This specification covers the materials, mix design, formwork, placement, consolidation, curing, and testing of cast-in-place structural concrete for buildings and structures. All concrete work shall conform to the requirements of ACI 301-20, Specifications for Concrete Construction, and where structural performance is specified by design, to ACI CODE-318-25, Building Code for Structural Concrete. The requirements of this standard govern construction-level execution; structural design requirements shown on the contract drawings and in the structural specifications take precedence over default values where a more stringent requirement is expressed.
Cast-in-place concrete is not a single uniform material but a family of performance-engineered mixtures, each tailored to its structural function, exposure conditions, and service environment. The consequences of defective concrete — hidden within formwork until stripping, and often structurally irreplaceable without demolition — are severe. Requirements for submittals, mix qualification, placement sequencing, consolidation, curing, and field testing are not administrative formalities; they are the controls that prevent defective work from being permanently incorporated in the structure.
Coordinate reinforcing steel procurement, fabrication, and placement with Concrete Reinforcement. Coordinate below-grade moisture management with Below Grade Waterproofing. Coordinate thermal performance of concrete wall and roof assemblies with Building Thermal Insulation.
This standard does not govern precast or prestressed concrete elements fabricated off-site, shotcrete, or concrete masonry.
All materials, production, placement, testing, and quality control shall comply with the latest adopted edition of the following standards. Where the contract documents, building code, or a referenced standard conflict, the more stringent requirement governs unless the Engineer of Record directs otherwise in writing.
| Standard | Title |
|---|---|
| ACI 301-20 | Specifications for Concrete Construction |
| ACI CODE-318-25 | Building Code for Structural Concrete — Code Requirements and Commentary |
| ACI 117-10 (Reapproved 2015) | Specification for Tolerances for Concrete Construction and Materials |
| ACI 305R-20 | Guide to Hot Weather Concreting |
| ACI 306R-16 | Guide to Cold Weather Concreting |
| ACI 308R-16 | Guide to External Curing of Concrete |
| ACI 347R-14 | Guide to Formwork for Concrete |
| ASTM C33/C33M | Standard Specification for Concrete Aggregates |
| ASTM C39/C39M | Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens |
| ASTM C94/C94M | Standard Specification for Ready-Mixed Concrete |
| ASTM C138/C138M | Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete |
| ASTM C143/C143M | Standard Test Method for Slump of Hydraulic-Cement Concrete |
| ASTM C150/C150M | Standard Specification for Portland Cement |
| ASTM C172/C172M | Standard Practice for Sampling Freshly Mixed Concrete |
| ASTM C173/C173M | Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method |
| ASTM C231/C231M | Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method |
| ASTM C260/C260M | Standard Specification for Air-Entraining Admixtures for Concrete |
| ASTM C494/C494M | Standard Specification for Chemical Admixtures for Concrete |
| ASTM C618 | Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete |
| ASTM C989/C989M | Standard Specification for Slag Cement for Use in Concrete and Mortars |
| ASTM C1064/C1064M | Standard Test Method for Temperature of Freshly Mixed Hydraulic-Cement Concrete |
| ASTM C1077 | Standard Practice for Agencies Testing Concrete and Concrete Aggregates for Use in Construction |
| ASTM C1157/C1157M | Standard Performance Specification for Hydraulic Cement |
| ASTM C1231/C1231M | Standard Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Cylindrical Concrete Specimens |
| ASTM C1240 | Standard Specification for Silica Fume Used in Cementitious Mixtures |
Contractor shall submit the following for the Engineer's review prior to procurement and placement. No concrete placement shall proceed on any element until the corresponding submittals have been reviewed and accepted. Submittals shall be complete at the time of submittal — piecemeal submissions shall be rejected.
Prior to placement, Contractor shall submit:
Contractor shall provide the following at substantial completion before concrete work is accepted:
All concrete shall be furnished by a ready-mix producer qualified in accordance with ASTM C94/C94M. The producer shall operate a batch plant that has been inspected and meets the requirements of a recognized concrete plant quality program such as those administered by the National Ready Mixed Concrete Association (NRMCA) or an equivalent state or regional program, or shall demonstrate equivalent quality controls. Contractor shall furnish the Engineer with evidence of producer qualification before the first delivery.
The Owner's independent testing agency shall meet the requirements of ASTM C1077 and shall be under the technical direction of a registered professional engineer. Field sampling and testing technicians shall hold current certification in accordance with ASTM C1077, specifically covering ASTM C172, C31, C39, C138, C143, C173 or C231, and C1064. Testing agency reports shall be submitted to the Engineer within 24 hours of each test.
