Tradition And Innovation
Joint optimisation in DfMA Concrete Infrastructure
FLI Carlow Semi-Precast Jointing Philosophy.
Simply put, we pour wet concrete against hard concrete – the way it’s always been done. While in a DfMA context this approach might be interpreted as old-fashioned, with the right production technology and the capacity to manufacture precast units with reinforcement projecting from almost every face this approach brings enormous benefits. Rather than being old-fashioned it reflects the state of the art.
With full continuity of the horizontal reinforcement the moment capacity of the wall is developed in both directions. This reduces structural mass in many applications by allowing two-way bending in the analysis and design without compromise. By executing bond stress calculations for the joint, we can minimise the joint width. A detailed structural design of the joint is included in our standard calculation pack.
As we’re pouring wet concrete against hard concrete the design process for our joint is the normal reinforced concrete design process. The requirements for strength and crack-width control at the interface can be calculated and achieved. The joint complies with all the requirements for the design life of the structure as a whole – no exception. Design life of 100 years + to first significant maintenance is now routine.
The important second line of defence is the C.I. Kasei Company Limited ‘Hydrotite’ hydrophilic strip. This now has an independently verified design life of 100 years (determined by Tokyo Underground for use in it’s tunnelling projects). There is a no more robust self-healing DfMA solution to waterproofing integrity.
Our unique approach to DfMA is to prepare the precast unit eliminating as many as possible of the site operations required for construction. Obviously this includes the projecting reinforcement but less obvious is the scabbling, hydrophilic rebate, lifting sockets and factory-fitting of the hydrophilic strip.
The MKK cone anchor system engages the formwork clamps. Also, we integrate the stability footing to ensure that the most difficult part of the construction is streamlined quickly producing the body of the structure on which the closing operations are relatively simple.
Our products may be up to 11.0m high and weigh 20 tonnes. Some tolerance in the assembly is essential to ensure a smooth installation process. The in-situ joint enables these products to be aligned and connected in a manner which doesn’t rely on a perfect fit or pulling together in a mechanical way which could induce stress. The concrete in its wet state accommodates all the variations resulting in no induced stress before the application of service stress.
Each water-retaining surface of the unit is prepared with it’s continuity reinforcement, scabbled to maximise bond, has a smooth rebated surface to which to adhere the hydrophilic strip and smooth outer edge strip to ensure a smart interface between the unit and the infill in the completed works. By forming a rebate for the adhesion of the hydrophilic strip, fresh concrete is protected from its expansive force in the near surface direction.
Thermal and Drying Shrinkage
In a wall application, typically 70% of our surface is precast and 30% cast in-situ. This means that 70% of the wall has completed its thermal and drying shrinkage before it arrives on site. Only the remaining 30% shrinks on construction. Whereas in conventional in-situ concrete structures joints are located at 6m – 8m centres, we have two interfaces at a maximum of 500mm centres. The actual strain at each interface is only 7% of that in cast-in situ structures. The task the joint has to perform is hugely reduced which is a significant part of the success of the concept.
On inflation, the hydrophilic strip exerts up to 3N/mm2 expansive stress. That’s almost half the crush strength of a conventional concrete block. This ensures a very effective seal against the carefully formed surface on the manufactured product and the wet concrete poured against it on site. This is the optimum use of the hydrophilic although it can be used against roughened surfaces.
While the precast component is relatively expensive, the cast in-situ joint is sourced from local concrete suppliers at approximately £35 per tonne. This reduces significantly the cost of an otherwise fully precast solution by reducing the manufacturing, transport and craneage costs by this percentage.
It’s not the joints you fear, it’s the distance between them. Anyone who has experienced failure at a joint in concrete works might presume that the increased number of joints represents a correspondingly increased risk. We believe not. It’s not the number of joints that represents the risk, but the distance between them, particularly in conventional concrete construction. With significantly reduced thermal and drying shrinkage strains at the interfaces and the self-healing capability of the hydrophilic, our Customer experience of the success of the joint has been vastly improved on in-situ concrete construction methods.
Hydrotite – We Don’t Use Anything Else
We’re the products best ambassador. We get the best product on the market with exceptional technical support for our supplier. There are options but we don’t use them. Traditional non-reactive methods which improve waterproofing capability by increasing through path cannot compare with the effectiveness of the expansive method. Keeping it simple is key and the DfMA approach of doing all the critical preparation offsite in a controlled and monitored environment is paramount.
Units are typically placed in their service positions from the delivery vehicle. The projecting reinforcement is configured in such manner as to completely overlap without conflict. A small number of vertical bars are usually required designed to be placed outside the overlapping cage for simplicity.
Once these bars are fixed, formwork units are attached to the face of the adjoining units by engaging system braces to the MKK Cone anchor connectors. Formwork is lightweight, re-useable and designed to be torsionally flexible so as to match perfectly the leading edges of both adjoining precast units. The key advantages of the MKK cone system are that they’re simple to engage and are fully embedded in the concrete section so that there is no water path through the unit. They are easily dressed on completion to give a neat regular finish comparable with the unit and the joint.