With the foundation designs complete, the team can now estimate the total cost of using a Tella Firma foundation and compare it to the incumbent over-excavation process. The first step in this comparison is determining which costs are likely to change and which will stay the same when transitioning from over-excavation to Tella Firma. It is obvious that the cost of over-excavation will be removed, as that is the purpose of using the Tella Firma system. There will also no longer be the need for a typical slab on grade foundation. That cost will be replaced by a post tensioned, elevated slab foundation, which is a staple in the Tella Firma system. With this more technical foundation, additional engineering oversight is required, which is an added cost to the system. Finally, the Tella Firma foundation requires the introduction of two entirely new costs: drilled piers and the physical Tella Firma lifting mechanisms. Both of these items will incur substantial costs to the project.
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Some costs will remain but change substantially in value, which also must be considered in the cost analysis. The amount of rebar used in the foundation will change to accommodate the new type of foundation system being used. The project schedule will also change significantly, which is related to price through the opportunity cost of liquid capital and the cost of labor.
Other costs will not be affected at all by the change from the over-excavation process to the Tella Firma system. A geotechnical report of the site is required in either situation, as is permitting for the development. The Tella Firma foundation is not usable for infrastructure, so roads and utilities will still need to be over-excavated. Since the team only received data on the costs to over-excavate the residential lots, it can be assumed that the cost to over-excavate roads and utilities is independent of this analysis and will not change materially. Lastly, formwork is required for a concrete slab, no matter what foundation type is used. For simplicity, it is assumed that the formwork cost for over-excavation is identical for the formwork cost for the Tella Firma slab. Table 7 provides a breakdown of the cost assumptions just discussed.
Table 7: Cost Comparison Breakdown
Costs Saved
Costs Incurred
Over-Excavation
Standard Foundation
Pier Drilling
Post Tension Foundation
Tella Firma Lifting Mechanisms
Engineering Oversight
Costs Unchanged
Costs Changed
Geotechnical Report
Roadway Over-Excavation
Permitting
Formwork
Schedule
Rebar
Labor
The cost comparison between over-excavation and Tella Firma was calculated using a unit price method. By using a unit price method, the same price assumptions could be used for across each foundation design. The assumptions used to calculate the unit price for each component of the cost comparison are outlined below.
Over-Excavation: The unit price for over-excavation was determined based on the price paid by Sterling Ranch to over-excavate the residential lots on Filing 3. The average cost was reported to the team as $2.75 per cubic yard of over-excavated soil. This price tag covers all costs of the over-excavation process. It does not, however, include the cost to over-excavate utilities to the residential lots. As mentioned before, utilities and the roadways they run under will have to be over-excavated in the Tella Firma process as well. For this reason, it benefits the analysis to not include utilities or roads in the over-excavation price and allow the cost comparison to be completely independent of this process.
A benefit of over-excavation is that the building site can be prepared for construction while the soil is also being remediated. Without over-excavation, there is an additional need for site preparation before the Tella Firma foundation can be built. This site preparation is minimal, such as removing trees or debris and setting the grade, but it is not free. Site preparation for the Tella Firma process is estimated at $1.25 per square foot of land.
Standard Foundation: The cost for a standard foundation was broken down into two material components: concrete and rebar. Concrete is estimated at $90 per cubic yard while rebar is estimated at $700 per ton. The cost of both concrete and steel can vary significantly based on geographic location and economic cycles. As the two primary components of a foundation system, a drastic swing in the price of concrete or steel could change the outcome of this cost analysis and should be monitored closely.
Pier Drilling: The presence of piers in the Tella Firma foundation system contributes to a need for additional concrete and steel reinforcing. It also creates a need for an auger to drill the holes for the piers. In a conservative estimate, it is assumed that a truck mounted auger is rented for an entire day for each Tella Firma foundation that is built. This cost comes to a price of $2542 per 8-hour workday.
