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Structural Concrete Characteristics

Joints in concrete

Joints are pre-planned cracks to accommodate the expansion and shrinkage of concrete, caused by changes in moisture and temperature. Although irregular cracks are unsightly and difficult to maintain, they generally do not affect the integrity of the concrete. Cracks in concrete can be controlled and minimized by properly designed joints. There are three types of common joints: Contraction joints, Isolation or expansion joints, Construction joints

Concrete cracks

Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete, this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16" for 10ft (0.4cm in 3m). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when its volume changes.

Different types of joints and there usage 

Isolation/Expansion Joints: Isolation joints are used to relieve flexural stresses due to the vertical movement of slab-on-grade applications that adjoin fixed foundation elements, such as columns, building and machinery foundations etc. Expansion joints are used primarily to relieve stress due to the confinement of a slab. If a slab is placed adjacent to structures on more than one face, an expansion joint should be placed to relieve stress. For example, if a slab were placed between two buildings, an expansion joint should be placed adjacent to the face of at least one of the buildings. Confinement on three faces would normally be handled by placing expansion joints on all three faces, and confinement on four faces should be isolated on all faces. This allows for thermal expansion and contraction without inducing stress into the system.

Contraction (control) joints: Contraction (control) joints are placed to control random cracking and should be placed at 2 times the slab thickness in feet for a maximum aggregate size of less than ¾”.

For example for a 5” slab with a ¾” coarse aggregate the maximum joint spacing would be 10’. When the maximum coarse aggregate size is greater than ¾” the spacing could be increased to 2 ½” times the thickness. For the prior example this would increase to 13’.

Applications that require thick slabs of 8" or more and good load transfer across joints, due to heavy loading, should be limited to a 15' contraction joint spacing to ensure aggregate interlock.

Construction joints are stopping places in the process of construction. Construction-joint types (a) and (b) are also used as contraction joints.

Strength ranges of concrete

Concrete can be proportioned to meet a wide variety of strength requirements. It is important to note that there is more than one type of strength property used to design concrete projects. The most commonly used design properties are:

  • Flexural strength: used for design of pavements (slab-on-grade)
  • Compressive strength: used for design of foundations, building elements (walls, columns, slabs), bridges (abutments, columns, decks) etc.
  • Flexural Strength: Flexural strength increases in direct proportion with compressive strength. This property is used specifically for pavement design. The flexural strengths of interest fall in a range of 3.9MPa (570psi) to 5.1MPa (750psi). These flexural strengths correspond approximately to compressive strengths of 28MPa (4000psi) to 48MPa (7000psi). While concrete can attain much higher flexural strengths, it is not required for pavements and use of higher strengths would have an adverse effect on the economics of the project with little benefit in performance.
  • Compressive Strength: The compressive strength of structural concrete begins at 17MPa (2500psi) and can be produced commercially at 138MPa (20,000psi) or more. Residential and light commercial building projects typically use concrete strengths ranging from 17MPa (2500psi) to 34MPa (5000psi). It is important to bear in mind that the lower strength concrete is only appropriate for mild environmental exposures and interiors protected from the elements. Severe environmental exposures (freezing and thawing cycles and deicer chemical exposure) require a minimum strength of 4000psi to assure durability. Local codes commonly provide guidance for the minimum requirements, but in many cases do not address long-term durability issues.

Heavy commercial and special structures (high rise buildings, long span bridges, slabs exposed to heavy abrasion etc.) typically require concrete strengths of 28MPa (4000psi) or more. The structural loading, durability requirements, special property requirements (low permeability, high abrasion resistance, etc.) or a combination of these factors control the actual required strength. Concrete design professionals should always be consulted for guidance regarding these structures.