The Versatility of ECC Concrete

Conventional concrete versus bendable concrete made with ECC

ECC (Engineered Cementitious Composite) is a specialized form of concrete developed in the early 1990’s by Dr. Victor Li, a professor of Structural and Materials Engineering at the University of Michigan. Dr. Li wanted to produce a form of concrete that wasn’t brittle, and that retained its strength even after it cracked.

ECC has properties and characteristics not shared by other forms of fiber reinforced concrete (FRC). These include:

  1. Tensile properties greater than conventional FRC
  2. Easy mixing and casting
  3. Low fiber volume (compared to GFRC)
  4. No weak planes in the cast concrete

Not Your Average Concrete

ECC is a mix that provides flexibility and high bending (flexural) strength.

ECC is not just ordinary concrete with fibers in it. It is a carefully tailored marriage of:

  • cementitious binder
  • very fine aggregates
  • PVA fibers

The types and amount of fiber play an important role, but so too does the formulation of the concrete itself.

ECC depends upon the micromechanical interaction between a strong, fine-grained homogeneous matrix, and well-dispersed structural microfibers. This engineered composite is carefully tailored to create a form of concrete that can bend and crack without losing strength. And because of the highly dispersed microfibers, cracks tend to be small, and sometimes even invisible.

ECC bends without developing large cracks, but it can have visible cracks.

The basic ingredients, composition, and proportions of ECC are similar to, but not the same as, the base mix used for GFRC. ECC mixes have less sand than GFRC, and the sand gradation itself is more important in ECC. The greatest difference between the two is in the fibers.

ECC is a mix that provides flexibility and high bending (flexural) strength, similar to what GFRC provides. While GFRC uses high volumes of large AR glass fibers, ECC uses comparatively low volumes of small synthetic PVA (polyvinyl alcohol) fibers.

Like AR glass fibers, PVA fibers have excellent structural properties that are ideal for ECC and set them apart from the ordinary fibers used in flatwork.

Ordinary fibers like nylon, polypropylene, and cellulose are either too stretchy, weak, or both, and these ordinary fibers function as stabilizing agents and are there only to help control plastic shrinkage. None of these add any strength after the concrete hardens.

In contrast, PVA fibers have some structural strength and can also be used for shrinkage control. They improve the mechanical properties of cured concrete, boosting its strength.

Comparing ECC with GFRC

Casting is faster and more efficient.

GFRC uses bundled alkali-resistant (AR) glass fibers in high doses (typically 3% for premix GFRC, and 5% and higher for spray-up applications). AR glass fibers are typically 19mm long, and the large, 200 filament bundles can be very visible if the fibers become exposed. This aesthetic drawback is why GFRC usually has a face coat, which is a decorative veneer layer that often is fiber-free.

GFRC is a material originally designed and optimized for the efficient casting of single-sided pieces, where one face of the casting is decorative, and the opposite side remains unseen.

While wet casting GFRC with only a flowable backer is possible (it’s called direct casting, and foregoes a face coat), the large AR glass fibers just below the surface are sometimes noticeable, and, the presence of the highly visible fibers in the concrete preclude grinding and polishing. Thus, direct casting is limited to pieces where an as-cast surface is desired, and the visible presence of large fibers is acceptable.

PVA Fiber for Concrete Countertops

PVA Fiber for Concrete Countertops

What makes ECC different from GFRC is that the PVA fibers are mixed into the whole mass of the concrete, instead of just in the backer layer. PVA fibers are nearly invisible when properly dispersed during mixing, unlike GFRC fibers which need to stay in large, very visible bundles.

Since PVA fibers are transparent, short (only 6-8mm long), and their diameter is a fraction of the diameter of a human hair, they disappear in the mix. This greatly simplifies mixing and casting, as there is no need for a separate face coat.

ECC can be made stiff and hand-packable, or, it can be made fluid and vibrated. Casting is faster and more efficient, as molds can be filled in one continuous pour, rather than in individual layers. This versatility makes ECC a smart choice for both precast and cast-in-place applications.