Post Tension Foundation: The Tella Firma foundation requires the same type of concrete and rebar used in the standard foundation. These unit prices will remain constant at $90 per cubic yard of concrete and $700 per ton of rebar. The Tella Firma foundation also uses post tension reinforcing, which is a different steel and comes at a higher cost. The cost for post tension cables is estimated at $1.65 per pound.
Tella Firma Lifting Mechanism: The technology that makes the Tella Firma foundation successful is the proprietary lifting mechanism that connects the piers to the concrete slab. A lifting mechanism is needed at each pier location so design is key to minimizing the total cost through the number of piers and lifting mechanisms. RMG Engineers advised the team that each Tella Firma lifting mechanism will cost $175 per mechanism when bought in bulk.
Engineering Oversight: The complexity of a post tension elevated slab foundation requires engineering oversight that is not necessary with a standard slab on grade foundation. An engineer will be present at the stressing of the post tension cables as well as the lifting of the slab. The engineer’s time is estimated at a final cost of $150 per hour. This price will also be highly variable based on locality and the firm chosen to oversee the design process.
Rebar: As mentioned, the amount of rebar needed changes based on the type of slab being used. The Tella Firma slab is primarily reinforced by the post tension cables, so it will likely need less standard rebar than the traditional slab. On the other hand, the piers in the Tella Firma foundation will require additional rebar, unless they are designed as plain concrete piers. This means the quantity of rebar needed will factor into the cost comparison but it does not change the unit price for rebar, which is taken at $700 per ton.
Schedule: One of the big claims of the Tella Firma technology is that it can save considerable time over the over-excavation process (A schedule comparison is presented in the next section). Since time is valuable, this schedule difference is also considered in the cost comparison, in the form of opportunity cost. Opportunity cost is calculated as the interest that could be made if the total cost of the project was invested in the market rather than the project, for a time equal to the project timeline. The investment vehicle chosen to represent the market was conservatively taken as the average annual rate on a 30 year mortgage, approximately 3.5%. No compounding was considered. For example, the opportunity cost of a project that cost $100 and took half a year to complete would be 100*(1+(0.035/2))-100 = $1.75. In this case, each foundation was considered as its own project, independent of the rest of the development process.
Labor: Labor is another time related cost; the construction workers must be paid for each day of construction. The cost of over-excavation includes the labor of that process, so labor in this analysis only represents the crew working on the foundation. For simplicity, it was assumed that both foundation types require the same daily manpower. This manpower was estimated using a 5 man crew with each member getting paid an hourly wage of $16.00. For an average 8 hour work day, this results in a total daily labor cost estimate of $640.00.
Cost of Capital: Cost of capital is the cost associated with taking a risk on a new investment such as the Tella Firma foundation. Cost of capital serves as a hurdle rate of return on investment; if the rate of return on the Tella Firma foundation is greater than the cost of capital, it is worth pursuing. If not, there is not enough incentive to go through the trouble of changing to the new process.
The cost of capital rate was provided by Sterling Ranch at 8%. This rate was factored into the cost comparison by multiplying the total cost of the Tella Firma foundation times (1 + cost of capital rate) or 1.08. This weighted Tella Firma cost is then compared to the unweighted cost of over-excavation. If the Tella Firma cost is still cheaper than the over-excavation cost, it has overcome the cost of capital rate and should be pursued.
With several costs dependent on the project schedule, it is important to get an accurate representation of the total time needed to build a foundation using the over-excavation method and the Tella Firma method.
Over-excavation followed by a traditional slab on grade foundation is a lengthy process. Based on the experience of previous Sterling Ranch filings, over-excavation takes about six months to complete, only after which can the foundation construction begin. Over-excavation is done all at once so, following the six month process, all lots in a filing are ready for foundations. Additionally, grubbing and grading can be completed during the over-excavation process, removing the need for future site preparation. Still, the traditional foundation process takes around 4 weeks. This adds up to a total over-excavation timeline of 210 days from the day ground is broken until the day a lot is ready for framing above the foundation.
