This article has been modified from its originally published version. The manufacturer of one of the products reviewed made threats of legal action to both to CCI and to another manufacturer named in this report. Furthermore, note that none of the manufacturers knew that this study was being conducted, and that CCI spent several months and thousands of dollars doing it.
We deeply apologize to those who are being denied access to the relevant, useful and truthful information that we worked so hard on. GFRC is a highly specialized form of concrete designed and optimized for making large, thin panels and lightweight 3D objects.
The key property of GFRC that makes this possible is its high flexural bending strength. Unlike conventionally-reinforced concrete where compressive strength is important, it is the bending strength of GFRC that is all-important.
Not only is ultra-high compressive strength irrelevant, ultra-high compressive strength concrete is well known for being brittle, a characteristic opposite to what makes high-quality GFRC. Typically GFRC needs to remain internally moist for at least 7 days in order to achieve adequate strengths.
Premature drying will slow or halt curing, leaving the concrete soft, porous and weak. This is impractical for most applications, so instead a polymer curing admixture is used. The polymer does this by essentially forming an internal curing membrane, slowing moisture loss. As with all concrete, curing is vital to achieving the desired physical properties.
Flexural and compressive strength, stiffness, porosity and mechanical toughness are all dependent upon the cement paste remaining moist so it can continue to hydrate. We call this curing, and the longer concrete cures the better it gets. These requirements are to ensure GFRC product quality, and to ensure that the curing compounds are tested by an independent laboratory to demonstrate that: The recommended quantity of polymer curing admixture in GFRC mix with no moist curing equals flexural properties of GFRC cured 7 days moist when both are tested at 28 days.
The long-term durability of the dry-cured polymer admixture modified composite, verified by aging tests, is equal or greater than the durability of GFRC cured 7 days moist. The polymer exhibits durability, ultraviolet stability, and oxidation resistance and stability in a high-alkaline environment.
Essentially, these requirements ensure that the polymer is subjected to independent testing that proves that it is an acceptable substitute for 7 day wet curing. Some key property requirements mandated by the PCI specification are: Without polymer, GFRC tends to become more brittle and weaker over time, and extensive testing accelerated and real-time aging over many years has shown the benefits of polymer in GFRC.
These two polymers comply with the PCI standard. Over the past several years, the use of GFRC in small scale architectural concrete such as concrete countertops, furniture, etc. The simplicity of forming, the ability to create complex three-dimensional pieces with relative ease, and the significant reduction in overall weight have made GFRC the material of choice for many artisans. One of the keys to being successful using GFRC is using the right ingredients and understanding their purpose and function.
The polymer plays an essential role to achieving the strength and durability expected from GFRC. Using the wrong polymer, or using a polymer incorrectly, can result in inferior concrete that is weak or exhibits cracking or curling. In addition to a carefully tailored chemistry designed to aid curing and preserve long-term flexural strength, GFRC polymers also contain defoamers and shrinkage reducing additive.
These additives improve the strength of the material by reducing trapped air and by eliminating micro cracks formed by drying.
Two common uses for other kinds of polymers are surface bonding agents and polymers used in overlays and micro-toppings. Recently dry powdered polymers versus liquid polymers have become favored in our industry, claiming equal effectiveness, simplicity of use and the cost-saving from not having to ship heavy water-based liquids.
Equal effectiveness and cost savings are addressed later in this article. To address ease of use, consider the following. We also verified requirement 3. We did not test requirements 2 or 4, and to our knowledge no independent laboratory has performed testing of those requirements. CCI tested three polymer systems: Forton VF Dry polymer 1: Test samples made using Forton also used two different pozzolans which were used as partial cement replacements: Dry Polymers The two dry polymer systems differ in how they are formulated.
Both claim to have a dry polymer curing admixture blended with other additives, such as a defoamer, shrinkage reducing admixture and wetting agents, plus other specialized additives unique to each company. Note that the BRCP dose was much higher because it includes more ingredients, including white silica fume pozzolan. The basic mix designs for the three different polymer systems were: SCC samples were made fluid, poured into the molds in one layer and gently shaken to level the mix. Care was taken to minimize disturbance or manipulation of the mix.
Two test panels were cast for each candidate mix design called a test series. The samples were cast, cured under plastic overnight and then demolded the next day. Demolded samples were then allowed to air cure on racks that permitted free air-circulation around all sides of each sample. Prior to 28 day testing, the samples were flattened by grinding, cut into standard size coupons and soaked in water for 24 hours, in compliance with ASTM C testing practices.
Cutting samples from test panel. Flexural Testing To investigate the strength and effectiveness of two dry and one liquid GFRC polymer systems, The Concrete Countertop Institute conducted an extensive array of flexural tests to determine whether the two dry polymers functioned as polymer curing admixtures. These select samples were cast from the same batch of concrete on the same day by the same person and the flexural tests were performed on the same day. The flexural test results from the 18 samples tested in NC and CA were statistically identical, verifying that the test results from the in-house testing at CCI were real and accurate.
