Concrete is one of the most versatile materials available to architects. It can be modeled, shaped, and precast, in addition to being used for structural and non-structural applications.
- Joseph Aspdin developed Portland Cement in 1824. For the better part of 50 years it was used mostly for industrial buildings.
- Francois Coignet added steel rods and helped Portland Cement reach more widespread use in home construction between 1850 and 1880.
- Robert Maillart is a swiss engineer who did Cement Hall, 1939 in Zurich. He is credited with helping expose the versatility in the material.
Basic Materials & Terms
Concrete: Concrete is a combination of:
- Cement (Binding Agent) (Portland Cement)
- Iron Oxide
- Alumina (Aluminum Oxide)
- Fine and coarse aggregates
These 3 components are mixed together in proportions designed to reach certain strengths and malleability. Raising or lowering ratios of one or the other has various effects on the characteristics of the concrete.
Binding Agent: To hold everything together, a binding agent is needed. Combining the four ingredients listed above under strictly controlled conditions, the Portland Cement chemically interacts with water to form a paste that binds the other aggregates.
Hydration: Water is needed to hydrate the cement and makes it possible to mix and place concrete into forms, however, too much water can decrease a concrete’s strength.
- Need about 4-5 gallons of water per 94 lbm of concrete.
- Water is one of the most important factors in determining strength.
Laitance: When too much water is present in concrete.
All concrete is designed to have a compressive strength of concrete after it has hardened for 28 days. This strength at the 28 days of cure, is known as the Design Strength.
- Typical compressive strengths are 2000 to 3000 psi. There are psi available up to 20,000 for special applications but they are more expensive and used only when needed.
Higher than Needed
For some applications, where you need a design strength of 10,000psi at 28 days, and you may only be able to let the concrete cure for 7 days before loading, you can use much higher strength concrete to achieve the 10,000psi at 7 days to continue with construction as planned. This concrete will cure to the higher compressive strength even though it is not necessarily needed. The trade off to paying more for the concrete is a shorter construction time.
Types of Cement
|TYPE I||Standard Cement / Normal Cement||Used for most general construction where special properties of other types are not needed. Some cities like New York, where labor is so expensive, this type will be less likely to be used since it takes longer to set.Typically concrete typically weighs about 150 lbm/ft3Lightweight mixtures can weigh only 50 lbm/ft3|
|TYPE II||Modified Cement||Used in places where a modest amount of sulfate resistance is needed and the heat of hydration needs to be controlled. This is a specialized type that is less common.Examples: Dams, or other massive structures|
|TYPE III||High-early Strength Cement||When a quick set is needed.Also is good for cold weather since it has a higher heat of hydration. This type is typically used where labor costs are high and the speed of the project makes sense to pay for this type (to keep working quicker rather than waiting).|
|TYPE IV||Low-heat Cement||Used in massive structures to minimize cracking and is very slow settingNo longer commonly used|
|TYPE V||Sulfate-resisting Cement||Used for structures that will be exposed to water or soil with high alkaline contentExamples: Bridges|
Aggregates are available in a multitude of sizes and shapes. Because cement is the most expensive component of concrete, the best mix is one that uses a combination of aggregate sizes that fill most of the volume, so that the minimum amount of cement is used to achieve the desired strength. Not using enough cement could lead to a concrete that isn’t strong enough, honeycombing, or other problems with the mixture.
- Fine Sizes: Fit through a no 4 sieve (four openings per linear inch)
- Coarse Sizes: ¾” to 1” diameter.
Aggregate Rules of Thumb
Aggregates typically occupy about 65-70% of the total volume of concrete.
- Generally, the largest aggregate cannot…
- Be more than ¾ (three quarter) the smallest distance between rebar
- Be more than ⅕ (one fifth) the smallest dimension of forms
- Be more than ⅓ (one third) the depth of slab
Aggregate Ratio / Proportioning
The proportions of the aggregates with the water will vastly change the concrete’s strength, but will also have ripple effects on other aspects of the concrete.
- Example: Changing the water content to be lower will make for stronger concrete, but will reduce workability in the concrete and make it harder to get into complex forms.
- The goal is to achieve the best compromise with all the admixtures, aggregates and water to maximize function, cost, and strength.
Admixtures are items that can be added to the concrete to impart particular qualities. These can be either chemicals or other materials, and typically have to do with strength or workability.
- Typical primary reasons: speed hydration, slow hardening, improve workability, add color, and improve durability.
- Typical secondary reasons: corrosion resistance, underwater use, alkali-silica resistance, shrinkage reducers, and super retarders.
|Air-entraining Agent||Increase workabilityIncrease durability||Forms tiny, dispersed bubbles in the concrete. This increases workability and durability of the concrete and improves its resistance to freezing and thaw cycles. The agent also helps reduce separation of the components as the concrete is poured into forms|
|Accelerator||Speeds up hydration||Helps the concrete achieve strength faster. This allows quicker construction and reduces length of time needed for protection of the concrete in cold weather.|
|Plasticizer||Reduces water needed||Maintains the correct consistency needed for correct placement and compaction but reduces the amount of water needed. This makes it possible to mix higher-strength concrete.|
|Retarder||Slows setting time||This slows down the setting time to help reduce the heat of hydration. Also allows workability over a long period of time in case the distance to job is far away or the concrete needs to be pumped a long distance.|
|Waterproofing Agent||Decreases Permeability||Decrease the permeability of the concrete and as the name states, it makes it more waterproof.|
Supplementary Concrete Materials
Supplementary Cementitious Materials (SCMs) are added to the concrete as part of the total cementitious system to impart other desirable qualities into the concrete.
