Contraction joints

The contraction joints are constructed in the transverse direction to relieve tensile stresses that developed because of thermal contraction and force the slab to crack at a pre-determined position. 

In case the width of the groove (AL) for a contraction joint is to be calculated, the equation below may be used (AASHTO 1993)  

In an unreinforced concrete slab, the contraction joints are spaced approximately every 4 to 5 m, depending on the slab thickness and type of aggregate. In a jointed reinforced concrete slab, contraction joints are typically spaced every 20 to 25 m, depending on the amount of reinforcement. The designer is advised to strictly follow the instructions of the pavement methodology used with regard to the spacing of the contraction joints. 

It is common to place dowel bars at the position of the contraction joint. The dowel bars assist the transfer of loads over the contraction joint and are placed approximately in the middle of the slab. Dowel bars, since no bonding with concrete is required, are smooth and covered by corrosion-resistant coating. Their diameter is typically 20 or 25 mm and they are usually spaced every 300 to 400 mm. 

Dowel bars may not be used when the joint spacing is smaller than 4 to 4.5 m or when traffic volume is low. 

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Test Method for Tack Free Time of Caulking Compounds and Sealants Specification for Nonbituminous Inserts for Contraction Joints in Portland Cement Concrete Airfield Pavements, Sawable Type Specification for Joint Sealant, Hot Poured, Elastomeric Type, for Portland Cement Concrete Pavements... 

Contraction joint A joint between building components where the only movement to be expected is from the shrinkage of either or both components. 

The mortar may be applied to properly prepared concrete or steel substrates by usual acid-resistant bricklaying methods. An epoxy, urethane asphalt, bi-tumastic, polyester or vinyl ester membrane is required behind the block to ensure corrosion protection of the substrate. Because the mortar joints are rigid, a system of expansion/contraction joints, usually filled with ceramic paper, must be designed to prevent cracks. 

Figure 2, Damages of airfield concrete pavement caused by stress concentration due to slab pressure (a) and degradation of bituminous mass filling contraction joints (b, c)...

Observations demonstrated that bonding of concrete slabs with polymer PM reduces vertical and horizontal deformations of them of about 50%, allowing for safe work of airfield pavements. It should be noticed that contractions joints deform horizontally of about 5 mm between winter and summer and similar value was measured in cases of vertical deformation. An example of 24 hours measurements of vertical displacements of airfield slab is presented in Fig. 16 (for original slab with bituminous filling and slab bonded with polymer PM). Comparison of vertical displacement shows that the use of polymer joints in contradiction to bituminous mass reduces warping deformations of the comers about two times, what is very advantageous in aspect of airfield exploitation. 

Contraction stresses develop when slab/environmental temperature decreases. Contraction tensile stresses should be defused otherwise, contraction cracks will develop. To avoid contraction stresses, adequately spaced contraction joints are provided. 

The spacing distance between the contraction joints is very important and critical. When it is greater than required, cracks will develop, and hence, the joint does not serve its purpose. 

In order to address the stresses described above, joints are provided, which are distinguished into three main categories (a) contraction joints, (b) expansion joints and (c) warping joints (see Figure 14.5). 

Figure 14.5 Contraction, expansion and warping joints, (a) Contraction joint (b) expansion joint and (c) warping joint.

The latter is obtained by reducing the effective depth of the slab, inducing a sawn groove. The depth of the groove is related to the thickness of the slab the width of the groove may be determined, but in any case its minimum value is 3-4 mm. The groove induced is filled with a suitable sealant material. Details of a typical contraction joint are shown in Figure 14.7. 

Figure 14.7 Construction details of a typical contraction joint with sawn groove. (Adapted from Highways Agency, The Manual of Contract Documents for Highway Works [MCDHW], Volume 3 Highway Construction Details, Department for Transport. London Highways Agency, 2006c [ Highways Agency].)...

Although contraction joints are constructed perpendicularly to the direction of traffic in most cases, some designers choose to construct them at an angle, approximately 10° to 15° off the transverse direction, skew joints. By using skew joints, contraction joints are not stressed simultaneously along the full length and the repetitive noise produced by the joints is minimised. 

At the contraction joint, there is a complete discontinuity of concrete slab. The gap width introduced depends on the slab thickness, the coefficient of thermal expansion of concrete, the developed friction with the sub-base and the spacing between two expansion joints. The gap width is usually 20 to 25 mm. The top part of this gap is always filled with appropriate sealant. The spacing between expansion joints is determined by the pavement design methodology followed. 

The dowel bars for expansion joints are slightly longer than those used in the contraction joints, typically 500-600 mm, and are usually placed every 300 mm. The bar diameter is always greater than that of the dowel bars placed in the contraction joints, usually 25 or 32 mm. As in contraction joints, the dowel bars are made of smooth steel and coated by a corrosion-resistant material. 

The proper anchorage of the tie bars is achieved either by welding the tie bars on longitudinal supportive steel bars to be placed at a certain depth (or height) or by bending both ends. Suffice it to say that the tie bars are not covered with a corrosion-resistant coating at their full length, in contrast to the dowel bars in the expansion or contraction joints. 

In URCPs, usually every third contraction joint is replaced by a warping joint. 

The maximum transverse contraction joint spacing is 4 m, when slab thickness is <230 mm, while the maximum transverse expansion joint spacing is usually 40 m. 

Placing of the expansion, warping and longitudinal joints and slab reinforcement (if used) are carried out manually before concreting and after laying the separation membrane, in the case of jointed reinforced concrete. The contraction joints are formed soon after concreting. 

Sealants are elastomeric substances used to seal (or caulk) an opening or expansion/contraction joints in building structures against wind and water [40]. 

Renault s new automotive plant in Tangier, Morocco (annual capacity of 170,000 vehicles with possible increase to 400,000) also employed steel-fibre concrete for jointless floor slabs. 165,000 m of SFRC was the technical solution to (1) comply with the fast-track construction programme demanded by Renault, as reinforcement was added directly in the concrete and (2) eliminate the need for sawcut contraction joints and dramatically reduced year-to-year maintenance costs. Class C25/30 and C30/37 concretes were used with 35 0 kg/m dosages of steel fibres (Lazzari, 2012). 

The epoxy adhesives produce, with limited contraction, joints whose main characteristic is an absence of creep at high temperatures (mechanical strength is maintained up to temperatures of 100-150° C). Epoxy groups exhibit a high reactivity with many antagonistic functions, mainly amines, but also alcohols, anhydrides, and carboxylic acids (O Fig. 14.14). 

Chapter 24 presents models to describe the control of human movement, primarily the dynamics of human extremities. Presented are the challenges associated with control methods for multiple input/ output systems, modeling muscle contraction, joints and other components of the human motor system, and parameter estimation. This chapter presents several models for limb movement control. 

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