Patent Application
20150048543
The present invention relates to shuttering and concrete laid with it.
Concrete is laid in shuttering, or formwork, which is often timber-work, which is removed after the concrete has set to the green state. This is a state which is hard to touch, but is not yet cured to full strength. As is well known, concrete is strong in compression, but less strong in tension and shear.
Traditionally for concrete slabs laid at ground level timber shuttering has been used. Once the shuttering has been removed when the laid concrete is has set to its green state, a neighbouring slab can be laid against a previously cast slab. There are limits to the sizes of slab that can be laid, largely because as concrete cures to full strength, it shrinks. Thus gaps develop between adjacent slaps. At these gaps sharp edges between the horizontal face of the slab and its vertical face at the opening gap. The sharp edges are known as arrises.
Application of vertical load to a horizontal slab face close to its free vertical face results in shear stress at the respective arris, which stress can be higher than the concrete can resist. The result is cracking of the arris, also known as spalling. We have produced a variety of so called “joints” which reinforce arrises, whilst at the same time allowing the concrete to shrink. Most joints include complementary arris protection members firmly connected into the concrete on their sides, with gaps arising between the complementary members.
Much of the stress in unprotected arrises arises due to vehicle wheels dropping slightly into the gaps between the slabs and impacting the arris towards which the wheel is rolling. The individual wheels drop little, but the cumulative fatigue stressing can result in comparative rapid spalling, in particular where tyres are solid as opposed to pneumatic.
In our recently filed patent application No 1115940.7, we have proposed a free movement, arris protection, construction joint for dividing two concrete slabs to be poured on opposite side of it, the joint comprising:
We proposed the regular wave shape to reduce the ability of a wheel to drop into the gap between the arris protection members. Without the drop, the wheel does not impact the members so severely, lowering their task in preserving the concrete from shear stress.
We now believe that for certain applications, a regular wave shape gap between two arrises can restrict the shear stresses to levels such that anis protection members are not required.
Accordingly, in a first aspect, we provide a method of laying concrete slabs comprising:
In use of the slabs by wheeled vehicles, load transfer from one slab to the next is progressive as opposed to step like. The shear stress at anyone point in the concrete along the wave shaped arris is significantly lower than it would be in a straight arris scenario.
We anticipate that a wide range of regular wave shapes will be effective, including wave shapes having straight portions, such as triangular, trapezium and square waves. We prefer that such wave shapes be given rounded corners to avoid stress concentration.
Whilst we prefer for the shuttering to impart the regular wave shape to the full depth of the slab, we can envisage the wave shape being tapered in its peak to peak extent with depth through the slabs.
According to another aspect of the invention there is provided concrete slab shuttering for use in the method of the first aspect, the shuttering having a regular wave shape along its length.
Whilst the invention provides that the shear stress at the arrises is restricted, the overall loading of the slabs remains the same and with it the tendency of one stab to be depressed whilst loaded and then the other as the load is passed to it. To restrict relative movement and transfer load from one slab to the next, it is known to provide dowels, both in rod and plate form, the dowels extending from one slab, across the gap to the next one.
Such dowels can be provided for in the shuttering of the invention. A variety of manners for this provision can be envisaged, in particular housings in the shuttering in which dowels are installed prior to casting of the first slab, with the shuttering being drawn horizontally away from the wave shaped slab face after curing to green state. Alternatively the shuttering can be provided projections onto which plastics sleeves can be fitted prior to casting of the slab. On removal of the shuttering, the sleeves remain in place and dowels can be inserted into them, to have the second slab cast around their protruding portion. Where the shuttering cannot be withdrawn horizontally and vertical movement is required as well, the sleeve carrying projections can be removable from the shuttering in order to allow such withdrawal, leaving the sleeves in place.
Typically, corners of the slabs to be cast will be formed at intersections between different lengths of shuttering.
It is envisaged that at corners of the slabs to be cast, the shuttering may be provided with short median flats and for strength of the corners, the shuttering waves may be arranged for the first waves to be directed in the same clockwise or anti clockwise direction with respect to the point of intersection of the short median flats.
Alternatively, it is envisaged that at corners of the slabs to be cast corner abutments may be provided between the lengths of shuttering, in abutment with respective corner ends of the lengths of shuttering. Typically, a corner abutment has a depth equal to or greater than the depth of the slabs and an extent between its outer edges and through its centre which is equal to or greater than the “peak to peak” height of the shuttering waves. A corner abutment with such an extent between its outer edges located at a corner of a slab to be cast will intersect the shuttering lengths at any point along the wave shape. This intersection permits the shuttering lengths to be cut to size without requiring short median flats at the corner ends.
It is envisaged that the corner abutments may be any shape, preferably a regular shape such as square or triangular and are ideally circularly cylindrical, with an outer diameter equal to or greater than the “peak to peak” height of the shuttering waves.