The Tella Firma process requires no over-excavation, so six months was quickly cut off the schedule. The Tella Firma process then comes down to seven main steps, which are detailed in Figure 4. The first step is site work, which includes clearing debris from the site and preparing it for construction; this takes 1-2 days. Step two is to drill the holes for the foundation piers. It was conservatively estimated that this would take 1 day per lot. Next, the piers must be reinforced and poured. Since the soil is the form, the process can continue once the piers have set enough to support the installed Tella Firma lifting mechanisms, approximately 3 days after the piers were poured. The next 2 days are spent preparing for the slab. This is when formwork is installed and post tension as well as standard rebar installed.
Step 5 is to pour the slab, which is estimated to take 1 day. Then the concrete slab must cure. Tella Firma foundations utilize post tensioning, which allows the slab to be lifted away from its formwork a few days after it is poured. This saves precious time over traditionally reinforced, elevated structures. After only 3 days of curing, the post tension cables can be stressed and the slab lifted. These two steps can be completed in the same day, bringing the total timeline of a Tella Firma foundation to 13 days from groundbreaking to start of framing.
Several steps of the foundation process require good weather, such as pouring the slab. Inclimate weather can delay the foundation process. However, the effects are the same on both the over-excavation and Tella Firma foundations. While these estimated schedules cannot be taken as guarantees of the foundation timeline, they are comparable.
The final step in the cost analysis is combining the foundation design with the unit price estimates and schedule estimates to prepare a final cost estimate of the Tella Firma foundation. By designing the size of the slab and piers needed to structurally support each house, the team has also determined the amount of each material that will be needed in the cost estimate. A simple calculation of material needed times unit price creates a total cost of the Tella Firma foundation broken down by component. These cost breakdowns are shown in Figure 1, 2, and 3. These graphs show exactly which aspects of Tella Firma are financially beneficial compared to over-excavation, and which aspects are not.
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When you need to rely on a stable foundation, you can almost always turn to concrete. It’s a reliable, sturdy material that’s used in just about every major construction project. Concrete slabs are one of the most commonly constructed flat surfaces and serve as foundations for homes, patios, sheds, upper floors, and more. A concrete slab could be precast before installation or built in place. And though they may seem like a fairly simple feature, many factors play into determining concrete slab costs that range from $3,600 to $7,200. Any or all of these qualifying aspects could affect your total price: the size of the pad, labor rates, grading, reinforcement, thickness, or any upgrades like radiant heating or finishing designs. As you plan your next project, use the information we’ve compiled to help plan for concrete slab costs.
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A concrete slab is a flat, horizontal surface made of a wet cement mix and crushed stones that hardens or cures into concrete. This curing process is a chemical reaction that ensures strength and durability against weather, weight, and use. A slab can be built directly into the ground by several inches as a foundation, or it can be suspended above the ground to form a flat floor for a multistory building.
If you’ve used the terms “concrete” and “cement” interchangeably in the past, you’re not alone. They’re commonly mistaken as the same material, but cement is actually the dry binder that makes up part of the concrete material. While cement could be used on its own to create a cement slab, it’s not advised. Cement slabs are prone to cracking and breaking since they don’t undergo the same curing process that concrete slabs do. The cost of a cement slab is $1 to $5 per square foot.
Concrete is made of a mixture of water, cement, and aggregate material like gravel or sand. Cement typically makes up 10 to 15 percent of this mix. As the mixture hardens, a chemical process cures the material to set it, making it reliably strong for foundations of all kinds. On average, concrete slabs cost $4 to $8 per square foot.
Photo: depositphotos.comYou might have only considered labor, materials, and size as cost factors when you’re thinking of having a concrete slab installed. But other variables will affect the price, such as the kind of base needed, grading, the pad’s thickness, the preferred finish, and whether the pad will be cast on-site or off-site. The average cost of $5,400 for a concrete slab accounts for many of these standard options.
