Base Materials

Contents

- Base Materials for Fastening
- Concrete
- Natural Stone
- Masonry Materials
- Solid Clay Units or Brick
- Hollow Clay Units or Clay Tile
- Mortar
- Gypsum Wallboard

Base Materials for Fastening

The design of modern buildings requires fastenings to be made in a wide variety of base materials. To meet this challenge fastener manufacturers have developed many products specifically targeted to certain types of base materials. There is hardly a base material in which a fastening cannot be made with a Hilti product. The user must carefully match the type of fastener with the base material to obtain the desired results. The properties of the base material play a decisive role in the suitability and performance of a fastener.

Concrete

Concrete is a mineral building material which is made from three basic ingredients; cement, aggregate and water. Special additives are also used to influence or change certain properties. Concrete has a relatively high compressive strength compared to its tensile strength. Thus, steel reinforcing bars are cast in concrete to carry the tensile forces, and this combination is referred to as reinforced concrete.

Cement is the binding agent which combines with water and aggregate and hardens through the process of hydration to form concrete. Portland cement is the most common cement and is available in several different types, as outlined in ASTM Specification C-150, to meet specific design requirements.

The aggregates used in concrete consist of both fine aggregate and coarse aggregate graded by particle size. Different types of aggregates can be used to create concrete with specific characteristics. Normal weight concrete is generally made from crushed stone or gravel. Lightweight concrete is used when it is desirable to reduce the dead load on a structure. Lightweight aggregates are made from expanded clay, shale or blast-furnace slag. Lightweight insulating concrete is used when thermal insulating properties are a prime consideration. Lightweight insulating aggregates are prepared by expanding a variety of products such as perlite, vermiculite, blast-furnace slag, clay or shale. The appropriate ASTM specification and unit weight for each of these concretes is summarized as follows:

The type and mechanical properties of concrete aggregate have a major influence on the behavior of drill bits used to drill anchor holes. The harder aggregates cause higher bit wear and reduced drilling performance.

The hardness of concrete aggregates also affects the quality of powder actuated fastenings. Driven nails or studs can penetrate "soft" aggregates, but hard aggregates near the surface of the concrete can adversely affect the penetration of a stud or nail. As a result, the holding power of the fastening can be greatly reduced.

The holding power (ultimate load) of fasteners is generally given in relation to the 28-Day compressive strength of concrete. In view of the significantly lower strength of green concrete, it is recommended that anchors and powder actuated fastenings not be made in concrete which has cured for less than 7 days, unless accomodation is made for the reduced capacity of the fastening. If an anchor is set in green concrete, its holding power should only be based on the actual concrete at the time. If an anchor is put in place and loaded later, the holding power of the anchor can be based on the strength of the concrete at the time of loading. For powder actuated fastenings the ultimate load must be based on the concrete strength at the time of driving a nail or stud.

Cutting through concrete reinforcement when drilling holes for anchors should be avoided. If this is not possible, the responsible design engineer should be consulted first.

Natural Stone

Natural stone has been used as a building material predating concrete and masonry. The stone may be used in block form in buildings, bridges, perimeter walls, or other structures. Stone is a prefered material for monuments. Stone is used for floors, stairs, and for exterior hard surfaces. Buildings are clad with stone panels. Fastening to stone always involves a post-installed anchorage of some concept. Stone is drilled in the same manner as concrete and masonry. The concerns are similar. Rotary hammer drills and carbide bits perform well in granite. Wet coring with diamond core bits can prevent brittle fractures. The diamond coring process may outperform rotary hammer drills in very hard rock. In some cases, such as fastening to thin marble cladding, care must be taken not to damage the stone when rotary hammer drilling.

Adhesive anchor systems are usually the best choice anchoring to natural stone. Voids or cracks in the stone will affect the performance of anchors. Because of the wide variations in stone properties, site testing is required to determine actual anchor performance and suitability.

Masonry Materials

Masonry is a heterogeneous building material consisting of brick, block or clay tile bonded together using joint mortar. The primary application for masonry is the construction of walls which are made by placing masonry components in horizontal rows (coarse) and vertical rows (wythe). Masonry components are manufactured in a wide variety of shapes, sizes, materials and both hollow and solid configurations. These variations require that the selection of an anchoring or fastening system be carefully matched to the application and type of masonry material being used. As a base material, masonry generally has a much lower strength than concrete. The behavior of the masonry components, as well as the geometry of their cavities and webs, have a considerable influence on the ultimate loads of the fastening.

When drilling holes for anchors in masonry with hollow cavities, care must be taken to avoid spalling on the inside of the face shell. This could greatly affect the performance of "toggle" type mechanical anchors whose length must be matched to the face shell thickness. To reduce the potential for spalling, holes should be drilled using rotation only (i.e. hammering action of the drill turned off).

Hollow Masonry Units or Concrete Block

Concrete block is the term which is commonly used to refer to concrete masonry units (CMU) made from Portland cement, water and mineral aggregates. CMU's are manufactured in a variety of shapes and sizes using normal weight or lightweight aggregates. Both hollow and solid load bearing CMU's are produced in accordance with ASTM specification C-90.

CMU sizes generally refer to the nominal width of the unit (6", 8", 10" etc.). Actual dimensions are nominal dimensions reduced by the thickness of the mortar joint.

Reinforcing bars are sometimes installed into the cavity of CMU's to provide greater load capacity. Cavities with reinforcing bars are filled with grout conforming to ASTM specification C-476.

Solid Clay Units or Brick

Brick are prismatic masonry units manufactured from clay, shale or a similar earthy substance. They are shaped by molding, pressing or extrusion and heat treated (fired) at elevated temperature to provide strength and durability to meet the requirements of ASTM Specification C-62 for solid brick and C-652 for hollow brick. Depending upon the grade, brick has a compressive strength from 1250 to 2500 psi but only a very small tensile strength. As a base material for anchors, sleeve type mechanical anchors and chemical anchors typically work well. Wedge type expansion anchors should not generally be used in brick due to the extremely high expansion forces which could crack the brick.

Structural Clay load-bearing wall tile is made from clay or shale and heat treated (fired) at elevated temperature to develop the strength and durability required by ASTM Specification C-34. These units are manufactured in a variety of shapes and sizes with one or more cavities and develop a compressive strength of 500 to 1000 PSI depending upon the grade and type. These units typically have a 3/4" face shell thickness and 1/2" interior web thickness. Clay tile as a base material is somewhat more difficult to anchor to due to the thin face shell and low compressive strength. Adhesive anchors such as the Hilti HIT HY20 with a wire screen are usually recommended because they spread the load over a larger area and do not produce expansion forces.

Mortar

Mortar is the product which is used in the construction of reinforced and non-reinforced unit masonry structures. Mortar consists of a mixture of cemetitious material, aggregate and water combined in accordance with ASTM specification C270. Either a cement/lime mortar or a masonry mortar, each in four types, can be used under this specification. A summary of properties and guide for selection according to ASTM specification are shown in the tables.

Gypsum Wallboard

Gypsum wallboard consists of an incombustible core, essentially gypsum, surfaced with paper firmly bonded to the core. It is typically made in flat sheets four feet by eight feet or larger, and from 1/4" to 5/8" thick in accordance with ASTM specification C36.

Gypsum wallboard is attached to the wall studs and ceiling joist in residential and commercial buildings to form the base for the finished wall or ceiling treatment.

Gypsum wallboard does not have the capacity to accept high loads. Hilti offers several small anchors designed strictly for use in wallboard.

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