A construction joint, or day-work joint, is one where fresh concrete has to be placed on or against concrete that has already hardened.
This type of joint is different from contraction and expansion joints, which are used to accommodate movement, and from joints incorporating water bars. Some construction joints do not need to be fully bonded, the reinforcement across the joint being adequate to transmit tensile or shear stresses across the slight gap that may occur due to contraction. But many construction joints may require the concrete to be bonded so that shear and tensile stresses are transmitted across the joint, in which case the risk of a shrinkage gap is to be avoided.
In both cases it needs to be recognized that joints always show, no matter how well they are made, so they should always be made to form a clean line on the surface. If appearance is important, such as with high-quality finishes, there are advantages in making a feature of the joints.
1. Location of Construction Joints
The position of construction joints should be settled before any concreting begins. As a general rule, joints in columns are made as near as possible to the underside of beams; joints in beams and slabs are normally made at the centre, or within the middle third, of the span.
2. Preparation of Construction Joints
The first requirement for a good bond is that the hardened concrete surface must be clean, free from laitance and have an exposed-aggregate appearance. After concrete has been vibrated, bleeding occurs by surplus water rising to the surface. The bleed water brings with it a small amount of cement and fines, and these are left on the surface after the water has evaporated. This layer of laitance is weak and, as well as being porous and not watertight, will not give a good bond for fresh concrete.
Similarly, concrete cast against vertical formwork also has a skin of cement paste on the surface, which, although not quite as weak as that at the top of a horizontal joint, is still likely to affect the bond when fresh concrete is placed against it. Laitance from both horizontal and vertical surfaces must be removed if and when a good bond or water tightness is required of the concrete itself. However, if watertightness is to be achieved by the incorporation of a water bar, removal of the laitance may not be necessary.
3. Horizontal Surfaces
There are a number of ways of removing laitance from the top of cast concrete to provide a surface with an exposed-aggregate appearance:
|Figure 1: Washing and brushing to remove laitance about two|
hours after placing of concrete.
- The easiest way is to brush off the laitance while the concrete is still fresh but has stiffened slightly. The timing for this is critical because it depends on the weather and the concrete - in warm weather concrete stiffens faster than in cold weather and a rich concrete stiffens faster than a lean one. The best time will usually be about 1 - 2 hours after the surface water has evaporated. A small brush is used to remove the laitance while gently spraying the surface with water (Figure 1). It is worth having two brushes handy - one with soft bristles and one with harder bristles in case the concrete has stiffened more than expected. Brushing should be done gently so that pieces of the coarse aggregate are not undercut or dislodged - just the tips of the aggregate showing is correct.
- If the laitance has hardened but the concrete is still 'green' - say the following morning - a wire brush and some washing will usually be enough to remove it. The surface should be well washed afterwards to remove the dust.
- Pressure washing can be done up to about 48 hours after placing, but timing is again critical and will also depend on the pressure, otherwise the aggregate particles may be dislodged.
- If the surface has been allowed to harden, then mechanical scabbling must be used. Small hand-held percussion power tools such as those used for tooling exposed-aggregate finishes, or a needle gun, are the best to use. The danger with this method is that it can shatter and weaken coarse aggregate at the surface or loosen the larger particles, so it should not be done until the concrete is more than three days old, and then only carefully. It is a slow and expensive method.
- Wet or dry abrasive blasting is usually suitable only when large areas have to be treated, such as in floor surfaces.
A method used occasionally is to spray a retarder onto the surface of the concrete to 'kill' the set, so that the laitance can be brushed off the following day or later. This method is not recommended because it is difficult to be sure that all the retarded concrete has been removed - if not, fresh concrete cast against it will have a poor bond. The bond with reinforcement may also be affected.
4. Vertical Surfaces
|Figure 2: Stop-end detail at a vertical construction joint. The essential requirements are simplicity of construction and sealing to prevent grout loss.|
Vertical construction joints in walls, beams and slabs will usually have been formed against a stop-end. Stop-ends should be located where the reinforcement is least dense; they should be well made, easily strikable, and fixed to avoid grout loss. A typical stop-end detail is shown in Figure 2.
