GB2285641A

GB2285641A - Dowel bar sleeve

Info

Publication number: GB2285641A
Application number: GB9400611A
Authority: GB
Inventors: Rodney Malcolm Arnold
Original assignee: Permaban Products Ltd
Current assignee: Permaban Products Ltd
Priority date: 1994-01-14
Filing date: 1994-01-14
Publication date: 1995-07-19
Also published as: GB9400611D0

Abstract

A dowel bar sleeve (1) has positioning means, such as inwardly projecting lugs (3) on opposite sides or opposing pairs of ribs on its upper and lower inner faces, for centring the sleeve (1) transversely of a dowel bar (2) to define a gap (6) between the sleeve and each side of the bar (2) and is adapted to contact the top and bottom of the bar (2) in use. The sleeve (1) may be open on its lower side. The gap (6) permits lateral movement of the sleeve (1) relative to the dowel bar (2) with expansion and contraction of the concrete cast around the sleeve in use, while the contact between the sleeve and the dowel bar (2) at the top and bottom permits the transfer of vertical loads from the concrete to the dowel bar (2). <IMAGE>

Description

DOWEL BAR SLEEVE The present invention relates to sleeves for dowel bars Slabs of concrete used in the construction of floors, pavements and the like are susceptible both to contraction during drying and to expansion and contraction caused by subsequent changes in temperature and it is known to form joints between adjacent slabs which allow for their relative movement so that stresses caused by the expansions and contractions are relieved. It is also known to incorporate dowel bars which span the joint between two adjacent slabs of concrete, at right-angles to the joint, to disperse and transfer across the joint loads applied to the concrete To allow for such relative movement between slabs, the portion of a dowel bar in one of the two adjacent slabs must allow that slab to move relative to the dowel bar, or allow the dowel bar to move relative to the slab.

This is usually effected by treatment of all or part of the dowel bar to make it slippery, known as debonding, or by the fitting of a sleeve of smooth material to the dowel bar. Such a sleeve is known as a dowel bar sleeve.

However, problems arise with this known solution when expansion or contraction of the adjoining slabs tends to occur in different directions along the joints, for example, at intersections between joints where there are dowel bars at right-angles to each other, or when a second slab is cast adjacent an existing slab where contraction during drying of the second slab tends to cause relative movement of the slabs along the joint.

If conventional debonded circular dowel bars are used at such intersections, the concrete slabs cannot move relative to one another because they are effectively tied or pinned together by the dowel bars at right angles. Thus, the concrete is prevented from moving and cracks begin to appear in the concrete when the induced stresses exceed the level which the concrete can sustain.

Loads applied to the concrete are discharged to the dowel bars, mainly as shear stress and, in the case of large joint gaps, bending stress. In order to allow the transfer of the shear stress, there must be contact between the concrete and the dowel bar. In the case of a square or rectangular dowel bar, there must be contact between the concrete and the top and bottom surfaces of the dowel bar as the load is applied to the concrete in a vertical direction, but there need be no contact between the sides of the square or rectangular dowel bar and the concrete for this purpose. However, when a circular dowel bar is used, there must be contact with the concrete around the entire periphery of the bar because, in the absence of flat surfaces, the shear stresses are distributed around the whole periphery of the bar.

If a square dowel bar is used, the concrete can move laterally and longitudinally relative to the bar, whilst maintaining load transfer vertically and thus overcoming the tying or pinning effect of dowel bars at right angles to one another; this is not possible when a circular dowel bar is used. The advantage of allowing lateral movement with a square dowel bar has been recognised and previously this has been achieved by attaching compressible material to the sides of the bar by glue, tape or plastic clips. These measures are laborious, costly and prone to errors in fitting.

The object of the present invention is to provide a dowel bar sleeve, particularly for a square or rectangular dowel bar, which allows the concrete at a joint to move both laterally and longitudinally relative to the dowel bar and which can be fitted easily and accurately to the dowel bar.

