Seismic Design of Cast-in-Place Concrete Walls

Cast-in-Place (CIP) technology for constructing building walls is more than 100 years old. Thomas Edison was one of the first to recognize its features and benefits. Over the years, new construction methods have developed including forming systems, the use of materials for enhanced insulation, and improved methods for removable forms, most commonly in single-family housing.
CIP produces a very strong wall and has an intrinsic thermal mass which provides, along with the appropriate insulation, an energy efficient building. It also permits the application of traditional finishes to both the interior and external faces allowing an appearance similar to frame construction, but with much thicker walls.
Cast-in-place concrete procedures are also used for large building construction. The twin towers of Marina City in Chicago at 588 ft. in height, the One Shell Plaza in Houston built in 1970, and Chicago’s Lake Point Tower built in 1968 are just a few examples.
Place Concrete Walls
Seismic Considerations
Earthquake-resistant buildings are constructed with three basic structural elements: the foundation; the vertical framing elements; and the diaphragms. In a reinforced concrete building, moment-resisting frames or shear walls, make up the vertical elements and consist of the vertical load-resistant and the lateral (seismic and wind) resistant systems. The vertical load-resistant system is composed of the floor (horizontal framing) and the column and walls (vertical framing).
During an earthquake and ground shaking, a building shifts through multiple displacement cycles. The reinforced concrete structural walls are designed and proportioned to resist the resultant combination of shear, moment, and axial force.
The design requirements are specified by building codes which assure the construction of a wall capable of resisting strong earthquake trembling without unacceptable loss of strength or stiffness. Shear walls, moment resisting frames, braced frames, or a combination of these systems make up the lateral resistant system.
The Recommended Seismic Provisions specified in the National Earthquake Hazards Reduction Program (NEHRP) account for variations in building seismic risks despite the design and construction quality. The risk is determined by several factors including:

  • The ground shaking intensity and other effects the structure may experience during an earthquake.
  • The number of occupants affected by the structure’s failure.
  • Use requirements after an earthquake.

Structures are evaluated and categorized according to seismic risk specified in the Seismic Design Category (SDC). Six SDCs categories range from A, for structures posing a minimal seismic risk to F, for those with the highest risk.
To guarantee that all buildings provide a tolerable public risk, the NEHRP Provisions require progressively more rigorous seismic design and construction as a structure’s potential seismic risk increases. Strength, detailing requirements, and the seismic resistance expense also increase.
When cast-in-place or precast walls are implemented to combat seismic forces in new buildings assigned to SDC D, E, or F, the IBC requires special structural walls.
The Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) (ASCE 2010) specifies the design force levels, the design proportions, and details as defined in the Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary (ACI 2011).
Building Codes
Building code editions most commonly adopted by state and local jurisdictions undergo continuous revisions to introduce improvements in design and construction practices. The codes applicable to earthquake considerations for building construction are the 2018 International Building Codes (IBC), the 2016 edition of ASCE 7, and the 2017 edition of ACI 318.
The International Building Code is an indispensable tool to preserve safety and public health that provides safeguards from hazards related to the construction environment. All 50 states have adopted it.
IBC codes
The benefits of the IBC are numerous. Application of the codes results in efficient and flexible designs. They encourage the implementation of new and smarter technology. The IBC emphasizes strict engineered solutions and permits the use of verified traditional methods.
In addition to the emphasis on safety, the IBC embraces innovative new technology. Revised on a three-year cycle, the IBC is specifically linked to ICC codes.
ACI codes
The ACI Building Code administers concrete structures designed and built in the United States.
The design provisions of the Code specify the minimum strength requirements for safety and prescribe serviceability and resilience requirements. Factors including deflection, concrete cover, cracking and corrosion protection which influence the design of the structural system also impact the design of the exterior wall.
Place Concrete Walls
Sustainability and Energy
Heating and cooling energy savings make insulated cast-in-place walls an appealing method of building construction. Cast-in-place walls contain few joints and have 10 to 30 percent better air containment than similar framed walls providing more consistent interior temperatures for tenants.
Cast-in-place systems are also appropriate for recycled materials. Supplementary materials like a slag or fly ash can replace a portion of the cement. Crushed concrete (aggregate) can be recycled to reduce the need for new aggregate. Recycled reinforcement steel and some polystyrene made with recycled material contribute to energy savings. These techniques earn points toward green rating systems such as LEED®.
Design
The NEHRP publishes a 40-page Guide for Practicing Engineers: Seismic Design Technical Brief No. 6: Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams. The guide provides detailed information for the design of cast-in-place concrete buildings to meet seismic requirements.

