Concrete's Greatest Weakness is Time

0:00 On March 2, 1973, the Skyline Plaza tower

0:04 was under construction in a suburb of Washington, DC.

0:08 Crews had just placed a portion of the floor slab for the 24th story,

0:13 just two floors short of the project’s final height.

0:16 Shortly after lunch, workers noticed that the new slab was deflecting.

0:20 Suddenly, a portion of the building collapsed,

0:24 killing 14 and injuring many more.

0:27 The collapse left a gap in the building 18 meters or 60 feet wide,

0:32 essentially slicing it in two.

0:34 Investigators later found that workers had removed the formwork

0:37 and shoring for the lower floors too early.

0:40 Because of cold weather, the already-placed concrete in those lower floors

0:45 hadn’t gained strength as quickly as they expected.

0:48 Without the shoring transferring loads into the structure below,

0:52 the under-cured concrete was forced to bear the weight.

0:56 And it just wasn’t strong enough.

0:58 Concrete is an incredible material.

1:01 I’ve covered a lot of concrete topics in previous videos.

1:05 There are good reasons why we use so much of it in the built environment.

1:11 But, and this is hard for me to say, it’s not without its flaws.

1:16 Even putting aside the environmental issues, as a building material,

1:20 concrete creates challenges that are unique and, in many cases,

1:25 not that well-understood.

1:26 Most building materials, after they're fastened or put in place,

1:30 are immediately ready to use.

1:33 That’s not true for concrete, and even if it seems kind of obvious,

1:37 it creates some really interesting challenges for engineers,

1:40 architects, and contractors.

1:42 So I’ve cast some concrete cylinders in the garage,

1:46 and we’re going to break them to understand this weird property

1:49 of concrete and some of the ways we work around it.

1:53 I’m Grady, and this is Practical Engineering.

2:06 As soon as water meets the cement in concrete mix,

2:09 the clock starts ticking, and there’s basically no stopping it.

2:13 The working life of concrete consists of two key phases,

2:17 and they demand almost opposite properties.

2:20 Phase one has to be workable and easy to shape.

2:24 Concrete placement and finishing is a ton of work with a lot

2:28 of steps that each have to happen at the right time.

2:31 Of course, the second phase is strength;

2:34 no matter how beautifully formed concrete is,

2:36 it’s useless unless it can handle its designed load.

2:40 The process begins even before the concrete arrives on site.

2:44 Most large jobs rely on ready-mix batch plants,

2:47 where ingredients are measured and blended according to project specifications,

2:52 then loaded into rotating drum trucks for delivery.

2:55 Concrete is relatively cheap by weight compared to other building materials.

2:59 At its most basic, it’s just sand, gravel, cement, and water.

3:04 But placing it is labor-intensive, time-sensitive, and expensive,

3:08 plus many projects use a lot of it.

3:12 So it’s important that the right stuff makes it to the job.

3:16 Engineers often put strict specifications

3:18 not only the the ingredients themselves,

3:20 but how the concrete is handled on the way to the job site.

3:25 Some even put limits on the number

3:27 of drum revolutions allowed before the concrete is dispensed,

3:30 helping to prevent ingredient breakdown and loss of entrained air.

3:35 Once on site, the first task is getting the concrete into the forms.

3:39 At this stage, workability is everything.

3:41 It doesn’t need to flow like water,

3:43 but it should move easily enough to be placed quickly and completely.

3:48 You want some flow, especially for complex shapes

3:51 or when you have a lot of reinforcement.

3:55 Next is consolidation- usually with vibration or agitation-

3:59 to get rid of excess trapped air.

4:01 For slabs, workers screed the surface to level it,

4:05 then use floats to push down coarse aggregates and prepare for the final finish.

4:10 This is physically demanding work,

4:12 and every step has to be done before the mix becomes too stiff to work with.

4:17 We do have some tools to manage this process.

4:21 Admixtures can adjust the set time

4:23 and improve workability without adding extra water,

4:26 which would otherwise weaken the final product.

4:29 But the water in concrete isn’t a solvent that dries out.

4:33 Concrete cures through a chemical reaction called hydration.

4:36 The water becomes a part of the concrete.

4:40 And that hydration process can be

4:43 affected by jobsite conditions like temperature,

4:45 wind, or delays at the batch plant, which are out of your control.

4:50 That unpredictability can make a big concrete pour extremely stressful.

4:55 You don’t get do-overs.

4:57 Depending on conditions,

4:58 concrete typically reaches its initial set in about 2 to 4 hours.

5:03 That’s when the mix is firm enough that you can’t easily press a finger into it.

5:07 At this point, it’s ready for finishing,

5:10 whether that’s troweling for a smooth floor,

5:13 brooming for a textured sidewalk, or stamping for decorative work.

