In recent years the quality and durability of concrete has come to the attention of the public in the press. Parking structures collapsing, bridges collapsing, "chuck holes" in the spring hatching out like Mayflies. Is our concrete not as good as we think it is?
I am very sensitive to such impressions concerning accusations of poor quality concrete or the suggestion poor concrete was the cause of a collapse or a tragic failure of a structure due to some catastrophic failure. Your impression is not unique in the general public. The press does report on these catastrophes and the initial impression to the reader is the failure was due to bad concrete.
I do not know the reason for every catastrophe, but in my mind, it not just a matter of bad concrete. Our society is very fortunate to have concrete used in building our infrastructures. We make very useful things from concrete and individual designs that take advantage of the shape and cast ability of concrete. Concrete is the "liquid stone" mankind has been searching for, through out history. Concrete allows the human mind to design a shape and form the shape with common building materials and then pour the concrete into the form and shape a structural reality. Example: a column to hold up a bridge. The past would have had stone cutters cutting stone from a quarry and stacking the stones to make the column. The concrete form and cast system is much more economical and efficient in time and money. The column forming is built from common building materials and the form is filled with the fresh concrete. The forming material is removed and a structural column of concrete stands ready to support the bridge.
In the last 75 years there have been changes in the make up of concrete. This may be what the press omits in the sensationalism of the catastrophe report. Some of the changes, in my opinion, have been for the good. Concrete now includes mineral admixtures and chemical admixtures. Many of these admixtures reduce the amount of water required to produce a usable concrete.
My research takes me back at least 100 years and in general concrete was produced by following a simple formula. I suspect it was volume measured more than measured by weight. The formula I uncovered was 8 parts of stone or gravel, 4 parts of sand and two parts of cement. The unmeasured volume was the water. I suspect the tradesmen were adding the water to "taste." Water content in concrete was used as an adjustable volume and was increased to make the mix more workable or plastic and reduced to make the mix more stiff or less plastic. Generally, less water in concrete produces stronger concrete.
Following the end of World War I, cement began to be tied to the water content. The term water cement ratio became significant. One of the balancing acts a concrete user or concrete producer must perform is introducing enough cement to produce a paste with the sandy fine aggregates to coat the entire collection of rock and sand in the concrete mix. The process of balancing the cement content with the water content along with the amount of aggregate to be covered with paste is called mix designing.
The mix designing process is involved with the selection of the correct size of rock and sand in order to fill a void economically and yet be useful to the contractor as a concrete. None of this careful mix designing is mentioned in the press reports.
Continuing with the history of concrete, liquid and mineral admixtures came on the concrete scene in the 1930s. These items are generally called lignin of lye and a powder product called flyash. These two products, when in combination or separately, added to concrete, reduced the amount of water normally used in the concrete to make it workable. Remember, less water generally increases the compressive strength of the concrete.
The 1930s period continued to contribute to concrete. Concrete's surface durability improved with a liquid admixture in fresh concrete causing microscopic air bubbles to form in the cement paste. This allowed the concrete surface to resist destruction from freeze-thaw attack. Freeze-thaw attack is one of the destructive agents causing chuckholes.
Concrete used in bridges, buildings, canals and dams is carefully analyzed, designed and tested. The concrete is not "bad" concrete, it is typically "good" concrete. The catastrophes usually happen when concrete is loaded beyond its design strength, or allowed to deteriorate due to lack of maintenance. These causes are acts of man, not the performance of concrete.
The industry produces good concrete, and if professionals are involved in the design and placement of the concrete, the project should be a success. There have been oversights and careless actions in the placement of concrete, but they are rare and in the case of a properly inspected project, these problems would be discovered and repaired.
Concrete has gotten better over time and I have confidence in concrete for building the future.
Always design the concrete to do the job, always cure the concrete to gain its full potential in strength and always maintain concrete to protect it into the future. L&M has the knowledge and products to keep your concrete performing now and into the future.