Tips to Follow when Ordering Concrete for Your Job Site

Ordering Concrete
No single concrete mix design works perfectly for every applicationstatesAlan Sparkman, the executive director of the Tennessee Concrete Association. He adds “Some basic knowledge and good communication with your supplier will help you get the perfect mix for each job.”
Ordering mistakes can be costly, potentially reducing your revenue and delaying project completion. You risk your reputation if you order the wrong type and quantity of mix for the project.
The following tips will help you avoid these mistakes.
Ordering the Right Amountof Ready Mix Concrete
Determine what you need the concrete to do so you can order the correct amount. Measure the dimensions of the area you need to fill, including length, depth, and width.
Concrete pour ordersare measured in cubic yards. You multiply the width by the length, then multiply the result by the depth or thickness required. This will provide you with the cubic yards necessary for the task.
Several websites provide an online calculator specifically for concrete. Or contact a local company and have a specialist visit your site to help you determine the right volume of concrete blend required for the project.
You also must know the compressive strength before ordering. Reinforced concrete ranging between 3,500 to 4,000 psi is used on footing and slabs, 3,500-5,000 psi on suspended slabs, beams and girders, and walls and columns need 3,000-5,000 psi.
For non-reinforced concreteuse 2,500 psi for footing and slabs, and 4,000-5,000 psi is required for pavements.
Slabs exposed to freeze-thaw conditions should be a minimum of 4,000psi.
Purchasing too much concrete is a waste of money and entails finding an environmentally safe way to dispose of the excess. Buying too little forces you to place your project on hold. As a rule of thumb, order about 5- 10% more than calculated to compensate for wastage and grade variations, over-excavation, loss of entrained air, settlement and other changes in volume.
Specify the Aggregate
The American Concrete Institute Standard (ACI) establishes the maximum size coarse aggregate. Typically a structural engineer provides advice on aggregate sizing. Your supplier can also provideguidance.
Mixing On-Site
If you are going to mix the concrete on site, consider the types of additives you require. Remember too that your supplier has no liability for the quality of the product if you add something to the mix on-site.
For large projects, ready-mix is best. Small variations in the mixing process, amounts of water added, and delays in pouring all will affect the uniform quality of the concrete used.
You may find the concrete will set before you can pour it.
Transportation and Delivery
How you get the concrete to your site requires careful planning and coordination with your supplier. Provide your supplier with as much information as possible.The further away your supplier is from your site the higher your transportation costs. Pay attention to deliverytimes and avoid scheduling deliveries during the rush hour.
Order spin loads rather than using tub-type trailers to avoid water separation.
When choosing a company to supply you the concrete, those closest to the job site should have priority. Ready-mix concrete must be poured within 300 revolutions of the truck’s tank, or within 90 minutes of mixing. It will harden quicker if the weather is hot.
If you need plasticizer or fiber, inform the supplier ahead of time rather than adding it to the mix on site.
Don’t forget to consider what you will use to transport the concrete to the pour location (for example, wheelbarrow or pump) if it is a non-air or air mix, and the concrete mix ID number for reference.
How to Minimize Costs
Remember that weekend ready mix deliveries can increase costs by as much as 10%. Plan your pouring schedules accordingly.
You can save money by minimizing the number of short loads, small concrete deliveries.
Don’t procrastinate when doing the pours. Concrete companies generally allow 4-5 minutes per yard delivered. Longer than that will often cost you a per-minute extra surcharge.
Prep your job site before the delivery truck arrives. You will be charged more money if the driver must sit around and wait.
Provide a Concrete Washout Area
To prevent concrete and liquids seeping into surrounding areas provide a wheelbarrow or other container for the truck to washout.
Additional Post-Ordering Tips
Don’t forget about safety. Cement is caustic. Wear protective masks and clothing when pouring the concrete. Wash off the concrete dust immediately after you are finished.

