Design and Construction of Joints for Concrete …

1 Design and Construction of Joints for Concrete Streets To ensure that the Concrete pavements we are building age, temperature and moisture differentials, and applied now will continue to serve our needs well into the future, traffic loadings. If these stresses are not relieved, uncon- it is essential to take into account all Design and construc- trolled cracking will occur. tion aspects. This includes thickness Design , subgrade In determining a proper jointing system, the designer and subbase preparation, and jointing. This publication must consider climate and environmental conditions, addresses the Design and Construction of jointing sys- slab thickness, load transfer, shoulder/curb and gutter tems for Concrete street pavements. Two other ACPA Construction , and traffic. Past performance of local publications, Design of Concrete Pavements for City streets is also an excellent source for establishing joint Streets and Subgrades and Subbases for Concrete Pave- Design .

2 Moreover, improvements to past designs using ments, address city street thickness Design and subgrade/ current technology can significantly improve performance. subbase preparation. Proper and timely Construction practices, in addition to Typically street pavement slabs range from 5 to 8 in. proper Design , are key in obtaining a properly performing (125 to 200 mm) in thickness. The recommendations for jointing system for street pavements. Late or inadequate jointing in this publication are for pavements within this joint formation may cause cracks to develop at locations general range and purpose. Special considerations for other than those intended. In most cases, sealing is other Concrete pavement joint systems (highways, park- necessary to assure the proper function of street Joints . ing areas, and airports) are covered in other ACPA. publications. A proper jointing system for Concrete street Jointing for Crack Control pavements ensures that the structural capacity and riding Proper jointing is based on controlling cracks that occur quality of the pavement is maintained at the highest level at the lowest annual cost.

3 A proper jointing system will: from the natural actions of the Concrete pavement. Joints are placed in the pavement to control the crack location 1. control cracking. and pattern. Observing the slab behavior of unjointed 2. divide the pavement into practical Construction plain pavements in service for many years can illustrate increments. how Joints are used to control cracking. 3. accommodate slab movements. To attain adequate workability for placing and finishing 4. provide load transfer. Concrete , more mixing water is used than is needed to hydrate the cement. As the Concrete consolidates and The development of Concrete pavement joint Design hardens, most of the excess water bleeds to the surface has evolved from theoretical studies, laboratory tests, and evaporates. With the loss of water, the Concrete experimental pavements, and performance evaluations contracts and occupies somewhat less volume.

4 A sec- of in-service pavements. A careful study of the perfor- ond major source of early shrinkage is caused by the mance of pavements subject to similar traffic and environ- pavement's temperature change. The heat of hydration mental conditions as the proposed pavement is of great and temperature of the Concrete normally peak a short value and should be considered in the Design of slab time after final set. After peaking, the temperature of dimensions and jointing details. Concrete declines due to reduced cement hydration and lower air temperature during the first night of pavement life. As the temperature drops, the Concrete pavement Jointing Considerations contracts. The need for a jointing system in Concrete pavements The pavement's contraction is resisted by subgrade results from the desire to control the location and geom- friction, which creates tensile stresses in the Concrete etry of transverse and longitudinal cracking.

5 Cracking slab. These tensile stresses cause a transverse crack results from stresses caused by Concrete drying shrink- pattern like that shown in Figure 1. American Concrete Pavement Association 1992. Fig. 1. Initial cracking in unjointed pavement Fig. 2b. Properly jointed pavement Spacing of the initial cracks varies from about 40 to Adequate load transfer results in lower deflections, which 150 ft ( to m) depending on Concrete properties, reduces faulting, spalling, and corner breaks, thereby point-to-point variations in subgrade friction, and cli- increasing pavement life. Load transfer across Joints for matic conditions during and after placement. street pavements is developed either by aggregate inter- After the pavement hardens, curling and warping lock or dowel bars. stresses develop from temperature and moisture gradi- ents in the Concrete that also affect the cracking pattern. Aggregate Interlock Evaluating the combined effect of restrained tempera- Aggregate interlock is the interlocking action between ture curling and moisture warping is complicated due to aggregate particles at the face of the joint .