Typically corner abutments are provided between two or more shuttering lengths which are arranged to intersect to form corners of the slabs to be cast. Preferably corner abutments are provided at intersections of three or more shuttering lengths which are to form the corners of three or more slabs to be cast.
To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:
Referring to the drawings, a sub-base 1 is prepared for a concrete slab 2 to be laid on, with straight shuttering 3,4 on two adjoining sides and shuttering 5,6 of the invention on the other two sides, beyond which further slabs will the laid.
The shuttering of the invention comprises casting face member 11, which is of rounded-corner-trapezium, regular-wave shape when viewed from above. The “peak to peak height” of the wave is 300 mm, with flats 12 at the peak being nominally 150 mm in length along the shuttering with a 25 mm radius between them and the angled flats 14 between the peaks extending at 45° to the length of the joint. The shape of the wave is stabilised by upper and lower pieces 15,16 of concrete reinforcing bar (rebar) welded along the length of the shuttering on an outer surface 17 of the member. At 1 m centres, tubes 18 are welded to the outer surface in outer troughs 19. The tubes are stabilised by pieces 20 of rebar connecting the outer end of the tubes to neighbouring outer peaks 21. The tubes are cross drilled 22 for pins 23 locating removable supports 24 for sleeves 25 to extend into the concrete when cast. The pins and supports are chained 26 against loss. Triangulated jacks 27 are provided along the length of the shuttering. The jacks have vertically arranged screws threads carrying supports 28 engaging the upper rebar 15, The jacks abut the lower rebar and hold the shuttering upright. Adjustment of the jacks allows the shuttering to be adjusted and regularly supported level.
With the shuttering set up and sleeves 25 fitted to the supports, the slab 2 is pour and tamped. It is allowed to cure to its green state.
The pins are removed and the supports withdrawn, leaving the sleeves in the slab. The shuttering is lifted away from the slab and set up again one slabs length from its first position. Dowels 32 are inserted in the sleeves.
The exposed face of the slab is left with protrusions 33 having the regular trapezium wave shape of the shuttering. The protrusions extend through the full depth of the slab. When the next slab 34 is cast, complementary protrusions 35 will form in the troughs of the protrusions 33. They interdigitate. The dowels have their one ends in the sleeves of the first slab and their other ends cast into the second slab, whereby they can transfer vertical load from one slab to the next.
In
The wave shaped shuttering lengths have 60° trapezium waves 51 through out most of their length. At their ends they have one pitch 52 of 45° trapezium waves. These provide 45° flanges 53,54 for abutment at the corner opposite from the plain corner. At the corners where the wave shuttering meets the plain shuttering, the flats 55 of the 45° wave abut the transverse flanges 46,47. The end 45° flanges 56 abut the angle flanges 48. In this way, the four pieces of shuttering mate together to contain the concrete poured for the slab 2.
Referring to
On final shrinkage of the slabs, a gap will open up between them following the wave shape, since the slabs will bond to each other only weakly and such bonds will break in preference to cracks developing in the body of the slabs.
The process of laying slabs will have been continued with wave shaped shuttering of the invention having been used wherever one slab extends on from another.
In use, when the slabs have cured to full strength, a wheel rolling over the gaps between neighbouring slabs will progressively increase their area of contact with the next slab as they decrease the contact with the previous slab. Even non-pneumatic tyres can be expected to apply a reasonably uniform pressure across their tyre contact area to whatever is supporting them. Thus the load applied to one slab will be progressively transferred to the next slab, added to which the dowels hold the slabs level with each other. The result is the arrises at the gaps do not receive a shock loading—or at least such a large shock loading as would be the case in the absence of the interdigitation. The arrises are expected not to suffer serious spalling.
The invention is not intended to be restricted to the details of the above described embodiment. For instance, the wave shape shuttering can be provided with flanges in addition to the rebar for enhanced stiffening.
As shown in
Alternatively, as shown in
Referring to
The abutments 110 are positioned at the intersections between lengths of shuttering 115, 116. The shuttering 115, 116 is cut to size for abutment against the corner abutment 110 at their respective intersecting ends 117, 118. The position of the cut at ends 117, 118 is independent of the wave shape of the shuttering, and is at the flat at the peak at 117 for shuttering 115 and at an angled flat at 118 for shuttering 116. The outer diameter 111 of the abutment 110 is larger than the “peak to peak” height of the shuttering and so the abutment will be in abutment with the shuttering at any point along the wave shape.
In use, corner abutments 110 may remain in place throughout casting of each of the slabs with which they are in abutment. Once the slabs have been cast, each abutment 110 can be removed and further slabs cast in their place. Alternatively, an abutment 110 may be removed before casting of a final slab, such that a corner of the final slab is cast against the edges of the slabs which had been cast against the abutment 110.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1203477.3 | Feb 2012 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2013/000072 | 2/22/2013 | WO | 00 |