Accordingly, the present invention provides a dowel bar sleeve having positioning means for centring the sleeve transversely of the dowel bar in use to form a gap between the sleeve and each side of the dowel bar to permit lateral movement of the sleeve relative to the bar, the sleeve being adapted to be in contact with the top and bottom of the bar in use to permit the transfer of loads from the concrete to the dowel.

In a preferred embodiment, the sleeve is substantially rectangular in cross-section but it may be at least partially open on one major side (its lower side in use) and/or it may have other cross-sectional profiles capable of achieving the lateral gaps and top and bottom contact described above. The sleeve is intended to cover at least half of the dowel bar in a conventional manner but in some circumstances may cover the entire bar. The positioning means are preferably of a yieldable material so as to allow sideways movement of the concrete relative to the dowel, without exerting any undue stress on either the concrete or the dowel bar.

Preferably, that end of the sleeve remote from a joint to be formed between two slabs is capped so that, when wet concrete is cast around the sleeve to form a slab, the concrete is prevented from filling the gap. The cap also forms an extension of the sleeve beyond the end of the dowel bar to allow for longitudinal relative movement of the concrete and the dowel bar.

The sleeve is made from a material which will withstand the weight of the concrete and will retain its shape during the formation of the slab, and which will allow the vertical transfer of the load from the concrete to the dowel bar substantially without compression of the sleeve. Examples of such materials are steels and other metal alloys, plastics, rubbers (which, if thin enough, will be compressible only by a negligible amount) and cardboard.

The positioning means which hold the sleeve symmetrically about the dowel may be steel or plastics spring clips either at frequent intervals along the sleeve or at one or both ends of the sleeve if the sleeve material is sufficiently rigid along its length to maintain its shape. Alternatively, a locating collar may be used at one end of the dowel and the end cap at the other, the end cap being designed to centre the sleeve. Preferably, the sleeve is made from a material, such as plastics, rubber or aluminium, which can readily be extruded or moulded and the positioning means are then formed integrally during extrusion or moulding in the form of internal longitudinal lugs, ridges, flanges, vanes or series of nibs or the like on or adjacent opposite sides of the sleeve. The lugs, ridges or vanes may be continuous or discontinuous.

Preferred embodiments of the invention will now be described by way of example, with reference to the attached drawings in which: Figure 1 is a cross-sectional view of the sleeve according to the invention.

Figure 2 is a partial plan view of the sleeve according to the invention in use at the joint between two adjacent concrete slabs, Figures 3a-3f show six alternative possible embodiments of the positioning means of the sleeve according to the invention.

As shown in Figure 1, a substantially rectangular sleeve 1 surrounds a square dowel bar 2 and is held equidistant from the dowel bar 2, along its length, by two longitudinal lugs 3 which project inwardly from opposite sides of the sleeve 1 so as to define a gap 6 along each side of the bar 2. The upper and lower sides 4, 5 of the sleeve 1 are adjacent and in contact with the respective upper and lower faces of the dowel bar 2, for transferring vertically-applied loads directly to the dowel bar 2.

Figure 2 shows the sleeve 1 surrounding a dowel bar 2 at a joint 10. One portion of the dowel bar 2a is non-sleeved, while the other portion 2b is sleeved, the centre of the bar 2 being in correspondence with the joint 10. In the embodiment shown, a portion la of the sleeve projects beyond the end of the bar 2 furthest from the joint 10 to allow the bar 2 to move into the space thus formed. An end cap 11 is used at this end la to close the sleeve 1 relative to the dowel bar 2, and to prevent wet concrete from entering the sleeve 1 during formation of the slab.

In an alternative embodiment, the sleeve 1 may extend the entire length of the dowel bar 2, with clearance at both ends of the bar 2.