The Difference Between Exterior and Interior Concrete Preparation

When mixing concrete, the goal is to achieve a product that has superior durability and maximum strength. Mixing the bags of concrete is not a difficult process, and when it is done properly it should last a lifetime. The trick is to get the exact ratio of mix and water, along with any additives, to ensure the concrete has the greatest strength you can coax from it.
It can be tricky to determine what the crucial point is. It takes some practice to look at the mix and know just the right amount of water has been added. If you use too much water, the concrete will be weak. If you use too little water, the particles contained within the mix are unable to bind together.
There is a difference between interior and exterior concrete preparation.
Interior Concrete Preparation
Bagged mix
Any hardware store, lumberyard, or home center should have a good variety of concrete mix. They generally come in bag sizes of between 60 and 80 pounds. Depending on the scope of your project, the different features you can find include fiber reinforced, high early strength, and fast setting. For most DIY fixes, it is perfectly fine to use a standard concrete mix.
The back of the bag is a good source of information. It should contain usage recommendations that allow you to decide whether you need to choose a special mix. For both interior and exterior jobs, the actual mixing process is the same. What changes are the additives, if any, that are found within the bag, and the ratios of ingredients.
Mix your own
To produce a mix of concrete with a psi of roughly 3,000, the ratios you will be working with are 3 parts of sand, 1-part cement, and 3 parts aggregates. To make a yard of concrete, you need 1,560 pounds of sand, 517 pounds of cement, 1,600 pounds of stone, and 34 gallons of water.
Most concrete projects can easily be completed with the above mixture. At four-inch thickness, your mixture should cover 80 square feet. A cubic yard of concrete with 4,000 psi will need some changes to the recipe.While the amount of stone stays the same, you will now need 1,450 pounds of sand, 611 pounds of cement, and 35 gallons of water.
When less sand and more cement is used, the final mixture is extremely strong, making it perfect for use in exterior projects like patios, driveways, sidewalks, commercial garages, and pool decks.
Mixing procedure
Once you have an appropriate container available for mixing, weigh out all the dry ingredients and add them into the bucket. Blend the ingredients together until they are all evenly distributed. If there are any clumps you are unable to break up, throw them out.
Start adding in the water and mix well after each addition. Be sure to scrape the bottom and sides of the container as well. Add more water as needed until the final mixture resembles cookie dough with no dry pockets.
Slump test
Whether you are using your concrete for indoor or outdoor use, a valuable tool for those new to the properties of cement is the slump test. Slump cones can be purchased wherever you buy the concrete, or you can simply use a plastic cup. Ideally, your mixture will have a slump of 4″ if it neither too soft or too stiff.
Using a pen, poke a hole in the bottom of a plastic cup. This will allow any trapped air to escape. Fill the cup with your concrete mix and pack it firmly. Turn the cement filled cup upside down on a flat, hard surface.
Use your hands to gently vibrate the cup off and away from the cement without stopping. Lastly, use a ruler to measure the slump to make sure it is about 4″.
Shotblasting and scarifiers
For large surfaces (usually exterior), shotblasting does three preparation jobs in one: stripping, cleaning, and profiling. Shotblasters are often the preferred choice because they leave surfaces dry and ready to work on, and they give the rough texture you are looking for.
Also referred to as milling machines or surface planers, scarifiers are more aggressive and quicker at removing concrete than a grinder. High-speed rotations of multi-tipped flails chip at the concrete until it falls away.
Preparing surfaces
If you are planning to restore or overlay existing concrete, the most critical step is preparing the surface correctly. When this step is skipped or done incorrectly, you will not get good results.
To achieve the proper bonding of coatings and concrete overlays, you need to determine the CSP (concrete surface profile). The scale goes from CSP 1 to CSP 9. The rule of thumb usually adhered to is that the thicker the topping or overlay is to be, the higher the profile number should be. To get numbers in the seven to nine range will generally require the use of scarifying or shotblasting.
There are three main steps in the surface preparation process: cleaning, roughening, and repairing. Both interior and exterior surfaces need to be properly cleaned before any work commences.
Cleaning
For exterior surfaces, and interior surfaces when indicated, one of the quickest ways to clean the concrete is with the use of a pressure washer outputting at least 3,000 psi. The soil level is what determines the type of cleaning method used.
When water pressure power just isn’t capable of removing all the grease, dust, and grime, there are several common chemicals strong enough to do the job, including oxalic acid based cleaners, chemical strippers, and alkaline degreasers.
Roughening
With the temperature extremes and varied weather conditions, exterior preparation requires more than just a run with the grinder and a vacuum to achieve the ultimate bond. Although the grinder and vacuum method is perfectly suited to most interior projects, especially when the area is small, exterior projects should follow the pattern of acid etch, neutralize, high-pressure wash, and a thorough rinse.
Unless the exterior surface you are working on has an existing sealer or coating, skip the grinding.
Repair
For both interior and exterior projects, it is essential to the result that broken pieces, holes, and cracks in the existing concrete are fixed. If they are not repaired before new concrete or decorative work is installed, the integrity of the project can be compromised, which can result in work lasting for far less time than it should.
Final thoughts
Both interior and exterior preparation and mixing are quite similar. The biggest differences are the CSP and cleaning methods used to get the area ready.
Since there is no foolproof rule, it may take some trial and error to get the correct ratio of chemicals and machinery that work the best together for your specialized requirements.

How Temperatures Affect Concrete Pours

Concrete Pours

Concrete and cement are two different things. Water, aggregate, and cement make concrete. Aggregate is typically rock, gravel or sand depending on the desired use of the concrete. Cement binds the water and aggregate together through the hydration process, a chemical reaction forming the durable solid we know as concrete.

Making cement starts by mixing finely ground limestone and clay, and heating the mixture to a high temperature. This process forms a powder made up primarily of calcium, silicate, alumina and iron oxide. Different combinations of these chemicals, together with additives create different types of cement, for various applications.

Cement for residential construction, civil engineering or specialized cement for harsher corrosive, underground or underwater environments, are examples.

You need to consider the temperature of the environment when you pour concrete to assure a quality final product.

The Hydration Process

The hydration process is the chemical reaction that uses water and the components of cement to form crystals that bond tightly with the aggregate. The process has an initial dormant stage during which the concrete can be worked and a final setting stage that cures and hardens the mixture into its permanent state. It is an exothermic reaction, meaning it produces heat. However, the temperature of the concrete affects it while it sets. Too high a temperatureand the chemical reaction speeds up, too low and it slows down. The result, concrete that is brittle with a significant loss of strength.

The Ideal Temperature to Pour

Ideal concrete, for standard use in residential construction, doesn’t freeze, is not affected by most chemicals,and is watertight and resistant to wear. For concrete to have these characteristics, it needs to cure at the proper temperatures. These are between 50 degrees and 77 degrees Fahrenheit.

Cold Temperatures

In concrete setting terms, colder weather is any time the temperature falls below or is expected to drop below 40 degrees Fahrenheit. However, it’s not the temperature of the ambient air that is important but the concrete itself, which needs to stay above 50 degrees Fahrenheit.

