Foundations

Monolithic Slab vs. Stem Wall Foundation

Monolithic slabs

Monolithic slabs are foundation systems constructed in one single concrete pour, typically consisting of a 4 inch thick concrete slab with interior thickened portions under load bearing walls and also thickened at all perimeter edges, while the stem wall slabs are comprised of three components. The advantages of monolithic slabs include:

  • Faster overall construction time (typically)
  • Lower cost (typically)
  • Very strong, especially with the addition of steel and fiber-mesh reinforcement
  • Can be further reinforced on poor soil lots by adding post-tensioning

Stem wall systems

Stem wall foundation systems are constructed with three components:

1) A spread footing to transfer the loads to the underlying soil

2) Masonry foundation wall (typically masonry)

3) Poured concrete slab

Advantages of stem wall foundation systems include:

  • Less fill required for sloped lots (the foundation walls can be built at different heights to create a flat building pad on a sloped lot)
  • Greater free-board distance from the top of slab to the surrounding grade of the lot will give the home a more pronounced appearance
  • Less movement of slab edge and more accurate positioning of all items stubbed up through the concrete slab

Post-tension foundation

Soils: Since most homes and small commercial structures are built on soil supported foundations, the physical properties and behavior of the underlying soils significantly affects the performance of soil supported foundations. Most soils are soft and compressible, and the more load is applied to them the more they compress. Building foundation settlements occur when the building load exceeds the combined bearing capacity of the foundation and the supporting soil. Multi-story and high-rise buildings are typically designed to survive several inches of settlements, while most homes are light weight structures which apply lower loads that result in much smaller foundation settlements.

Clay soils are the most sensitive to moisture fluctuations. During wet weather the water soaking into the clay soils causes them to expand and create upward pressure under building foundations. The same clay soil during dry weather becomes dry and contracts, creating voids that leave portions of the building foundation unsupported. The higher the soil clay contents, the stronger and stiffer the foundation must be to remain undamaged.

Post-tensioning: One way to strengthen the slab is to construct a waffle-like system of supporting ribs with embedded reinforcing steel and post tension cables added to the ribs to increase its strength. Post-tensioned cables are twisted strands of high-strength steel, covered by a plastic sheathing similar to the electrical cables. After the concrete slab hardens the cables are tensioned with hydraulic jacks and anchored at the slab edges. The cables apply pressure to the slab and the grade beams from both axes, similar to vise jaws. No cracking occurs as long as the concrete is evenly compressed by all post-tension cables.

The size and the spacing of the concrete waffles and the required post-tension cable compression forces are determined by the projected pressure caused by shrinking and swelling clay soil cycles. Slab and foundation systems supported on high clay content soils will require more reinforcement than those constructed on low clay content soils. There are construction issues with post-tensioned slabs between the engineers and the contractors. Understanding the causes of these issues will help contractors to build better post-tensioned structures.

Top photo: Post-tensioning cables are draped inside the slab cross section. Bottom photo: Over-balancing of the concrete weight by post-tensioning will crack and lift the entire slab.

Post-tensioning issues: One of the main differences between a post-tensioned slab or beam and a standard steel reinforced concrete element is that the strands drape or profile inside the concrete, resulting in changes of their vertical location along the length of the element. The typically concave tendon profile will try to straighten itself out when stressed, creating an uplift load (also called a balance load) on the concrete. This counter force minimizes and often totally removes the dead concrete weight from the stress and deflection calculations. Since post-tensioning creates an active load on the structure it is imperative that all tendon locations precisely match the engineering drawings. Inaccurate tendon locations will greatly increase the uplift force on each member. The most common cause of incorrect tendon placement is a discrepancy between the structural drawings and the supplier’s shop drawings. Contractor and inspector must both verify that the layout exactly matches the structural drawings and not only the shop drawings.

Applying uplift loads larger than the weight of the floor causes problems during stressing when the tendons push up with a force larger than the concrete weight. This net upward force can result in very large tensile stresses at the bottom of the slab/column joint where there is typically little or no rebar, and can actually lift the slab. Unlike rebar, which activates only when loaded, too much uplift load (due to the number of strands or due to the increased drape) can significantly impact the slab.

Top: To prevent restraint, plastic is placed along the top of a wall which the slab crosses. Bottom: The black foam rubber around the first few inches of the dowels coming out of the adjacent wall allow the post-tensioned slab to compress during tensioning without cracking the slab.

Slip connections are critical: Post-tensioned slab shrinks 20% to 30% more than conventionally reinforced concrete. Post-tensioned concrete will move about 1 inch for every 100 feet of slab that is not restrained by a lateral system. If the edge of the slab is 25 feet away from the nearest shear wall, this edge will move in about 0.25” when post-tensioned. If this edge movement is restricted, either the slab or the restraining element will most crack under the additional force.

Slab restraint is usually caused by concrete or masonry walls that are connected at the perimeter of the structure. In addition to a larger movement, post-tensioned slabs contain substantially less rebar than conventional slabs, which is their main economic benefit. But since a post-tensioned slab does not have excess rebar to minimize and distribute cracking, the cracks will be larger and very visible. This is why the use and proper construction of slip details is critical to the performance and the aesthetics of post-tensioned concrete. Typical slip details use plastic, felt or building paper to eliminate the bonding of the slab to the walls. Many restraint cracks are created by engineers or contractors who underestimated the strength of the bond between a slab and concrete or masonry walls. When rebar is required between the slab and the walls, pipe insulation or foam rubber surrounding a portion of the dowel can be used to allow relative movement without activating the dowel. Reinforcement must always be discontinuous across the pour strip.

Pour strips: Pour strips are specific to post-tensioned members and are typically located at the mid-span or quarter point of the bay. To provide the crack control benefit the slabs on each side of the pour strip have to be completely separated when the tendons are stressed. Any reinforcement extending from one slab into the other will create a tension tie, restraining the relative movement of the two slabs, and very likely causing cracking. All rebar and post-tensioning must be lap spliced inside the width of the pour strip. Contractors must pay special attention whether the engineer specified the edges of the pour strip to remain fully shored after the tendons are stressed, but before the concrete has been placed to tie the two slabs together. The confusion happens because after a successful stressing, the majority of the slab (except the pour strip) is structurally stable and doesn’t require forms or re-shores for stability. But without edge re-shores, these mid-span pour strips can create large and heavy cantilevered slab sections on either side (before they are filled with concrete to tie them together). This will cause major deflections and severe cracking from the loads that were never intended nor reinforced by the structural engineer.

 

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