Rigid pavements are constructed with expansion and contraction joints to reduce the temperature stresses which needs proper design and construction. The expansion joints are provided with dowel bars and sometimes contraction joints as well. The size and spacing of the dowel are designed and provided as per the standard specification of the practical consideration. The article provides comprehensive information about the significance, design and usage of dowel bars in rigid pavements.
Dowel Bars
These are a piece of mild steel bars used in between the concrete construction joints to provide mechanical connection, especially in the concrete slabs without restricting horizontal joint movement. These bars are provided to the transverse construction joint in the rigid pavement to affect load transfer from one slab to the next adjacent slab while permitting relative longitudinal movement of the slabs.
Half the length of the bar is bonded in one concrete slab and the remaining portion is embedded in the adjacent slab, but it is kept free for movement during expansion and contraction of the slab, Khanna & Justo (p.388). It allows the opening and closing of the joint, maintaining the slab edges at the same level, and the load transference is affected from one slab to the other.
They increase load transfer efficiency by allowing the leave slab to assume some of the load before the load is actually over it. This reduces joint deflection and stress in the approach and leaves slabs. According to Rogers (p. 224), dowel bars used in expansion and contraction joints are usually positioned on metal cradles so that they will not move from their required position while the concrete is being placed and compacted. These cradles, however, should not extend across the line of the joint.
Also, read: What is Rigid Pavement?: 4 Types of Rigid Pavement
Purpose of Dowel Bars
It is used to serve the following purpose to the concrete pavement or slab:
- The impact load on the slab is reduced and transferred to the adjacent slab while slabs are moving independently.
- It reduces joint failure such as the development of cracks in the corner and the edge.
- The performance of the slab joint is improved.
Design of Dowel Bars
When the transverse joint in a concrete pavement lacks dowel bars (Figure 1), the loaded slab will deflect by dx1 under load P, and the adjacent slab will remain unaffected. Conversely, when the same load is applied to a slab with provided bars (Figure 2), it will deflect by dx2, and the adjacent slab will experience a deflection of dx3. If the bar is firmly embedded on both sides of the concrete slab, dx2 will be equal to dx3. However, dx3 < dx2 and this ratio is influenced by factors such as slab thickness, dowel size, diameter, and spacing.
The slab without dowels exhibits more significant deflection compared to the slab jointed with dowel bars. According to Friberg’s analysis, the pressure distribution along the bar under the load on one slab is depicted in Figure 3. The point of pressure reversal (point a) serves as the criterion for determining the length of dowel bars. In dowel design, the load transfer is calculated considering the dowel system’s capacity, which is dependent on variables such as pavement thickness, subgrade modulus, relative stiffness, and dowel size and spacing.
For designing the dowel bar system, the IRC recommends adopting Bardgury’s analysis for evaluating the load transfer capacity of a single dowel bar in terms of shear, bending, and bearing concrete as mentioned below.
For shear in the bar,
For bending in the bar,
For bearing on concrete,
for a single dowel, Where,
P’ = load transfer capacity in kg
d = diameter of the bar
Ld = total length of embedment of bar
w = joint width
Fs, Ff and Fb = permissible shear stress, permissible flexural stress and permissible bearing stress in kg/cm2.
Length of Dowel Bar
The minimum length of the dowel is calculated as (Ld + w). The load capacity of a single dowel bar is determined by selecting the lowest value among three calculated values for P’. The entire dowel system is assumed to have a load capacity equivalent to 40% of the design wheel load. To find the required load capacity for the dowel group, the total group capacity is divided by the capacity of one individual dowel.
Size of Dowel Bar
The size of the bars is usually between 25 mm to 40 mm in diameter as per the Indian Standard code 6509 – 1965. Stainless steel bars and epoxy-coated bars are used as dowel bars.
Spacing of Dowel Bar
The effective distance for load transfer through dowels is 1.8 times the radius of relative stiffness (l), extending from 1.0 under the load to zero at 1.8 times l. Capacity factors are calculated for the dowel system, assuming different spacings. The chosen design value is the spacing that aligns with the required capacity factor.