The critical issue with cold weather is that concrete can freeze before it sets, this results in a weak, granular concrete with a distinct lack of strength. Or the mix sets too slowly with the interaction between cement and water (the hydration process) slowing down or stopping altogether, again causing a weak structure.

The remedy is to shield or protect the concrete from the cold to allow it to set correctly. Adding hot water to the mix, or chemical accelerators like calcium chloride to speed up the hydration process are options. As is adding slightly more cement into the mix, which causes more heat during the setting process. Curing and insulating blankets can be used to keep the concrete warm and protected from the cold. As can other types of physical shields and heaters.

Preparation is important too. Never set concrete on the frozen ground, and remove all snow and ice from areas surrounding or in contact with the mix. Always thaw frozen ground first by using heat pipes and blankets.

Lastly continuously check the temperature of the concrete with a pocket or infrared thermometer. It needs to be kept above 50 degrees Fahrenheit to set correctly.

Hot Temperatures

In hot weather, the temperature of the concrete needs to be kept below 77 degrees Fahrenheit. Ambient temperatures of 90 degrees Fahrenheit or higher can cause concrete temperatures to be too hot, requiring precautionary measures.

When temperatures are too hot, the hydration process is speeded up, resulting in a weaker bond.
Precautionary measures include adding cold water to the mixture, cooling aggregates before mixing, shielding the concrete from direct heat using heat and temperature shields or fogging the area directly above the mix. Additionally, chemical additives can slow the setting process, assisting the curing of concrete in hot weather.
Just as for cold weather, preparation is essential. Wet the subgrade and all areas to come into contact with the concrete. Wetting needs to be substantial, enough to make sure water is not absorbed from the mix into the subgrade, leading to cracking. Saturate at least four inches of the subgrade to stop water absorption from the concrete while it is setting.

Again, check temperatures regularly and adjust your precautionary measures to maintain an optimized temperature for the concrete.

Summary

Concretes three main ingredients, water, aggregate, and cement set into a solid through a chemical process called the hydration reaction. This reaction uses water to form crystals with the cement that bond to the aggregate, ultimately creating a bulky and robust solid, the concrete we all know.

Just like all chemical reactions, success requires the right conditions. And in the case of concrete, the temperature is an essential factor. Temperatures between 50 degrees 77 degrees Fahrenheitare ideal for concrete to set. Outside these ranges, precautionary measures must be taken, to ensure a stable and robust concrete.

Best Concrete Curing System

Best Concrete Curing System
 

The curing or setting of concrete is an often misunderstood process. As concrete is wet when it is mixed and poured, many believe curing is a drying process that strengthens and sets the mixture in place. However, maintaining adequate moisture levels during the curing process is essential, with premature drying often leading to unwanted cracking.

The curing of concrete is a hydration process which takes time. By controlling temperature and humidity levels, you can help to create the ideal environment and optimize the hardness and strength of concrete.

Optimal conditions for concrete curing

As well as leading to cracking, improperly cured concrete is often weaker, less resistant to abrasion and can suffer from scaling. Concrete exposed to the elements can be particularly susceptible to these problems. Drying winds can reduce moisture levels, and fluctuating temperatures affect optimal curing conditions.

The strength of concrete increases over time, and it can continue to strengthen over decades. Around 90% of the strength, however, is typically achieved during the first four weeks, with the first three days of hardening being most critical. It is therefore essential to ensure the best curing conditions for concrete during this early phase, and there are several different curing systems to choose from.

But which system is best for you? Below we will describe some of the different curing systems available and their respective areas of use.

Curing of concrete specimens

Laboratories often require the curing of individual concrete specimens. Specific standards are set by the ASTM and AASHTO, along with recommendations for accepted curing methods. Curing tanks and moisture rooms are two such methods, each with their specific requirements. A comparison of the two will help you to decide which is most suitable to your situation.

Curing tanks

Curing tanks are one of the simplest and least expensive solutions for the controlled curing of concrete specimens. Water storage tanks such as those used for livestock feeding can effectively hold several smaller concrete specimens. This makes them perfect for shorter-term or temporary applications, or where space is limited.

Curing tanks are filled with calcium hydroxide-saturated water, which maintains moisture levels around the specimens. The calcium hydroxide prevents leaching of this chemical into the water. Additional equipment is then used to maintain optimal temperature levels in the water – monitoring, circulating and heating it as necessary. This ideal temperature is 23.0±2.0°C (73.4±3.5°F). They can even be interconnected to increase their capacity and the efficiency of the temperature regulation system.

Curing tanks are low-maintenance and easy-to-assemble yet provide all the conditions necessary for optimal curing of concrete specimens. Spillages from the tanks can occur, however, and the chemicals in the water prevent the specimens from being handled without protective gloves. The large surface area of water exposed can also lead to humidification of the rooms in which they are housed.

Moisture room

A more advanced solution for the curing of concrete specimens is a moisture room; a room made from moisture resistant materials in which temperature and humidity levels can be maintained. These can be pre-fabricated units or can be constructed in situ to the required size. This allows for the simultaneous curing of larger and greater quantities of specimens.