Most vertical construction joints do not require any surface treatment, plain smooth surfaces being quite satisfactory. Only when a joint is subject to high shear forces (and the engineer must decide this) or when a monolithic watertight joint is required will the surface need preparation. Suitable methods are as follows:
- If the stop-end can be removed some 4 - 6 hours after concreting without disturbing the main formwork and reinforcement, a spray-and-brush method as described in method 1 above for horizontal surfaces can be used.
- If the stop-end is removed the following morning, the concrete will usually still be green enough for the cement skin to be removed to a depth of about 2 mm using a wire brush. The surface should be brushed immediately after striking the stop-end.
- If the surface has hardened, then light scabbling, pressure washing or abrasive blasting are likely to be necessary to obtain the right texture.
- Expanded metal mesh, suitably framed, is particularly useful for stop-ends, especially when the reinforcement is congested. In some situations, such as where watertightness at the joint is not essential, expanded metal can be left in. If it is removed the following day by pulling it off, the surface should then be sufficiently rough and laitance-free for no further treatment to be required. Where the joint line and appearance are important, the expanded metal should be kept about 40 mm away from the face to avoid breaking off the corners or arrises along the face.
5. Concreting at Construction Joints
It is essential for the fresh concrete to be placed and compacted s that it bonds with the prepared surface. Poorly compacted concrete or honeycombed concrete at the bottom of a lift in a wall or column leaves a joint which is both weak and unsightly. First, any dirt, dust or rubbish (e.g. sawdust, pieces of wood, nails and bits of tie wire) must be removed from the surface of the concrete. This can best be done by blowing out all the dirt and rubbish with a compressed air hose. If compressed air is not available, then thorough brushing out is necessary. This cleaning out should be done before the formwork is erected, but if there is still some debris left after erection it should be removed by taking out one of the stop-ends.
On prepared concrete surfaces the use of mortars or grouts or wetting the face of a joint is not recommended for the following reasons:
- Tests have shown that the bond between the hardened and fresh concrete is not significantly increased.
- The restricted access to a horizontal joint at the bottom of a lift for which the formwork has been erected makes it difficult to ensure the grout or mortar has been uniformly applied; in any case, it would need to be scrubbed into the surface to be effective.
- It is virtually impossible to apply mortar or grout to a vertical joint - especially when the formwork is in place.
- There is a danger the grout or mortar will dry out before the concrete is placed; any drying out puts back the laitance, which had been carefully removed.
- The appearance of the joint may be spoilt by a line of different colour.
Succesful construction joints are achieved simply by careful placing and thorough compaction of concrete against a properly prepared surface.
The first layer of concrete must not be deficient in fine material. If there is any danger of losing mortar from the concrete by leakage while transporting or placing it, the first batch should ideally be made richer than subsequent batches by reducing the coarse aggregate content. With ready-mixed concrete there may be a tendency for the beginning of the discharge to be rather coarse, in which case the first barrow-full may be discarded.
The first layer should be spread uniformly over the surface to a thickness of only about 300 mm. For small columns it will probably be necessary to use shovels to avoid putting in too thick a layer. Discharging a 0.5 cubic meter(cum) skip into a 600 mm square column, for example, will result in honeycombing at the bottom. When casting columns and walls the poker should always be put in before the concrete goes in, and be drawn up slowly as the concrete is progressively placed; this avoids compacting the surface layer, which would make it very difficult for the air trapped lower down to be expelled upwards subsequently. If using a skip for walls it should be moved along the top. Baffle boards are useful to make sure the concrete is discharged cleanly to the bottom of the forms. The first layer of concrete must be thoroughly compacted by poker vibrators inserted at close centres, depending on the size of the poker and the consistence class of the concrete. Lighting may be necessary for seeing the concrete at the bottom of the pour to check that it has been properly placed and compacted.
It is usually undesirable to make concrete flow horizontally using vibration, but at vertical joints some flow of the concrete towards the joints helps to avoid possible lack of compaction. As the layers of concrete are placed in a wall they should be kept back about 150 - 300 mm from the vertical joint and the poker used to make the concrete flow towards the joint; this needs particular care and a welldesigned concrete if segregation is to be avoided.
Concrete on site - 7: Construction joints. 1993, British Cement Association, Crowthorne. Ref. 45.207. 8 pp.