The testing agency is engaged by and reports to the Owner (or Owner's representative), not the Contractor. The Contractor shall provide the testing agency with advance notice of each pour, safe access to the work, and cooperation during testing. Presence of the testing agency does not relieve the Contractor of responsibility for meeting specification requirements; all acceptance decisions remain the Engineer's.
A pre-concrete conference shall be held before the first structural concrete placement, attended by the Contractor's superintendent, the concrete producer's representative, the testing agency's project manager, and the Engineer. The agenda shall cover: mix designs and approval status, placement sequence and joint locations, hot or cold weather contingency plans, consolidation equipment and methods, curing materials and duration, and the testing plan.
No concrete mixture shall be used in the work until approved by the Engineer. Mix design approval does not transfer liability for structural performance from the Contractor to the Engineer; the Contractor remains responsible for proportioning, batching, delivery, and placement in conformance with the approved design.
Field test records are preferred because they reflect actual production variability; trial batches represent only laboratory conditions and typically overstate the quality of production concrete. Where field test records are used, they shall include at least 30 test results from the proposed mix within the past 12 months, or the Engineer shall require an increase in f'cr per ACI 301-20 Table 4.2.1.
The durability of concrete is primarily determined by the relationship between its mix design and the exposures it will encounter in service. ACI CODE-318-25 Chapter 19 establishes exposure categories that govern minimum w/cm and minimum specified compressive strength. Specifying strength alone without addressing exposure categories is the most common durability specification error and frequently results in concrete that meets structural strength requirements but degrades prematurely.
The required maximum w/cm and minimum specified compressive strength shall be determined from ACI CODE-318-25 Tables 19.3.2 and 19.3.3 based on the above categories. Where a structure contains elements with different exposures, each concrete class shall be selected to satisfy the most severe exposure applicable to that element.
Portland cement shall conform to ASTM C150/C150M. Type I/II cement, which meets both Type I and Type II requirements, is the default and is appropriate for the majority of structural applications. Type III cement (high early strength) shall be used only where accelerated strength gain is specifically required and approved; its use in flatwork shall be approached cautiously because Type III increases the risk of thermal cracking and rapid setting that complicates finishing.
Type V cement is required for concrete in contact with soil or water containing severe or very severe sulfate concentrations (S2 or S3 exposure categories). Type V shall not be substituted with ordinary portland cement in these conditions, even with slag or fly ash supplementation, unless the combination has been demonstrated equivalent by testing per ACI 201.2R.
Fly ash, ground-granulated blast-furnace (GGBF) slag cement, and silica fume may be used as partial replacements for portland cement where the mix design demonstrates compliance with all strength, durability, and workability requirements. Fly ash shall conform to ASTM C618, Class C or Class F. Slag cement shall conform to ASTM C989/C989M. Silica fume shall conform to ASTM C1240.
SCMs generally reduce permeability and improve long-term durability, particularly for sulfate and chloride resistance. Fly ash and slag slow early strength gain, which must be accounted for when establishing stripping and loading schedules. Silica fume substantially reduces permeability and is particularly effective in C2 exposure (chloride environments) but requires careful water control and immediate curing due to its high surface area.
Coarse aggregate shall conform to ASTM C33/C33M. The nominal maximum aggregate size shall comply with ACI CODE-318-25 Section 26.4.2.1, which requires that aggregate size shall not exceed three-quarters of the minimum clear spacing between reinforcing bars, one-third the depth of slabs, or one-fifth the narrowest dimension of a member — the least of these governs.
The 3/4 in. maximum size is appropriate for the majority of structural concrete placements in buildings. Larger sizes reduce water demand and can improve economy in mass elements, but shall only be used where member geometry and reinforcing congestion permit. Smaller sizes may be required in thin sections or elements with closely-spaced reinforcement.
Fine aggregate shall conform to ASTM C33/C33M. The fineness modulus shall be between 2.3 and 3.1. Fine aggregate with organic impurities, deleterious materials, or clay content in excess of ASTM C33 limits shall be rejected.
Where aggregate sources are suspected of alkali-silica reactivity (ASR) or alkali-carbonate reactivity (ACR), aggregates shall be evaluated in accordance with ASTM C1260 or ASTM C1293 before use. Aggregate that tests expansive in accordance with these procedures shall not be used unless the mix design includes measures demonstrated to mitigate expansion, such as adequate SCM content or use of low-alkali cement.
Mixing water shall be potable water or water conforming to ASTM C1602/C1602M. Non-potable water may be used only where tested and approved in accordance with ASTM C1602. Seawater shall not be used as mixing water in any concrete containing reinforcing steel or other embedded metals.
Chemical admixtures shall conform to ASTM C494/C494M. Only admixtures that are tested and shown to be compatible with the cementitious materials, other admixtures, and aggregate shall be used. Admixture dosages shall be within the range tested in the mix design; field adjustments beyond that range shall require re-evaluation by the Engineer.