According to specifications humidity levels in a moisture room must be kept at or above 95% relative humidity. There are various systems for creating the inside atmosphere in a moisture room. One way is to use fogging humidifiers in combination with temperature regulation equipment to maintain levels. As with curing tanks, this temperature is 23.0±2.0°C (73.4±3.5°F).

A more advanced system involves atomizing spray heads which release temperature-controlled water into the room, providing both heat and humidity at the same time. Both systems are likely to incorporate an externally mounted control panel which regulates the temperature and humidity in the room. This allows for easy monitoring of the curing specimens.

Moisture rooms allow for easier and more effective storage of specimens. By using racks or shelving the specimens can be maximally exposed to the humid air. Organization and handling are also much easier than in curing tanks. Moisture rooms, therefore, allow for more effective curing of greater quantities of specimens but can be more expensive to build and maintain.

Final Thoughts

Which curing system you choose ultimately depends on your circumstances. If only a few specimens are to be cured, or you only require a temporary installation, then curing tanks will likely provide the most cost-effective solution without sacrificing on quality. When larger or a great number of individual specimens are to be cured, it may be necessary to take advantage of a moisture room’s greater capacity. For longer-term applications, the opportunities for more effective organization and easier handling can also make moisture rooms an excellent choice.

 
 
 
 

Concrete vs Steel

High-rise buildings dominate the skylines of our cities, and in the race to have the highest or the most modern, the boundaries of architectural engineering are pushed even further.
The materials that make this possible – concrete and steel, each have their strengths and weaknesses. So, which is the better material?
Safety Considerations
Safety Considerations
Concrete
The concrete industry has always maintained that concrete is safer than steel. It requires no additional fireproofing to meet fire codes and has performed well during natural disasters.
Buildings made of cast-in-place reinforced concrete can withstand winds of more than 200 miles an hour and can withstand flying debris.
Cast-in-place concrete provides very good resistance to impacts and explosions.
It can resist extreme temperatures from fires without losing its structural integrity.
Steel
Reports issued recently by the National Institute of Standards and Technology show the reduced structural integrity of steel was to blame for the collapse of the twin towers in NYC. The fierceness of the jet-fuelled fires contributed to the collapse. With measures such as spray-on fireproofing, buildings made of structural steel are capable of withstanding greater temperatures.
The strength of steel, and its ductility, along with design and engineering, make it a good choice in areas of seismic activity. It can bend without breaking in high winds.
Design
Concrete
You can create and shape anything out of concrete, making advanced design possibilities a reality.
Concrete has the advantage of offering extra space possibilities. Cast-in-place reinforced concrete can give more rentable space, due to the lower floor-to-floor heights. Donald Trump’s architects switched from steel to concrete so they could add two extra stories to the new building at the former Chicago Sun-Times site.
Steel
Steel remains the popular choice for office and multifamily developers. Use of girder slab, castellated beam construction, and staggered truss enables much lower floor-to-floor heights than expected in structural steel buildings.
Steel has the highest strength-to-weight ratio of any construction material.
Cost Considerations
Concrete
Prices of construction material have increased, but the cost of ready-mix concrete has remained stable.
Insurance companies look favorably on cast-in-place reinforced concrete buildings because the safety and structural integrity reduce liability on their part. Owners and developers of such buildings can save nearly 25% annually on the costs of property insurance.
Steel
Structural steel represents less than 20% of all the steel used in building construction. The increase in costs of the structural framing system represents less than 2% of the 10% increase in project costs.
 
Costs for concrete framing systems have gone up equivalently to the costs of a steel framing system, despite what concrete experts say, according to John P. Cross, vice president of the American Institute of Steel Construction in Chicago.
Environmental Impact
Concrete
Concrete is often locally sourced and needs less energy to transport to construction sites.
Rebar for concrete is often made from recycled steel.
At the end of its life, concrete can be crushed and recycled, but it can’t be used for new building concrete, whereas steel can.
Steel
85% of steel is recycled, according to the British publication Building.
New steel made from scrap uses about one-third of the energy necessary to make steel from virgin materials.
Steel fabrication is often completed at a significant distance from the construction site, which increases the use of energy needed to transport it.
Construction Scheduling
Concrete
Buildings with concrete can almost always be constructed faster – sometimes twice as fast, according to Alfred G. Gerosa, president of the Concrete Alliance Inc, in New York City.
On a 2-day cycle, workers can pour up to 20,000 square feet of floor space every two days.
Steel
While steel can’t beat concrete’s 2-day turnaround; it does provide benefits of its own. John P. Cross says he believes structural steel framing systems are the way of the future, and they result in a faster construction schedule.
CAD programs can pass their information straight through a database as a 3-D model, and send it to detailing and shop floor fabrication programs. These productivity increases help to ensure the future of steel as a viable construction material.