Application:
Slabs with small thickness do not qualify for the usage of dowel bars. Thus it is not recommended to provide slabs less than 150 mm thick.
The minimum requirements of dowel bars for different thicknesses of highway pavements are given in the table below as per IS 6509 – 1965.
| Pavement Thickness | Dowel Diameter | Dowel Lenght | Dowel Spacing |
|---|---|---|---|
| 150 | 25 | 500 | 200 |
| 200 | 25 | 500 | 300 |
| 250 | 32 | 500 | 300 |
The recommended minimum requirements of dowel bars for various thicknesses of airfield pavements are given in the table below as per IS 6509 – 1965.
| Pavement Thickness | Dowel Diameter | Dowel Lenght | Dowel Spacing |
|---|---|---|---|
| 200 | 25 | 500 | 300 |
| 250 | 32 | 500 | 300 |
| 300 | 36 | 550 | 350 |
| 350 | 36 | 550 | 350 |
| 400 | 40 | 600 | 350 |
Also, read: Types of Steel Reinforcement Bars
Advantages of Dowel Bars
- The tension and deflection are decreased by using dowel bars.
- These bars help the slabs bear more weight by strengthening them.
- These bars also improve the initial pavement durability.
- Improvements in joint flexibility among the structural members.
- It increases the project’s initial cost while lowering the
Disadvantages of Dowel Bars
- Enhanced intricacy: The process of installing dowel bars can be more intricate compared to traditional joint construction, necessitating specialized equipment and skilled personnel.
- Elevated expenses: Incorporating dowel bars in construction can lead to higher costs, attributed to the added materials and labour needed for their installation, which can contribute to an overall increase in project expenses.
- Maintenance demands: Although dowel bars can enhance pavement durability, they may require more maintenance over time due to potential issues like corrosion, leading to additional upkeep requirements.
- Environmental implications: The production and installation of dowel bars can have adverse environmental impacts, consuming energy and resources during manufacturing and transportation.
Difference between Dowel bars and Tie bars
| Dowel Bars | Tie Bars |
|---|---|
| Are made of stainless steel bars and epoxy-coated bars are used as dowel bars. | Deformed epoxy-coated steel bars are used for tie bars. |
| Are provided to transverse joints. | Are provided at longitudinal joints which are generally at right angles to the centre line. |
| It helps in transferring load from one slab to another. | Prevent lanes from separation and differential deflection. |
| Reduce joint faults and corner cracking | Reduce traverse cracking in the pavement slab |
Also, read: How To Calculate Unit Weight of Steel Bars
FAQs:
Q: What is the maximum length for a dowel bar?
Answer: The maximum length of the dowel bar is 500 mm for the rigid pavement or plain cement concrete slab. However, the length increases from 500 mm to 600mm when used in the airfield pavement.
Q: What is the size of the dowel bar?
Answer: The diameter of the dowel bar varies from 25 mm to 32 mm depending upon the thickness of the concrete slab and is spaced 300 mm from centre to centre.
Q: How is the length of the dowel bar calculated?
Answer: Dowel Length Calculation: The dowel length is calculated as the sum of the distance on either side of the load position, taking into account the dowel spacing and the radius of relative stiffness. The formula for calculating the dowel length (Ld) can be represented as:
Ld = 1.8 X rX 2 + w
where:
Ld = Dowel length,
r = Radius of relative stiffness,
w = Width of the pavement or distance between the joints.
References:
- Standard, Indian. (1960). IS 6509:1965. Code of Practice for Installation of Joints in Concrete Pavements. Indian Standard. BIS. New Delhi 110002
- Khanna, S.K. & Justo C.E.G. (2001). Highway Engineering (8th ed.) Nem Chand & Bros., Civil Lines, Roorkee.
- Rogers, M.(2003). Highway Engineering. Blackwell Publishing
- Fwa, T.F. (2006).The Handbook of Highway Engineering. Taylor & Francis Group, LLC
