2 of the most challenging aspects of constructing with precast concrete are the lifting/handling and connections. In this section, we will deal with the dos and don’ts of lifting, moving and handling of precast concrete elements with regard to safety, productivity and cost-effectiveness.
This article will not cover topics such as installation or troubleshooting as those topics are already covered quite comprehensively by the individual suppliers in their literature. Instead, we will focus on the bigger picture issues that are preventing the industry from moving forward.
Still to this day, reinforcement U Bars and Post Tension Wires are still used as lifting points for precast concrete elements, but those using these types of lifting points do not understand the dangers that they are exposing the workers to. Most lifting designs do not have any built-in redundancy, so a failed lifting point will most often result in an element crashing down to the ground. Even if there are no casualties, a dropped element is often then scrapped as the damage resulting in a dropped element is too much to repair them.
The cost of using a proprietary lifting anchor is approx. 0.5% of the cost of a typical element and therefore can be considered as very cheap insurance against losing a piece of precast concrete. Not to mentions all the likely liquidated damages that could be incurred as a result.
The reason why rebars and post-tension wire are not suitable as lifting points is simply due to the fact that they are not designed for it. Both steels are designed to work in tension and although rebars can be bent, the ductility of the steel is always reduced after the bar is bent due to yielding in the extreme fibres of the steel. So much so, that there are discrete limits on the size of the radius that you are allowed to bend them to. Generally between 4d to 6d depending on the code and the diameter of the bar. Attaching a crane hook to a U Bar will result in the partially yielded bar to be further bent around a radius that is even small than those allowed in the rebar codes. Therefore the risk of failure increases significantly using either rebar or post-tensioning wire.
There are some proprietary lifting systems available in the market that seem to call this suitability in to question but to understand it fully, you need to cut through the wire and look at the cross-section. The wire products sold for lifting have a 7 strand configuration, but the central core strand is made of a fibre core and not a steel core in the case of a post-tension wire. This allows the wire to be much more flexible and bend around a smaller radius than its steel-cored cousin.
There are 2 very common ways of lifting 2-dimensional panels, namely, face lifting and edge lifting.
Face lifting is where a number of anchors are placed into the flat top of an element and the concrete is lifted in a flat position. This is commonly used for staircase landings, man-hole covers and floor units.
Edge lifting is where a number of anchors are placed along the thin edge of an element so that the concrete can be tilted up into a vertical position for storage and installation. This type is commonly used for shear walls, infill walls and façade panels.
The challenge with lifting and handling all precast components is the rigging required to attach the element to the crane but still allowing for the load to be equalised across all the anchors. Some real no-no’s for this would be if fixed-length chains or slings are used, or if you try and use a multiple of 3 anchors in any one lift, so 3, 6 and 9 anchors should only be used if absolutely necessary or with very advanced rigging to ensure the anchors are fully equalised.
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As we have moved towards PPVCs and PBUs the challenges associated with lifting have changed and in many ways become much more complex.
Often the element being lifted have a number of boundary conditions that we need to respect such as wall thickness, concrete weight and centre of gravity.
One of the biggest challenges associated with the lifting of these 3-dimensional elements is the weight of the element due to the availability and range of cranes available on site. There are a number of precasters exploring lightweight aggregates or the use of fibre reinforcement to reduce the weight of the elements, but many prefer to simply use thinner and thinner walls. The industry seems to be settling on a 90mm thickness for a PPVC wall, so back to back elements can provide a 200mm thick finished wall by backfilling the remaining 20mm with a cementitious grout.
The wall thickness will often mean that we need to use a lot more of a smaller capacity anchor rather than just a few larger capacity anchors. This, in turn, makes the equalisation of the loads even more challenging and most precasters prefer to use fabricated lifting frames to assist with the equalisation.
However, as most architects prefer to have a few different geometries in their buildings, it means that one standard lifting frame gets used for a variety of different geometries. Providing the frame is well designed, this can be a good way of minimalizing the capital outlay for a project, but excessive out of plane lifting should be avoided at all costs as this can result in the thin concrete wall spalling away and causing damage to the concrete or, worse still, a failure of the lifting point. Therefore the lifting frame should be designed to be as closely aligned to the wall geometry as possible.
Another challenge for 3-dimensional shapes that needs to be considered is the relative position of the anchors to the centre of gravity (COG). Providing the centroid of all the lifting points coincides with the COG when viewed in plan, then the element will be lifted flat with no tilting. However, as each PPVC might be slightly different, it is not practical to do this 100% of the time and some slight lifting or rotation is deemed to be acceptable.
One thing to be avoided wherever is that the centroid of lifting is lower than the COG. There are a number of exceptions to this, but in general, the element will be much more unstable than it would be if the centroid of lifting was above the COG.
Lifting In Thin Elements
The thin walls do create a challenge for lifting as you are required to place lifting anchors together with any required supplementary rebar (an additional bar near to, but not in direct contact with the lifting anchor) or complementary rebar (additional bar in direct contact with, or passing through the lifting anchor) into a very think section whilst respecting concrete cover and avoiding the mesh.
For Singapore, using supplementary rebars is not favoured by professional engineers, with most of them preferring to use anchors with complementary rebar, but the problem of placement still exists. According to SS EN 1992-1-1 the covered required to ensure bond transfer is just 1x bar diameter, so we only need a cover of 13 or 16mm depending on the capacity of the anchor being used. This makes it easier to fit inside the wall and transfer the lifting stresses into the concrete via bond transfer using the development lengths defined in SS EN 1992-1-1 Clause 8.4. One common trait that has been adopted by some professional engineers and needs to be stopped is the insistence on the complementary bars being the full height of the PPVC and returned under the slab at the base. This consumes an additional 7m+ of steel per lifting anchor which results in unnecessary and additional cost and additional embodied CO2 into the element which is bad for the bank balance and the environment. For a PPVC with 8 lifting points is results in about 90kgs of additional material with a cost of more than $70 per element that could be eliminated.
Health & Safety
The final, but the most important, piece of the puzzle is health and safety. At the moment, their elements are placed into position using a crane and then we need operators to climb up to the top of the element (3m+) just to disconnect the clutches and hooks. There are a number of remote release systems available on the market that contractors and precasters alike could consider and significantly improve the productivity and safety of precast operations on site.
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