Investigation of Minimum Local F-Numbers
(The views expressed in this post are the views of Jonathan Matson and not necessarily those of Somero Matson Group.
There is a source of confusion around the F-number system. This problem is the concept of minimum locals. This report will identify: the original problem, the current solution, the confusion with the current solution and finally the author will also propose a new solution.
Historical Problem
A source of confusion in the F-number system stems from a term known as “minimum local”
Minimum Local F-Numbers can be defined as:
A number or way for the owner to control how bad the worst section of the floor will be. [1]
They are better explained as a workaround to a concern generated by the industry when the F-number system was first introduced. But to understand this we will have to back up and explain the problem.
As is known by many, the F-number system was created to eliminate the confusion caused by the ambiguous “gap under straightedge” method that was the primary method for checking the flatness and levelness of a floor for as long as anyone can remember. This method was beset with many inherent problems, chief of which is stated in:
R4.8.6.2: At the time the document was prepared, no nationally accepted specification has been developed to govern the evaluation of a floor surface using this procedure; therefore, minimum sampling requirements have been established in this section. [2]
The ineffectiveness of the “Gap Under Straightedge” solution was such that it became the impetus for Allen Face to create a new more methodical way to test floor surfaces for levelness and flatness known as the F-Number system as established by ASTM E-1155. This system revolutionized floor flatness testing and concrete flatwork itself, driving the adoption of the laser screed and facilitating a more scientific approach to improving floor flatness and levelness performance in the forming, placing, and finishing process, allowing the industry to progress to the current state where desired Floor Flatness and Levelness can now be practically requested by owners and delivered by contractors.
Current Solution
However, in the implementation of the F-Number standard, there was a significant amount of education and teaching that had to be done before the industry would embrace it, there were many hurdles to overcome and objections. This is partly due to inertia “this is the way it has always been done” but also because to our mind, the gap under straightedge seems more intuitive and easier to understand versus a statistical process which is the backbone of the F-Number system. However, our intuition is faulty here and the industry had to be persuaded to adopt the new system. This was accomplished, however, along the way a compromise was made to encourage the adoption of it. This compromise was the creation of a term known as minimum local F-numbers. Minimum locals are not a part of the ASTM E-1155 standard and were created independently by Allen Face to solve a doubt that the industry had in the beginning of the usage of F-Numbers.
Those who are new to measuring floor flatness and levelness discover something interesting when they begin to read the ASTM E-1155 document. Though countless floors are measured with the minimum local and countless contractors and owners live in fear of the minimum local, there is no such thing as a minimum local in the ASTM E-1155 document. The minimum local was created by Allen Face when introducing the ASTM E-1155 test standard to the floor flatness industry in order to satisfy the two interested parties.
The problem was that since the sections of the slab's FF FL numbers are statistically averaged together to obtain an overall FF FL number for the job, owners worried that place and finish contractors could pour an extremely un-flat and un-level section in one area of the floor then compensate by pouring extremely flat and level sections around it. This would leave the owner with a floor that passed the overall but had a section in it somewhere that is potentially unusable. Allen Face’s solution was then to suggest creating a minimum local FF FL number for each section of the slab that would have to be met regardless of the overall number specified. This would be the minimum acceptable level of flatness and levelness expected by the owner. This satisfied the owner. It has left us with an interesting problem though, that a contractor can perform to the minimum acceptable standard on every section and yet fail the overall for the floor, which seems illogical.
This has created confusion in the industry as it is easy to imagine the situation where a naïve contractor is told he passes the minimum local every pour only to fail the overall at the end of the job. There are other reports of owners using minimum locals then not accepting the use of the Overall terminology in the ASTM E-1155 document, as it is reported that they state that if a minimum local is the minimum quality acceptable then the contractor cannot be held liable for not hitting a number that is higher than the minimum quality acceptable.
