Chemical anchors: how to fix bolts and rebar into concrete
To fix a bolt or threaded rod into concrete with a chemical anchor, drill a hole of the size the data sheet gives for your rod, clean every bit of dust out of it, inject resin from the bottom of the hole until it is about two-thirds full, then twist the rod in and leave it untouched until the loading time has passed. In hollow blocks or perforated bricks you must use a mesh sleeve first, or the resin simply runs into the voids. Polyester resin such as Akfix C900 suits general fixings; a styrene-free epoxy acrylate such as Akfix C920 is the choice for heavier loads, rebar and indoor work where odour matters.
What a chemical anchor is
A chemical anchor is a two-part resin in a twin cartridge. As you squeeze, the resin and hardener meet in a static mixing nozzle and start to cure. In the hole, the resin fills every gap around the rod and bonds to the concrete, so the load spreads along the full depth of the hole instead of pushing outwards like an expansion bolt. That makes it useful close to edges and in older or weaker concrete.
Typical jobs: handrails, gates, canopies, satellite dishes, signs, racking, machinery and starter bars.
Polyester or styrene-free: which cartridge
Akfix C900 Polyester Chemical Anchor
Akfix C900 is a general-purpose polyester injection mortar for solid and hollow base materials. Akfix lists concrete, solid and hollow masonry, hard natural stone and solid rock, and typical jobs such as canopies, handrails, balcony fences, signs, satellite dishes, racking and machinery. Its data sheet lists a European Technical Assessment, ETA-24/0115. We sell it as a 300 ml cartridge at KSh 2,400, or KSh 27,000 for a box of 12.
Akfix C920 Styrene-Free Chemical Anchor
Akfix C920 is a styrene-free epoxy acrylate resin with very low odour. Akfix publishes it for heavy load-carrying fixings in solid stone and concrete, for anchor rods and reinforcing bars, and for medium loads in hollow brick. The low odour makes it the better choice inside occupied buildings. It costs KSh 2,850 a cartridge, or KSh 32,400 for 12.
The gun
Twin cartridges need a double-barrel gun that pushes both chambers together so the mix stays in ratio. The Akfix AKT351 double-barrel gun (KSh 5,000) is made for the Akfix chemical anchor cartridges. A standard silicone gun will not do this properly.
Step by step: solid concrete
1. Drill the right hole
Use the drill size and depth from the data sheet. Akfix's C900 table, for example, gives:
| Rod | Drill diameter | Embedment depth | Approx. holes per 300 ml (theoretical) |
|---|---|---|---|
| M8 | 10 mm | 80 mm | 55 |
| M10 | 12 mm | 90 mm | 35 |
| M12 | 14 mm | 110 mm | 21 |
| M16 | 18 mm | 125 mm | 11 |
| M20 | 24 mm | 170 mm | 5 |
Akfix notes that on site you usually inject more than the theoretical amount, so expect fewer fixings per cartridge, especially on small holes. The same table gives edge distances and spacings (for M12, 110 mm from an edge and 220 mm between anchors). Rebar connections are a structural design matter: follow the engineer's hole size and embedment.
2. Clean the hole: blow, brush, blow
Dust left in the hole is a common cause of weak fixings. Akfix's instruction is to remove water, dust and loose material with a round brush and a blower or compressed air. The usual site routine is blow out, brush the full depth with a round wire brush, then blow out again, and repeat until no more dust comes out. A hand pump works for small holes; use compressed air for large or deep ones. The hole should be dry before you inject.
3. Prepare the cartridge
Remove the cap, cut the foil clip end where the cartridge requires it, screw on the mixing nozzle and load the gun. Squeeze out and discard the first part of the mix until the resin comes out an even light grey; Akfix says to avoid using the first 10 ml.
4. Inject from the bottom
Put the nozzle to the bottom of the hole and inject while slowly withdrawing it, so you do not trap air. Akfix says to fill a solid hole about two-thirds full.
5. Insert the rod with a twist
The rod must be clean and free of oil. Push it in with a slow twisting motion to the full depth, so resin coats the thread and comes up to the surface. Hold it with a wedge if needed; you can only adjust it during the working time.
6. Wait for the loading time
Do not tighten the nut or load the fixing until the full loading time has passed. Akfix gives the times by the temperature of the base material, not the air. From Akfix's data sheets:
| Base material temperature | C900 working / loading | C920 working / loading |
|---|---|---|
| 5 °C | 25 min / 120 min | 25 min / 2 h |
| 10 °C | 15 min / 80 min | 15 min / 1 h 30 min |
| 20 °C | 6 min / 45 min | 8 min / 45 min |
| 30 °C | 4 min / 25 min | 4 min / 20 min |
| 35 °C | 2 min / 15 min | 2 min / 15 min |
On a hot afternoon you may have only a few minutes of working time, so drill and clean every hole before you start injecting. Check the data sheet for your batch, as figures change between revisions, and tighten only to the torque it gives.
Hollow blocks and perforated bricks
In hollow material the resin runs into the cavities and never builds up around the rod. The fix is a perforated mesh sleeve: insert the sleeve, fill it with resin, then insert the rod. The resin pushes through the mesh and locks behind the block's inner walls. Akfix's data sheets give sleeve sizes of 16 × 85 mm for brick and 16 × 130 mm for concrete block. Both C900 and C920 are published for hollow material, C920 for medium loads. Do not expect the loads from the solid concrete tables in hollow masonry.
Common mistakes
- Skipping the brushing; blowing alone leaves a dust film.
- Using the first squeeze of resin before it has mixed to an even colour.
- Filling from the top and trapping air at the bottom of the hole.
- Loading the fixing before the loading time, especially in a cold early morning.
- No sleeve in hollow block.
Where to buy
All three items are in our chemical anchors collection, with the Akfix data sheets attached to the C900 and C920 product pages; the rest of the range is in our Akfix collection. Contractors and steel fabricators buying by the box can request a trade quote on our trade page. Collect from our Nairobi trade counter at Royal Business Park, opposite Complast, Old Mombasa Road, Monday to Saturday, 8:00 to 17:30, or order for countrywide delivery at a flat KSh 350, free on orders over KSh 15,000. For a quick check on which resin suits your job, message us on WhatsApp.
Common questions
Is a chemical anchor stronger than an expansion bolt?
Not automatically, but it bonds along the full depth of the hole without pushing outwards on the concrete, which is why it is used close to edges, in weaker concrete and for rebar. The load a fixing can take depends on the resin, rod, depth, concrete and edge distance. Use the data sheet tables or an engineer's design for anything safety-critical. Our guide to wall plugs and raw bolts covers the expansion fixings and the loads each one suits.
Can I use a chemical anchor in a wet hole?
Akfix's instructions say to remove water and dirt from the hole before injecting. Dry the hole out with compressed air. If your job is underwater or the hole cannot be dried, ask us before buying.
Can I use a part-used cartridge later?
Yes, within limits. Akfix's instruction is to unscrew the mixing nozzle and replace the cartridge cap after use. When you restart, fit a new nozzle and discard the first part of the mix until the colour is even. Akfix says C920 should be used within a month once opened.
Which is better for rebar?
Akfix publishes C920 for reinforcing bars and heavy loads in concrete. Rebar sizes, depths and loads should come from the structural engineer.