Many fasteners used in the construction fields are unique in shape and size as well as application, so it should be of no surprise that there is a void when it comes to specific knowledge of construction trade fasteners. As an industry, we know they are there, but we never talk about them.

Everything has a touch of concrete in it, from a basement wall, porch and driveway to entire structures. The road signs that you drive past are anchored into concrete bases, as is the bridge you drove over and the corner gas station's canopy.

The material is a mixture of sand, various aggregates and cement. Cement is a calcined mixture of clay and limestone that has been baked at a high temperature to form a powder. When water is added to it, cement becomes a binder (the term is mortar) for the other ingredients. Aggregates are various additions that give the material body and strength. They can be gravel (fine to coarse sized), shale, chopped steel, glass, steel fiber, and even plastic-like materials.

The finished material is brittle, and its strength lies mainly in the compression mode. Designing and fastening through, into and onto concrete takes fasteners that put the material into compression. Tensile forces much above 5Mpa will, with most general concrete projects, result in pullout fastenings. Special techniques and mixtures can triple this figure, but at great cost.

Pre-stressed concrete refers to a process whereby rods of steel are tensile loaded (pulled axially) and the concrete poured over them in molds to form sections. When the tensile forces are released, the compression on the concrete by the steel rods increases the strength of the material to a point where thinner sections can be used. This is often done on bridges, stadiums, and places where excessively heavy sections would affect overall structure and appearance.

Fasteners for concrete work are not necessarily threaded. To give structural rigidity to the project, to prevent cracking due to sag, flexure forces from sideways movement and expansion cracking from temperature and earth movement, steel bars are wired together into a skeleton that is embedded in the mix. These fasteners, called "rebars," come in a variety of hardness and materials, depending on the requirements of the job at hand.

Where sideways bulging may be a problem, or to hold sides of a project together (or apart), threaded rods are fastened through the concrete sections and are tightened with nuts and plates on the outside of the project. These are often found on bridges and caissons to prevent internal weight from bulging out the sides and, conversely, in mine shaft and tunnels to prevent internal collapse from the same forces.

Since not all concrete projects are one-piece, poured-at-one-time operations, other fastening devices are often used. These are loosely classified into "in place" anchors and post-installed anchors. Shapes vary from a long rod, partially buried by the poured concrete, to "L" shaped thread rods and studs. The amount of part internally placed into the concrete is critical to correct pullout strength. It is estimated that a pull-out strength of about 150 percent of the expected forces is required for a good joint. The length will vary from about 18 in. for a ½-in. diameter rod to about 36 in. for a 1-in. diameter part.

Also important is the degree to which the concrete bonds to the rod. For this, correct mixture and moisture content is always a concern. To assist the joint strength, rods are often made with grooves, spirals, and other retaining features. Cast-into-place anchors, unfortunately, often are pushed out of place during the pouring. Also, if precise locations are required, they may not fit the bill.

The problem of setting a second piece atop the first and finding that they are out of alignment by an inch or more needs no explanation.

Exact placement of beams is generally done with an anchor device. After the concrete is poured and hardened, the location of the anchor point is marked and a hole is drilled.

The anchor is available in several varieties. One is placed in an undercut hole and, when tightened, the end bells out to fill the space. Grouting is forced in the remaining spaces to seal the joint further. A second type is driven by a hammer. The anchor is placed in the hole and the protruding end hit with a hammer. This forces the center rod into the casing, which bells out, in similar manner as the other type. Grout is used to fill in any space left. Third is a threaded rod, which is placed in the hole, then filled with grouting to seal it. This type has the lowest pull-out strength.

Powder-actuated rivets are driven into concrete to place signage, lightweight components (cable and wire conduits), and other non-structural pieces. Lag screws and anchor inserts can be used for similar attachments, especially where repair and frequent removal may be involved.

One unique fastener used in building construction is the tension washer. The washer is inserted into the joint to determine if the correct amount of clamping force is applied. The mechanisms vary, but some variation of a collapse feature is present in each. When the joint is tightened, a bump, a wavy flange, etc. will collapse flat at a known clamp load value. A quick check with a shim gage will determine if the right load is present on both new installations and as a check on current builds.

Construction trade fasteners, while sometimes considered a bit of an afterthought, are invaluable on the site. Your knowledge of the fasteners, and their characteristics, can be the difference between a proper fastening, and the before-mentioned one inch or more miscalculation.

Thomas Doppke is president of Technical Presentations Co., a fastener educational and consultant company specializing in all phases of fastener technology, problem solving, engineering and design. Contact Thomas at techpres@earthlink.net.