep and gasket compression can cause 10-20 percent preload loss. Acceptance per GB 50205 (Code for Acceptance of Construction Quality of Steel Structures): check anchor projection length, centre offset and elevation deviation on every bolt; for chemical anchors perform on-site pull-out tests per JGJ 145 at not less than 0.1 percent and not fewer than three anchors, with ultimate capacity at least 1.5 times the design value. Need anchor bolts, chemical anchor kits or foundation fixing design? Contact JIALI Industrial (164.hk) with equipment loads, base dimensions and environmental conditions for matched selection and quotation.

Related Reading

• Stripped threads in aluminium? Wire thread insert selection, installation and inspection essentials
• How to use GO / NO-GO thread gauges: thread inspection and incoming acceptance essentials
• JIALI launches Stainless Steel zone with full 304 / 316 range

← Back to News