ULTRA HIGH PERFORMANCE CONCRETE CRC: DUCTILITY

Ductility was one of the key elements when Hans Henrik Bache developed CRC in 1986.

CRACK CONTROL AND DUCTILITY

We are often asked why we use fibers in addition to conventional reinforcement. The answer lies in crack control and ductility.

Ductility was one of the key elements when Hans Henrik Bache developed CRC in 1986. The high-strength matrix had already been developed a few years earlier when he invented and patented the DSP concept (Densified Cement / Ultra-fine Particle materials) in 1978, which later became the foundation for the company Densit. With the development of CRC, closely spaced reinforcement was combined with a high-strength matrix and rigid, strong fibers, allowing for steel-like behavior in concrete, with the added benefits of durability, fire resistance, and more. The fibers provided "local" ductility, while the dense reinforcement offered "global" ductility by acting as a stiff frame. The reinforcement divided the fiber-reinforced matrix into small "cells," each capable of developing multiple cracks without losing coherence.

By using very high fiber contents (up to 12 vol.%—or more than 900 kg per m³—of very short, thin fibers) and very dense reinforcement, it was possible to achieve extraordinary results. An example is shown in the image below, the result of a test conducted with the Swedish military, where a grenade was fired at CRC panels with a thickness of 20 cm.

IMPACT

Today, we use a scaled-down version of Bache's design. We typically do not produce products requiring the performance levels achieved in the initial tests. We have reduced the fiber content and the amount of reinforcement to improve workability—and to make the elements cheaper and more competitive. However, toughness remains very important for all our products, enabling us to achieve a multi-crack system and thereby ensure very small crack widths. This gives us minimum requirements regarding fiber content and reinforcement distribution. The effect of this is perhaps best illustrated by two videos filmed at the University of Ottawa in Canada. Professor Hassan Aoude from Ottawa has conducted several projects using a so-called "shock-wave tube," which can simulate explosions and test the response of different types of elements. Several papers have been presented, and two of his students (Sarah de Carufel and Christian Melancon) received a Best Paper Award at the UHPC Symposium in Des Moines in July 2016 for their paper on singly reinforced slabs tested in the shock-wave tube (a link can be found below for those interested).

http://www.extension.iastate.edu/registration/events/UHPCPapers/UHPC_ID60.pdf

The two videos were made as part of a test on seismic design, comparing a self-compacting concrete (SCC) without fibers with a CRC mix with fibers. As seen in the videos, there is a significant difference in the behavior of the two columns, even though both columns have exactly the same reinforcement—including stirrups placed with short spacing. The SCC column exhibits a brittle failure, where the concrete outside the central failure zone remains mostly intact, while the CRC column demonstrates much tougher behavior, with the column being activated along its entire length to absorb the load.

EN 14651 TEST

We would have liked to perform shock-tube tests as part of our standard testing, but unfortunately, we do not have access to such equipment and have to settle for simpler tests. We use the test described in the standard EN 14651 (4-point bending on a notched beam) as our standard test. This test measures the corresponding values of the load on the beam and the opening of the notch (crack width at the bottom of the notch) that is cut in the middle of the beam.

As mentioned in a previous post, this test is not particularly suitable for our type of concrete, as we use very short fibers, but it still provides some interesting information regarding the effect of our fibers. We have a few basic requirements for the properties of our CRC concrete. The LOP (limit of proportionality) must be relatively high—specified in the test as the maximum stress at a crack opening of less than 0.05 mm. Additionally, the fR,1—stress measured at a deformation of 0.5 mm—should ideally be higher than the LOP, and the ratio between fR,3 (stress at a deformation of 2.5 mm) and fR,1 should preferably be higher than 0.7. Finally, a rough approximation of fracture energy—based on the area under the curve—can also be useful for evaluation. There can be significant differences in how the curves appear for different test samples, and we use this type of test as an initial screening of fiber types and fiber content—followed by various tests where the test samples contain reinforcement bars.

In the test shown above, we see satisfactory behavior of beams with short steel fibers, while the other fiber type would not be suitable for CRC. Although ductility is an important and integral part of the behavior in CRC, we do not use it directly in our calculations. However, crack control is important for durability, stiffness, deflections, and aesthetic reasons. Ductility is also part of the reason for the effect we see in our "cold bending" tests.

Since we need to sell our products every day, we cannot just be satisfied with finding the fiber type and content that gives us the highest strength and toughness. If that were the case, Bache already achieved excellent results back in 1986/87. We need to find the best compromise between performance and price in each case (with a certain minimum performance) to be competitive— and performance in this case also relates to workability. Some fibers that provide good mechanical properties result in very poor workability (something I will address in a later post). Over the years, we have tested many different types of fibers and will continue to do so to take advantage of developments in the field. This knowledge is important for designing the right material for each task.

Based on this knowledge—and the work we have done over the years in this field—we want to be able to decide for each project which fiber type and content are best suited. This sometimes brings us into conflict with the requirements of a specific project, where the provider (or their advisor) specifies a certain fiber content without considering the fiber's dimensions and length/diameter ratio.

We are always open to discussion—but unfortunately, this is not always the case for the provider. Let us know if you have had similar considerations—or if you prefer relatively simple rules that are easy to relate to and perhaps provide better security. We also welcome your comments or questions. This post has been a (relatively) brief review of some of the parameters we consider when ensuring the necessary toughness for a construction, but we hope it has still been of interest.