Concrete Slip Form and Properties of Materials 1

[DanGalbraith]

I am new to this forum, if this is not the correct place to ask this question please forgive me.

Background

The service I provide is to produce Digital Terrain Modeling for Construction Machine Control. I also provide detailed construction support services such as existing condition surveys, stakeout computations for the contractors personnel and so on. As a surveyor for the last 32 years, I have performed my share of construction staking, but never got involved in the construction process.

The current project is a concrete slab replacement for the local airport. The contractor is using a slip form for the pilot lanes and will hand form the lanes in between.

The contractor is allowing me free access to the process so I can learn more about it. As a surveyor there are 6 things I concern myself with which are x,y,z, roll, pitch and yaw.

Question

As the concrete exits the down stream side of the slip form, the concrete expands latitudinally. I am told that this is a constant predictable action.

My questions are:

1. Why can the expansion be predicted?
2. What is the formula to determine the expansion?
3. How is the expansion affected by the water to cement/aggregate ratio?
4. How is the expansion affected by the depth of the material?
5. Why does the mixture expand latitudinally and not in the z plane?

There are a few good references on line and one of the best is the Pavement Tools Consortium at Washington University (

http://training.ce.washington.edu/PGI/),

but they don’t get into my questions in very much detail.

Thanks in advance

Dan Galbraith

 

[Drumchaser]

Hey Dan

I am a relative new comer to this forum as well. You might want to post this at the "Concrete Job Site technique Engineering" section. That having been said I will comment on your concerns with an emphasis on the practical application and leave the forensic and thermal comments for some of the other folks in this forum that are surely more versed in such matters.

Your comment on the latitudinal or "transverse" expansion to the center line of the slab as the slip-form moves forward:
depending on the slump of the concrete (how wet or dry it is) will be a big factor on how much "edge sluff" or transverse expansion, you will encounter as the paver moves forward, the usual spec. for slip form concrete slump is about 3/4". If in fact this is what you are referring to.

The two major types of Concrete pavements "here" are CRCP ( Continously Reinforced Conc. Pavt.) and CPCP (Contraction Design) both with varying depths depending on your pavement design. I would guess your plans require 12"-15" CRCP, with double mat reinforcing steel (just a guess)
The slip form process is most prefferred for many reasons (mostly speed) as you will encounter on the formed "hand pours".
My experience is of the highway variety so your spec.s may be a little different, although it is my understanding some of our spec.s were derived from airport runway type construction with regard to the texture applications ( astro-turf drag and tining of 1/8"-3/16" in depth)
Back to your concerns...........which appear to be the cross slope and vertical profile. Once the offsets are in place and the contractor sets up the stringline(s) we pull a string line transverse and measure down to check depth and location of the re-steel mat(s)to be in compliance with the tolerances given. Also the multi-piece tie-bars need to be graded out along the side(s).
If the offsets along the side(s) of the paving are correct and the slip-form machine is fuctioning properly (no hiccups)you should have true cross-slope and vertical control established, with the finishers using 10 ft long handled straight edges to check the trueness of the slab behind the operation.

It has been my experience that the "YIELD" (over or under-run) of the concrete usually gets to be a BIG area of conflict (finger pointing) if the subgrade is not true.

Your project most likely has a "ride" or profile spec. relating to the smoothness and trueness of the vertical profile (when driving down the lane) transverse constr. joints etc. With penalties being diamond grinding of the areas where the pavt. that does not meet spec.

We do not get concerned with the expansion of the longitudinal joint other than if the edge does not stand up vertically, which means the slump exceeds the spec. and the edge will have to be hand worked or pulled up. If the edge cannot be fixed we have the edge sawed out and repoured.
Hope this helps.

 

[concretemasonry]

When you get into the low slump concrete, there are factors that normally have much more effect than typical poured concrete. You have a greater reliance on the mechanical properties of the mix (aggregate SHAPE and gradation - especially fines shape, cement content, moisture content and vibration - if any).

This is somewhat similar to the classic soil compaction and density with a reliance on the material and moisture content, but to greater extent lesser degree.

Angular aggregates will hold a shape better as will drier mixes. Increased cement contents will require more moisture, which works against the mechanical strenth that is required to hold the shape of "green" concrete until the hydration process provides the "chemical" bonding.

The area of very low slump concrete placement is in an in-between area, is unpredictable because of the variable conditions and lack of real uniformity of the concrete mix. Certainly, a tradtioonal concrete mix with just less water is not the solution.