Water-reducing admixtures (Type A or F) are strongly encouraged in all structural concrete because they allow the target w/cm and strength to be achieved at reduced water content, which improves long-term durability and reduces shrinkage. Adding water to increase workability in the field is the single most destructive quality compromise in concrete construction; a superplasticizer dosed at the plant is the correct response to workability needs, not water addition at the job site.
Retarding admixtures (Type B or D) shall be required for placements where concrete temperature is anticipated to exceed 80°F at time of delivery or where extended haul distances risk initial set before placement is complete. Accelerating admixtures (Type C or E) shall be evaluated for cold weather use when ambient temperatures are below 50°F, in coordination with the cold weather protection plan.
Calcium chloride shall not be used as an accelerating admixture in concrete containing reinforcing steel, prestressed elements, aluminum embeds, or galvanized steel, because chloride ions initiate corrosion of embedded metals. Non-chloride accelerators conforming to ASTM C494 Type C shall be used instead.
Air-entraining admixtures shall conform to ASTM C260/C260M. Air entrainment is required for all concrete exposed to freezing and thawing (F1 or F2 exposure) and for all concrete exposed to deicing chemicals. Air entrainment improves durability by creating a system of closely spaced microscopic air voids that relieve the pressure generated when pore water freezes and expands; without this relief, freeze-thaw cycling progressively destroys the concrete surface and ultimately the structural section.
Air content has a direct inverse relationship with compressive strength — each percentage point of air above baseline reduces 28-day strength by approximately 3 to 5 percent. The mix design shall be established to achieve the required f'c at the specified air content; the strength reduction caused by entrained air shall be accounted for in the mix design, not compensated for by adding water.
The specified compressive strength f'c is a structural design value that represents the minimum acceptable strength of the hardened concrete. Because concrete production has inherent statistical variability, the mix must be proportioned to achieve a required average strength f'cr that is higher than f'c, providing a safety margin against low individual test results. The required average strength shall be determined in accordance with ACI 301-20 Section 4.2. The Contractor shall not proportion concrete to f'c — proportioning to f'c rather than f'cr is a frequent source of failing tests.
The maximum w/cm governs concrete durability. The Engineer of Record shall determine the required maximum w/cm for each exposure class based on ACI CODE-318-25 Table 19.3.3. The Contractor shall not exceed the specified maximum w/cm under any circumstances; adding water at the job site to increase slump increases the w/cm and is grounds for rejection of the batch.
Slump is the primary field measure of concrete workability. Slump shall be selected to allow proper placement and consolidation without segregation or bleeding. Higher slump should be achieved by adjusting admixture dosage, not by increasing water content.
Maximum slump at point of discharge shall be as specified above. Concrete delivered with slump exceeding the maximum shall be rejected; the Contractor shall not add water to reduce slump. Where pumpability is required, the concrete producer shall proportion the mix for pumpability within the specified slump range; the Contractor shall not request water additions to improve pump performance.
Concrete temperature at point of delivery shall be measured in accordance with ASTM C1064/C1064M.
Where ambient temperature at time of placement is anticipated to exceed 90°F, or where concrete temperature is at risk of exceeding the maximum, the Contractor shall implement hot weather concreting procedures per ACI 305R-20 and shall notify the Engineer at least 48 hours before such placements.
Where lightweight structural concrete is required for reduced dead load on elevated slabs or other elements, the mix shall be designed using lightweight aggregates conforming to ASTM C330/C330M. The equilibrium density and specified compressive strength shall be as shown on the structural drawings. Lightweight aggregate shall be pre-wetted in accordance with the aggregate supplier's recommendations to prevent absorption of mixing water after batching, which would otherwise cause a rapid slump loss and apparent workability reduction that leads to field water additions.
Formwork shall be designed, engineered, and constructed to support all dead and live loads incident to the work, including the full weight of fresh concrete, construction loads, and any lateral loads from concrete pressure and wind, without excessive deflection or failure. Formwork design shall be performed by a registered professional engineer licensed in the jurisdiction of the project, in accordance with ACI 347R-14 and applicable building codes. The Contractor bears full responsibility for formwork safety and performance; the Engineer of Record's review of formwork drawings is limited to confirming that loads imposed on the structure are as assumed in the structural design and does not extend to approval of the formwork system design.
All form surfaces that will be in contact with concrete shall be clean and free of dirt, sawdust, mortar droppings, ice, snow, and standing water at the time of concrete placement. Form release agent (form oil) shall be applied to all form surfaces before placing reinforcement to prevent adhesion of concrete. Form release agents shall be compatible with any specified surface treatment, coating, or adhesive that will be applied to the finished surface; silicone-based or reactive release agents shall not be used where paint, epoxy, or adhesive bonding is required.
