REINFORCEMENT IN DRILLED SHAFT DEEP FOUNDATIONS
John B. Turner, P.E.
The foundation of a building or other structure is designed and constructed to transmit forces from the structure to the soil. Under typical conditions, these forces are the result of gravity – the weight of the building, occupants, and materials within the building – and the result of wind, earthquakes, flowing water, and other environmental exposures.
The design of all foundations takes into account downward loading on the foundation element and the ability of the soil to resist that load. In a drilled shaft foundation, this transmission of downward forces is generally through compression of the shaft of the foundation, often with the stress in the pier becoming less with depth as the surrounding soil takes the load through skin friction. In instances of uplift on a deep foundation, a pier resists upward movement through a combination of sustained superstructure loads, pier self-weight and by the friction of the pier shaft against the adjacent soil. In some soils, most or all of the downward force is resisted at the bottom of the embedded end of the shaft (the tip). The estimated capacity of this resistance is called end bearing. Where a drilled shaft for a pier is widened at the bottom of the boring, the pier is said to be underreamed or belled. The bell may be intended to increase downward capacity by increasing the area of the tip of the pier, or may be intended to resist uplift on the pier, acting as an anchor by engaging the surrounding soil.
In the absence of battered foundation elements, piers must also resist the horizontal component of lateral forces through bending of the pier shaft and bearing along the sides of the pier against soil. Software (such as LPILE by Ensoft, Inc.) is typically used to compute the bending forces in the pier and the interaction of the pier with the surrounding soil.
In expansive soils – those that expand when moist and contract when dry – the shaft may also be required to resist the uplift created when the upper soil strata go through moisture cycles. In these soils, as soil dries, it can shrink away from the shaft and sink downward. Precipitation on the soil can then flow into the space around the shaft, soak into the soil, causing swelling of the soil. As the soil expands, it may grip the shaft and then, as the soil continues to expand, the soil exerts upward force on the surface of the drilled shaft. These moisture cycles may be seasonal fluctuations of precipitation or multi-year droughts. The geotechnical engineer typically estimates the depth of this fluctuation in soil moisture and specifies a depth over which the designer ignores skin friction. The designer then assumes that this specified length of pier is not providing friction resistance against forces in the pier. In addition, the geotechnical engineer may specify an amount of upward force that should be anticipated so that the shaft is designed to resist this upward force (uplift).
The extent to which a cast-in-place pier is reinforced depends upon the loading of the pier and the character of the surrounding soil. In a simple case, the designer may determine that only the portion of a pier is subject to net tension, based on the weight of the building and pier and the ability of skin friction to transmit the load to the Earth. In such a case, a deep pier may be required because some load combinations result in greater downward force than upward force. In some situations, the permanent loads may necessitate deeper foundations to reduce/prevent long-term settlement. In such cases, a designer may specify that the area of reinforcement be reduced with depth or discontinued below a specified depth.
Where soils are not capable of providing adequate lateral resistance agoinst buckling along the pier length, reinforcement may be needed to confine the concrete and prevent splitting of the concrete under compression. Reinforcement may also be required through the full depth of a pier if the soil is potentially subject to seismic liquefaction. Piers that are underreamed to resist uplift will require substantial reinforcement to be continuous from the top to the bottom of the pier.
In very heavily loaded piers, reinforcement may be required to increase the strength of the pier, just as with above ground concrete columns.
Concrete Cover
In all cases where reinforcement is required, concrete cover around all bars in necessary over the length of the reinforcement. ACI 318 Building Code Requirements For Structural Concrete, 2014 edition (ACI 318-14) and ACI 301 Specifications For Structural Concrete, 2016 edition (ACI 301-16) both specify that three inches of concrete cover is required between the outermost reinforcement and the soil against which concrete is placed as a forming surface. This specified cover is subject to a tolerance that typically reduces this to a minimum cover requirement of two inches. ACI 117 Specification for Tolerances for Concrete Construction and Materials, 2010 edition (ACI 117-10) provides the permissible tolerances on concrete cover and other variables than may affect cover thickness. ACI 336.3R Report on Design and Construction of Drilled Piers section 5.2.1 states that reinforcement should be “accurately placed and supported in the correct locations” and protected from exposure to soil when casings are removed.
The basic construction requirements for securely locating reinforcement within forms or against soil before concrete is placed are specified in ACI 301-16:
3.3.2 Placing
3.3.2.1 Tolerances:
Place, support, and fasten reinforcement to maintain its location during concrete placement in accordance with Contract Documents. Do not exceed tolerances specified in ACI 117 before concrete is placed.