However, the complexity of ECC makes it impractical to prescribe a from-scratch mix formula. (In contrast, GFRC is a very simple mix easily made from scratch.) For this reason, The Concrete Countertop Institute does not have a from-scratch mix calculator for ECC and recommends that you buy a preblended ECC mix such as the Buddy Rhodes ECC Blended Mix product.

Glow in the dark concrete countertops in the Cayman Islands

In July, I traveled once more to Grand Cayman to help my student Terry Wilson with a cool project. This was a 190 sq ft concrete bartop for the new Georgetown Yacht Club. It incorporated recycled glass (beer bottles), sliced coral and conch shells and glow in the dark aggregate!

The owner wanted to use sustainable materials, and he had heard several years ago about concrete countertops and how recycled glass can be incorporated into them. For the past 2 years, he has been saving every beer bottle he and his friends drank. He built his own concrete countertops for his office reception area and the washrooms for the marina. They look really great for a do-it-yourselfer! He showed me the countertops as an example of the look he was going for.

One of the DIY bathroom countertops

The client pointing out glass pieces he liked

He wanted to use glow in the dark aggregate to create a special effect and also to cut down on lighting energy. I advised using Glow Stone from Ambient Glow Technology, as it has the best luminosity and is available in a variety of colors and sizes. He had obtained some samples from another company (at exorbitant prices!); you will see later the clear difference in luminosity.

The client selecting Glowstone color and size

It REALLY glows!

The owner decided on 3/4″ Glow Stone in all 3 colors. The next order of business was to slice the coral and conch shells (easily available on the island) and to break up the beer bottles. There were 27 cases! Breaking them was not as easy as you might think. We had to crush them in small batches in a box, using a cast iron tamper.

Crushing beer bottles

Coral and conch slices ready to go

We used beach sand from nearby, and for the pozzolan used a recycled glass pozzolan similar to VCAS. We also used every bag of white cement on the island! When Terry told me about the project, I told him to buy up every bag of white cement he could find, because I know from last summer how long it can take to get supplies onto the island.

Pour day went great. The pour was about 3 yards. We used ice to retard the mix. It’s always about 88 degrees (31 C) in Grand Cayman.

Adding ingredients to the truck

Ice to cool and retard the concrete

The concrete truck arrives


After screeding, we seeded the top with all the decorative aggregate and accents and screeded it in. This was a harsh mix! In the photo below, the section in the back has been floated, and the section in the front has not yet been.


The next day, we started grinding the concrete countertops to expose the recycled glass aggregate and other decorative features. At 1 day, the compressive strength was already 2150 PSI. The 3-day strength tested at 3730 PSI, and at 7 days it was 6750 PSI. Not the strongest concrete I’ve ever made, but as you know from other blog entries, compressive strength is not the most important property.

Terry grinding the edges

Hot, gritty and sweaty

All the grinding, honing and polishing was very hard work, but it paid off. The client was thrilled, and the details in that bartop are amazing!

Finished bartop

Closeup of beer bottles

Closeup of conch shell

One section of the bartop

The Glow Stones looked amazing too. In the photo below, you can see some smaller stones from the client’s sample of the other “glowing” aggregate. They are much dimmer than the stones from Ambient Glow Technology!

Larger AGT Glow Stones contrast with other “glowing” aggregate

It glows!

In all, the project took about a week. The forms were built ahead of time. We poured on a Tuesday, spent Wednesday through Friday grinding, honing, grouting and polishing, and then Terry applied a simple penetrating sealer that weekend.

I can’t wait to go back and see the finished yacht club and have a beer at this bartop!

For even more photos, see the album on the CCI Facebook page.


When does a GFRC concrete countertop look just like a cast in place concrete countertop?

I’ve heard people say that they don’t like GFRC because it looks too uniform, like plastic solid surface. Nonsense! GFRC can have any look any other form of concrete can. It is concrete, after all.

Here’s a great example. In summer 2011, I spent 3 months in the Cayman Islands helping my student Terry. We did a gazebo bartop using a cast in place technique and a mix delivered in a truck from the local batch plant.

Forming for the cast in place gazebo

Mix being delivered from batch plant

The finished look

We also did round and rectangular tables for a restaurant, and the owner wanted exactly the same look as the gazebo, plus some amber glass. But, the tables couldn’t be cast in place, and they couldn’t be too heavy.

