Q: 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?
A: 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. LATICRETE® has the knowledge and products to keep your concrete performing now and into the future.
Concrete Street Expansion
Q: I was interested in finding out about the possible causes of concrete street expansion, other than heat itself. We have experienced numerous problems of over-expansive joints, causing the individual sections of concrete to force one another to push up and crack. I was wondering if you had heard of any reasons or studies aiding in this phenomenon involving any chemicals in the concrete or anything similar to this. Any information about expansive concrete or street blowups/blowouts would be great.
A: I do not have formalized studies about concrete paving blowups to share with you, try the Federal Highway Administration in Washington D.C. But, I intend to share with you the experiences I have.
Slab on ground concrete construction with proper jointing in the transverse direction, usually perform very well even in hot conditions. Pavement abutting bridges is a separate matter and will not be addressed here.
You are correct, there are situations that bring about blow ups. I have seen the following causes:
Contraction joints are crammed full of incompressible matter, sand, soil, calcium efflorescence, etc. This occurs when the joint filler is absent or not in good repair. The slab or panel of concrete is bearing directly upon the abutting panel. The original design did not permit that. The original design allowed for a clear space to develop between the panels due to shrinkage during setting. The contraction joint was detailed to be filled with bitumen sealant and to remain free to move horizontally.
The freedom of horizontal movement is denied by the joint's fill of incompressible material. The free joint space designed in by the owners staff is no longer available and the two or more weather heated panels begin to push against one another. The more heat or the longer the heat spell the more the expansion of the flat panels.
All the above is taking place and then we can become site specific, an intersection is very vulnerable, due to multiple directions of the expansive forces. A slab sloping down a hill and jamming against an other set of panels going in the 90 degree direction from the down hill slope will cause a lot of energy to build at the interface between the slope panel and the flat intersection panel.
Last, dowels and other embedded items cause point stresses that contribute to the build up of energy in a particular place. Dowels are supposed to permit freedom of movement in a horizontal direction and of course prevent vertical dislocation at a particular joint detail. Dowels must be dead level to perform this function, and one directional end of all the dowels needs to be bonded in a panel and the opposite side needs to be isolated to allow freedom of horizontal movement. Real world, dowels are installed not level, dowels are disturbed in the paving process, and sadly, dowels are not isolated on one end and they actually transfer the expansion energy through to the abutting panels.
Result, the heated panels push directly upon one another and rare up at the joint interface and "blow up." Sometimes violently. The 6:00 p.m. news makes a big deal out of it and everybody says Wow! Bad concrete! We know, it is not bad concrete it is poor joint maintenance or a bad dowel installation, or maybe a set of man holes in an intersection and the man holes are arranged in a wedge, and the wedge shape is blocking the natural movement of the concrete panels, but it is not bad concrete.
The thing I did not mention is the subgrade under the concrete. there are times when expansive clay will bulge or uplift a section of concrete and it is blamed upon heat, but in fact is a subgrade related problem.
High Strength Concrete Mixes
Q: We have a bridge division in our construction company. We need a product to prevent the rapid crusting and drying of the high strength concrete mixes we use to cast the bridge decks and the approaches to the bridge. What do you have?
A: We have a monomolecular film product called E-CON™. E-CON™ is not a cure. It is a liquid and the liquid is mixed with clean water. The ratio of the mixture is one part of E-CON™ to 9 parts of water. This dilution ratio allows your crew to haul a small amount of product to the project and dilute it to a much larger volume for their use on site.
Your question contained a lot of detail concerning flyash and silica fume in the mixtures your crew works with on the bridge decks. The silica fume is difficult to finish and leads early plastic surface cracking. one process to control this surface plastic surface cracking is to suspend water fog nozzles over the slab or deck. This is very difficult to do. The next best thing is to spray E-CON™ upon the fresh concrete surface. Your crew can finish right over E-CON™ and the product can be reapplied repeatedly as the surface needs subsequent protection from rapid drying and crusting.
E-CON™ is unique because the formula is composed of water loving molecules that attach themselves to the damp surface of the concrete and water hating molecules that are repelled by the dampness of the surface of the concrete. This causes it to produce a protective shield over the surface of the fresh concrete. This protective shield is effective for about an hour at a time and needs to be reapplied to benefit from its continued protection. The E-CON™ shield can be tooled over and finished over and not disturb its protection. The product has a bright marker color to reveal the areas treated with E-CON to the finishers and as the color lessens, more can be applied.
Always cure concrete once the finishing of the surface has been completed, E-CON™ is to be used prior to curing. It can be used upon the concrete approaches your crew installs leading up to the bridges.
Bridges and Suspended Concrete
Q: What is available to protect our clients' bridges and suspended slabs of concrete? There will be car and truck traffic and there will be salt applied during the winter.
A: The product to protect your exterior concrete is AQUAPEL™ and in severe cases AQUAPEL PLUS™. AQUAPEL™ is a new generation reactive waterborne sealer for concrete and, in some cases, masonry. The formula is clear and non-staining.
It penetrates the concrete and masonry surfaces and chemically bonds directly to the concrete. There is no slick membrane or vulnerable skin to wear off.
The results of using the product are a surface that is highly resistant to moisture penetration and damaging deicing salts. Nothing protects concrete better than a good, durable concrete mix design combined with AQUAPEL™.