Unlike admixtures, SCMs have cementing properties to them, and therefore they can be used as replacements for portland cement. Because of this, some SCMs can greatly increase a buildings sustainability, such as fly ash.
|Fly Ash||Waste material obtained from coal-fired power plants||Fly ash improves workability, reduces temperature rise, minimizes bleeding, permeability, inhibits alkali-silica reaction (ASR) and enhances sulfate resistance.Fly ash can reduce the amount of portland cement needed and can help attain sustainability points.|
|GGBFSGround-Granulated Blast-Furnace Slag||Produced from the material formed from molten slag that is a byproduct of iron and steel manufacturing.||25-50% substitution for portland cement is common.Slag improves workability, decreases the need for water, and increases setting time in concrete, all of which can be a benefit in large pours and during hot weather.It also reduces bleeding, improves resistance to sulfate and chloride attack and can prevent damage from ASR.Concrete that contains slag develops more slowly, particularly the first 7 days. It continues gaining strength after the initial 28 days and has a higher ultimate strength than normal cement.|
|Silica Fume||Collected by filtering smoke created by the production of silicon and ferrosilicon metals.||Consists of mostly particles of silicon dioxide about 1/100th the size of a grain of cement.Silica fume is also available in liquid form. It is added to concrete at a proportion of 7 to 10% by weight of the cement.Silica fume decreases permeability, increases compressive strength, improves abrasion resistance, and reduces bleeding.|
|Pozzolan||A siliceous or aluminum siliceous material that, in finely divided form and in the presence of moisture, reacts chemically with calcium hydroxide released by the hydration of portland cement. This forms various cementitious compounds.||Pozzolan is used as a partial replacement for Portland Cement and to decrease permeability, increase strength, and improve resistance to ASR and sulfate attack. The Romans used volcanic ash, which was another form of pozzolan.|
Other Concrete Products
|Autoclaved Aerated Concrete (AAC)||AAC requires less material and results in less construction site waste. It provides sound control, thermal retention, and has greater air tightness than wood stud walls.Resistant to insects and mold.|
It does not have the strength of standard concrete, so construction is limited to nonbearing walls or other structures designed for this. Must also be protected from the exterior with plaster, masonry, or some other exterior finish.
|Precast concrete product manufactured by adding aluminum powder to concrete, hardening it in molds, and then curing the molds in a pressurized steam chamber (autoclave)|
The blocks have approximately ⅕ (one fifth) the density of conventional concrete. They are typically manufactured in blocks 10” high by 25” long and thickness of 4, 8, and 10”.Blocks are laid with thinset mortar, and can be worked with woodworking tools.AAC panels are also produced for the floor, roof, and wall panels.
|Self-Consolidating Concrete (SCC)||Concrete mix that can be placed by means of its own weight without the use of vibration.||SCC Placement finishes faster, requires less labor, and increases productivity. This type of concrete flows easily around dense reinforcement and provides a smoother, more uniform surface than standard concrete, so less time is required to make cosmetic repairs.Forms can be stripped sooner since the concrete hardens quicker and develops its strength faster.More expensive but will produce a better aesthetic exposed finish, therefore its commonly used for cylindrical columns.|
|Carbon Fiber Concrete||Uses epoxy coated carbon fiber mesh in place of standard steel mesh for secondary steel reinforcement. It’s used to make precast panels thinner and lighter. Because carbon fiber is non corrosive, less concrete cover is required to protect the rebar.||The resulting panels require smaller foundations and support structures, reduce transportation costs, and speed the erection process.Carbon fiber is manufactured by extruding industrial-grade carbon into ultrathin fibers. The fibers are bundled together to form pieces resembling yarn, called TOWS.Typical grid of carbon fiber mesh has about 7 times the tensile strength of standard steel mesh.|
|Ultra High Performance Concrete (UHPC)||Type of concrete characterized by high strength, low water absorption and high resistance to waterborne and airborne chemical degradation||Compressive strength ranges from 17,000 psi to 25,000 psi with flexure strength from 3,600psi to 6,000psi. Strength is achieved by using really small particle sizes and unique particle chemistry that is mixed, vibrated and cured in a factory under controlled conditions.The material is generally used as precast and not mixed on the job site.The precast panels can be up to ⅝” thin for simple panels. (Regular precast panels are about 4-6” thin)Glass fiber reinforced concrete panels are 1.5” to 2” thick.|
|Poured Gypsum Decks||Used for roofs and similar to concrete in that a liquid mixture is poured on a reinforcing material.||Provides a highly fire resistant roof deck.Typical application is fiber plank or rigid insulation is placed, with wire mesh reinforcement on top of that. The gypsum is placed over that in a minimum depth of 2.5”|
Common Concrete Problems
Alkali–Silica Reaction (ASR): More commonly known as “concrete cancer”, is a swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates, given sufficient moisture.