Precast / CIP slab mix 2

[msquared48]

Anyone recognize the name of this type of concrete CIP/Precast slab construction from the 60's?

Looks like concrete block (hollow core) infill used as forming material/chase/flat ceiling, with spaces in between for reinforced CIP joists. Joist reinforcing is cast into a precast concrete section that is also used as a bottom form between the hollow core planks. Has a 2.5" +- topping slab over all.

Kinda looks like a predecessor of the PT hollow core slabs.

The project is an 8 story circular tower that is owned and managed by the local Housing Authority.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[concretemasonry]

That is a floor filler system that has been used for decades (mainly in Europe).

The most common masonry units today are concrete masonry units, although in some areas they still use hollow clay units. The concrete units are either conventional shaped units (nominal 6" or 8" thick or a variety of special shapes with cutouts for the beams depending on the system.

The beams/joists may be precast, pre-stressed beams about 4" or so in width at the base that carries the main reinforcement and has a smaller "bulb" on top for minor and handling steel. The beam casting beds are steam heated steel beds up to 150' long where the strands are pulled. They are usually a maximum of nominally 6" or 8" high. The other beams consist of a special truss that is cast into a concrete base.

The beams are set and shored in the case of longer spans and the masonry units are place between the beams with the block cut-outs resting on the bottom "bulb" or concrete of the beam. Concrete is usually poured between the beams to tie everything together and as a 2" topping over the the block/beam assembly. After a few days curing, the shoring is removed. The bottom/ceiling is usually plaster as a finish coat.

There are many similar systems depending on the manufacturer, who often makes the beams (truss or pre-stressed) and special shape units, so concrete fillers have become the most common fillers. I have seen these systems using AAC lightweight block and even a wood fiber concrete block.

The most unusual application I saw was the use of the system without topping for steep sloped roofs on homes. The most common use of the systems are for single family homes and low rise apartments where the maximum span is 4 or 5 meters. Beams can be made to any length, may be carried in stock and the need for large cranes is eliminated and material handing is easier than erecting large concrete precast units. Odd shaped or curved structures or buildings with chopped up floor plans make for good applications of the system.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[bookowski]

Have seen similar things, but not exactly like that - try doing a google search for 'beam and block floor'.

Check out this link, on the last page of the pdf they show something very similar to what you have:

http://www.concreteconstruction.net/images/cost-effective concrete floors_tcm45-590177.pdf

 

[msquared48]

So, in this case, with the 2.5" slab and joists cast as one unit over the precast planks as shown in the plans in all the sections, and no special reinforcing showing up in the planks on these same plans, I would have to assume that the planks are just for forming purposes with no structural contribution to the floor system capacity.

That may not be the actual case, but without verification of the reinforcing, would be a conservative approach. I'll have to run with this. Thanks...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[TXStructural]

The floor is one of many types of filigree systems. Geography will give a hint as to the origin of the particular system, as they varied by location. Modern "voided slab" variations include systems entirely CIP and others using precast panels and thicker toppings to produce voided flat plate floors with smooth exposed soffits.

Some general information is in Wiki:

http://en.wikipedia.org/wiki/Filigree_concrete

 

[hokie66]

Mike,
I don't see that the floor would work at all without the bottom bars in the precast planks.

 

[concretemasonry]

There are no individual bars in the bottom planks. since they are pre-stressed or trussed.

I think a previous post got lost regarding the use of that system that is commonly used on multistory residences and low rise apartments for decades internationally.

In past practice, the key is that the pre-stressed or precast beams beams/joists are supported at intervals BEFORE the beams are set. The beams/joists are set and then infill block are placed to reduce the preliminary dead load and provide a system to accept the weight of the grout/concrete. After the cementitious materials have cured enough, the supports are removed to allow more construction. They key is the initial temporary supporting beams for the spans for the use if deflection is problem for long spans.

Usually, the cementitious materials are a "grout-like" for complete bonding and filling that is topped with a concrete flooring mix with the same small pumps. This creates a hybred composite pre-stressed/reinforced system that has had performance for decades that is difficult for modern engineers to grasp if it is not looked at in total concept. The compressive strength of the topping is not of great consequence, but the German standards do require impact resistance for the infill materials (AAC and wood shred/ concrete) for resistance and durability.

It is just a proven international method proven to bond materials together in a flooring system for many decades that has survived and is used, despite some of the "filler" block materials. A good engineer can sort out the various materials, construction/procedures order to determine the acceptability.

For odd shaped plans and curvatures there has to be some professional recognition of lateral load distribution to sign of on a textbook/code project.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

Dick, the system which M^2 is talking about did not involve prestressed elements. Have a look at his attachment. It has bars in the planks, and trusses which are incorporated into the cast in place part of the system.

 

[ishvaaag]

The whole of Spain buildings is made of this kind of arrangements. Usually, as hokie66 says, when some kind of truss is involved, the base is not prestressed, but there are as well inverted tee prestressed elements that serve to similar purpose. The trussed ones use to need be shored at thirds or fourths prior to concreting, the prestressed ones may need the same or neither at all depending upon the condition. The flooring system is almost invariably finished with gypsum in the underside, except where hanging ceilings are used.