The amount of labor needed differs from project to project. Labor involves preparing the foundation and forms, laying the base or reinforcement, mixing the concrete, pouring, and finishing. The average concrete slab cost of $4 to $8 per square foot includes labor. Heavy equipment could be required if significant grading needs to happen during the site preparation. Larger slabs will also require more equipment to handle the size of the job.
Some grading is included in most jobs, although it may be closer to leveling than grading. Suppose the area you’re planning to lay a slab on is not flat at all. In that case, the contractor will evaluate the site to see whether i can be appropriately leveled or if a structural engineer will be needed to determine the correct slope to hold the weight of whatever is planned for the concrete slab. Building a slab on a grade typically costs about the same rate of $4 to $8 per square foot.
You may be able to build your concrete slab directly on the ground, but the slab could shift over time. Most concrete foundations are built on a base of gravel or crushed stone adaptable to the earth’s slight shifts below. This kind of base is best for unstable ground or ground that doesn’t drain well. Base costs will vary based on local material prices.
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To ensure the concrete slab lasts longer, a concrete specialist will determine what kind of mix is best for your project. It could impact the price slightly if your mix needs a higher amount of cement or aggregate than average. Also, if you need to add a concrete wall on the edge of the slab, you’ll pay more to have the shape formed and poured.
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Concrete slabs come in any thickness that suits the project. The standard thicknesses are 4 inches and 6 inches, which work well for driveways, patios, sheds, and more. For a 24-foot by 24-foot, 6-inch-thick garage floor slab, you’ll probably pay $3,460 for labor and materials. An epoxy seal would cost an extra $1,430 to $2,960.
As you consult with a concrete contractor, you might learn that the design and function you have planned for your concrete slab will require thicker edges than the rest of the area. This could be due to the size of the pad or the weight of a structure that it will need to hold. Thicker edges will cost an extra $1 to $1.50 per square foot.
The cost of concrete could differ from one city to the next. If there is high demand in your area for concrete installations, prices rise to meet the demand. Consult with a contractor or do your own research to learn what material prices look like in your area.
There are several techniques that concrete specialists use to make a plain concrete slab look like faux wood or paver stones. Additionally, you could opt to add a splash of color to create a distinctive look. These finishes come with associated costs, so it’s not worth adding a special finish if the slab is planned to simply hold up a shed. For $8 to $12 per square foot, you can have the slab textured, colored, bordered, or stenciled. If you prefer engraving, staining, or scoring, you’ll pay $12 to $18 per square foot. Any advanced or highly customized designs cost $18 or more per square foot.
Construction costs differ widely from one region to the next, but especially between urban and rural areas. Higher living costs affect labor and material rates for metropolitan areas. Still, rural locations could have fees for travel or higher costs due to demand if there aren’t many contractors available.
Precasting a concrete slab is a helpful option in many situations where the slab is a unique size or design, and it can be placed easily with the proper equipment. For instance, parking garages, roof tiles, concrete pavers, and fire pits are just some examples of a precast slab. Casting in place is more common in standard projects like driveways or patios where the forms are set to the project’s exact specifications, then the concrete is poured in place. Concrete prices are typically lower when casting in place.
Photo: depositphotos.comThere are a few more possible considerations that can affect the overall price. Opting for radiant heating is an upgrade that appeals to many homeowners, and some slabs will need extra reinforcement or additives to remain durable.
Every slab will require some kind of mesh to help create a stable base for the concrete to adhere to as it cures, but if extra stability is needed, then a wire mesh can be added for another $0.35 per square foot. Concrete slabs that will hold a heavy load or be built on soil in poor condition could require steel rebar reinforcements. The rebar is built in a grid structure and costs $2 to $3 per square foot on average.