When you go a step further into the zero slump concrete you are in a different world where the mechanical strength of the mix is critical because of the abilty to produce extremely unform concrete, control vibration and control curing. - Examples are some precast elements, concrete pipe, concrete block and high strength concrete pavers. In this area, a gradtion is just a very rough measure of the materials and vibration and moisture are much more ctitical. The cheapest way to get strength is adding water, but it is contrary to quality controls and the end cost. There have been millions of dollars spent internationally on vibration and the differences between uni-directional and bi-directional vibration in addition to the frequency variations.

Your area is really an in-between area where there is very little real research and formulas due to the lack of controls.

You have a definite challenge!

Dick

 

[DanGalbraith]

Thanks Drumchaser and Dick, not only do I appreciate the thoughtful detailed answers, it’s actually very refreshing to be able to discuss these issues on this level.

There are more questions, but first a bit more background.

This project is the first successful application of fine grading with Machine Control this contractor has experienced. However it was not without it’s problems.

We began by using Laser augmented RTK GPS to do the sub-base work. Generally we can depend that 0.1’ vertically is expected with standard RTK and much better with Laser augmented RTK, but it did not work for several reasons. I could write pages on the theory and application, but for this purpose suffice it to say that the 25’ radar tower ‘got us’.

My recommendation to the contractor was to go to Robotic Total Stations to control the grader in this application. In a survey environment a vertical precision of 0.01’ is very doable if the procedures are adhered to. The error budget in the machine environment puts us more in the 0.05’ range vertically. (error budget = Instrument set up, accuracy of instrument measure up, precision of instrument, distance from instrument to measure point, accuracy of measure up at the blade, condition of the joints or pivot points on the machine, operator experience, precision of the cross slope sensors, type of material, and probably a few more less significant things)

The contractor actually used 2 robots, the first one on the machine, the second one in a traditional survey use as a grade checker. On the sub-grade material (local soils) we were finding the 0.02’+/-, on the base material (crushed concrete) it was a little less. I produced 3 surface models for this project. One for the model design (AutoCAD 2008 with Carlson Civil 2009), one for the machine (Topcon based) and the last one for the grade checker (Tripod Data Systems, (TDS)). The base material is designed to be 6” deep and the inspector told me that it is at 6” on about 99% of the job with a few anomalies at 5 ½” to 7”.

On the DTM, I chose to go with a TIN model and not a grid model. The grid model might actually produce a smoother model and is the recommended model type by some Machine Control system manufacturers. In this case we have a design point at each joint elevation and the TIN was quicker to produce.

The contractor, very predictably is watching this project like a hawk with an eye towards errors or conditions created by others that affect the schedule and profitability of the project. As for me, I wouldn’t have it any other way. Once the desired outcome is realized, I will have a client for life.

The next big technical issue is to introduce the contractor to ‘Stringless’ Machine Control technology. It’s out there and is being used successfully on both airport work and roadway. The cost of the system is not staggering, but it is hefty. If the Stringless technology can be used to eliminated the strings...well, lets just say the contractor can be much more profitable.

Hence, my interest in transverse expansion in the extruding concrete becomes a possible factor in the placement of the ‘virtual stringline’ model. It may well be that the properties of the concrete are not a concern of any kind for me, but as a life long student of the technical aspects of my work, I just want to know as much as possible. I guess I never got over my R&D days at the Corps of Engineers.

There are very important design issues that translate into the construction of the project like smoothness, thickness and finish. All of which is very important, but not what a surveyor does for a living.

The specification of the concrete is that it is PCP with doweled expansion, contraction and construction joints. We are connecting to existing concrete slabs which are also dowelled. The typical slab width is 12’6” and the slip form is placing 25’ lane widths. The intermediate slab joints are dowelled with tie bar baskets. The concrete is 16” deep.

I do not have the actual concrete mix specifications, but asked the senior inspector about the slump value and he said 2. I didn’t ask him 2 what.

Questions

When using a slip form, can we expect the transverse expansion of the pilot lanes to be uniform within say, ½” given the mix and slump to be constant?

Assuming the slip form is used between the pilot lanes and if for some reason the actual placed concrete is varied in the existing pilot lanes between 24’-11” and 25’-1” in width can we set the side walls at 24’-11” (or at the clearance that would accommodate a 24’-11”) to allow for clearance and still be able to rely on the mix to fill the 25’-1” width.

The slip form runs on strings or existing material at the 4 corners of the machine and has a horizontal and vertical sensor bar at each location. How much actual adjustment on the width of the extruded concrete does the slip form accommodate? In other words, given the previous scenario; if the side walls on the slip form are set at a clearance to accommodate a 24’-11” lane width, how much transverse adjustment do we have?

 

[Drumchaser]

Hey Dan

My early background was construction layout of the
"throw chain" and hand calc.s variety. Sounds like your in the high-tech cutting edge variety!

