material.

但是，我之前沒有談到它最大的弱點

But, I didn't mention its greatest weakness.

嘿!我是Grady這是Practical Engineering頻道

Hey I'm Grady and this is Practical Engineering.

今天的單元，我們要繼續混凝土這個主題，並討論鋼筋混凝土

On Today's episode, we're continuing the series on concrete with a discussion of reinforcement.

這個影片是由Skillshare贊助

This video is sponsored by Skillshare - more on that later.

要了解鋼筋最大的弱點之前，我們首先要先了解一下材料力學

To understand concrete's greatest weakness, first we need to know a little bit about mechanics

白話文說就是"材料在壓力下會有什麼行為"

of materials which is the fancy way of saying “How Materials Behave Under Stress.”

這裡我們說的壓力不是焦慮絕望這種壓力

Stress, in this case, is not referring to anxiety or existential dread but rather the

而是材料內部的壓力

internal forces of the material.

壓力(應力)主要有三種模式:壓力(互相擠壓)、拉力(互相扯開)

There are three fundamental types of stress: compression (pushing together), tension (pulling

和剪力(互相滑動)

apart), and shear (sliding along a line or plane).

不是每種材料都能平均承受這三種應力

And, not all materials can resist each type of stress equally.

結論而言，混凝土非常能夠承受壓力，可是對拉力卻沒那麼在行

It turns out that concrete is very strong in compression but very weak in tension.

當然我這麼說你不一定要馬上相信

But, you don't have to take my word for it.

來看看現場示範吧

Here's a demonstration:

這兩個圓筒是同一批號生產的

These two concrete cylinders were cast from the exact same batch, and we'll see how

讓我們來看看他們能承受多少重量

much load they can withstand before failure.

首先，壓力測試

First, the compressive test.

這個混凝土棒在450公斤下碎裂

(Hand pump gag).

對混凝土來說，這個重量滿輕的，因為我剛才混凝土加了很多水去攪拌

Under compression the cylinder broke at a load of about 1000 lb (that's 450 kilo).

我的拉力測試就沒那麼複雜了

For concrete, that's pretty low because I included a lot of water in this mix.

我加了一些螺栓進去這個樣本，然後現在我把它吊起來

The reason is my rig to test the tensile strength isn't quite as sophisticated.

我把這個桶子加滿礫石，但還是不夠重

I cast some eye bolts into this sample, and now I'm hanging it from the rafters in the

所以我又加了啞鈴，讓水桶掉下來

shop.

這個水桶的重量大概36公斤，這不到壓力測試重量的10%

I filled up this bucket with gravel, but it wasn't quite enough weight to fail the sample.

這些告訴我們什麼? 你不應該拿混凝土去製作繩子

So, I added another dumbell to push it over the edge.

事實上，若沒有辦法矯正這個拉力的弱點，你不應該用混凝土

The weight of this bucket was only about 80 lbs or 36 kilos - that's less than 10% of

去製作任何結構物，因為很少結構物只單純承受壓力而已

the compressive strength.

現實生活中，幾乎所有結構物會承受綜合的應力行為

All this to say, you shouldn't make a rope out of concrete.

結構樑就屬一個典型的例子

In fact, without some way to fix this weakness to tensile stress, you shouldn't make any

這支樑是我手工做的，用混凝土在我的車庫裡做的

kind of structural member out of concrete, because rarely does a structural member experience

現在讓樑承受應力

just compression.

這就是樑會變成的樣子: 上層承受壓力

In reality, almost all structures experience a mixture of stresses.

下層承受拉力

That's no more apparent than in a classic beam.

我持續加重量，你大概猜得出來哪個部分

This particular classic beam is homemade by me out of pure concrete here in my garage.

會開始裂開

Applying a force on this beam causes internal stresses to develop, and here's what they

這個裂開幾乎是瞬間的，但你可以看到裂痕從底層開始

look like: the top of the beam experiences compressive stress.

底層是拉力最強的地方，然後再往上裂

And the bottom of the beam experiences tensile stress.

你知道我要表達的是什麼了，混凝土本身並不是一個很棒的結構材料

You can probably guess where the failure is going to occur on this concrete beam as I

混凝體一個人沒辦法承受各種拉力

continue to increase the load.

所以，大部分的情況，我們加強化物質進去增強

It happens almost instantly, but you can see that the crack forms on the bottom of the

強化物質加在混凝土裡面，合體變成一個複合物

beam, where tensile stress is highest, and propagates upward until the beam fails.

混凝土抗壓力，而強化物質抗拉力

You see what I'm getting at here: concrete, on its own, does not make a good structural

最常見的強化物質就是鋼筋

material.

我做了一個新的樑，把螺紋鋼筋放在底部

There are just too many sources of tension that it can't resist by itself.

螺紋的作用可以讓混凝土抓在鋼筋上面

So, in most situations, we add reinforcement to improve its strength.

壓力下，你可以首先注意到這支樑比剛才強多了

Reinforcement within concrete creates a composite material, with the concrete providing strength

我們現在的重量已經遠超過剛才的重量了

against compressive stress while the reinforcement provides strength against tensile stress.

第二件事注意到，裂得慢了一點

And, the most common type of reinforcement used in concrete is deformed steel, more commonly

你可以在樑斷掉前明顯的看到裂痕的產生

known as rebar.

這其實是鋼筋混凝土非常重要的一個部分

I made a new beam with a couple of steel threaded rods cast into the lower portion of the concrete.

之前純混凝土很脆弱，毫無預警的就斷裂了

These threads should act just like the deformed ridges in normal rebar to create some grip

但鋼筋混凝土卻有韌性，你可以在斷裂之前先清楚看到裂痕

between the concrete and steel.

