# How monolitic is concrete when falsework is being crafted for parts of future structure only?

Suppose we craft a reinforced concrete pylon for a cable-stayed bridge. It's very tall and rather thin. No one builds falsework for the entire pylon - it'd be very impractical. Instead, a part of falsework is built for the lowest part of the pylon, filled with concrete, then some time later that falsework is removed and the next part is built for the higher part of the pylon.

So the pylon in not really a single piece of concrete, it a combination of pieces seamlessly united together because the concrete surface of the earlier part is kept clean until falsework is moved up and so new portion of concrete just sticks to it pretty well.

How "monolitic" is such monolitic concrete? How much does it differ from concrete put into falsework size of the entire structure?

• "so new portion of concrete just sticks to it pretty well" is misleading. The two pieces also usually have common rebar that is left sticking out of the first piece. – Chris Mueller Jan 18 '16 at 13:09
• @ChrisMueller Yes, but now amount of rebar helps if you paint the earlier concrete with alkyd paint. – sharptooth Jan 18 '16 at 13:12
• I believe that when one layer of concrete has set before the next layer is poured it is known as a 'cold joint.' You will find plenty of literature about them, but the general point is that if they are planned for (w/ continuity of rebar and rough surfaces) they aren't a big deal, but if they happen unexpectedly at a bad location, they can be a problem. – Ethan48 Jan 18 '16 at 14:18
• I'm resisting the urge to change the title of this question to "Can concrete poured in multiple stages be considered monolithic?". I think it better describes the question, but it's not as if the current title is wrong, so I don't feel comfortable going for it. – Wasabi Jan 18 '16 at 15:39
• Also consider that most concrete codes require some sort of roughening or other processing at a cold joint in concrete so the subsequent layer can "stick" to the previous one. – grfrazee Jan 18 '16 at 15:45

Now, pre-cast beams are often placed over consoles as in the figure below. Consoles, unlike cantilevers, suffer almost exclusively shear forces and must therefore have "shear friction" reinforcements. The book Design of Concrete Structures (as well as many others) has a description of the method in section 4.9 (pg 160). In new structures these consoles (and the column core between them, of course) should be poured all at once to avoid any joints. That being said, this isn't always possible. If you are reinforcing an existing structure, you might need to create such a console on a column which doesn't have one. You therefore have to deal with a joint between the new concrete of your console and the old concrete of the existing structure. This is an obvious problem since it creates a preferential shearing plane. The shear friction method considers such cases with a friction coefficient $\mu$ which is a function of the type of shearing plane (fully monolithic; new concrete on old, roughened concrete; new concrete on old, non-roughened concrete; steel on concrete). The book linked above has the appropriate coefficients listed on pg 161.