ACI 318-14 Building Code Requirements for Structural Concrete contains the following provision imposing a similar requirement:
26.6.2.2 Compliance requirements:
Reinforcement, including bundled bars, shall be placed within required tolerances and supported to prevent displacement beyond required tolerances during concrete placement.
ACI 301 section 3.3.2.4 references ANSI/CRSI RB4.1 Support For Reinforcement Used In Concrete and requires compliance with its provisions.
The Concrete Reinforcing Steel Institute (CRSI) originally issued CRSI RB4.1 in 2014. This is a mandatory (code) language document that formalized the provisions of the CRSI Manual of Standard Practice. This document describes the material and use requirements for reinforcing bar supports. RB4.1 sets the basic requirement in the following provision:
3.1.1.
All reinforcement shall be accurately located in forms or against earth, and held firmly in place before and during the concrete placement by means of reinforcement supports.
More specifically for drilled shafts, CRSI contains this provision:
3.2. Side-Form Spacers
3.2.1. Side-form spacers shall be used when needed to maintain side concrete cover on the reinforcement against a vertical form or excavation, including drilled shafts.
ACI 336.R3-93 (2006) Design and Construction of Drilled Piers section 4.4.3 states that reinforcement should not touch the sidewall of the excavation and minimum concrete cover of 3 inches should be maintained through the use of spacers.
ACI 336.1-01 Specification for the Construction of Drilled Piers 3.4.6 specifies that the minimum side cover in piers should be 3 inches to soil and should be at least 4 inches in cased piers where the casing is to be removed. The cover is to be maintained using roller-type side form spacers.
Based on these industry codes, standards, and specifications, reinforcement required for structural reasons in a drilled shaft, whether placed against a casing or with exposed soil, should be positioned using side form spacers. Further, because corrosion of reinforcement may adversely affect the integrity of a pier shaft, even where the reinforcement is not required for structural purposes, all reinforcement should be supported to maintain the required cover.
The purposes of concrete cover include:
- Protection of the reinforcement from initiation and progression of corrosion,
- Confine the reinforcement to improve bonding to the concrete, and
- Confine splices of deformed reinforcement at lap splices
Protection of reinforcement from corrosion by the concrete cover is the result of two characteristics of concrete: pH of concrete and low permeability of the concrete to air and water.
Fresh concrete is alkaline (basic), with a pH greater than 12. When concrete is initially placed against the steel reinforcement, the surface of the steel is said to be passivated. This passivation inhibits corrosion, effectively preventing corrosion until the pH of the concrete diminishes with age. This process is known as carbonation because it is typically the result of airborne carbon dioxide reacting within the concrete matrix. The rate of this reduction of pH by carbonation is a function of the environment of use, the thickness of the concrete cover, and the porosity of the concrete. Concrete typically protects the encased steel reinforcement until the pH at the surface of the steel reaches about 10 to 12. This pH threshold for initiation of corrosion is reduced by the presence of chlorides, with initiation of corrosion beginning as soon as chloride level reach sufficient concentrations.
Once corrosion initiates, the relatively slow rates of permeation of air and moisture through the concrete matrix limits the rate of corrosion of steel within the concrete. The thicker and more dense the cover, the slower corrosion will occur after it is initiated. If any portion of the reinforcement cage is exposed to soil, corrosion will reduce the effectiveness of the reinforcement over time.
The corrosion of bars confined in the concrete results in expansion of the steel volume as rusting occurs. This force of this expansion is sufficient to crack concrete and open up additional pathways for moisture and oxygen to reach the reinforcement, accelerating the corrosion processes. If the corrosion occurs in the pier above the level at which reinforcement is required for strength, the capacity of the pier can be compromised. Where earthquake resistance or uplift is anticipated, or overturning is a factor, such as for highway structures, maintaining pier strength is critical to safety and performance. Because of the relative ubiquity of chlorides around highways, concrete cover is a critical protection for foundations beneath these structures.
Concrete cover also provides confinement necessary for lap splices to function and bars to develop composite action with the concrete. ACI 318 and ACI 301 both specify that three inches of concrete cover is required between the outermost reinforcement and the soil against which concrete is placed as a forming surface. For most applications, this specified cover is subject to tolerances specified in ACI 117. These tolerances typically reduce the specified three-inch cover to a minimum cover requirement of about two inches. As part of this requirement, it is understood that the soil will have an irregular surface and the concrete cover will vary. It is the contractor’s responsibility to maintain the cover thickness within the specified tolerance.
The use of side form spacers is required to maintain this side cover and reduce the tendency of the cage to drag against drilled shaft walls as the reinforcement is inserted into the shaft. Unless the shaft is lined to exclude water or control the flow of wet or loose soil into the shaft, the dragging of the cage against the soil may slough soil into the shaft and end up coating the ties or spirals with wet soil.