這讓你有機會去避免可怕的災難，讓你在災難發生前就做修正

Under the press, the first thing you notice is that this beam is much stronger than the

螺紋鋼筋在大部分的情況下都是一個很好的強化物質

previous one.

它相對便宜、經過測試、能理解

We're already well above the force that failed the unreinforced sample.

但它確實有一些缺點。其中之一就是它的被動性

But the second thing you notice is that the failure happens a little bit slower.

鋼筋有延伸性，所以鋼筋無法抗拉

You can easily see the crack forming and propagating before the beam fails.

如果他沒有機會伸展開來

This is actually a very important part of reinforcing concrete with steel.

通常這意味著混凝土會在鋼筋吃掉所有拉力前裂開

It changes the type of failure from a brittle mode, where there's no warning that anything

混凝土裂開並不全然是壞事，畢竟我們是要求混凝土

is wrong, to a ductile mode, where you see the cracks forming before a complete loss

抵抗壓力，而它有裂痕的情形下還是能夠做好這份工作

of strength.

但有些時候你還是希望能夠避免裂痕

This gives you a chance to recognize a potential catastrophe and hopefully address it before

或者因為鋼筋而導致的變形

it occurs.

這些情況，你也許會需要考慮一個有主動性的強化物質

Rebar works great for most reinforcement situations.

也就是預力混凝土

It's relatively cheap, well-tested, and understood.

預力的意思就是事先提供(兩端的)壓力

But it does have a few disadvantages, one of major one being that it is a passive reinforcement.

(去抵消由混凝土部份拉力)

Steel lengthens with stress, so rebar can't start working to help resist tension until

一種方式是張拉鋼筋再澆灌混凝土

it's had a chance to stretch out.

鋼筋到達一定強度後，拉力會在裡面

Often that means that the concrete has to crack before the rebar can take up any of

鋼筋回縮時會擠壓混凝土，使混凝土獲得預壓力

the tensile stress of the member.

大部分橋梁都是這樣預鑄的

Cracking of concrete isn't necessarily bad - after all, we're only asking the concrete

瞧瞧樑的下層就知道了

to resist compressive forces, which it can do just fine with cracks.

另一個製作預力混凝土的方式叫做"後張法"

But there are some cases where you want to avoid cracks or the excessive deflection that

後張法是在混凝土養護完之後才施壓

can come from passive rebar.

這個樑模型，我置入塑膠套管

For those cases, you might consider going to an active reinforcement, also known as

鋼筋可在裏頭滑動

prestressed concrete.

混凝土凝固後，我固定螺帽加壓

Prestressing means applying a stress to the reinforcement before the concrete is placed

在壓力下，這支樑並沒有比傳統鋼筋混凝土強壯

into service.

但它在裂痕出現前，確實能承受更多壓力

One way to do this is to put tension on the steel reinforcement tendons as the concrete

不僅如此，不像之前鋼筋戲劇性的斷裂

is cast.

這次是螺帽附近的螺紋先消失

Once the concrete cures, the tension will remain inside, transferring a compressive

我希望這些示範能夠展示為什麼大部分的混凝土應用都需要加上強化物質

stress to the concrete through friction with the reinforcement.

為了加上抗拉力以及為了提高韌性

Most concrete bridge beams are prestressed in this way.

就像上一支影片，針對這個複雜的主題，我只是說明了一些皮毛而已

Check out all that reinforcement in the bottom of this beam.

許多工程師的終其職業生涯中都在研究、設計混凝土結構

Another way to prestress reinforcement is called post-tensioning.

但是，我玩混凝土玩得很高興，也希望你們覺得有趣

In this method, the stress in the reinforcement is developed after the concrete has cured.

我很樂意繼續混凝土這個主題，如果你們有任何問題

For this next sample, I cast plastic sleeves into the concrete.

在下面留言囉

The steel rods can slide smoothly in these sleeves.

也許我可以在下一支影片中回答

Once the beam cured, I tightened nuts onto the rods to tension them.

謝謝收看! 讓我知道你的想法

Under the press, this beam wasn't any stronger than the conventionally reinforced beam, but

謝謝Skillshare贊助播出

it did take more pressure before the cracks formed.

透過線上教學和影片，我學會這座這個頻道的每個影片

Also, this one wasn't quite as dramatic because instead of failing the actual steel

我們都知道內容品質的差異有多大

rods, it was the threads on the nuts that failed first.

Skillshare讓我可以向不同領域的專家學習新的技能

I hope these demonstrations helped show why reinforcement is necessary in most applications

製作高品質的課程，就像世界知名圖像設計師Aaron Draplin一樣

of concrete - to add tensile strength and to change the failure mode from brittle to

我製作很多技術模型去傳達複雜的主體

ductile.

所以向AJD這種專家學習新技巧對我來說非常寶貴

Just like the last video, I'm just scratching the surface of a very complicated and detailed

如果你正在嘗試學習新的、增進舊的技能

topic.

廢話不多說直接點選下方連結(點選顯示完整資訊)並開始學習

Many engineers spend their entire career studying and designing reinforced concrete structures.

與Skillshare一起

But, I'm having some fun playing with concrete and I hope you are finding it interesting.

前1000個報名者可以拿到兩個月的免費使用

I'd love to continue this series on concrete, so if you have questions on the topic, post

再一次，謝謝收看，讓我知道你的想法!

them in the comments below.

Maybe I can answer them in the next video.

Thank you for watching, and let me know what you think!

Thanks to Skillshare for sponsoring this video.

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I make a lot of technical illustrations on Practical Engineering to communicate complex

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If you're trying to learn a new skill or improve on an existing one, cut through the

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