The most typical problem of these floors (counting that the structural members are sound) is some cracks appearing in the finishing gypsum between the member and the filling part, this due either torsional behavior badly accounted in the design or thermal (usually expansion) movement. Caring for that, the system works.

It is usual to include some bracing inset in the guise of a false or true transversal member at the center of the one way members when the span reaches 6 m. Spans range from 0 meters to about 6.5 meters usually, and very rarely 7.5 meters.

 

[msquared48]

The spans on this system range from abpout 7 feet at the interior to about 25 feet at the exterior. The 4" wide CIP concrete joists were spaced at 2'-4" on center with the precast planks being 24" wide and just under 8" thick.

As near as I could tell, no reinforcing was shown for the planks on these particular polans. It appears from one note on the drawings that the plank design was a delayed submittal, a yet to be design at that time to be approved later by the City. So, without those submittal drawings, or forensics, the reinforcing is an unknown. Thats why, for my structural modification, I'm staying away from the floor and roof slabs and only bearing on the walls that support the floor and roof.

I'm really trying to grasp where the economics of this system comes into play here. Were the planks designed for the full live and dead loads or only the slab dead load with the live being carried by the CIP joists and thin CIP slab. From an economic standpoint, it seems ridiculous not to design the planks for the full dead plus live and abut them together as is done today. I just must be missing something here...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[concretemasonry]

I was describing a "family" of very similar flooring systems.

Some use trusses embedded in a concrete base to carry the "fillers" before the concrete was cast to tie everything together and top the system that usually had wire mesh or light steel.

The other common systems in the same "family" used slender, usually prestressed, beams also made or cut for the spans that support the fillers.

Both systems are supported prior to concrete placement, so the system of separate elements acts as a unit after curing and the supports are removed.

isshvaaag -
You are right the truss system is very common in Spain. Our sister company (I worked for the managing partner company) made special block fillers for projects in southern Spain and also worked closely with several other Spanish manufacturers and I got "dragged"(LOL) into providing technical assistance and visited many projects from Seville to Valencia and south to the coast. I never saw "gypsum" used in the construction, but it could possibly used in some way. For the residential units we built in Marbella, we used a high slump concrete to insure full bond between the beams/trusses and the fillers (pumped with small portable pumps) and cycled the placement in a group of units and followed immediately with a stiffer layer of concrete after the first layer had consolidated somewhat, but still was able to be vibrated into the first layer. This is very similar to the usual masonry grouting methods that allow the wet materials where the excess moisture is absorbed by the drier materials (fillers and possibly beams) to increase density and bond before a final layer to final lift/layer is completed. Timing of the layers was critical, especially in the hot Spanish sun during the hot season. - It was not a fully engineered process since most floor filler systems internationally are based on engineering and tradition, common sense backed with past performance. Visiting Spain at least 4 times (4-5 days) a year for 15 years was a perk for being the only engineer available that enjoyed the country and people that could meet with engineers, contractors, accounting firms and be a return courier, so I always scheduled my trips over a week-end so allow me to go to see "Toro" on Sunday. Fortunately, a later job allowed me to return to Spain frequently to see old friends and keep reasonably current on changes and compare the systems that were used in northern and eastern parts of Europe.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[BAretired]

That system (Beton Precast Joist System) was used by an Alberta firm by the name of Du-Al Blocks (no longer in business). They made a variety of lightweight blocks using wood fiber and cement. The joists were cast-in-place concrete on a 1.5"x 6" wide or 1.5" x 8" wide precast Beton Joist. The filler material between the joists consisted of Du-Al Soffit Tiles made of the same material as their standard block.

Later, they came up with a Beton Joist system and a Beton Plank system which did not require as much or any shoring.

BA

 

[hokie66]

Mike,
Trying to figure what you are saying. Your OP showed bottom bars in the precast planks, varying from 3-#3 to 3-#7. Are you talking about the filler blocks being unreinforced? They are just permanent formwork, not part of the structural system.

 

[msquared48]

Hokie:

That appears to me not to be part of the planks themselves, but a separate precast section of concrete enclosing the bars you mention plus the webbing. It also seems to serve as a form for the bottom of the CIP joists.

Believe me, Hokie, I can fully inderstand and appreciate your confusion here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

I see, you are calling the blocks planks, and I was calling the precast bit the planks.

I think the assumption for the blocks is that they just arch across. As concretmasonry and others said, these systems are common in other parts of the world. I saw a lot of it in Turkey, where they use clay blocks to space the joists. But then there is a lot of construction in those parts that makes you wonder...when a big shake comes, and it happens there a lot, down she comes.

 

[bookowski]

Photo attached of a floor that I came across a year or two ago that is an early version of this type of system. This used terra cotta blocks to create a concrete joist system, the blocks remain in place to create the flush ceiling. An early voided slab as others have noted. As you can see this one didn't fair too well.

As others have noted this is still common in some places outside the US. I know that in Haiti they still do a similar thing using CMU.