Four-point bending tests measuring peak load at break were performed, yielding Modulus of Rupture MOR data, which is the peak flexural strength of the material measured at sample failure. GFRC sample in 4-point bending. The average MOR data for the three polymer curing admixture systems are shown below, along with data for similar GFRC made without any curing admixture and moist cured for 7 continuous days prior to air curing.
All data shown are 28 day values. Each colored bar in the chart above represents the average MOR for a particular test series, and each test series consisted of up to 12 individual flexural test samples cast from the same batch of concrete.
The chart summarizes the results from over individual flexural tests. Variations in MOR for each test series are shown by the error bars which show one standard deviation of variation. Larger error bars represent greater MOR variation within a test series.
After careful analysis, we determined that the magnitude and variation of strength was mainly due to the presence and distribution of entrapped air bubbles within the GFRC matrix, and partially to the non-uniform orientation and distribution of the glass fibers within the samples. More entrapped air resulted in lower strength. This is in alignment with PCI requirement 3 about unit weight i. Variations in the amount of entrapped air in adjacent samples cut from the same test panel.
Each test series is labeled with a shorthand code that reveals details of the particular mix design tested. A 7 instead of a 1 refers to the second panel, since each panel had 6 flexural test coupons cut from it. The polymer curing admixture and its dose are next.
For direct comparison one BRCP test series was tested at a 1: The fiber volume fraction and size are listed next, which were 19mm long AR glass fibers. Fiber volume fraction is defined as the weight of fibers divided by the total weight of concrete, including the weight of fibers plus all other mix ingredients, wet and dry.
The final number is the water to cementitious ratio. Conclusions from Flexural Testing As seen in the chart above, all three polymer curing admixtures were able to achieve the average equivalent 28 day flexural strength of GFRC moist cured for 7 days: The average performance for each of the 3 polymers dry cured was roughly equivalent to the performance of no polymer wet cured for 7 days shown by the gray bar. Some mix parameters SCC, or a different pozzolan for instance yielded individual strengths lower or higher than the 7 day moist cure average strength, but each polymer curing admixture was capable of achieving that strength for some of the test series.
No particular polymer curing admixture was significantly superior or inferior in terms of the ability to achieve adequately high flexural strengths.
All showed marked variation in strength despite being cast and compacted as identically as possible. A common reason cited for using dry polymers is the cost savings due to not shipping water. To evaluate the actual cost of making GFRC using each of the three different polymers, it is necessary to look at the total cost of making a known amount of GFRC, rather than looking at the cost of obtaining only the bulk polymer curing admixture.
This is because each of the three polymer systems use mix formulas which specify different recommended doses, water to cementitious ratios, pozzolans and pozzolan doses, and sand to cementitious ratios. Note that continuous wet curing for 7 days with no polymer achieves the same necessary flexural strength. The greatest cost savings would be achieved simply by not using polymer.
However, wet curing for 7 days is not realistic for most creative concrete artisans, hence the need for polymer. Shipping can greatly influence the cost for ingredients, and more importantly, the actual cost to produce GFRC. The influence that shipping has on cost can only be revealed when actual material use rates when making GFRC are taken into account.
Simply comparing the cost to ship a bucket of one material versus a bag of another reveals nothing about what your actual costs are. Likewise, simply comparing the bulk price of one ingredient to another provides a skewed perspective on how costly or inexpensive it is to buy and use one ingredient versus another. To reveal the actual cost involved in buying, shipping and using different polymer and pozzolan systems, typical amounts for each ingredient were selected that would be used to make GFRC backer using those ingredients.
The sand and white cement used in each of the mixes were locally sourced and are based on real pricing for Federal white cement and bagged 30 silica sand locally available in Raleigh, NC.
AR glass fibers, like the polymer and pozzolans, often must be shipped and are not often locally available. Fibers are usually purchased and shipped from the same source as the polymer and pozzolan. The material and shipping costs for the fibers were included in the calculations but are not reported, only the cost differences between the polymers and pozzolans are shown below, since the same fibers were used in both mixes.
Zone 2 represents a short shipping distance where the delivery address is close to the shipper. Zone 8 represents the farthest distance between shipper and delivery address, equivalent to shipping across the country. Labor, tax and membership discounts are not included. While the bulk material and shipping costs for each polymer system varies, the magnitude of the cost for making GFRC using any one of the three polymer systems is quite low compared to the retail price of the products made from GFRC.
And the magnitude of the cost differences between any of the three polymer systems is small and relatively insignificant: Liquid polymer can freeze. It is possible to thaw out and may be usable after one freeze, but precautions should be taken, including no-freeze shipping which increases cost.