Properties of Reinforced Concrete

Reinforced concrete refers to concrete that has reinforcements embedded inside. These reinforcements are necessary because concrete cannot withstand tension and shear stresses encountered in the everyday operation of the structure. Environmental factors such as strong winds may cause large vibrations of a concrete structure and possible damage if it does not have reinforcement material inside.
Although concrete has existed for thousands of years, reinforced concrete did not appear until recently. In fact, reinforced concrete was first used by Jean-Louis Lambot in 1848, when he embedded iron bars and wire mesh in some concrete rowboats.
Today, reinforced concrete is found on most construction sites.
Reinforced Concrete
Why Use Reinforcements?
The invention of reinforced concrete in the 19th century revolutionized the construction industry. By placing steel bars and plates inside the concrete, the ability of the concrete to sustain heavier loads greatly increased. This invention allowed for the construction of high-rise buildings.
When you embed reinforcements in the concrete, the concrete and the reinforcing material work together to resist any applied forces. Compressive and tensile forces push and pull on the concrete over its working lifespan.
Today, reinforced concrete is used in almost all construction projects. It has also surpassed wood to become the most commonly used construction material.
Steel Reinforced Concrete
All kinds of materials can be used to reinforce concrete. But steel is the most popular choice due to its ability to expand and contract at the same rate as the concrete. This feature ensures the steel will not crack the concrete during fluctuations in temperature.
Reinforcing steel is usually embedded in the concrete in the form of steel rods, steel bars, or steel meshes. The steel is tied together to make a skeletal structure before the concrete is poured over top.
After the concrete sets, the newly created reinforced concrete will have superior resisting properties. It can withstand high tensile and compressive stresses for long periods.
Fiber Reinforced Concrete
Fibers are a new type of reinforcement used to make reinforced concrete. These fibers are rated by their aspect ratio, which is a ratio of the fibers’ lengths to its diameter. They are randomly dispersed throughout the concrete and stirred well to create a uniform mix.
The most common material used to make the fibers include glass, synthetic, and even steel. Other materials used to make the fibers that are not so common include asbestos, which is economical, and carbon, which has great mechanical properties.
Properties of Reinforced Concrete
Concrete by itself is an aggregate mixture of cement and stone. This mixture does not stick together when dry but forms a rigid structure when water is added and then stirred.
With the addition of steel inside the concrete, the newly formed reinforced concrete has a coefficient of thermal expansion that is similar with that of both the steel and the concrete. As a result, internal stresses resulting from variations in the environmental temperature are almost non-existent.
Additionally, when the concrete cement hardens, it corresponds to the surface features of the steel. This allows the stresses acting around the reinforced concrete to be efficiently spread between the two materials.
Another desirable property of reinforced concrete is the development of a thin film on the surface of the steel. This occurs due to the alkaline environment caused by lime. As a result, the steel becomes extra-resistant to corrosion as moisture cannot penetrate this layer of lime easily.
Problems with Reinforced Concrete
The most common problem associated with reinforced concrete is corrosion. In the presence of moisture, the embedded steel may corrode and chip away. Corrosion results in extensive damage to the inner core of the concrete. For large structures such as high-rises, bridges, and dams, this presents a serious problem, as the failure of such a large structure would be catastrophic.
Techniques have been developed to detect the extent of the corrosion in the embedded steel. This information can be used to calculate the lifespan of large reinforced concrete structures so they can be rebuilt at the end of their life cycle.
Final Thoughts
Reinforced concrete refers to concrete that has embedded materials inside it. This material is often steel or fiber, and they work together to increase the tensile and compressive strength of the composite structure. As a result, reinforced concrete has superior qualities compared to plain concrete. Reinforcement makes them an attractive option for building projects, and they can be found on almost every construction site today.