Figures 3a-3f show possible alternative positioning means such as: a series of nibs 31 along the lower (or upper) inner face of the sleeve, adjacent each side thereof; two opposing pairs of lugs 32 extending inwardly from near the corners of the sleeve; guides 33 in the form of a pair of continuous inner walls intended to be in contact with the dowel bar; upstanding parallel flanges 34 projecting from the base of sleeve, which may be open between the flanges 34; two V-shaped ridges 35 which extend centrally from the opposite side walls of the sleeve; and opposing pairs of parallel ribs 36 on both the upper and lower inner faces of the sleeve. The lugs 32, the flanges 34, the ridges 35 and the ribs 36 may be continuous or discontinuous.

In use, a first slab of concrete is formed by inserting the portions 2b of a plurality of dowel bars 2 into respective capped sleeves 1 and then pouring wet concrete over the sleeved portions 2b of the dowel bars so that the portions 2a of the dowel bars 2 extend beyond the edge of the slab where the joint 10 will be formed, at right angles to the edge, and are spaced apart along the line of the joint to be formed. A second slab of concrete is cast by pouring wet concrete over the other portions 2a of the dowel bars 2 up to the joint at the edge of the first slab. The positioning means 3 ensure that each dowel bar 2 is held in the correct position in its sleeve 1, that is, that the sleeve 1 is equidistant from the dowel bar 2 along its length.

Alternatively, a first slab may be cast around the non-sleeved portions 2a of the dowel bars 2, the sleeves 1 then being placed over the portions 2b which, in this case, extend beyond the edge of the slab and the concrete of the second slab being cast around the sleeved portions 2b of the dowel bars 2.

However, it is also possible, with small slabs, that a first slab may be cast around capped sleeves 1 (dowel bars 2 being inserted in the sleeves 1 during casting if necessary to prevent the sleeves from collapsing and then being removed when the concrete has set) and a second slab may be cast around dowel bars 2 so that a portion 2a of each dowel bar 2 projects beyond the edge of the slab. The projecting portions 2a of the dowel bars 2 in the second slabs are then inserted into respective sleeves 1 in the first slab when the two slabs are brought together.

Movement of the concrete across the joint 10, that is, movement of the slabs together or apart, is permitted by the movement of the dowel bar 2 inside the sleeve 1 and the portion la of the sleeve as the joint 10 "opens" or "closes", and movement along the joint 10 is permitted as the dowel bar 2 moves laterally within the sleeve 1 due to the presence of the gap 6 created between the sides of the sleeve 1 and the sides of the dowel bar 2. Thus, the slabs of concrete are able to move relative to one another in all directions, even at joint intersections, without causing stress cracks in the concrete.

Claims

1. A dowel bar sleeve having positioning means for centring the sleeve transversely of a dowel bar in use to form a gap between the sleeve and each side of the dowel bar to permit lateral movement of the sleeve relative to the bar, the sleeve being adapted to be in contact with the top and bottom of the bar in use to permit the transfer of loads to the dowel bar from the concrete cast around the sleeve in use.

2. A dowel bar sleeve according to Claim 1, in which the sleeve is substantially rectangular in cross-section.

3. A dowel bar sleeve according to Claim 1 or Claim 2, in which the sleeve is at least partially open on one major side.

4. A dowel bar sleeve according to Claim 3, in which the sleeve is open on its lower side in use.

5. A dowel bar sleeve according to any one of the preceding claims, in which the positioning means are of a yieldable material.

6. A dowel bar sleeve according to any one of the preceding claims, in which the sleeve is made from an extrudible or mouldable material and the positioning means are formed integrally with the sleeve as internal projections on or adjacent opposite sides of the sleeve.

7. A dowel bar sleeve according to any one of the preceding claims, in which the positioning means comprise opposing pairs of parallel ribs on the upper and lower inner faces of the sleeve.

8. A dowel bar sleeve according to any one of the preceding claims, including a cap for closing one end of the sleeve in use, the cap forming an extension of the sleeve beyond the end of the dowel bar to permit longitudinal movement of the sleeve relative to the bar in use.

9. A dowel bar sleeve substantially as herein described with reference to, and as shown in, the accompanying drawings.