Concrete's Greatest Weakness is Time

On March 2, 1973, the Skyline Plaza tower was  under construction in a suburb of Washington,   DC. Crews had just placed a portion of the floor  slab for the 24th story, just two floors short   of the project’s final height. Shortly after  lunch, workers noticed that the new slab was   deflecting.

Suddenly, a portion of the building  collapsed, killing 14 and injuring many more.   The collapse left a gap in the building 18 meters  or 60 feet wide, essentially slicing it in two. Investigators later found that workers had removed  the formwork and shoring for the lower floors too   early.

Because of cold weather, the already-placed  concrete in those lower floors hadn’t gained   strength as quickly as they expected. Without  the shoring transferring loads into the structure   below, the under-cured concrete was forced to  bear the weight. And it just wasn’t strong enough.

Concrete is an incredible material. I’ve  covered a lot of concrete topics in previous   videos. There are good reasons why we use  so much of it in the built environment.

But,   and this is hard for me to say,  it’s not without its flaws.   Even putting aside the environmental  issues, as a building material,   concrete creates challenges that are unique  and, in many cases, not that well-understood. Most building materials, after they're fastened  or put in place, are immediately ready to use.  

That’s not true for concrete, and even if  it seems kind of obvious, it creates some   really interesting challenges for engineers,  architects, and contractors. So I’ve cast some   concrete cylinders in the garage, and we’re going  to break them to understand this weird property   of concrete and some of the ways we work around  it. I’m Grady, and this is Practical Engineering.

As soon as water meets the cement in  concrete mix, the clock starts ticking,   and there’s basically no stopping it. The working  life of concrete consists of two key phases,   and they demand almost opposite properties.  Phase one has to be workable and easy to shape.  

Concrete placement and finishing is a  ton of work with a lot of steps that   each have to happen at the right time.  Of course, the second phase is strength;   no matter how beautifully formed concrete is, it’s  useless unless it can handle its designed load. The process begins even before the concrete  arrives on site.

Most large jobs rely on ready-mix   batch plants, where ingredients are measured  and blended according to project specifications,   then loaded into rotating drum trucks for  delivery. Concrete is relatively cheap by   weight compared to other building materials. At  its most basic, it’s just sand, gravel, cement,   and water.

But placing it is labor-intensive,  time-sensitive, and expensive, plus many projects   use a lot of it. So it’s important that the right  stuff makes it to the job. Engineers often put   strict specifications not only the the ingredients  themselves, but how the concrete is handled on the   way to the job site.

Some even put limits on  the number of drum revolutions allowed before   the concrete is dispensed, helping to prevent  ingredient breakdown and loss of entrained air. Once on site, the first task is getting  the concrete into the forms. At this stage,   workability is everything.

It doesn’t need  to flow like water, but it should move easily   enough to be placed quickly and completely.  You want some flow, especially for complex   shapes or when you have a lot of reinforcement.  Next is consolidation - usually with vibration   or agitation - to get rid of excess trapped air. 

For slabs, workers screed the surface to level it,   then use floats to push down coarse aggregates and  prepare for the final finish. This is physically   demanding work, and every step has to be done  before the mix becomes too stiff to work with. We do have some tools to manage this process. 

Admixtures can adjust the set time and improve   workability without adding extra water,  which would otherwise weaken the final   product. But the water in concrete isn’t  a solvent that dries out. Concrete cures   through a chemical reaction called hydration. 

The water becomes a part of the concrete. And that hydration process can be affected  by jobsite conditions like temperature,   wind, or delays at the batch plant,  which are out of your control. That   unpredictability can make a big concrete pour  extremely stressful.

You don’t get do-overs. Depending on conditions, concrete typically  reaches its initial set in about 2 to 4 hours.   That’s when the mix is firm enough that you can’t  easily press a finger into it.

At this point,   it’s ready for finishing, whether  that’s troweling for a smooth floor,   brooming for a textured sidewalk,  or stamping for decorative work.   Each technique has to happen during a short  window between the initial and final set,   when the concrete is firm enough to support  workers but still soft enough to shape. On big projects, timing is critical.