Advantages and Disadvantages of Reinforced Concrete

Reinforced concrete is a popular material to help ensure your structure will remain strong and durable for many years. It’s made from relatively inexpensive materials, easy to pour into shapes and it’s extremely strong. It’s lighter than it looks, about a third as dense as steel and only slightly denser than glass. Although it’s extraordinarily strong in compression ratings, it is weaker in tension applications. It cracks or snaps if bent or stretched, but can be reinforced with steel to resist these factors.
Let’s review some of the advantages and disadvantages of reinforced concrete.
What Is Reinforced Concrete?
Reinforced Concrete
Reinforced concrete is made by casting wet concrete over a cage of steel reinforcing bars. The concrete then dries and hardens around the bars.
Advantages
High Compressive and Tensile Strength
When the concrete sets and hardens around the bars, the reinforced concrete stands up to tension and compression. The concrete provides the compressive strength, and the steel provides the tensile strength. Steel is used for the reinforcement because it expands and contracts in heat and cold almost the same way as concrete, which means it won’t damage the concrete by expanding at a different rate. This reinforcement allows the construction of tall buildings and other structures.
Cost-Effective
Reinforced concrete is very cost-effective. It is cement mixed with aggregates (rock, gravel, or sand fragments) and water, with a relatively small amount of steel added for reinforcement.
This combination makes concrete much cheaper to use than steel and other building materials. In fact, this material is considered the most cost-effective way to complete large construction projects.
Fast Construction
Not much can compare to the speed of a project based on reinforced concrete. Although typically mixed on-site and poured into shape, it can also be supplied in pre-cast pieces. Large construction projects with identical sections can be inexpensively assembled from pre-cast sections shipped to the final location.
Modular concrete has allowed many major cities to construct complex blocks, beams and wall sections to build high-rise buildings, bridges, and roadways in rapid time.
Versatile
Scientists and engineers have studied concrete additives and can alter the mixture based on application. Adding certain materials can make concrete set faster or remain viscous while working, be resistant to the effects of extreme temperatures or environments that might otherwise compromise its strength.
Reinforced concrete provides the versatility that allows engineers to design and build the highest skyscrapers, longest bridges, and deepest tunnels.
Engineers can also impact the features of a pour by using vibration, pour methods and additives that accelerate or retard setting to adjust the strength and compaction with physical changes.
Applying tension to the steel cage before the wet cement is poured results in the concrete compressing as it sets, making it stronger. Tension while setting inhibits cracking and allows less concrete to be used in an application which would otherwise require more.
Weather resistant
Water reducing additives that create air cells make concrete resistant to freeze-thaw cycles because the microscopic chambers of air relieve pressure when water expands as it freezes. Water reducers can be added to slow the setting rate of the concrete, which also makes it stronger using less concrete and more resistant to high temperatures.
Corrosion resistant
Water reducers can also be used to create low permeability, which helps keep out corrosive elements when concrete will be exposed to marine environments or de-icing chemicals. Engineers use this to keep water from seeping in and creating cracks or corroding steel reinforcements.
Fire Resistant
Wood, metal, and many other materials can’t handle the same temperatures as reinforced concrete without catching fire or deteriorating. The material’s low rate of heat transfer means the interior remains cooler than the surface. It is also chemically inert, making it impossible to catch alight.
This heat resistance makes it ideal for structures that need to regularly withstand high-temperatures, such as a factory or engineering workshop or apartment buildings where fire safety is important.
Disadvantages
Vulnerable to Temperature Changes
Changes in temperature can cause reinforced concrete to shrink and expand. If not carefully mixed and poured, cracks can appear and leave the steel reinforcement bars vulnerable to rust. What exacerbates the issue is that water can seep into these cracks and then freeze in winter. Freezing and thawing cause even more shrinking and expansion, which leads to more cracks and faults occurring.
Natural Deterioration Gradually Occurs Over Time
Reinforced concretes experience a natural decay over time. A chemical reaction occurs where alkalis from the cement react with silica from its aggregates. The corrosion causes a gradual deterioration throughout the structure of the concrete. Cracks occur from within, and flaking wears away at the surface.
Steel Rusts
If moisture manages to seep in through cracks in the concrete, it can make its way to the steel reinforcement. The moisture causes the steel to rust. The rusting process causes expansion leading to more structural damage.

Tremie Underwater Concrete Method

A cement tremie is a large, telescoping funnel used to precisely deposit aggregate mixtures on underwater surfaces. There are several reasons why concrete might need to be applied underwater. For example, subaqueous foundation and tunnel repair where surrounding water cannot be removed.
Although there are various methods for applying concrete underwater, the tremie underwater concreting method is among the best methods of placing a large volume of concrete in this situation.

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The Origins

The word “tremie” originates from the French language and translates to “hopper” in English. However, the original use of tremies for underwater cement remains unknown. The earliest use of tremie methods on record was from the mid-1800s, but the tremie method gained popularity at the beginning of the 20th century–both the Detroit River Tunnel and Pearl Harbor used tremie methods in their construction.
The Concrete
Tremie concrete is different from traditional concrete because of its self-compacting nature. This means no mechanical vibration is necessary for compacting the concrete. Since compacting equipment cannot be used in most underwater situations, this self-compacting nature is important to the product’s workability. Tremie concrete falls somewhere between the workability of conventional and super-workable concrete.
The Equipment
A hopper is positioned above the water’s surface, and a long pipe connects it to an underwater surface. This pipe is also known as a tremie pipe. Telescoping tremie pipe sections allow the cement mixture to travel to subaquatic surfaces without being compromised by turbulent water flow. This eliminates the problems associated with washout using other underwater concreting methods.
Since tremie pipes are can be very large, they are often moved using a crane or derrick. Where a derrick and crane cannot be properly deployed, a barge hoist tower or cofferdam can also be used for large tremie pipe movement. Commercial tremie pipes are made from steel and are at least six inches in diameter. Many are much wider. Small pipes often restrict aggregate flow and clogging can become an issue.
The Seal
Tremie systems are unsuccessful if cement and water mix in the tremie pipe segments. Therefore, it is vital to create a water-free pipe before placing cement. Several methods of purging the pipe’s water exist, those used are at the discretion of the contractor.
One common method is preventing water from accumulating in the pipe. This is accomplished by plugging the pipe’s end with a rubber sealed board. When the cement-filled pipe is in place underwater, the board is removed. The cement flow then seals the pipe from water pressure.
Sometimes other methods are used for plugging the pipe, and both improvisational and mechanical methods exist. For instance, a burlap bag stuffed with straw can be put into the hopper before the cement. A foam rubber plug, or pig, can also be used. The bag or pig plugs the hole and forces the water out as gravity pushes the cement down the tremie pipe.
A cone valve and rotary valve are examples of mechanical methods of plugging a tremie pipe: A cone valve is a control operated steel plate that allows operator control by the hopper. A rotary valve is a compressed air mechanism used within the pipe itself. This compressed air system forces water out of the tremie pipe when needed.
The Application
Concrete is fed from a mixer into the hopper. Once the form is filled with concrete, outward concrete flow buries and seals the delivery end of the pipe. The tremie pipe’s delivery end is kept submerged at least three feet in the cement overflow mass to maintain the water seal, depending on the project.
Maintaining a continuous concrete flow rate during application is ideal. Both increased and decreased flow rates can translate into potentially compromising the delivery pipe’s seal.
Ideally, the concrete placement will be precise, and any disturbances afterward will be minimal. Also, controlling the flow rate to have a bulging or layering morphology is ideal. Maintaining an ideal slump is desired. An ideal slump is about five to seven inches and is considered the standard, although wider slumps are also acceptable.
Occasionally, the tremie system will require lateral movement. If lateral movement is required, removing and repositioning the tremie system is recommended. The tremie pipe must be resealed between each repositioning. Without proper repositioning, the final product could lose some of its structural integrity.
The Advantages
The greatest advantage of using the tremie method for underwater cement placement is relative ease of application, and the ability to place large volumes of cement aggregate underwater at relatively deep locations in a short amount of time.
Also, the tremie system helps improve curing conditions for underwater projects since it gives the concrete a dry vessel to prevent water contamination. The vessel also allows applications in places that might otherwise require dewatering, where possible.
Finally, the tremie system helps cure underwater concrete placement of air pockets or honeycombing within the aggregate mixture. It also helps prevent cement loss due to turbulent water flow and dilution in certain environments.