We have seen presentations (footage)using some forms of "fine grading with Machine Control". It looked very efficient, however somewhat cost prohibitive if the machinery was not actively working on a feverish schedule in order to pay for itself.

We routinely see the 'Stringless' Machine Control
technology on some of our contractor's ACP laydown Pavers, but have yet to see a slip-form application. We have had good results.

Questions

When using a slip form, can we expect the transverse expansion of the pilot lanes to be uniform within say, ½" given the mix and slump to be constant?---- I think so...however adjustment of the side plates may be required. This seems to be very important to you.......not knowing the layout of your paving plan I can only guess you are trying to make sure it all fits correctly with-in the pay limits of your plan, steel placement, and existing pavt. limitations. The paving contractor who is providing and operating the paving spread should have knowledge in detail about the variation of the adjustment and variation of the edge plate(s). The two major equipment suppliers of this machinery that come to mind are Gomaco and Miller. Usually these folks will discuss and provide support in great detail on their products. Video's provided at request.

"If" I am understanding your subbase correctly, it sounds like you are placing your conc. paving on crushed aggregate. We have always placed ours on ACP in order to allow for expansion or movement. Concrete pavt. that is not allowed to move or slide does not sound like good construction practices.
Also it sounds like your spec.s are of the dowel bar basket flavor....transverse every 15 ft or so? no re-steel other than tie-bars to existing and along the sides of the new pavt.? If the tie-bars are pre-placed instead of wet-set (longitudinal) then I can relate, somewhat, to your edge variation worries.

The next two questions would be a for the experienced paving sub and or the paving equipment supplier.

After our subbase and ACP "Bond breaker" is placed we profile the proposed area of placement (old school) with a level and rod. Any areas of concern we massage (raise PGL is a last resort) as needed with concerns for X-slope and thickness tolerances. Then provide FILLS for the contractor to set stringlines, and begin finetuning from there. Once the first slab is placed you are somewhat locked in on your YIELD in other matching lanes. Again, this is usually a prime area of concern to the one paying the bill for the concrete.

Hope this helps.

 

[DanGalbraith]

Thanks again.

I’m finding the successful use of Machine Control can be improved with an interest in the entire project flow. Just as there are differences in construction practices, there are little differences in the data that I provide to the contractor that make the data more user friendly.

The theories behind the ‘old school’ methods have not changed. It is the tool set and the computing power by the use of data controllers that we put in the hands of field technicians that has changed. The latter actually concerns me in a lot of cases because we have a generation of field technicians that are not required to learn the theories, but can rely on the data controller to do the computation.

The efficiency of ‘standard’ Machine Control is said to result in a grading project that can be performed in about ½ the time. Of course, these numbers come from the manufacturers and the sales reps. In practical use, I find that an experienced operator can certainly cut the time to grade a site. It is all relative and not all sites are the same. Even a 20% increase in efficiency on a $500k job will just about pay for a retro fitted system. The cost can be said to be $125k and is spread between the hydraulics, cross slope sensors and generally an RTK (Real Time Kinematic) GPS unit.

GPS is a very useful tool, not anywhere the panacea that we would be lead to believe. RTK by specifications alone can never be any more precise than 15mm (0.05’) per unit in the vertical. At best I never let my contractors believe the solutions they will get to be better than 0.1’. Adhering to a stricter use of the error budget and the procedure the vertical solution is actually more in the 0.2’ range.

RTK is based on a Base unit set on a known position and a Rover unit which can be at the machine or on a rod in a more traditional survey use. The biggest advantage to GPS is that 1 Base can handle multiple Rovers. It does not require much concern about the control geometric relationship (strength of figure) and the learning curve is quicker.

Robotic Total Stations are the real key to precision. The biggest draw back is that 1 robot can only control one rover. It also requires more attention to the control layout design, i.e., interconnected control points that have strong geometric relationship one to another.

The only ‘Stringless’ system that is successfully controlling the traditional string machines (C&G, asphalt and slip form pavers, (etc.)) to my knowledge is based on robotics. This is especially so for slip forms.

The setup for slip forms is based on 3 robots used in a continuous chain. The front pair of robots controls the slip form. One prism is mounted on the front and one on the rear thereby controlling the yaw component of the movement. The roll or cross slope is controlled by the cross slope sensors and the pitch or forward grade is controlled by the robots.

The third robot has been positioned upstream of the machine as the machine moves forward. As the back robot of the controlling pair is getting too far out of range to control the machine, the software transitions to the upstream robot and the back robot is moved to the front and repositioned.


Daniel E. (Dan) Galbraith

https://www.gulfsurveyors.com/