e [Music] from the houses we live in to the buildings we work at to all the places in between we use concrete structures every day even when you can't always see it concrete is a part of every structure built you may think that concrete is just concrete but the concrete for this highrise this retaining wall this road and this storage tank is all very different even the concrete for this driveway in Florida is very different from this one in Chicago how can a material that makes all these structures so similar be so different the answer starts with the ingredients that go into a concrete mix in its most basic form concrete is a mixture of Portland cement water and Aggregates such as Rock and Sand the Aggregates serve as the filler material in concrete while the Portland cement and water form a paste which serves as the glue that holds the Aggregates together through the chemical reaction called hydration this paste hardens and binds the Aggregates into a strong durable Mass we call concrete concrete basically consists of three simple ingredients and by varying the types and proportions of these ingredients we get different concrete why would concrete need to be different to meet the unique design and construction needs of a structure a suitable concrete mixed design can be determined but we have to ask ourselves what will the concrete be used for where will the structure be located how will the concrete be placed and when will it be placed the answers to these questions all affect the final ingredients that go into a concrete [Music] mix different structures require different concrete mix designs because their uses vary for this high-rise building the concrete must support heavy loads it needs strong concrete this pavement will be exposed to cold weather and deicing chemicals its concrete must be durable for this water storage tank watertight concrete is a must and for this Warehouse floor the concrete must withstand constant abrasion how do we meet these different needs when designing a concrete mix strong durable watertight concrete begins with a key ingredient water water is used in a concrete mix to properly mix and coat all the ingredients for hydration to occur and for the concrete to be workable the right amount of water in a concrete mix is critical a concrete mixed with insufficient water won't have enough paste it'll be hard to place and will harden with a rough honeycomb texture a mix with too much water will be smooth and easy to place but the hardened concrete May lack strength and durability excess water dilutes the strength of the concrete paste making the hardened concrete weaker more vol able to cracking and also more permeable to salts and other destructive chemicals how do we ensure that the concrete has the proper amount of water in it at a ready miixed concrete plant cement water and Aggregates are carefully proportioned to meet the strength and durability requirements of the structure the stronger the more impermeable the concrete needs to be the less water we would start with to make sure the mix has the proper amount of water the the consistency of the mix is checked before it's placed by performing what's called a slump test the slump test may show that the mix has the proper consistency has too much water or that the mix doesn't have enough water it may have lost water due to Extended Transit time or unexpected delays in the placement of the concrete some additional water may have to be added prior to placement but this water must only be added in the ready miix truck drum and must be uniformly mixed with the proper number of revolutions according to ASM [Music] c94 strong durable watertight concrete begins with a low water content but the right amount of water to cement in a mix the water cement ratio largely depends on how the concrete will be placed a mix with a low water content happens to be ideal for this slip formed curb which is formed by a machine and needs little finishing but what if we need a low water cement ratio mix that needs to be pumped take this multi-story parking garage the specs require the high strength and durability of a low water cement ratio mix construction practices limit the concrete placement to a pump the stiff mix used for the curbs certainly won't be fluid enough for pumping is it is it possible to have a low water cement ratio mix that's fluid at the same time sure but not by using Portland cement water and Aggregates alone chemical ad mixtures give concrete properties it normally wouldn't have to make a mix more fluid a water reducer can be added water reducers make the concrete flow easier from the pump through the hose to this elevated slab in this Warehouse without increasing the water content if a water reducer is not used us pump hoses can become clogged and blocked delaying the placement of concrete water reducers are also used for concrete in areas that are difficult to place such as around congested reinforcing steel are you starting to see how the concrete for this vertical retaining wall and this sidewalker different they have different strength requirements and they're also placed quite differently the water reducer needed for the Pumped retaining wall wouldn't be needed for this hand-placed sidewalk with we've seen how a structure's final use and how the concrete will be placed affect our choice of ingredients but what about when the concrete will be placed how do weather conditions and seasonal changes affect our mixed [Music] design it's summer now while the concrete is being placed in these column forms but suppose construction is delayed until fall or even winter can the same concrete mix be used throughout the year probably not without some adjustment to the mix to get Quality Concrete in hot or cold weather other chemical ad mixtures may be required to understand why they might be needed let's talk about how concrete sets and hardens as concrete dries the water that doesn't chemically react with the cement slowly Rises to the surface this water is called bleed water and eventually evaporates in hot weather water evaporates much faster when this happens the surface can dry out and crack now you may think we can simply add more water to the mix in hot weather but adding more water actually increases shrinkage cracks to slow setting time and minimize cracking in hot weather a retarding ad mixture should be added to the mix in very cold weather hydration and strength gain may slow or stop completely to place concrete for this highrise in Winter an accelerating ad mixture may be needed to speed the concrete set time and strength gain hot or cold air temperature along with concrete temperature humidity and winds all affect how concrete sets and dries these varying