Types of Concrete Foundations

For a vast number of people, the construction industry is almost a complete mystery. Even if you walk by construction sites every day of your life, the chances are if you haven’t worked in construction – or don’t know someone who has – the whole thing will remain a closed book.
However, the world of construction is a fascinating place, and well worth knowing about even for people who have never swung a hammer in their lives. As for people with an interest in DIY, or who simply want to get to know the building they live in a little better, knowledge of construction can be invaluable – and one of the best places to start with that is with the foundation.
There are as many foundations as there are buildings. The specific needs and characteristics of every home and every construction site are different, so naturally, the foundations will vary hugely in every case.
However, in most cases foundations for different buildings will fall into one of a few different categories, depending on the building, the ground, and the climate. Here are some of the main categories of concrete foundation used in most residential and commercial buildings.
Types of Concrete Foundations
Slab-on-Grade Foundation
In a slab-on-grade foundation setup, rather than being supported by a footing or another intermediary layer, the concrete foundation slab is poured directly onto the ground. The edges of the slab are designed to be significantly thicker than the rest, which makes for an extremely strong base of support for the home.
The fact that the concrete is in direct contact with the earth means this foundation type is not best-suited to all areas and soil conditions. Generally, it is best used in areas with warmer, drier climates, where the ground is not likely to freeze. It may also make the property more vulnerable to flood damage.
On the other hand, it is cheap and extremely stable, and less vulnerable to certain kinds of environmental damage, including infestations by pests such as termites.
Variations on direct slab-on-grade foundations include the floating slab formation – in which the slab is not in direct contact with the ground, but is secured by concrete layers several millimeters above it – and frost-protected slab-on-grade. Frost-protected foundations are fundamentally like direct slab-on-grade, but the concrete slab is insulated with two layers of polystyrene sheeting. This foundation style is more effective than slab-on-grade for colder climates, although the structure itself will still require insulation in winter months, and it is both tough and cost-effective.
Raft Foundation
Raft Foundation
Raft foundations, also sometimes known as mat foundations, are similar in some respects to slab-on-grade foundations, in that the structure ultimately rests on a single concrete slab.
However, unlike slab-on-grade foundations, in which the base slab only supports the weight of the lower part of the dwelling, raft foundations are designed to support the load of the entire structure. Columns transfer the weight from upper floors, while the slab itself supports the lower part of the building. The mat can also be reinforced by using beams or ribs built directly into the foundation.
Raft foundations are used particularly when the soil is especially poor-quality regarding load bearing. They are also ideal for sites with uncertain sub-soil water behavior, as the concrete mat itself is relatively easy to waterproof when compared to separate spread footing foundations.
T-Shaped Foundation
T-shaped foundations, also known as a spread footing foundation, are common in many homes and other residential buildings. As the name suggests, spread footing foundations are significantly wider at the base, to provide more stability and help distribute the weight of the load-bearing foundation wall more effectively.
T-shaped foundations are frequently used in cold regions and areas where the ground is particularly likely to freeze during the winter. The footing is built below the frost line, the foundation walls are constructed and placed, and the concrete slab is subsequently poured on.
T-shaped foundations are ideal for homes with a basement, and for small- to medium-sized buildings in locations where the soil is in moderate to good condition. They’re also relatively cheap and simple regarding construction, at least when compared with other foundation types.
Conclusion
Depending on the type of building you’re dealing with, foundations will vary considerably. For larger structures, such as skyscrapers, a far deeper foundation is necessary than with residential properties. This type of foundation may incorporate shafts, piles, caissons, or earth-stabilized columns to dig into the subsoil layer and provide extra stability for the larger structure. Other larger structures may have a monopile foundation, which uses a single massive structure driven to a significant depth to provide stability.
Whatever the foundation type you’re dealing with, the sheer variety demonstrates the level of experience and skill on offer in the construction and engineering communities. Concrete, after all, is a tricky business – it takes a delicate balance and a light touch.