Just as hardwood or tile floors can have radiant heating, concrete can, too. It’s a great way to keep feet warm on a cold surface while saving energy costs and avoiding the dust from forced air systems. As the concrete is being prepared, the installer will lay a pattern of tubes with hot water or electrical heating elements to heat the surrounding concrete. Concrete slabs have become a popular option for use as flooring in industrial-style homes or in businesses. But you could also opt to have radiant heat installed in a patio or driveway so that snow and ice don’t build up so easily in the winter.
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If you need an extra-strength concrete slab without rebar, ask about having proper additives added to the concrete mix. These special elements can reduce moisture content, maintain quality, and ensure a strong cure.
Are you replacing an old concrete slab? Then you’ll need to factor in the cost to break up and haul away the original slab before you build the new one. Concrete slab removal costs $500 to $1,800 on average.
Photo: depositphotos.comWhile most home projects don’t require any particular kind of concrete mixture, it’s best to be prepared with possible options that may apply if you’re working on a distinct project. We’ve included several types of concrete that could be used for concrete slabs.
This is the standard kind of concrete used for most projects, including driveways, patios, and sidewalks. It’s a basic mix of cement, aggregate, and water that cures over a period of several days to form a strong foundation.
When a concrete slab is used to hold more weight than average, like a multistory house or heavy balcony, the concrete is mixed at a higher strength to withstand the added pressure. High-strength concrete can hold over 6,000 psi.
A high-performance concrete is the next step up from a high-strength concrete. It performs well against frequent foul weather and can withstand 8,000 psi.
Ultra high-performance concrete is specially formulated with fibers and extra additives like limestone or quartz flour to make an extremely strong structure. It’s capable of withstanding over 17,000 psi without the use of rebar.
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You can choose to finish any of these types of concrete with a stamped finish. It’s a great way to spruce up a flat surface to look like wood, stone, or brick. The contractor has a limited window of time in which the freshly poured concrete can be stamped by hand. Since it’s a labor-intensive process, the cost averages $12 to $18 per square foot.
Once water is added to a concrete mixture, it must remain in constant motion; otherwise, it will harden in the container it was mixed in—a disaster inside a concrete mixer. If your project has a hard-to-reach area or structural support made of concrete, the installer will probably use self-consolidating concrete, which flows more quickly and doesn’t have to be agitated constantly. It’s best for tricky areas or special structures.
If you love pebbled beaches or sea glass, then a glass concrete slab, steps, or border is an option to consider. You can have the concrete mixed with recycled glass to create a visual masterpiece. The glass pebbles create a textured surface that’s sealed with epoxy to hold the pebbles in place against weather and foot traffic.
A special mix of asphalt and concrete is an option for commercial use like roads, airports, parking lots, or even dams. It combines some of the asphalt slurry mix with any number of crushed stones like gravel, sand, or mineral powder. Asphalt can be used for driveways just as concrete can, but it comes with its own pros and cons.
If you’re interested in a more environmentally friendly option, then permeable, pervious, or porous concrete should be on your list. It’s mixed without fine aggregates, so a honeycomb structure forms as it hardens. Rainfall, air, and heat can move between the layers unlike modern concrete. It’s not as smooth as traditional concrete, but it can be just as durable.
As the prefix “mono” suggests, a monolithic concrete slab is a single structure that’s poured all at once. This is often done when pouring a house foundation where the footing is usually lower than the floor area. The cost of a monolithic concrete slab completely depends on the total size and design, but an average range is $4,600 to $20,000.
Also known as an engineered slab, a post-tensioned slab is basically a monolithic slab but with steel cables laid inside the concrete. The ends of the cables are stretched and attached to their designated hooks to help add strength to resist cracking.
If the concrete slab needs to include a foundation wall, then the concrete will be poured directly into a pre-built wall instead of the forms. Piers or post holes are often added for additional support.
Photo: depositphotos.comAlmost every construction project, building, or support system uses concrete—more even than wood. It’s a robust and durable material that can be customized to fit a project’s exact needs. Beyond being necessary support structures, they’re also a great way to create stable flat surfaces indoors and outdoors.