No problem! With GFRC, I was able to create a face coat (like a mist coat, but with aggregate in it and obviously not sprayed) that exactly duplicated the look of the cast in place mix. Then I just applied a backer coat to that. The result: Just the right look and a happy client. GFRC made it possible.

Lots of buckets! This is the "batch plant" for GFRC.

The round tables, showing backer coat

The finished look

Rebar in a cast in place concrete countertop

I recently got the following question regarding my blog post “Mix design for cast in place concrete countertop in Cayman“:

Q: What is the size of the rebar? Where was it placed in the slab?

Here is what the rebar looked like:

rebar in gazebo

The rebar size and location were dicated by local practices (and the local structural engineer). While I know from practice (and from being an engineer myself) that the rebar used was grossly oversized, politics were the over-riding influence on the reinforcing schedule.

All the rebar was 1/2″ bar, located in the slab so that there was 2″ of cover measured from the top surface of the countertop. The 2″ cover depth is common where corrosion is an issue. Since this is an outdoor countertop on an island, corrosion is always a concern with concrete exposed to the elements.

Note that too little cover depth can lead to problems other than corrosion. See this article about rebar size in concrete countertops.

Mix design for cast in place concrete countertop in Cayman

I wrote recently about the planning stages for an outdoor gazebo bartop I’m doing here in the Cayman Islands. We’ve now poured the project, and I wanted to give you some insight into the mix we used and why.

This project required a large volume of concrete (nearly 8000 lbs, or 2 cubic yards), so I chose to have the local (and neighboring) concrete batch plant mix and deliver the concrete.

loading truck

However, the concrete was based on white portland, and I felt that a standard construction mix would have been too inferior for the demands of a high quality concrete countertop. Therefore I chose to modify one of the batch plant’s mixes to suit my needs.

I spent some time with the plant’s engineer to learn more about their basic slab mix. Because the aggregates were local, I felt that using a mix that they had extensive experience with was smarter than trying to force fit an outside mix design that may or may not have worked the way I wanted it to. All I did was add a few admixtures to modify the base mix (and of course use white portland instead of gray portland). In addition to the base mix, I added pozzolan, a retarder, pigment and a shrinkage reducing admixture. I didn’t even need to use a superplasticizer, although we had one on hand if we needed to tweak the mix at the job.

Cast-in-place concrete needs special attention because it’s more difficult to control the environment during curing, and the concrete is exposed to greater environmental extremes (heat at least, but not cold here in Grand Cayman) than precast concrete used inside a home. Unlike sidewalks and floors, outdoor concrete countertops still have to look good and remain crack free.

I was casting a single slab over 45 feet long that wrapped around a hexagonal concrete gazebo. The thick concrete columns that hold up the roof penetrate the bartop, which wraps completely around each column. Crack control was very important, so I had to use a shrinkage reducing admixture to control the root cause of cracking, rather than hope fibers would do the job. I did not use fibers since ordinary fibers don’t prevent cracking anyway.

Here in the Caymans it gets hot, but not super hot. Average daily temperatures are 88 to 90 every day, all summer. It’s also very humid, which is a good thing, since the concrete doesn’t dry out as fast as when it’s hot and dry, like in a desert climate. To control early set I used a retarder, which gave me plenty of working time.

Although the truck delivered the concrete 10 feet from the job, it still had to be wheelbarrowed up to the bar and hand placed in buckets.

truck parked

The workability of the concrete was around a 4″ slump when we started, and it was unchanged when we finished pouring 2 hours later.

mix consistency

It took another 30 to 45 minutes to begin to firm up, but we left the job before it got to a hard trowel stage. Since we planned on grinding the concrete to expose the aggregate, all we did was screed and float in some decorative coarse aggregate and sliced coral into the top.

adding coral

For more photos of the project, view the entire photo album on the CCI Facebook page.

Click here for a free seminar including a specific cast in place mix design.

Even in the Cayman Islands, concrete countertops can be challenging

I love my job. I get to meet interesting people. I get to travel the world and help people be more successful with their concrete countertop businesses. And I make cool concrete.