Another problem arises, and that is defining what the area size for a minimum local should be. One can do a thought experiment and quickly see that if one shrinks the minimum local to the smallest testable surface area, if there is a problem with that area and it generates a low F-Number it will have much more weight with a smaller amount of runs to average with and cause the area to fail the minimum local. This could be a negative for the contractor.
The positive side though is that if the area is small it makes remediation cheaper for the contractor. If we take our thought experiment the other direction and have the minimum local area extend to as large as possible, if there is a bad spot that a random run goes over in the area it may average out as it will have less weight with more runs, hiding it, allowing the contractor to pass the minimum local test. The downside for the contractor is that he would have a much bigger area to repair unless one applied investigative methods to narrow down the worst parts.
As a contractor, it would seem the best approach would be to have as large of areas as possible tested and then use a methodology to try to find and isolate the bad areas of the failed minimum local area.
In my opinion, the most awkward part of this solution is the creation of a term that is not in the ASTM E-1155 document. This lends itself to being interpreted and understood differently by every party involved in the industry. The different proposals put forward do not address this and seem to add even more terms that are not in ASTM E-1155. However, they all seem to be reaching towards the same end, which is already hidden within ASTM E-1155. We can achieve our goal of simplifying the F-Number system while hewing closer to the original wording of the document.
Proposed Solution
The approach is as follows:
In ASTM E-1155 the assumption is always that the test surface is defined as the entire floor of a building. It is even assumed by many that the test surface can be stretched to be all the surfaces of different buildings on a given job site and/or all the floors of a given building on a high rise. Thus, the overall F-Number is the statistical average of all these floors and separate building floors for a given job site, assuming they have all been specified with the same F-Number. However, is this how a test surface and overall F-Numbers are defined in ASTM E-1155? Let’s look:
3.1.11 test surface—on any one building level, the entire floor area of interest constitutes the test surface, with the limitations listed in 7.1. [3]
7.1 Test Surface—On any one building level, the entire floor area of interest shall constitute the test surface. [3]
7.1.1 When this test method is used to establish compliance of randomly trafficked floor surfaces with specified FF Flatness and Fl Levelness tolerance, each portion of the surface which has a unique specified set of tolerances must be treated as a separate surface. [34]
9.15 Within each test surface, combine each test section’s F-Numbers by area-weighting to obtain overall F-Numbers as follows: [3] *
*ASTM E-1155 does not provide an actual definition of overall F-Numbers, however, it is implied by 9.15 to be the area-weighted sum of all the composite F-Numbers for each section.
As one reads through these definitions a fact will pop out, A test surface is very specifically defined as not “entire floor area” as seems to be commonly thought, but “entire floor area of interest” [3].
This is the key. The creation of minimum locals came about from assuming a test surface should always cover the most square footage of a job site possible, which is not necessarily what a test surface is according to ASTM E-1155. A single slab whether it be a million square feet, or a few thousand square feet can have more then one test surface.
The test surface is only the area we are interested in. This allows us to use the ASTM E-1155 document like it seems it was originally intended. On a given placement day, the only area I am interested on that job is the recently placed concrete. That is my test surface. Perhaps it is too large of a placement, I can break the placement into multiple surfaces if I please each with its own test sections or just one test section each. This also prevents us from having to adopt new terms and go outside of ASTM E-1155 like SSI’s proposal [4] while achieving similar goals.
Thus, what this accomplishes is that an owner specifies an overall F-Number for the test surfaces that are to be measured. This is the same as what we now refer to as minimum locals which do not exist in ASTM E-1155 and are defined by third parties. The area of interest is the test surface which has an overall F-Number specified. This test surface has either one or multiple test sections as needed or desired. There is now no confusion. The owner is free to specify the entire job’s test surfaces with the same F-numbers and/or specify different F-numbers for different test surfaces as allowed in 7.1.1.
This solution seems rather elegant and brings the application of the F-Number system back to its original document
The problem with defining the size of the area for a test section for minimum local also is now the problem of identifying the right size of a test section. This is one that a lot of discussion is generated about, but it would be hard for ACI to create universal guidelines. The size of the test section is rather dependent upon individual jobs, contractors, owners, situations, the size of test surfaces, etc. There is not a good one-size-fits-all area for a test section.