conditions require different mix designs and different chemical ad mixtures [Music] all too often we think Quality Concrete means strong concrete strong concrete is a must for these core walls and elevator shafts in this multi-story commercial building but strength alone isn't always the most important property of concrete take these driveways in Chicago their concrete doesn't need to be as strong as the multi-story building but they do need something else durability against the effects of the environment Concrete in cold wet climates under goes many freeze thaw Cycles during the fall winter and spring what happens during a freeze thaw cycle no matter how low the water cement ratio concrete is somewhat porous water is able to enter the spaces and freeze when water freezes it expands when there is no room for this expansion stresses develop inside the concrete the concrete relieves these stresses by cracking over many freeze thaw Cycles these stress cracks appear as surface cracks or scaling for Quality Concrete and freeze thaw areas an air entraining ad mixture should be added to the mix entrained air provides millions of microscopic bubbles of stress relief when water freezes and expands air entrainment is what makes this weather exposed Chicago driveway different from the floors not exposed to the weather in this multi-story building and even this driveway in Florida which isn't exposed to the many freea [Music] cycles we know that different concrete is needed depending on where it's located when it will be placed how it will be placed and what it's going to be used for in addition to adjusting the water content using chemical ad mixtures and air entrainment we can further Custom Design concrete for different applications by using different types of cement there are five types of Portland cement produced in Mills like you see here these different types of Portland cement can be used to suit various design and construction needs almost all uses of concrete can be made with type one a general allpurpose cement suitable for buildings Bridges floors Pavements and pre-cast structures but some concrete structures require special needs these pre-cast pipes are exposed to sulfates in the ground a type 2 Cement gives the concrete moderate resistance to sulfate attack the concrete for this winter placement will set and Harden slower a Type 3 cement can be used to give higher strength concrete at an earlier age than type 1 its application is similar to using a type 1 with a chemical accelerating ad mixture the concrete for this Dam will generate a tremendous amount of heat during hydration using a type 1 cement a Type 4 cement minimizes the heat for these massive structures this sewage treatment plant is exposed to high concentrations of sulfates a type five CIT gives the concrete protection against these sulfates different cement types can be Blended together for a combination of other needs and are often mixed with mineral ad mixtures mineral ad mixtures act like cement in the presence of water and they include fly ash silica fume and ground granulated Blast Furnace slag these mineral ad mixtures generally the concrete set time but increase workability and strength fly ash is most often used to reduce the cost of a mix silica fume is commonly used to decrease the concrete's permeability it was used for this parking garage in this northern city that needed protection from the salts carried in by cars when concrete is more permeable these salts along with air and water corrode steel reinforcement is shown here on the underside of this parking garage floor in addition to different types of Portland cements which may be blended with different mineral ad mixtures we can further tailor a concrete mix with our choice of aggregate types from from the Quarry size shape proportion and gradation must also be considered in choosing these Aggregates for this Warehouse floor strong hard Aggregates were chosen to add wear resistance fine Aggregates like sand make pumping and finishing easier for this retaining wall and this third story slab for this Chicago driveway soft spongy Aggregates were specifically excluded from the mix because they absorb water and cause popouts during freeze all Cycles no matter what their use Aggregates must be clean and free of chemicals and particles that could affect hydration and bond of the cement [Music] paste achieving Quality Concrete goes beyond simply designing the concrete for a specific structure location and use the concrete must also be properly placed and Consolidated correctly finished and jointed and adequately cured let's examine each of these three processes and their role in Quality Concrete once the mix arrives at the job site it must be placed as soon as possible the type of placement will depend on the use of the structure and the construction requirements unless the concrete is placed by Machine it needs to be Consolidated why do we consolidate concrete during mixing and transporting concrete entraps large air voids if these voids are not removed the concrete will harden with a rough honeycomb texture concrete is vibrated to consolidate spaces within the mix and bring air voids to the surface to adequately consolidate the concrete a vibrator with a correct amplitude must be used at specific ific intervals the amount of vibration will depend on how workable or stiff the concrete mix is over vibration and attempts to move the concrete horizontally must be avoided and can cause non-uniformity and settlement of the co Aggregates Quality Concrete also depends on proper finishing all too often concrete has been properly designed placed and Consolidated only to have its surface damaged by proper finishing when should concrete be finished after consolidation concrete is struck off and bull Flo to a level smooth surface this must be done before water in the mix Rises or bleeds to the surface once bleed water appears it's important not to continue final finishing until it has evaporated the more water in a mix the more bleed water and longer this waiting time will be finishing with bleed water on on or near the surface can cause defects like spoiling adding water during finishing has similar effects and must be avoided always remember that excess water dilutes the strength of the paste making the hardened concrete weaker more permeable and often lighter in color during finishing the concrete must be properly jointed why does concrete need to be jointed as concrete dries it loses water and shrinks and also changes temperature shrinkage and thermal change cause internal stresses to develop to relieve these stresses concrete cracks we place joints where we anticipate the concrete will crack a joint is really just a neat control