concrete is as much a part of the urban landscapeas trees are to a forest. itâ€™s so ubiquitous that we rarely even giveit any regard at all. but, underneath that drab grey exterior isa hidden world of complexity. hey iâ€™m grady and this is practical engineering. on todayâ€™s episode - itâ€™s concrete 101. this video is sponsored by brilliant. more on that later. concrete is one of the most versatile andwidely-used construction materials on earth. itâ€™s strong, durable, low maintenance, fireresistant, simple to use, and can be made
to fit any size or shape - from the unfathomablymassive to the humble stepping stone. however, none of those other advantages wouldmatter without this: itâ€™s cheap. compared to other materials, concrete is abargain. and, itâ€™s easy to see why if we look atwhat itâ€™s made of. concrete has four primary ingredients: water,sand (also called fine aggregate), gravel (aka coarse aggregate), and cement. a recipe that is not quite a paragon of sophistication. one ingredient falls from the sky, and therest come essentially straight out of the ground.
but, from these humble beginnings are bornessentially the basis of the entire worldâ€™s infrastructure. actually, of the 4, cement is the only ingredientin concrete with any complexity at all. the most common type used in concrete is knownas portland cement. itâ€™s made by putting quarried materials(mainly limestone) into a kiln, then grinding them into a fine powder with a few extra herbsand spices. cement is a key constituent in a whole hostof construction materials, including grout, mortar, stucco, and of course, concrete. a lot of people donâ€™t know this, but everytime you say cement when you were actually
talking about concrete, a civil engineerâ€™scalculator runs out of batteries. iâ€™m just kidding of course, and you canhardly be blamed for not knowing the difference if youâ€™ve never mixed up a batch of concretebefore. even if you have mixed some concrete, goodchance it was in a ready-mixed bag where all the ingredients were already portioned together. but, each ingredient in concrete has a specificrole to play, and cementâ€™s role is to turn the concrete from a liquid to a solid. portland cement cures not through drying orevaporation of the water, but through a chemical reaction called hydration.
the water actually becomes a part of curedconcrete. this is why you shouldnâ€™t let concrete dryout while itâ€™s curing. lack of water can prematurely stop the hydrationprocess, preventing the concrete from reaching its full strength. in fact, as long as you avoid washing outthe cement, concrete made with portland cement can be placed and cured completely under water. it will set and harden just as well (and maybeeven better) as if it were placed in the dry. but, you may be wondering, â€œif water pluscement equals hard, whatâ€™s the need for the aggregate?â€
to answer that question, letâ€™s take a closerlook by cutting this sample through with a diamond blade. under a macro lense, it starts to become obvioushow the individual constituents contribute to the concrete. notice how the cement paste filled the gapsbetween the fine and coarse aggregate. it serves as a binder, holding the other ingredientstogether. you donâ€™t build structures from pure cementthe same way you donâ€™t build furniture exclusively out of wood glue. instead we use cheaper filler materials - graveland sand - to make up the bulk of concreteâ€™s
volume. this saves cost, but the aggregates also improvethe structural properties of the concrete by increasing the strength and reducing theamount of shrinkage as the concrete cures. the reason that civil engineers and concreteprofessionals need to be pedantic about the difference between cement and concrete isthis: even though the fundamental recipe for concrete is fairly simple with its four ingredients,there is a tremendous amount of complexity involved in selecting the exact quantitiesand characteristics of those ingredients. in fact, the process of developing a specificconcrete formula is called mix design. and i love that terminology because it communicatesjust how much effort can go into developing
a concrete formula that has the traits andcharacteristics needed for a specific application. one of the most obvious knobs that you canturn on a mix design is how much water is included. obviously, the more water you add to yourconcrete, the easier it flows into the forms. this can make a big difference to the peoplewho are placing it. but, this added workability comes at a costto the concreteâ€™s strength. to demonstrate this balancing act, iâ€™m mixingup some ready-mix concrete with different amounts of water. for the first sample, iâ€™m using just enoughwater to wet the mix.
you can see itâ€™s extremely dry. a mix like this is certainly not going toflow very easily into any forms, but you can compact it into place. in fact, dry concrete mixes like this areused in roller-compacted concrete which is a common material in the construction of dams. for the next three samples, i used increasingamounts of water up to what is pretty much concrete soup. after the concrete has had a week to cure,i cut the samples out of the molds. itâ€™s time to see how strong they are.
this is actually more or less how concreteis tested for compressive strength in construction projects. obviously iâ€™m not running a testing labhere in my garage, but i think this will give us good enough results to illustrate how watercontent affects concrete strength, plus these cylinders look like they might attack at anytime, and we need to deal with them. i made three cylinders of each mix, and iâ€™llbreak each one, watching how much pressure the cylinder was applying at the moment offailure. and this experiment was too cool not to invitemy neighbors over to help. we started with the samples that used themost water.
it was no surprise that it took almost nopressure at all to break them, on average about 700 psi or 5 mpa. you can see how crumbly the concrete is evenafter having a week to cure. all that water just diluted the cement pastetoo much. the next two samples used the range of watersuggested on the premixed concrete bag. these were much stronger, breaking at an averageof 1600 psi and 2200 psi or 11 mpa and 15 mpa for the high and low end of the watercontent range. and you can really see the difference in howthe concrete breaks. finally we broke the samples with the leastwater added to the mix.
you can see how rough these samples were,because there wasnâ€™t enough water for the concrete to flow smoothly into the molds. but, despite looking the worst of the four,these were the strongest samples of all, breaking at an average of around 3,000 psi or 20 mpa. on this shot you can even see the crack propagatingthrough the cylinder before it fails. it just goes to show how important mix designcan be to the properties of concrete. even varying the water content by a smallamount can have a major impact on strength, not to mention the workability, and even thefinished appearance of the concrete. itâ€™s impossible to state how much i am justscratching the surface here.
there is so much complexity to the topic ofconcrete partly because it has so many applications: from skyscrapers to canoes and everythingin between. in fact, no matter where you are, youâ€™rerarely more than a few feet from concrete - a fact that is inexplicably a source ofgreat comfort to me. but, i took less than 10 minutes to describewhat is literally the foundation of our modern society. so iâ€™m dedicating at least the next fewvideos to dive deeper into the topic of concrete. the next video will be about its greatestweakness. if youâ€™ve got questions about concrete,put them down below in the comments and maybe
i can get them incorporated into the nextvideos. thank you for watching, and let me know whatyou think! if you are watching my videos, you probably share my passion for understanding how things work and wanting to apply that knowledge to your everyday life. to do that requires not only learning core concepts, but developing intuition. brilliant is a problem solving website that teaches you how to think like an engineer. their courses are designed to help you gain a deep understanding of the topic. i love this one on solar energy that goes into lots of detail on the intricacies of these systems. to get started developing your intuition, go to brilliant.org/practicalengineering
or just click the link in the description and sign up for free. the first 200 people will get 20% off the annual premium subscription. again, thank you for watching, and let me know what you think!