However, by eliminating the minimum local something interesting happens. Let us examine a hypothetical job: The owner requests a certain overall F-Number for a building slab for each area of interest. The contractor, owner, and test agency work together to create a test plan for the building which as the placements are large involves splitting up the day’s placements into multiple surfaces with multiple sections that are a reasonable compromise of size given the constraints set by the tolerances of the floor needed, and the demands of the schedule.
If the contractor performs there are no problems. However, if there is a bad placement, and the overall for one of the test surfaces fails. Now a problem isolation and remediate process can be kicked off. It would go something like this: What surface failed? Which is the worst section within that surface?
That is identified by the lowest composite F-Number. The process can stop here, and the contractor can rip out and replace or give back money etc. or it can now go deeper. One way would be for the test agency to provide the location of the worst runs in that test section. These can be verified. Now by examining the elevation plots of the run the bumps or dips can be identified. These can be verified and marked. Now the tester can fan out, running parallel and perpendicular lines to the original at 5, 10, and 25 ft intervals and quickly isolate and mark a bump to grind and/or a dip to fill or replace. This is all done without minimum locals. This process could play out in many ways, perhaps ACI should specify a process for identifying and repairing the floors?
Contractors may interject here who are used to the overall being for an entire job (which is not found in the ASTM document) that they will be forced to maintain their quality standards for every pour which is only logical. If the contractor wants “bad day” pours that will have to be in the contract where a certain percentage of the floor’s test surfaces can fail and be ignored and/or have techniques in place to isolate the failed areas to remediate or replace them.
However, this method simplifies the identification of and remediation of bad spots, as now one can use the test sections composite F-Number to find the bad areas. This solves all the awkwardness of having to use individual runs to pinpoint bad areas that everyone seems uncomfortable with. Thus, the bad section can be identifying with that day’s test surface or area of interest and replaced or repaired.
What the contractor will find though is that finally placement prices may be allowed to go up as contractors will no longer be able to game the F-Number system by arguing with the owner about minimum locals and how they are not even part of the ASTM E-1155 test method (if they fail some minimum locals )and then by turning around and arguing with the owner about Overall -F-numbers not being necessary if they met the minimum standard of quality if they had passed the minimum locals on every section and failed the overall at the end of the job.
There are quite a lot of concrete contractors who do this, maybe some on this committee if we are honest, and there are quite a few consultants and test agencies who take advantage of this as well and get paid to do the arguing by either the owner or the contractor and at the end of the day, the owner is unhappy, the contractor is unhappy and the consultants are buying boats.
Along this line of reasoning, if a dispute arises and the contractor self-checks after the test agency and/or a second test agency is brought in to check the floor, they should be required by the contract to run in the same lines as the first test agency. This is due to the fact, that switching patterns from cross hatch to diagonal for instance can give a small bump in the F-Numbers if the new lines cut across the bumps or dips at a shallower angle than the initial runs causing them to look more gradual then they did on the first test.
As far as the Face company giving golden trowels, they can easily generate their own overall composite F-number for a job by averaging the test surface numbers together. As the golden trowel competition is not part of the ASTM 1155 specification they can do as they please.
Therefore, my recommendation as to the definition of minimum local is to eliminate it entirely. There will be some work involved on the part of test instrument manufacturers to change software and educate current users, however, in the end, it will be a much more intuitive measurement system with less controversy in application.
Sources
[1] Dipstick Floor Profiler Operators Manual February 2013
[2] ACI Manual of Concrete Practice. Detroit, Mich. American Concrete Institute.
[3] ASTM Standard E1155, 2008, "Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers," ASTM International
[4] SSI Engineering Bulletin, Floor Flatness and Levelness Testing-The Complete Specification, Wayne W. Walker et. Al. 10/11/2018
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