Anchor Fastening Technical Guide , Edition 22
2.0 ANCHOR FASTENING TECHNOLOGY
2.1 BASE MATERIALS
2.1.1 BASE MATERIALS FOR FASTENING
The wide variety of building materials used today provides different anchoring conditions for anchors . There is hardly a base material in or to which a fastening cannot be made with a Hilti product . However , the properties of the base material play a decisive role when selecting a suitable fastener / anchor and determining the load it can hold . It is the responsibility of the designer to carefully match the type of fastener with the base material to obtain the desired results . For base materials not listed , contact Hilti , Inc . ( US ) at 1-800-879-8000 or Hilti ( Canada ) Corporation at 1-800-363-4458 .
2.1.2 CONCRETE
Concrete is a synthetic stone consisting of a mixture of cement , aggregates , and water . In many cases , special additives are used to influence or change certain properties . Concrete has a relatively high compressive strength compared to its tensile strength . Thus , steel reinforcing bars are frequently cast in concrete to carry tensile forces , and this combination is referred to as reinforced concrete .
Cement is a binding agent which combines with water and aggregates and hardens through the process of hydration to form concrete . Portland cement is the most commonly used cement and is available in several different types to meet specific design requirements ( ASTM C150 ).
The aggregates used in concrete consist of both fine aggregate ( usually sand ) and coarse aggregate graded by particle size . Different types of aggregates can be used to obtain concrete with specific characteristics . Normal-weight concrete is generally made from crushed stone or gravel , while lightweight concrete is obtained using expanded clay , shale , slate , or blast-furnace slag . Lightweight concrete is used when it is desirable to reduce the dead load on a structure or to achieve a superior fire rating for a floor structure . When thermal insulating properties are a prime consideration , lightweight aggregates are manufactured from perlite , vermiculite , blastfurnace slag , clay or shale . Finally , sand lightweight concrete is obtained using lightweight aggregate and natural sand . In general , all concretes with a unit weight between 85 and 115 pcf are considered to be structural lightweight concretes . The ASTM specifications related to concrete type and weight can be summarized as follows :
ASTM
Concrete Type
Aggregate Grading Specification
Concrete Unit Weight pcf
Normal Weight ASTM C33 145-155 Sand Lightweight ASTM C330 105-115 All Lightweight ASTM C 330 85-110
Lightweight Insulating Concrete
ASTM C 332 15-90
The type and mechanical properties of concrete aggregate have a major influence on the behavior of drill bits used to drill anchor holes . Harder aggregates , in fact , cause higher bit wear and reduced drilling performance .
The hardness of concrete aggregate can also affect the load capacity of power-actuated fasteners and anchors . Driven fasteners or studs can generally penetrate “ soft ” aggregates ( shale or limestone ), but hard aggregates ( like granite ) near the surface of the concrete can adversely affect the penetration of a fastener or stud and reduce its load capacity . The effect of aggregate mechanical properties on anchor performance is less well understood . In general , harder / denser aggregates ( i . e . granite ) tend to result in higher concrete cone breakout loads , whereas lightweight aggregates produce lower tension and shear capacities .
Concrete is typically assumed to crack under normal service load conditions or , more specifically , when tensile stresses imposed by loads or restraint conditions exceed its tensile strength . Crack width and distribution are generally controlled through the use of reinforcement . With consideration for the protection of the reinforcing steel , crack widths , per ACI 318 , are assumed to be less than approximately 0.012 in ( 0.3 mm ). Under seismic loading , flexural crack widths corresponding to the onset of reinforcing yield are assumed to be approximately 1-1 / 2 x static crack width = 0.02 ” ( 0.5 mm ). Both ACI 318 and the International Building Code conservatively assume cracked concrete as the baseline condition for the design of cast-in-place and post-installed anchors since the existence of cracks in the vicinity of the anchor can result in a reduced ultimate load capacity and increased displacement at ultimate load compared to uncracked concrete conditions . Design for uncracked concrete conditions is permitted by the model Building Codes only for cases where it can be shown that cracking of the concrete at service load levels will not occur over the anchor service life . For cases involving design for seismic actions , post-installed anchors must be demonstrated as being suitable for use in cracked concrete as well as for seismic loading .
Anchor Fastening Technical Guide Edition 22 | 2.0 ANCHOR FASTENING TECHNOLOGY | 2.1 BASE MATERIALS Hilti , Inc . 1-800-879-8000 | en español 1-800-879-5000 | www . hilti . com | Hilti ( Canada ) Corporation | www . hilti . ca | 1-800-363-4458
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