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Foundations must be strong, durable, and resist breaking down from stress or weather. That’s why concrete is such a popular choice as a foundation material. When it’s set below the frost line, it stays in good condition to hold the structure above. Concrete also allows room for any kind of reinforcement without losing structural integrity.
Whether a thick or thin slab is needed, concrete makes a great floor surface that can also be reinforced and supported with steel cables or other elements. These are often precast and can be used at multiple levels beyond the ground floor.
Wooden pool decks require heavy maintenance or replacement since the chemicals in the water break down the surface more quickly. On the other hand, concrete pool decks remain the popular choice since they’re durable and can be customized to suit many styles.
If you need a flat space for outdoor living, concrete ranks high on the list of material choices. You could even choose a prefabricated slab if your location doesn’t have any unique circumstances. These are usually thinner, so install a concrete patio on-site if you need more weight capacity. You can ask a contractor about the concrete cost per yard for small patios.
Outdoor utilities need a flat surface to be able to operate properly. Air conditioning units are the most common reason for pouring a concrete slab. The slab needs to be thick enough to support the weight and about 4 inches wider on all sides than the unit itself. Don’t plan this slab until your HVAC technician has given you the exact specs of the unit that’s best for your house.
If you already have a patio but want to add a hot tub, you’ll need to pour another concrete slab. Be sure to ask how heavy the hot tub will be when it’s full so that the slab is built to support it without shifting or sinking unevenly. Also, you may want to create a space for a basketball hoop, and a concrete slab is a cost-effective way to create the flat surface you need for a quick pickup game.
Working with concrete involves more tools and equipment than you think, and if you get into a jam, you could end up with some ruined concrete-covered clothing—or worse. If you are familiar with the kind of concrete you need and how to operate a small mixer, you could try to pour your own small slab for an air conditioning unit; just know that the slab must be level so the condenser can work properly. Beyond minor jobs that don’t need special concrete or reinforcement, you’re better off handing the job over to a pro with plenty of experience.
Preparing large areas for concrete takes time, effort, and coordination to make sure it’s leveled and shaped correctly. Making certain the concrete forms are straight isn’t as easy as it looks, and if the concrete ends up sloping toward your house on accident, you’ll end up with foundation problems from pooled water. In some cases, the project will require a structural engineer’s assistance to ensure the slab can withstand the weight, slope, or design. Saving a few thousand dollars on labor isn’t worth it if a poor job results in significant damage later. A concrete contractor should also provide a warranty or guarantee on their work.
Photo: depositphotos.comMany want to save money on big projects since every extra feature can ratchet the price higher. Some of the concrete slab costs for your project will largely depend on the terrain and structural needs, which are usually out of your control. But there are still some ways to save on concrete slab costs.
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The concrete slab you’re planning to build will probably be used frequently. Since concrete lasts for decades, such a frequently used feature should be built to last by a reputable contractor. Use these questions to help guide your search for a concrete contractor who is licensed and insured.
Every variation on a concrete slab installation changes the price, whether it’s size, thickness, shape, design, finish, or reinforcement. The more standard or common the project, the easier it will be to estimate concrete slab costs. Use these answers to frequently asked questions to help guide your budgeting process.
Assuming that the slab does not require extra grading, reinforcing, or thickness, you can expect to pay $865 for a 12-foot by 12-foot concrete slab.
A 1,200-square-foot slab measures 30 feet by 40 feet, which is a large design. The average cost is $7,800 to install a slab this size.
A concrete slab is made of a stronger material that’s specifically designed to withstand heavy weight or tension. It’s made of cement, water, and some mixture of crushed rocks or sand. A cement slab is made of cement and water, making it less durable and more prone to cracking.
On average, concrete costs about $110 per cubic yard.
Sources: HomeAdvisor, HomeGuide, Thumbtack
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