6 It relies on the their opposing nature. For instance, when the top of the shear interaction between aggregate particles at the slab is warmer than the bottom, causing the top to irregular crack faces that form below the saw cut. This expand, the bottom of the slab usually has a higher form of load transfer has been found to be most effective moisture content causing it to expand. Hence, the on roadways with short spacings and low truck volumes. amount of stress developed in the slab will be less than A 1985 study by the Minnesota DOT1 found that aggre- the stress due to temperature or moisture gradients gate interlock load transfer provides acceptable pave- alone. Repetitive traffic loads compound the problem. In ment performance when truck semi-trailer volumes are any case, restrained curling and warping in combination fewer than 80 to 120 trucks per day per lane. In dry, non- with loads will cause additional transverse cracks between freeze environments, greater truck volumes may be the initial contraction cracks shown in Figure 1.

7 Also, a accommodated. Other studies have indicated that at longitudinal crack will form along the approximate least four to five million AASHTO 18,000-pound Equiva- centerline of pavements with two lanes of traffic. The lent Single Axle Loads (ESAL's) are required to produce resulting crack pattern is shown in Figure 2a. The interval objectionable faulting in undoweled ,3. between transverse cracks is normally about 10 to 20 ft Laboratory 4,5,6,7 and field studies 3,7 have shown that ( to m), depending on factors such as pavement aggregate interlock load transfer is improved with thickness, shrinkage properties of the Concrete , sub- subgrade strength; size, angularity, and durability of the base and subgrade conditions, and climatic conditions. aggregate; joint face roughness; and slab thickness. Figure 2b shows a proper jointing system used to control Furthermore, these studies found that shorter slabs in- slab cracking.

8 Crease aggregate interlock effectiveness by reducing the movement and opening at each joint . Load Transfer Dowel Bars For jointed Concrete pavements to perform adequately, traffic loadings must be transferred effectively from one Dowel bars are round, smooth, steel bars placed across side of the joint to the other. This is called load transfer. transverse Joints to transfer loads without restricting horizontal joint movements due to thermal and moisture contractions and expansions. They also keep slabs in horizontal and vertical alignment. Dowels reduce deflec- tions and stresses due to traffic loads. This in turn prevents or reduces faulting, pumping, and corner breaking on roadways that carry a large number of trucks and/or have longer joint spacings. The use of dowel bars for minimizing faulting and pumping should be considered when the slabs are longer than 20 ft ( m), when truck semi-trailer traffic exceeds 80 to 120 per day per lane, or when the accumu- lated Design traffic exceeds four to five million AASHTO.

9 ESAL's per lane. Typically, this truck traffic level requires Fig. 2a. Crack pattern in unjointed pavement due to an (200-mm) slab or greater. Since most city environmental and load stresses streets do not experience these truck traffic levels and 2. recommended joint spacings are not greater than 15 ft 3. Longitudinal Joints : Joints parallel to the pave- ( m), dowels are generally not necessary. For 8-in. ment centerline that control cracking and (200-mm) slabs or greater, dowels and/or the following delineate lanes of traffic. methods are recommended for most highway applica- 4. Isolation and Expansion Joints : Joints placed to tions8: allow movement of the pavement without dam- aging adjacent pavements, intersecting streets, 1. Thicker slabs drainage structures, or other fixed objects. 2. Stiffer subbase/subgrade (higher effective k- value). 3. Less erodable subbase ( cement-treated or Transverse Contraction lean Concrete subbase).

10 4. Edge support ( tied Concrete shoulders or Joints tied/integral curb and gutter). 5. Coarse grained subgrade soils (improved Transverse contraction Joints primarily control the natural drainage). cracking in the pavement. Their spacing, saw cut depth, 6. Longitudinal edge drains. and timing of joint formation are all critical to the Joints '. performance. Proper transverse joint Design for both If jointed reinforced pavements are used, dowels should plain and reinforced pavements will specify the joint be placed across Joints to assist in load transfer. Due to interval that will control cracks and provide adequate the longer joint spacing, the joint opening is wider and load transfer across Joints . load transfer by aggregate interlock becomes ineffec- tive. Recommended dowel dimensions and spacings for joint Spacing jointed reinforced pavements with joint spacings greater than 20 ft ( m) are given in Table 1.