I’m currently spending the summer down in the Cayman Islands helping out my student Terry with several large projects. (I’ve been there before – here’s a photo of the project I helped him with in December.)

Terry Grand Cayman

The first project I’m working on this summer is a 100 sq ft outdoor bar/gazebo. It’s a pretty simple job – just a hexagonal bartop.


But this experience is showing me that even when things seem simple on the surface, the little details which appeared insignificant at first suddenly become the critical factor that brings everything to a screeching halt.

This outdoor bar is a pet project of a local developer. They and their architect drew up plans in January for a hexagonal-shaped bartop in an outdoor gazebo. The design was unchanged up until a week ago, and we were given the go-ahead to start forming for pouring right away.

 forming gazebo

On the morning we started forming the architect halted everything and said the client changed the bar profile, reducing the overhang distance on the patron side and increasing the overhang on the bartender side. This was to allow for the beer taps to be installed in the bartop instead of on kegerators.

So new forms were cut and assembled. When asked about the details on the taps, the response was that they were pretty sure which ones were going to be used, but the details were not finalized. A few days later the tap details emerged, but it turns out the tap, its drip tray and the bartop don’t actually work together. Oops!

And guess what: The architect didn’t discover this. The owner didn’t. Terry and I did – the concrete countertop guys.

The morals of this long story are:

1. What seemed like a simple project that really was a lot more complex than it appeared.

2. You, the concrete guy/gal, need to understand a lot more than just concrete.

The fact that the plans were drawn (and re-drawn) before the taps were selected highlights the need for someone with experience, skill and an awareness of how everything will come together. The architect isn’t that person. The owner isn’t that person. It’s you. You must deal with the details that matter.

Sure, you have to know the basics of forming and casting and making concrete. That’s the easy part that everyone who offers training covers. But it’s the little details that have nothing to do with making concrete that are what separates those who are successful and make things work the first time from those who learn the hard way, through mistakes, expensive re-do’s and frustrated clients.

This is why I spend so much time on sinks, faucets and installation in my intensive class. The devil is in the details. Even in the paradise of Grand Cayman.

PS – Keep track of my Cayman adventures on Facebook. I’ll be posting lots of photos and updates.

PPS – Actually, “Hell” is in Grand Cayman. Here’s a photo.

Hell Grand Cayman

Good concrete countertop mix design for cast in place

Key characteristics that define a good cast-in-place concrete countertop mix are finishability and shrinkage resistance. However, these two characteristics are at odds with each other and must be carefully balanced in order to produce a good cast in place concrete countertop mix that is a joy to finish and does not curl or crack.

Other beneficial characteristics worth mentioning include workability and flexural strength. While high compressive strength is not necessary (though it is impressive), high quality concrete, often a byproduct of creating a high compressive strength mix, is also beneficial and desireable. Additionally, adequate work time, high early strength and a good appearance add to the list of desireable characteristics.

The two key characteristics that are very important, finishability and shrinkage resistance, are often determined by the aggregate gradation and the cement to aggregate proportioning.

Finishability, that is, the ease of trowelling the concrete into a smooth, even, high-quality surface relies on a sufficient amount of cement paste and very fine aggregate to create enough cream to trowel. Cream is the fine portion of concrete that is floated to the surface early in the casting process and is worked and reworked during trowelling.

Shrinkage resistance is also influenced by the water-cement ratio, by the cement paste content and by the amount of fine aggregate. Whereas finishability benefits from more cement paste and fine aggregate, shrinkage resistance benefits from less cement paste that has a lower water-cement ratio, since that is what actually shrinks. Minimizing the fine aggregate preserves workability when the cement paste volume is reduced, because fine aggregate (sand) has much more surface area than coarse aggregate, so more cement paste is needed to coat and separate fine sand than is required for a coarse blend of aggregates.

A poor cast in place concrete countertop mix would have large aggregate of one size, say 3/8″, mixed with fine sand. This is a case of “gap grading”. A good mix will have well-graded aggregate.

aggregate gradation in concrete countertop mix

Click here for a free seminar including a specific cast in place mix design.

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