Concrete is the most widely used construction material, over 3 billion tons of cement, the main ingredient of concrete, was produced in 2010¹, and North American construction companies have become master’s in its various uses. Formwork, steel reinforcement and fresh concrete placement have all improved significantly since the first reinforced concrete skyscraper construction, The Ingalls Building built in Cincinnati, Ohio in 1903². Although concrete is heavy, requires forms for placement and producing it create a significant amount of pollution, it gives architects infinite flexibility in their design, is widely available and cost effective.
Most concrete structure today are built following the cast in place methodology in which, carpenters build a form, the mold, according to the engineered drawings, ironworkers install steel reinforcement and masons pour fresh concrete in the newly built formwork.
Some of the most effective methods of forming today include the use of modular forms systems, like the one produced by PERI, DOKA, Aluma or Duraform. Slipforming can be very efficient, if the structure has been designed to be build that way, usually long structure without too many changes in dimensions. In regions requiring lots of insulation ICF (Insulated Concrete Forms), are getting more and more popular. With this technique, there is no stripping of the form, which stays after the concrete is poured and constitute the insulation of the building. The main difference in the way you approach forming work is related to whether the concrete member is vertical, think walls, columns and pillars, or horizontal work, like bridge decks, slabs or beams. In both ways, you need panels, with a nice flat surface, since your concrete will be imprinted with the same pattern than your form surface and a supporting structure, all the frames, braces and support, to support all the weight of the fresh concrete.
If the design allows it, meaning it is repeatable from component to component, you can use pre-fabricated formwork and just “jump” the formwork, so you don’t need to repeat the whole assembly every time. In that case, the frame of the formwork system stays assembled and the panels are retracted from the freshly poured concrete. You then move the assembly to the next location by mechanical means, usually a crane or hydraulic cylinders, oil the panels, for a better surface finish and reposition the formwork assembly according to the drawings. Self-moving forms, with a hydraulic system, would be generally used on tall vertical structure, like a core of a high rise, or a long horizontal one, like tunnel construction, the main advantage of this technique is that you don’t need a crane to move the forms.
For more custom applications, modular components are widely used, in which, lightweight panels and structural members, all neatly organized in metal racking, are manually installed to create the formwork needed. In any case, there is always lots of filler pieces, block outs and embedded pieces of metals, that have to be built by hand, with material like plywood, dimensional lumber and sometime, steel.
The tensile strength of the concrete is created with the addition of steel reinforcement, which is installed once some, or all of the forms, depending on the structure, have been put up. The forms being used as a support for the rebar assembly. Most of the rebar come on site, cut to length, bended and sometimes, in fully assembled cages. Large steel companies use automated machines to bend and cut to length the individual steel bars, based on structural drawing.
When building tall continuous structural members, the rebar can come in pre-assembled sections joined to the previous one with special couplers that basically make 2 steel bars act like, one long bar. This is generally used on large diameter rebar in highly reinforced construction, because of the otherwise, very long lap length. The use of special connections to join 2 components of a building is one way to make pre-fabrication more effective.
The third element of concrete work is the placement of fresh concrete. Large concrete pump trucks are widely used, delivering concrete to remote location, thanks to special admixtures, modular hoses and high pressures pumps. Concrete is generally brought to site by ready mix trucks of various sizes. Large work areas or high buildings sometimes require the use of concrete placement booms. Basically, an articulated steel boom mounted above the workplace, connected to a pump, with rigid steel hoses, staged in an accessible location for ready mix trucks. As the work progress, you relocate the placement boom to suit your needs.
As for the concrete placement in itself, it’s generally done by a crew of masons and laborers. With the help of the concrete pump, they pour the mixture at the right location, to pre-determined heights. Some sites use remote controlled screeds but, most site do it with manual labor. As soon as the concrete start setting, the finishers come in to float and trowel the surface with various types of trowels, gas or hand powered. This step of the process requires great skills and a “feel” for the state of the concrete.
Often forgotten, because it’s hard to see the value of it, proper curing of fresh concrete is desired if you want to attain specified strength level, increased durability and a good surface finish. The idea is to keep the concrete in ideal conditions for hydration, with controlled temperature and humidity levels. There are many ways to do this but, keeping the forms in place for a few days while spraying water mist on top of the surface is generally sufficient. Extreme concreting conditions, like very hot or cold temperatures, require extensive preparation, including enclosures, heating and cooling systems. Sensors embedded in the structure are now available and can help greatly in monitoring the conditions of the concrete.
This article is intended as a quick review of current industry practices, I know there is many companies out there doing things differently. The point is, to know current techniques and processes so we can have a better vision for the future.
¹The Concrete Conundrum, www.ChemistryWorld.org ²https://en.wikipedia.org/wiki/Ingalls_Building