We’ll look at one of my favorite materials - concrete. The baseline condition of construction is the building codes, or providing “life-safety.” For concrete used in a structural capacity, the integrity of the tensile strength reinforcement is the key issue. Typically, this reinforcement is steel bar (rebar). The problem is one of durability. When steel gets wet , it corrodes. What causes it to get wet is when concrete cracks, the moisture can penetrate to the steel. So there are a few possible solution paths: reduce cracking, find a tensile reinforcement which doesn’t have moisture problems, or protect the entire assembly from getting wet. The solutions may overlap, but it helps to remember that the key problem is durability.#1 - Reduce Cracking - While concrete has a high compressive strength, it has a very limited tensile strength. To overcome this limit, rebar is used in tension in the structure. Extra steel can help reduce cracking, but steel is expensive and is still vulnerable during the curing and drying process. That problem can be addressed by a mix design with a low water content, and a slow even curing time, both of which are can be achieved with the use of alternative pozzolans, such as fly ash. Another option to reducing cracking is by increasing the concrete ductility, for example with the use of crumb rubber in the concrete mix. While this actually decreases the tensile strength, it also increases the energy absorption, and makes it more resistant to thermal changes.
|Self-healing concrete by bacteria|
Another option is to address the cracking as it occurs. “Self-healing” concrete is a “Smart” building material that, in theory, is more durable because it repairs itself. This is especially useful in areas that might not be accessible for concrete repair. Apparently, self-healing concrete is a big research field right now, but the success of the research will ultimately be measured in the economic feasibility for commercial production. There seem to be two leading contenders. One is a concrete matrix embedded with sodium silicate capsules which rupture when the concrete cracks. The sodium silicate reacts with the calcium hydroxide, already present in the concrete, forming a calcium-silica-hydrate gel, which heals the cracks and blocks the concrete’s pores. The other self-healing concrete recipe comes from Delft Technical University, in the Netherlands, and is a limestone producing bacteria that is mixed into the concrete and is activated when corrosive rain works its way into the structure.#2 - Non-corroding tensile strength. The second approach is to avoid the corrosion problem by finding a replacement solution for gaining the tensile strength with a material that won’t corrode. Carbon fibers can do the trick, either added in to the ready mix, or as a grid mesh used in pre-cast panels. The acceptance of this option is a function of the cost equivalent of steel, and also the reluctance by code officials to accept fibers in lieu of rebar. This may explain the trending of use in the pre-cast panels, which are “engineered” and thus relieve code officials of the liability of responsibility. Fiber-reinforced concrete can also be used for concrete repairs, which goes back to addressing Solution #1 – reduce cracking.
|ICF House still standing after Katrina tidal surge|
#3 - Protect the Assembly . The third approach is to protect the assembly from getting wet. After all, the compressive strength of concrete, as well as the tensile strength of rebar, is largely independent of concrete cracks. For example, Insulating concrete forms (ICFs) can protect the concrete in a wall from surface moisture. A capillary break is needed to provide protection from wicking moisture from the footing, and of course a roof protects the top of the ICF wall. This is a “field-tested” method, as there are some classic examples of houses which got the full force of Katrina ‘s tidal wave, but didn’t suffer any long-term structural damage in the ICF walls.I am a great fan of good ‘smart’ technology. And an even greater fan of building smart.