A very good example of this can be found in the specification for non-shrink grout. In the past, all too often non-shrink grouts were selected solely on compressive strength. The highest strength got the nod. But over time we've learned this about cement aggregate systems (including structural grouts): as the cement content is increased the compressive strength goes up, but the system also becomes more brittle. This can be very problematic in systems that are exposed to dynamic loads.
Consider the case of machine base grout pads. As the loads placed on the grout pad change in velocity, intensity and even in direction, a very high strength grout can fail in fatigue and lose its load-carrying capability. The conclusion? It could be said that the high strength brute has strong muscles but brittle bones.
The Economics of Cement
As was pointed out in Carl Bimel's article in the Fall 2001 edition of L&M Concrete News, there are reasonable concerns regarding unnecessary high strength. He states, "ACI 302 actually suggests that since there is no direct correlation between compressive strength and water/cement ratio, the two should not be combined in a specification for interior floors."
My understanding of this statement is that a given water/cement ratio will not necessarily produce the same compressive strength using raw material from different sources. Specifications that are strength-driven may require a different amount of cement than those that are water/cement ratio driven. If both are specified, there is a good chance that more cement may be used than is needed.
It has long been known that as the cement content increases, drying shrinkage becomes more of a problem. We are now learning the importance of optimizing cement content for maximum performance. In other words, we are learning more about the economics of cement.
In the area of concrete repair, the high strength brute can also lead us astray. Here again, all too often the brute gets the specification nod. Yet, when selecting a repair material, not only must strength be considered, but also bonding capability and the compatibility of the repair mortar with the substrate that is being repaired.
Generally speaking, as the compressive strength increases, the modulus of elasticity will also increase. For those not familiar with the term, the modulus of elasticity is determined by applying an increasing load to a specimen and determining how much the specimen is compressed (shortening of the specimen) at different load levels. It should be noted that the modulus of elasticity discussed in this article is determined when a sample is placed under longitudinal compressive stress per ASTM C 469. (See illustration below)
Modulus of elasticity is the ratio of normal stress (in pounds) to corresponding strain (in inches).
The modulus of elasticity is expressed as millions of pounds per square inch. Most concretes and cementitious mortars will have a modulus of elasticity in a range of 4x106 psi to 6x106 psi. As the modulus of elasticity increases, the material becomes increasingly brittle and rigid.
Looking for a better match in the modulus of elasticity
It should be pointed out that concrete in its environment will undergo volume change from time to time. These changes are caused by a number of actions such as changes in temperature, moisture content and loads placed on the concrete. In response, the concrete will undergo movement. It is most important that the repair materials respond to these activities in similar manner as the concrete being repaired.
If the modulus of elasticity of the repair material is not matched to that of the concrete being repaired, there is a high probability that delamination will occur. If materials are mismatched, they will not respond in a similar way when acted upon by outside forces. If not closely matched, one material may expand or contract more than the other under the same conditions. This disparity oftentimes creates a force that can pull the two materials apart at the bond plane.
So, how can you prevent such mismatching? We recommend that when selecting a cementitious repair mortar, choose one that has a compressive strength in the same range as the concrete to be repaired.
Epoxy grouts: The problem with creep
When selecting an epoxy grout for the support of equipment and machinery, compressive strength and modulus of elasticity become issues-but not for the same reasons as for cementitious materials. The issue in this case is load-carrying capability.
An epoxy grout may have a compressive strength as high as 15,000 psi but its load-carrying capability may only be 7,000 or 8,000 psi. The load-carrying capability of a grout is the load that a grout can support over time without the grout specimen shortening in length. This shortening in length is defined as "creep." The difference between compressive strength and load-carrying capability is due to the elasticity of the epoxy grout and the way in which compressive strength is determined. It must be pointed out that epoxy grouts are more elastic (rubber-like) than cementitious grout. Cementitious grouts are more glass-like.
Compressive strength is determined by placing a specimen under test in a machine that applies a load on the specimen. This load is increased over a few minutes until the specimen fails. In the case of cementitious specimens, the point of failure is met with a violent and quick rupture of the specimen. The maximum applied to the specimen is the load-carrying capability of the specimen.
In the case of epoxy grouts, as the applied load is increased there will come a point in the load cycle where the specimen will profoundly shorten in length (creep). It is at this point that the specimen has reached its load-carrying capability. At this point, the machine continues to increase the load on the specimen until a slow rupture occurs with the compressive strength reported as the highest read.
Cementitious grouts fail in rupture and epoxy grouts fail in creep. The point can be made that cementitious grouts reach their creep limit at the point of rupture and epoxy grouts meet their creep limit well before rupture. It is at the creep limit that the specimen is no longer capable of supporting the load placed on it. It has been found that epoxy grouts with a minimum modulus of elasticity greater than 3x106 psi have performed quite well.
It's time to kick our strength addiction
The new technology is bringing forth a new school of thinking. As a result concretes, mortars and grouts are being designed for compatibility with their total environment.
By the use of modern additives, we are changing the very nature of these cementitious products. We have learned how to increase the abrasion resistance without increasing brittleness. We are making products that have very low drying shrinkage and reduced creep. We are learning more and more about the economics of cement and more about modulus of elasticity matching.
All this being said, compressive strength is still important. But in order to enter the brave new world of the new technology, we must be willing to kick our addiction to the idea that higher compressive strength automatically means a better product.
© 2004 L&M Construction Chemicals, Inc. | ConcreteNews Spring 2004.