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Contraction joints in rigid pavement explained in civil engineering/highway engineering/ road design
Typology: Assignments
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Made by : Ali Hamid Kadhim Supervised by Dr. Nabeel Saleem
-Introduction Joints are formed in concrete slabs as part of the process of constructing rigid paving for roads. Joints are discontinuities in the pavement slab that are necessary to allow for expansion, contraction and warping. Rigid paving consists of a reinforced or unreinforced insitu concrete slab laid over a thin granular base course. The rigidity and strength of the pavement enables the loads and stresses to be distributed over a wide area of the subgrade. Joints are spaced depending on a range of factors: 1-The amount of reinforcement used. 2-The proposed traffic intensity. 3-The slab thickness. 4-The frictional restraint of the subgrade. 5-The temperature at which the concrete is laid. Joints comprise a filler which separates the slabs, and a sealing compound which is used to fill the top 25 mm of the joint to prevent the entry of water and grit. Suitable jointing materials include impregnated fibre board, cork, sheet bitumen, and rubber. The joint sealing compound must have good adhesion to concrete, extensibility without fracture, resistance to flow in hot weather, and durability. A system of dowel bars is introduced between slabs to prevent slab movement and ensure load-transfer. Dowel bars are positioned at mid-depth of the slab at centres of 300 mm. The diameter of the bar usually ranges from 20-30 mm but varies with the slab thickness. A plastic sleeve 100 mm-long is inserted on one end of the dowel to allow free movement of the slab. The sleeve should contain a pad of compressible material at the end.
Concrete is subjected to shrinkage due to loss of moisture in it at the time of drying, which results in the formation of cracks in the structure. To avoid these cracks, contraction joints are sometimes formed in concrete structures such as roads, retaining walls, floor and lining of tunnels and canals. The interval of these joints is from 5 m to 10 m. Contraction joints may not be necessary if sufficient reinforcement is provided to distribute the shrinkage cracks uniformly throughout the structure so that the width of each crack will be negligible.
Contraction joints are also called control joints. Control joints are the joints which allow horizontal movement of the slabs. If no joints were used, random cracking in the slab would occur when the tensile stress exceeds the tensile strength of concrete. These joints also eliminate random cracking due to thermal volume changes. In general, contraction joints are made purposely by creating a vertical plane of weakness in the slab. The cracks, if any, can thus occur only at this weak ended plane rather than at random locations. Figure 1. Rigid Pavement Showing Contraction Joints
Design involves the length of the slab given by :
Typically, a contraction joint is a sawed transverse joint normally placed every 18 ft to control cracking due to pavement contraction caused by shrinkage and temperature fluctuations. The plans for the particular contract are required to be checked to verify the proper joint placement. The minimum/maximum joint spacing is reviewed with the Area Engineer or District Construction Engineer so that joints may be established in the initial pours that will complement adjacent pavements. A dowel bar assembly (Figure 5), commonly called a "basket", is placed on the subgrade before paving at the location of the contraction joint. The steel bars of this assembly are large and, once incorporated into the concrete pavement, transfer the vehicular load from one slab to another, eliminating differential settlement at the joint. Figure 5. Dowel Bar Baskets
The Technician is required to monitor the locations of the reinforcing steel, placed as tie bars in the edges and at the center line of the pavement, to ensure that the tie bars do not interfere with the operation of the contraction joint. Tie bars placed over a basket assembly are adjusted longitudinally so that they do not hamper the movement designed into the joint. The longitudinal adjustment required is seldom more that 1 ft (Figure7). Figure 7. Basket Adjustment All joints are required to be sealed prior to opening a section of pavement to any traffic, other than construction traffic, or before the end of the current construction season. The steps in constructing a contraction joint (Figure 8) include the following:
Figure 8. Contraction Joint
