Bond can be thought of as the shearing stress or force between a bar and the surrounding concrete. The force in the bar is transmitted to the concrete by bond, or vice versa. Bond is made up of three components: (a) chemical adhesion, (b) friction, and (c) mechanical interaction between concrete and steel. Bond of plain bars depends primarily on the first two elements, although there is some mechanical interlocking due to the roughness of the bar surface.?Deformed bars, however, depend primarily on mechanical interlocking for superior bond properties. This does not mean friction and chemical adhesion are negligible for deformed bars, but that they are secondary.

With the use of deformed bars, bond failures could result from concrete crushing at the bearing face of the deformations; by shearing of the concrete around the outer extremities of the bar; by longitudinal splitting of the bar; or by a combination of these three failure modes.

The bond of deformed bars is developed mainly by the bearing pressure of the bar ribs against the concrete. Bars having ribs with steep face angles (larger than about 40° with the bar axis) slip mainly by compressing the concrete?in front of the bar rib. The concrete is crushed, and a concrete wedge forms in front of the bar rib. Bars with flat ribs (less than 30°), however, slip with the ribs sliding relative to the concrete.

With little confinement, large deformed bars may fail in bond by splitting of the concrete along the plane of the bar. This type of failure?depends primarily on the load on the concrete and not much on the bar stress and the bar diameter or the bar perimeter. As the confinement around a bar improves, usually by the use of external concrete or transverse reinforcement, the ultimate load per unit length depends increasingly on the bar diameter. Small bars, top cast bars, or bars that are confined to the extent that bond failure generally occurs by shear failure of the concrete keys instead of splitting, will carry a maximum unit load proportional to the bar perimeter ( hence to the bar diameter).

Based on the above reasoning, the maximum bond force per unit length depends primarily on the bar diameter, the amount of enclosure provided by transverse reinforcement (hoops, ties, or stirrups) placed around the bar, or confinement provided by external concrete?(concrete cover), as well as the concrete tensile strength.

The bond of welded wire fabric embedded in concrete is dependent on the ability of the welded transverse wire to provide anchorage. The important variables are the size ratio between the transverse and longitudinal wire of the fabric and the quality of the weld connecting them. For deformed wire, which has been developed recently, crushing of the concrete against the deformations or the shearing of the concrete core at the outer periphery of the wire may be more critical than splitting. The bearing area also is important for controlling slip at a given load.

The development of bond in prestressed concrete is also attributed to adhesion, friction, and mechanical resistance. Friction between concrete and tensioned tendons(or grout and tendons) is the principal factor responsible for the transfer of the prestress force from steel to the concrete, and the other two factors are of less importance in bond development.

When the external prestressing force in the tendon is released, the tendon diameter tends to increase, thus producing high radial pressure against the concrete, which in turn produces high frictional resistance in the transfer zone. Prestress transfer bond is present from the ends of a prestressed member to the beginning of a region in which the steel tension is constant. The length over which this transfer is made is termed the prestress transfer length, and is a function of the perimeter configuration, area and surface condition of the steel, the stress in the steel, and the method used to transfer the steel force to the concrete. Tendons with a slightly rusted surface can have an appreciably shorter transfer length than clean tendons. However, the danger of careless rusting allowing localized pitting should be considered. Gentle release of the tendon will permit a shorter transfer length than abruptly cutting the tendons. There is relative movement of steel and concrete, and accordingly adhesion cannot contribute to prestress transfer. Mechanical resistance probably contributes little to prestress transfer in the case of individual smooth wires, but it might be argued that strand will offer some mechanical resistance because of the helical grooves in the stranded configuration, but this type of deformation may not be depended upon for any appreciable contribution to the development of bond. A significant innovation in prestressing strand is the development of deformed strand, which is desirable for reducing the transfer length in some structural elements.

An increase in wire tension due to flexure reduces the diameter, relieves the radial pressure, and reduces the bond near the ends of a beam. Bond failure in prestress concrete also may occur due to too close spacing of the tendons.

Bond to concrete may be prevented for some pretensioned reinforcement in the end regions. Bond may be prevented by various?means; one method is the use of plastic tubing which is often referred to as "blanketing. "

Metal reinforcement should be free from loose, thick rust, mill scale, mud, oil, grease, paint, and loose dried mortar at the time concrete is placed, as these materials adversely affect or reduce bond.

A normal amount of rust increases bond. Normal rough handling of bars generally removes most of the loose rust and mill scale.?However, in some instances it may be necessary to rub with a coarsely woven sack or to use a wire brush. Metal reinforcement, except prestressing steel, with rust, mill scale, or a combination of both, can be considered as satisfactory, provided the minimum dimensions, including height of deformations, and weight of a hand wire brushed test specimen are not less than applicable ASTM specification requirements. Prestressing steel should be clean and free of excessive rust, oil, dirt, scale and pitting. A light oxide coating that can be removed by a soft dry cloth is permissible.

Slip caused by relative movement ( in addition to that caused by crushing) also occurs when the frictional properties of the rib face are reduced by grease. The extent to which slip properties are affected by grease depends on the face angle; ribs with flatter face angles are more affected by poor frictional properties.

Bond is increased by a tight adherent cement paste or mortar coating on the bars. In some cases, bond of hot dip galvanized reinforcement may be reduced due to the attack of fresh concrete on the zinc and evolution of hydrogen with the resultant formation of gas pockets next to the bars. Suspensions of bentonite and water are commonly used for construction convenience to support side walls of foundation trenches.?Reinforcement in cages immersed in the?suspension and then surrounded by tremie placed concrete may suffer?reduced bond strength.