5:17 Each technique has to happen during a short

5:20 window between the initial and final set,

5:23 when the concrete is firm enough to support

5:26 workers but still soft enough to shape.

5:28 On big projects, timing is critical.

5:32 Standardized tests are often used to measure set times and guide

5:36 trial batches so that each task can be scheduled precisely.

5:40 After final set, the next phase begins: waiting.

5:43 I cast a bunch of concrete cylinders to show you exactly what I mean.

5:49 It’s 24 hours later, so let’s get these on the hydraulic press.

5:52 I’ve got Brady in the shop supervising the process.

5:55 And my scale isn’t calibrated,

5:57 so we’ll do all the comparisons in arbitrary units of force.

6:01 Some people suggested kilogradys last time I used this, so let’s go with that.

6:06 Even without looking at the scale,

6:08 you can tell these samples aren’t very strong.

6:11 Under the press, they kind of crumble more than break apart,

6:15 and this is pretty typical.

6:18 After a day, concrete’s strong enough to walk on.

6:20 And, depending on the structure,

6:22 this could be a good time to strip off the formwork,

6:25 but you’re not going to get away with much more than that.

6:28 I broke 3 cylinders, and we’ll plot them on the graph like this.

6:32 Let’s fast forward to 7 days.

6:34 For large projects, the concrete specifications

6:36 often require a test at this point.

6:39 It’s the same idea as what I’m doing here,

6:42 just with more sophisticated equipment.

6:44 Samples collected on site are put in cylindrical or cubic molds,

6:48 taken to a lab, and cured in controlled conditions.

6:52 Then they’re put into a press much more complicated than

6:56 this, and the force required to break them is measured.

6:59 The idea behind a 7-day test is that, if

7:02 the concrete isn’t going to reach its required strength,

7:05 you want to know as early as possible.

7:08 Let’s put these test results in on our graph.

7:11 The average was 9300 kilogradys so, a 3X increase from the 1-day breaks.

7:17 Strength gain usually follows a predictable curve,

7:21 so early results can be extrapolated with reasonable confidence.

7:25 If something’s wrong, you can often tell early and start planning accordingly,

7:30 even if that means tearing out a pour and resetting the schedule.

7:34 As costly as it sounds,

7:35 it’s nothing compared to the consequences of trusting concrete

7:39 that isn’t as strong as the engineer assumed in design.

7:43 This highlights one of the biggest challenges with concrete:

7:47 you can’t fully test quality until after installation.

7:50 Most building materials go through inspection before arriving on site.

7:55 With concrete, you can test the raw ingredients and even make trial batches,

8:00 but the real test is whether the mix you

8:03 placed in the formwork meets strength requirements after it cures.

8:07 That uncertainty adds risk.

8:09 To hedge against it, suppliers often design mixes with extra strength margin

8:14 to make sure that, even with some random variation,

8:18 strength will never come in too low.

8:20 Sometimes, waiting longer can help a borderline mix catch up.

8:24 But in some cases, a failed strength test

8:27 really does mean tearing everything out and starting over.

8:31 Another complication is where samples are cured.

8:34 Standard lab specimens are kept in tightly controlled environments.

8:37 This helps verify that the supplier met the required mix specifications.

8:42 But it doesn’t always reflect conditions in the actual structure,

8:47 where temperature, humidity, and weather can vary wildly.

8:50 That’s why many projects also include testing of field-cured samples,

8:55 which gives a more realistic picture of the in-place strength.

9:00 If this had been done at Skyline Plaza,

9:02 the cold-weather delays in curing might have been caught,

9:05 preventing a costly and deadly failure when shoring was removed too early.

9:10 On a well-run job, a good 7-day

9:13 result gives confidence that everything is on track.

9:17 Even though the concrete hasn’t reached its target strength yet,

9:20 you have a solid indication that it will.

9:23 I also broke some 14-day samples, not typically required on jobs,

9:27 but useful for seeing the big picture.

9:39 The graph shows that strength continues to rise,

9:42 though the rate is already slowing.

9:44 Let’s jump ahead two more weeks.

9:46 28 days is a fairly arbitrary,

9:49 but widely used benchmark for when the rate of hydration flattens out.

9:56 Usually, when we talk about the compressive

9:59 strength of concrete- 4000 psi or 28 MPa, or 10,000 kilogradys per square smoot,

10:05 or whatever it might be- we’re talking about the minimum 28-day strength.

10:10 A significant amount of concrete engineering is based on this strength.

10:16 The goal is that 28 days after placement,

10:18 you can feel confident that the structure will perform

10:22 up to the maximum loads as it was designed.

10:25 My 28-day samples broke at an average force of about 11,000 kilogradys,

10:30 about 20 percent stronger than the 7-day ones.

10:34 Pretty close to the rule of thumb that concrete

10:37 reaches around 75% of its final strength after one week.