Standardized  tests are often used to measure set times and   guide trial batches so that each task can be  scheduled precisely. After final set, the next   phase begins: waiting. I cast a bunch of concrete  cylinders to show you exactly what I mean.

It’s 24 hours later, so let’s get these on  the hydraulic press. I’ve got Brady in the   shop supervising the process. And my scale isn’t  calibrated, so we’ll do all the comparisons in   arbitrary units of force.

Some people suggested  kilogradys last time I used this, so let’s go   with that. Even without looking at the scale, you  can tell these samples aren’t very strong. Under   the press, they kind of crumble more than break  apart, and this is pretty typical.

After a day, concrete’s strong enough to walk on. And,  depending on the structure, this could be a   good time to strip off the formwork, but you’re  not going to get away with much more than that.   I broke 3 cylinders, and we’ll plot them on the  graph like this.

Let’s fast forward to 7 days. For large projects, the concrete  specifications often require a test   at this point. It’s the same idea as what  I’m doing here, just with more sophisticated   equipment.

Samples collected on site are put  in cylindrical or cubic molds, taken to a lab,   and cured in controlled conditions. Then they’re  put into a press much more complicated than this,   and the force required to break them is  measured. The idea behind a 7-day test is that,   if the concrete isn’t going to reach its required  strength, you want to know as early as possible.

Let’s put these test results in on our  graph. The average was 9300 kilogradys so,   a 3X increase from the 1-day breaks. Strength  gain usually follows a predictable curve,   so early results can be extrapolated with  reasonable confidence.

If something’s wrong,   you can often tell early and start planning  accordingly, even if that means tearing out   a pour and resetting the schedule. As costly  as it sounds, it’s nothing compared to the   consequences of trusting concrete that isn’t  as strong as the engineer assumed in design. This highlights one of the biggest challenges with  concrete: you can’t fully test quality until after   installation.

Most building materials go through  inspection before arriving on site. With concrete,   you can test the raw ingredients and even  make trial batches, but the real test is   whether the mix you placed in the formwork  meets strength requirements after it cures.   That uncertainty adds risk.

To hedge against it,  suppliers often design mixes with extra strength   margin to make sure that, even with some random  variation, strength will never come in too low.   Sometimes, waiting longer can help a  borderline mix catch up. But in some cases,   a failed strength test really does mean  tearing everything out and starting over.

Another complication is where samples are  cured. Standard lab specimens are kept in   tightly controlled environments. This helps  verify that the supplier met the required   mix specifications.

But it doesn’t always  reflect conditions in the actual structure,   where temperature, humidity, and weather can vary  wildly. That’s why many projects also include   testing of field-cured samples, which gives a  more realistic picture of the in-place strength.   If this had been done at Skyline Plaza, the  cold-weather delays in curing might have   been caught, preventing a costly and deadly  failure when shoring was removed too early.

On a well-run job, a good 7-day result  gives confidence that everything is on track. Even though the concrete hasn’t  reached its target strength yet,   you have a solid indication that it will. I also broke some 14-day samples, not typically  required on jobs, but useful for seeing the big picture.

The graph shows that strength continues to rise,   though the rate is already slowing.  Let’s jump ahead two more weeks. 28 days is a fairly arbitrary, but widely used  benchmark for when the rate of hydration flattens out.

Usually, when we talk about the compressive  strength of concrete - 4000 psi or 28 MPa, or  10,000 kilogradys per square smoot, or  whatever it might be - we’re talking about   the minimum 28-day strength. A significant amount  of concrete engineering is based on this strength.   The goal is that 28 days after placement,  you can feel confident that the structure   will perform up to the maximum loads as it  was designed.

My 28-day samples broke at an   average force of about 11,000 kilogradys, about  20 percent stronger than the 7-day ones. Pretty   close to the rule of thumb that concrete reaches  around 75% of its final strength after one week. But you see the problem here.

