Construction professionalsmust put on their six thinking hats when deciding which of the two mostdemanding materials in terms of energy required to produce them is to be usedto ensure a smaller carbon footprint.
The genuine initiatives aboutmigrating towards sustainable concrete building pre-casting is an excellentone, must face the inevitable burden of using two of the most demandingmaterials in terms of energy required to produce them, hence with an accentuatedinfluence on the carbon footprint.
The logic put in place bythe strategies contributing to mitigating such influence are always based onimproving the current status of things. For example, using cement CEM II ofless contain of clinker than the CEM I, improving the properties of thestructural concretes, the partial recycling, avoiding the building wastages.
The lateral thinking termcast by the doctor and writer Edward de Bono, is a tremendous boost in tacklingthe problems in different angles. One of the techniques is the dual approach.This article’s purpose is submerging the professionals of precast concretebuilding in a challenging situation:
Which design would beadopted, what building scheme, system, and techniques would be used if theamount of available cement was limited? Say there is a need for developing a ten-acreland into an apartments zone, and there is a reduced tonnage of cement tocomplete it; or even better, the same amount of cement is allowed to differentdevelopers to see which one can build a larger carpet area with the same G+scheme.
This would be a harsh wayof tackling the sustainability by the root, and most probably the prevailingindicator on how efficiently we were proceeding would necessarily be: thecontribution of each kilo of cement, hence each kilo of steel as well, to theload bearing demand of a building.
In Singapore, one of theMinistry of National Development boards established a ratio called ‘ConcreteUtilisation Index’ (C.U.I.) obtained as:
C. U. I. = Cubic meters ofconcrete / Carpet area
This already tells of theattention drawn on the interest in using lean building systems.
When we go deeper in thesearch for an increased structural performance brought per kilo of cement orsteel, we must study the superstructure and irremediably each separate member wouldpre-casting be used. Either with load bearing walls or frame structures, therearen’t many ways to reducing the quantity of materials employed, but on thefloors.
Figure 1: Buildingof the Amazon automatic warehouse in Barcelona.
When pre-stressed concretehollow core slabs are used to make the floors/roofs, there is an average 25 to40 per cent concrete weight reduction depending on the slab thickness. We mightbe saving around 10 kg/m2 of steel as well on the floors, for whatthe HCS are very attractive members.
As we further dig intotheir value to obtaining a higher load bearing contribution per kilo of cementscoring, we realize that since the floors are lighter, the rest of thesuperstructure can be leaner. Longer non-supported spans are achieved, thusreducing the number of columns required.
Figure 2: Non-supported20.0 meters spans using 50 cm hollow core slabs.
Since the weight of thefloors made with pre-stressed hollow core slabs is less, the depth of thesupporting beams can be reduced.
Following the logic ofthis article, the next step would be to optimize the choice of the HCS.
As we are applying the‘lateral thinking’ to pre-casting, only a higher score in the contribution ofeach kilo of cement to the structural demand matters. This is the sole drivingvariable in the search of the ideal HCS.
In the first place, ourhollow core slab should be cast available with any thickness. Otherwise, whatwould be the point in using HCS to save weight if in case a 7.0 inches slab isneeded? Only the 8.0 inches can be sourced from the pre-casting factories. Thepre-caster must adapt their production to the needs of the builders, and notthe other way around.
Figure 3: Any HCSthickness must be made ready to optimize the floors execution, instead of havingto use the next thicker one available.
Secondly, we wish crosssections with optimized number of cores. A few cores as big as possible is across section design convenient to certain OEMs, but then we are handicappingthe optimization of the HCS choice, because:
- Lessnumber of cores as big as possible doesn’t necessarily mean lighter slabs.
Figure4: #19 cables distributed in 07 positions, or #11 cablesdistributed in 05.
- With asmaller number of cores, there are less theoretical positions for thepre-stressed wires or strands, for what the bending moment capacity associatedwith the area of steel is inferior to when the cross sections offer a highernumber of cores.
Figure 5: Ideally,given a HCS thickness, it can be cast with different possible numbers of cores.
- When thehollow core slabs are installed on the horizontal supporting members, they arenot always simply supported in isostatic mode. In fact, it is frequent to seethem mounted pined or in continuity. This requires opening some of the coresfrom the top of the slabs.
Figure 6: Pinningof the slabs at the horizontal supporting members. It means a restrainedassembly offering negative moment capacity as it happens when the slabs aremounted in continuity.
Trying to use hollow coreslabs with big cores rises a concern on the retraction of the concrete poredwhen opened cores are refilled. Not surprisingly, the International Federationof Concrete (in French: Fédération International du Béton, fib, Lausanne –Switzerland) in his Bulletin 6.0 clearly states that to connect the hollow coreslabs with added steel bars on the negative, it is better a HCS cross sectionwith a higher number of cores.
- We arealso definitely demanding hollow core slabs which can be cut at any length andcast at any width. Otherwise, when the floors are not multiple of thestandardized slab width 1.2 meters, the slabs are cast at full width and, lateron, cut longitudinally, thereby wasting the fraction not used.
Figure 7: Fractionalwidth hollow core slabs to avoid wastage.
- In thesearch for high performance hollow core slabs for precast floors, we arelooking for slabs integrated in the various building systems. It must bepossible to work the concrete surfaces out, so that they can offer an addedcapacity to mobilize shear at the interface with concrete cast at differenttimes, as it happens at the joints.
- The sideindenting on the hollow core slabs makes them suitable for their use in seismiczones. In non-affected areas, they work in diaphragm action even withoutcompression layer.
Figure 9: Theadditional friction obtained with the side indenting exceeds the same broughtby the compression layer.
Would the bending momentor the shear capacity be compromised by eliminating the compression layer, thethickness of the slab may be increased, still hollow.
Figure 10: A sideindented hollow core slab at the site to complete a floor without compressionlayer due to the execution of a radiant floor.
The precast floors madewith hollow core slabs eases enormously to a building execution where everykilo of cement delivers the maximum possible contribution to the structuralneeds of the buildings.