10:41 But you see the problem here.

10:42 A month is a long time, and time is money in the world of construction.

10:46 There are some things you can do

10:48 in the interim- maybe install anchors or apply light loads.

10:52 For a sidewalk or driveway that rarely sees heavy vehicles,

10:55 concrete might be strong enough at 7 days.

10:58 But for applications where the margin between

11:01 expected loads and material strength are tighter, you just have to wait.

11:05 And this can be a real problem in some cases.

11:08 Think about concrete roadways.

11:10 How long are you willing to wait to keep a lane closed after a repair?

11:14 Tall buildings have a similar problem.

11:17 If you wait 28 days for every floor to cure,

11:20 it’s going to be a long and slow project.

11:23 You can see how concrete cure time turns into a serious

11:26 bottleneck and can often become the critical path on a construction schedule.

11:31 Luckily, there are a few ways to speed things up.

11:34 One is just to use a stronger mix.

11:37 The logic here is simple.

11:38 Say you need a 4000 psi concrete, but you don’t want to wait 28 days.

11:43 If you use a 5000 psi mix design, theoretically,

11:46 you’ll hit 4000 psi after just over a week.

11:50 This adds material cost, but the time savings can make it worthwhile.

11:55 Other strategies include using “high early strength” cement

11:58 that’s ground more finely to speed up hydration,

12:01 or altering the mix ratio by adding more cement or reducing water.

12:05 Heating the mix water or curing under blankets can also help.

12:10 Chemical accelerators are another tool.

12:12 Calcium chloride is a popular choice because it’s cheap, but it has drawbacks.

12:17 Chloride ions can speed up corrosion of steel reinforcement,

12:21 so lots of engineers won’t allow calcium chloride in concrete in their projects.

12:26 Non-chloride accelerators (or NCAs) have gotten better over

12:29 the years and may be a safer alternative, but they still pose challenges.

12:34 The curing of concrete is an exothermic reaction,

12:38 so faster hydration generates more heat,

12:40 which can lead to cracking as the concrete cools.

12:43 And, of course, it shortens the working time for placing and finishing.

12:47 I hope you can see the complexity in all this.

12:50 There is a lot we ask concrete to do, and because it hardens relatively slowly,

12:55 there’s a lot riding on how and when concrete gains strength.

12:59 It’s not just about stripping forms or removing shoring.

13:03 In many construction projects,

13:04 the strength gain of the concrete governs every downstream operation.

13:09 It determines when floors can support framing,

13:12 when roads can open, and when a project can move forward.

13:16 And there’s nothing magical about 28 days.

13:18 It’s just four weeks.

13:19 It’s a number of convenience that makes it

13:22 easy to talk about concrete strength and compare properties.

13:26 In fact, most concrete will continue to gain strength

13:29 for months or even years after that first four weeks,

13:33 depending on the mix design and steps taken during curing.

13:37 And many projects require that it does.

13:39 Compressive strength isn’t everything when it comes to concrete.

13:43 There are time- or exposure-dependent failure modes like shrinkage, creep,

13:48 and long-term degradation from freeze-thaw

13:51 that play an important role in design.

13:54 So some projects like dams and bridges often have 90-day requirements

13:58 to ensure that the concrete eventually reaches a strength to resist them,

14:02 even if it doesn’t need to happen right away.

14:05 But that 28-day convention gives

14:08 a hint about concrete’s greatest weakness: time.

14:11 Really, no other structural material requires you to wait

14:15 weeks before knowing whether it will actually perform as expected.

14:20 While most materials arrive on site ready to use,

14:23 concrete requires a leap of faith.

14:25 And then, a long pause.

14:28 Concrete is strong, durable, and incredibly versatile.

14:31 There’s nothing like it!

14:33 It’s a building material worth celebrating in many ways,

14:36 but only on its own terms.

14:38 You can place it quickly.

14:40 You can shape it into nearly anything.

14:42 But you can’t rush what happens next.

14:45 That’s the challenge and the art of concrete construction:

14:48 it’s a balancing act between acting fast and waiting long enough.

14:53 It’s a material that embodies both a sprint and a marathon.

14:58 A lot of people don’t think about concrete as an academic topic,

15:02 but because of its importance across the globe,

15:04 there are a lot of researchers who spend their entire careers studying it.

15:08 I read a lot of journal articles about concrete as research for this video,

15:13 most of them helpful if not particularly groundbreaking.

15:16 But every once in a while, an academic paper takes on a life of its own.

15:22 That’s the story told by my friend Kevin

15:24 of the “Bobby Broccoli” channel in the new documentary, 17 Pages.

15:28 A single paper sparked a scandal so big it was called the “Scientific Watergate.

15:33 17 Pages dives deep into one of the most

15:37 controversial science ethics cases of the 20th century.

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