Reinforcement positioning
In addition to protecting the reinforcement, the use of side spacing supports on drilled shaft reinforcement helps to maintain alignment of the reinforcement within the shaft. In most cases, the shaft is drilled vertically and the reinforcement is intended to be vertical. Reinforcement cages may seem rigid but long reinforcement cages set into drilled piers tend to warp because each bar is relatively loosely connected to the cage. As with individual bars, bars in tied cages that rest only on the bottom of the shaft follow Euler buckling with little adjustment for being in a cage. In most cases, the longitudinal bars will tend to all bend/buckle in the same direction rather than supporting one another. In battered shafts, it is even more important to adequately support reinforcement away from the interior of the drilled shaft as the bars tend to be deflected off axis by gravity.
While the need to keep bars straight within the drilled pier at first appears trivial, consider that the lateral location of an unsupported reinforcement cage would be able to vary up to six inches (three inches of cover on each side). As the cage attempts to bend, it also may twist, further complicating the follow-on work. In addition to allowing interaction of the reinforcement with the surrounding soil (and moisture), bending or twisting of the reinforcement results in shortening of the above ground extension of the reinforcement. Placement using properly spaced side form spacers/supports helps maintain proper placement.
In addition to side-form supports, in most instances, reinforcement requires support at the bottom of the pier. Supports installed on the bottom ends of longitudinal reinforcement reduce the intrusion of moisture and help distribute the weight of reinforcing bars into the soil without allowing them to sink into the soil.
Where the reinforcement does not extend to the bottom of the shaft, it is typically hung from a support across the drilled shaft. In this condition, supports maintain alignment with the shaft wall, assuring proper covers.
Support quality and use
CRSI RB4.1 also specifies testing of supports to assure that supports function as required. Under the testing requirements, materials used in supports and the configuration of the supports are required to be evaluated to assure that they both maintain the bar location during placement of concrete and do not reduce the durability of the concrete cover.
While not part of the requirements from CRSI, side form spacers used in drilled shafts need to resist dislodgement or breakage as the reinforcement cage is placed into the drilled shaft. No standard test method is currently available to evaluate these aspects. Experience shows that sled-type supports should be attached to the vertical reinforcing bars and should straddle ties or spirals to reduce the tendency to rotate on, or slide along, the vertical bars, thus becoming ineffective. Sled-type supports have been discontinued by most manufacturers because they are difficult to use and wheel-type supports have become the preferred support.
Wheel-type spacers are fastened around transverse reinforcement (ties or spirals). These supports outperform sled-types because rotation of the wheel results in less friction against the shaft wall, reducing dislodgement of soil where the spacer contacts the shaft wall. This rotation also lowers the forces on the spacer and may assist in placement of flexible reinforcement cages, particularly where the reinforcement might drag against irregularities along the shaft.
Despite these requirements and benefits, drilled shaft reinforcement is frequently placed without the use of side form spacers. While the selection of reinforcement supports is often a matter of “means and methods of construction”, it is important for engineers to specify in Construction Documents that supports be used. Within CRSI RB4.1, the load capacity ratings for supports gives a tool to designers and contractors who want to assure that the final structure meets the Contract Documents. Including reinforcement support specifications in Construction Documents assures that the contractor has received notice to use the correct bar supports. During bidding, contractors can then include appropriate compensation for the purchase and installation of these supports. During construction, because supports have been specified, it is less likely that they will be omitted because of an oversight.
ABOUT THE AUTHOR:
John B. Turner is a Professional Engineer with experience acquired from years as a structural design engineer with nearly twenty years of experience in mishap investigation, failure analysis, education, industrial operations, and construction safety. As a designer, he served on design teams for schools, hospitals, warehouses, office buildings, and government facilities. Mr. Turner recently worked with the steel reinforcement industry as it pursued code changes for the use of high-strength steel reinforcement and other new technologies. He has a Masters of Science in Civil Engineering from Texas Tech University and a BS in Safety Engineering from Texas A&M University. Mr. Turner’s professional affiliations include The American Concrete Institute, ASTM International, Structural Engineers Association of Texas – board member and past chapter president, and former Greater Southwest Regional Manager of the Concrete Reinforcing Steel Institute. He has served on several Technical Committees including ACI 301 – Specifications for Structural Concrete, ACI 117 – Tolerances, ASTM A1.05 – Steel Reinforcement, SEI – Disproportionate Collapse Mitigation of Building Structures Standards and Texas A&M University Commerce – Construction Engineering Industry Advisory Board.
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