A month is a  long time, and time is money in the world of   construction. There are some things you can do in  the interim - maybe install anchors or apply light   loads. For a sidewalk or driveway that rarely sees  heavy vehicles, concrete might be strong enough   at 7 days.

But for applications where the margin  between expected loads and material strength are   tighter, you just have to wait. And this can  be a real problem in some cases. Think about   concrete roadways.

How long are you willing  to wait to keep a lane closed after a repair?   Tall buildings have a similar problem. If  you wait 28 days for every floor to cure,   it’s going to be a long and slow project. 

You can see how concrete cure time turns   into a serious bottleneck and can often become  the critical path on a construction schedule. Luckily, there are a few ways to speed things up.  One is just to use a stronger mix.

The logic here   is simple. Say you need a 4000 psi concrete, but  you don’t want to wait 28 days. If you use a 5000   psi mix design, theoretically, you’ll hit 4000 psi  after just over a week.

This adds material cost,   but the time savings can make it worthwhile.  Other strategies include using “high early   strength” cement that’s ground more finely to  speed up hydration, or altering the mix ratio by   adding more cement or reducing water. Heating the  mix water or curing under blankets can also help.

Chemical accelerators are another tool. Calcium  chloride is a popular choice because it’s cheap,   but it has drawbacks. Chloride ions can  speed up corrosion of steel reinforcement,   so lots of engineers won’t allow calcium chloride  in concrete in their projects.

Non-chloride   accelerators (or NCAs) have gotten better  over the years and may be a safer alternative,   but they still pose challenges. The curing  of concrete is an exothermic reaction,   so faster hydration generates more heat, which  can lead to cracking as the concrete cools. And,   of course, it shortens the working  time for placing and finishing.

I hope you can see the complexity in all  this. There is a lot we ask concrete to do,   and because it hardens relatively slowly, there’s  a lot riding on how and when concrete gains   strength. It’s not just about stripping forms or  removing shoring.

In many construction projects,   the strength gain of the concrete governs  every downstream operation. It determines   when floors can support framing, when roads  can open, and when a project can move forward. And there’s nothing magical about 28 days.

It’s  just four weeks. It’s a number of convenience   that makes it easy to talk about concrete  strength and compare properties. In fact,   most concrete will continue to gain strength for  months or even years after that first four weeks,   depending on the mix design and steps  taken during curing.

And many projects   require that it does. Compressive strength  isn’t everything when it comes to concrete.   There are time- or exposure-dependent  failure modes like shrinkage, creep,   and long-term degradation from freeze-thaw  that play an important role in design.  

So some projects like dams and bridges often have  90-day requirements to ensure that the concrete   eventually reaches a strength to resist them,  even if it doesn’t need to happen right away. But that 28-day convention gives a hint about  concrete’s greatest weakness: time. Really,   no other structural material requires  you to wait weeks before knowing whether   it will actually perform as expected.

While  most materials arrive on site ready to use,   concrete requires a leap of  faith. And then, a long pause. Concrete is strong, durable, and incredibly  versatile.

There’s nothing like it! It’s   a building material worth celebrating in many  ways, but only on its own terms. You can place   it quickly.

You can shape it into nearly  anything. But you can’t rush what happens   next. That’s the challenge and the art of concrete  construction: it’s a balancing act between acting   fast and waiting long enough.

It’s a material  that embodies both a sprint and a marathon. A lot of people don’t think about concrete as  an academic topic, but because of its importance   across the globe, there are a lot of researchers  who spend their entire careers studying it. I read   a lot of journal articles about concrete as  research for this video, most of them helpful   if not particularly groundbreaking.

But every  once in a while, an academic paper takes on a   life of its own. That’s the story told by my  friend Kevin of the “Bobby Broccoli” channel   in the new documentary, 17 Pages. A single  paper sparked a scandal so big it was called   the “Scientific Watergate.

17 Pages dives deep  into one of the most controversial science ethics   cases of the 20th century. And if you want to  check it out, it’s only available on Nebula. Nebula’s a streaming platform built by and for  independent creators, including channels like   Strange Parts, Integza, Real Engineering, and  Hacksmith Industries.

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