Commercial Foundations & Structural Concrete

Commercial foundation selection in San Antonio is driven by three variables: the building's structural load, the underlying soil conditions as documented by a geotechnical investigation, and the required design life. Selecting a foundation system without a geotechnical report is a decision made without the most important piece of information available. In Bexar County, that information almost always changes the design.

Slab-on-grade with post-tensioning is the most frequently used commercial foundation system in San Antonio for buildings under four stories and without extremely heavy point loads. Post-tensioning applies continuous compression across the slab through high-strength steel tendons, allowing the slab to resist differential soil movement as a single rigid element rather than cracking into sections. For retail strip centers, restaurants, medical offices, and light industrial buildings on Bexar County clay soils, a post-tensioned slab-on-grade is often the most cost-effective structural solution available.

Drilled pier and grade beam systems are used when surface soils cannot provide adequate bearing capacity, when the building height or structural loads exceed what a slab-on-grade can support, or when the geotechnical report identifies the need to transfer loads to deeper, more stable soil layers. Drilled piers in San Antonio are typically bored to depths of 15 to 35 feet, depending on where the stable caliche or rock layer is encountered. Grade beams connect the pier tops and support the structural frame above.

Structural concrete is not a single product. It is an engineered mix design where compressive strength, water-cement ratio, aggregate size, admixtures, and curing method are all specified to match the application and environmental conditions. In San Antonio's summer heat, where ambient temperatures routinely exceed 100 degrees Fahrenheit during concrete pours, mix design and curing management are not secondary details. They are the difference between a slab that achieves its design strength and one that loses 20 to 30 percent of potential strength through premature moisture loss.

Water-cement ratio is the single most important variable in structural concrete quality. Lower ratios produce denser, stronger concrete with lower permeability directly reducing the risk of sulfate attack from Bexar County's sulfate-bearing soils, which are common in the region. Every additional gallon of water added to a commercial mix at the job site to improve workability reduces the final compressive strength of that pour. A reputable structural concrete contractor will not add water to the truck on site without retesting the mix.

Concrete is strong in compression and weak in tension. Reinforcement exists to handle the tensile forces that concrete alone cannot resist. In commercial structural applications, the reinforcement design rebar size, spacing, coverage depth, lap splice length, and whether post-tensioning is included is determined by the structural engineer of record and documented in the construction drawings. No substitutions or field changes to reinforcement should be made without engineering review and a written revision to the structural documents.

Conventional rebar reinforcement for commercial slabs in San Antonio typically uses Grade 60 deformed bars at #4 to #6 size, depending on slab thickness and design loads. Rebar spacing for two-way reinforced commercial slabs ranges from 12 to 18 inches on center in each direction. Concrete coverage over reinforcement the distance from the bar to the outer face of the slab must meet ACI 318 minimums and is typically 3 inches for slabs in contact with soil and 1.5 inches for interior slabs not exposed to weather.

Post-tensioning in commercial slab-on-grade construction uses 0.5-inch or 0.6-inch diameter seven-wire strand tendons stressed to approximately 33,000 pounds per tendon after the concrete reaches sufficient strength, typically 2,500 psi. The resulting compression counteracts the tensile forces from differential soil movement, live loads, and thermal expansion, allowing thinner slabs to perform under conditions that would crack a conventionally reinforced design. Post-tensioned slabs in San Antonio commercial construction are typically 5 to 7 inches thick, compared to 7 to 9 inches for an equivalent conventionally reinforced design.

Structural concrete placed on inadequately prepared subgrade fails regardless of how well the concrete itself is specified. In San Antonio's expansive clay environment, subgrade preparation for commercial construction involves multiple steps that are often underestimated by developers focused on above-grade construction costs. The subgrade and base course are the foundation beneath the foundation, and they are buried and inaccessible once the pour is complete.

Subgrade moisture conditioning is the first and often most overlooked step. Expansive clay subgrade must be pre-wetted to a depth of 18 to 24 inches and brought to a uniform moisture content before base material is placed. Placing a commercial slab on dry clay introduces the risk of post-construction swell as the clay wets up through seasonal rainfall, lifting sections of the slab unevenly. This is the primary mechanism of commercial foundation distress in Bexar County and it is almost entirely preventable with proper pre-wetting and moisture testing before base placement begins.

Base course for commercial structural slabs in San Antonio is typically 6 to 8 inches of compacted crushed limestone (caliche) meeting TxDOT Type A or B gradation requirements, compacted to 95 percent of maximum dry density per ASTM D698. The base distributes load from the slab to the subgrade, provides a stable working surface for formwork and reinforcement placement, and assists drainage under the slab. For warehouse and heavy industrial applications with fork truck traffic and racking loads, base thicknesses of 10 to 12 inches are common. A vapor retarder minimum 10-mil polyethylene is placed over the compacted base before reinforcement is set.

All commercial structural concrete in San Antonio requires a building permit through the City of San Antonio Development Services Department. The permit application for structural concrete work must include sealed structural drawings from a licensed Texas Structural Engineer (SE), a geotechnical report, a concrete mix design submitted by a qualified testing laboratory, and a Special Inspections Program (SIP) as required by the International Building Code (IBC) as adopted by the City of San Antonio.

The Special Inspections Program is the quality assurance mechanism the code uses to verify that structural concrete is placed in accordance with the approved drawings. For commercial structural concrete, IBC Chapter 17 requires continuous special inspection during reinforcing steel placement, concrete placement and consolidation, post-tensioning operations, and any high-strength concrete applications. A special inspector employed by an approved testing agency not the contractor's own quality control personnel must be present during these activities and must document their observations in daily inspection reports.

Cylinder break testing is a mandatory element of the Special Inspections Program. For commercial structural concrete, the standard protocol is a minimum of one test set (four cylinders) per 50 cubic yards placed or per each day's pour, whichever produces more sets. One cylinder is tested at 7 days to confirm early strength gain, two are tested at 28 days for design strength verification, and one is held as a reserve. If 28-day breaks fall below the specified design strength, the engineer of record determines the next steps which may include additional coring, load testing, or structural evaluation of the affected element.