Using screw anchors, also known as concrete screws or masonry anchors, in concrete is a common and effective way to hang objects from walls and ceilings. Screw anchors provide a strong hold in concrete by threading into a pre-drilled hole and expanding at the base. This article will discuss everything you need to know about using screw anchors in concrete, including the different types of anchors, proper installation techniques, weight capacities, and tips for best results.
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What are screw anchors?
Screw anchors, as their name implies, are anchors that screw into a material, typically concrete or masonry. They consist of an exterior threading that resembles a screw and allows the anchor to dig into the base material when turned. The key feature of screw anchors that makes them ideal for concrete is that they have an expanding mechanism on the non-threaded end.
This expansion is achieved through different methods depending on the anchor type, but it results in the base flaring out beneath the concrete surface. This creates resistance that keeps the anchor from being pulled out. Screw anchors are extremely versatile and can be used in many materials, but concrete and masonry provide the best holding power.
Types of Screw Anchors
There are several different types of screw anchors available that work well in concrete:
- Wedge anchors – These have a cone-shaped expansion sleeve that wedges into the concrete when the screw is tightened.
- Sleeve anchors – These feature an exterior sleeve that expands outwards when the inner screw is tightened.
- Hex head anchors – The hex head bears against a split expansion clip that opens up beneath the concrete.
- Hammer-set anchors – These are hammered into a pre-drilled hole before being turned to expand the base.
The main factors to consider when choosing an anchor type are the size of screw needed, the weight capacity, and whether it will be used in cracked or uncracked concrete (some types work better in one versus the other).
Can You Use a Normal Screw in Concrete?
While screwing a normal wood or sheet metal screw into concrete is possible, it does not result in a secure hold. Regular screws are not engineered to cut into and grip concrete. As you turn the screw, it will either spin freely in the hole without biting in or it will become lodged in the hole without any pulling resistance.
Either way, a normal screw will not have the strength to remain fixed in concrete under much load. The unique expanding design of screw anchors allows them to achieve up to several hundred pounds of anchorage strength in concrete, while standard screws provide almost no strength.
Concrete Screw Anchor Installation
Installing screw anchors properly is important to achieve maximum holding strength. Here are some tips to follow:
Drilling the Hole
Always start by drilling a hole for the anchor using a masonry bit the same diameter as the anchor. Drill the hole slightly deeper than the length of the anchor to allow room for expansion. Clean out any dust or debris from the hole.
Choosing Drill Mode
Set your drill on hammer mode when drilling concrete and masonry to create impact as well as rotation. This allows the bit to chip away the material more efficiently. Use a lower speed to avoid overheating.
Picking the Right Anchor Size
Match the anchor diameter and length to the size of your application. A good rule of thumb is to use an anchor with a diameter 1/4 the thickness of the concrete member and a length at least 1 1/4 times the thickness.
Pre-Drilling Tip
For heavier objects, pre-drill holes using a drill bit the same diameter as the screw thread (not including the expansion sleeve). This lets the threads engage before widening the base.
Proper Setting
Turn the anchor slowly at first until resistance increases as the expansion mechanism engages. Then continue turning approximately 2-3 revolutions to fully set the anchor. Do not over-torque or strip the anchor.
Allowing Proper Curing Time
Let newly poured concrete cure for at least 28 days before installing screw anchors. This allows the concrete to fully hydrate and gain its intended strength.
Weight Capacity for Screw Anchors in Concrete
The weight a screw anchor can hold depends on factors like the type/size of anchor, concrete strength, and thickness of the base material. Some typical weight capacities for anchor screws inserted in 2,500 psi concrete are:
| Anchor Diameter | Weight Capacity |
|---|---|
| 1/4″ | 100 lb |
| 3/8″ | 150 lb |
| 1/2″ | 400 lb |
| 5/8″ | 600 lb |
| 3/4″ | 850 lb |
Higher strength 3,000+ psi concrete and thicker material will allow for increased capacities. Always follow the manufacturer’s recommended load limits for the specific anchor type and size you are using.
Tips for Using Screw Anchors in Concrete
Here are some helpful tips to get the best results when using screw anchors in concrete:
Check for Cracks
Avoid using screw anchors in areas with cracks or damaged concrete. The expanding force can cause more cracking. Fill cracks with epoxy first if necessary.
Reinforce Thin Concrete
For overhead applications or thin concrete under 1 inch thick, use sleeve anchors and add washers and nuts to distribute force and prevent pull-through.
Lubricate the Threads
Waxing screw threads reduces friction and makes it easier to install anchors. Just avoid getting lubricant on the actual shank expansion area.
Ensure the Base Material is Solid
Concrete must be properly consolidated around anchors for maximum holding strength. Tap the anchor to test for solid integration into the base prior to loading.
Double Up for More Strength
When additional capacity is needed, use multiple anchors spaced close together to share the load. This effectively doubles, triples, or quadruples the anchorage strength.
Conclusion
Screw anchors provide an extremely secure and reliable way to fasten objects to concrete walls and ceilings. By drilling the properly sized hole for the anchor, taking care to set the anchor correctly, and following load limits, you can obtain heavy-duty holding strength. Just be sure to use screw anchors designed specifically for concrete, not standard wood or sheet metal screws. This will provide the best grip and prevent pull-out failure. With the right technique, screw anchors are an easy solution for hanging or attaching just about anything to concrete.