Form ties shall be of sufficient strength to resist the hydrostatic pressure of fresh concrete at the specified placement rate. Tie spacing and size shall be determined by the formwork designer. Snap ties, she-bolts, and coil-rod tie systems are all acceptable. After form removal, tie break-backs shall be recessed at least 3/4 in. from the concrete surface and the recess shall be patched as specified under Repair of Defects.
Soffit forms for elevated slabs and beams shall be cambered to offset the anticipated deflection under the weight of fresh concrete, unless the structural design has accounted for formwork deflection and the resulting slab thickness variation. Uncompensated formwork deflection is a common cause of slab thickness being less than specified and of visual waviness in finished slab soffits. The Contractor shall verify that camber and shoring stiffness provide a final slab thickness that meets the tolerances of ACI 117-10.
Multi-story shoring and reshoring shall be designed by the formwork engineer to account for accumulated construction loads in accordance with ACI 347R-14. Shoring shall not be removed until concrete has achieved sufficient strength to carry the imposed loads without distress. Where the Contractor proposes to remove shoring at ages earlier than the minimums of ACI 347R-14, field-cured cylinder break results shall demonstrate that the concrete has reached the design service strength under the actual temperature history of the pour.
Vertical formwork (walls, columns, and beam sides) may be removed earlier than soffit forms because the concrete self-supports upon removal. The minimum age for vertical form removal shall be determined by the ambient temperature and the concrete's strength development. Column and wall forms shall not be removed until the concrete can support its own weight and surface temperatures will not cause thermal shock.
Reinforcing steel procurement, detailing, fabrication, and placement, including bar sizes, grades, cover requirements, and splicing, are governed by Concrete Reinforcement. The Contractor shall verify, before any concrete placement proceeds, that reinforcing steel is in place in accordance with approved shop drawings, that cover is correctly maintained by listed plastic chairs, continuous bar supports, or other approved supports, and that reinforcement, embeds, and conduits are not displaced during placement and consolidation.
Minimum concrete cover for reinforcing steel shall be as shown on the structural drawings and shall conform to ACI CODE-318-25 Table 20.6.1.3. The minimum cover for cast-in-place concrete shall be:
The Contractor shall not accept field substitutions or modifications to reinforcement without written direction from the Engineer of Record. Tradesmen shall not use reinforcing bars as stepping or pulling points in a manner that displaces the bar from its designed position. Before placing concrete in any element, the Contractor's superintendent and the testing agency technician shall jointly verify that reinforcement type, size, spacing, and cover meet the approved shop drawings.
Immediately before placing concrete, the Contractor shall inspect and verify: formwork is tightly constructed and braced, all loose material is removed, reinforcing steel and embeds are positioned per approved drawings, anchor bolts and sleeves are templated and secured, the subgrade is damp (not muddy) for slabs on grade, formed surfaces are coated with release agent, and openings for cleanouts are closed. In cold weather, ice and snow shall be absent from all surfaces. Concrete shall not be placed until the above conditions are satisfactory and the testing agency technician is on-site.
Concrete may be placed by direct discharge from truck chutes, by buggy, by concrete pump, by crane and bucket, or by conveyor. The placement method shall not cause segregation, excessive velocity, or loss of entrained air that compromises the mix properties at the point of placement. Pump lines, chutes, and elephant trunks shall direct concrete to its final location without permitting free fall that exceeds 4 feet for reinforced concrete or 5 feet for mass placements, unless a segregation-prevention device is used.
Pump lines shall be primed with a mortar slurry of the same cement content as the concrete; the priming slurry shall be discarded and not incorporated into structural elements. Pump lines shall not be cleaned by injecting water back through the line into a placed element.
Concrete in walls shall be placed in lifts not exceeding 18 inches to prevent lateral pressure exceeding the formwork design load and to allow effective vibration consolidation. The vibrator shall be able to penetrate the full depth of each lift and into the preceding lift by approximately 6 inches to knit the layers. Slabs and footings may be placed in a single lift unless the Engineer directs otherwise.
All concrete shall be consolidated immediately after placement by internal mechanical vibration, except where the Engineer specifically approves self-consolidating concrete (SCC) that is designed to flow without vibration. Vibration is the single most important factor in achieving dense, void-free concrete; unconsolidated concrete characteristically shows honeycombing, cold joints, and cold layers that are among the most difficult structural defects to repair.
Internal vibrators shall be inserted vertically at regular intervals not exceeding 1.5 times the vibrator's radius of action (typically 18 to 24 inches for a 2-inch head). The vibrator shall be inserted quickly, held steady for 5 to 15 seconds until air bubbles cease rising, then slowly withdrawn at a rate that does not leave a hole. The vibrator shall not be dragged horizontally through the concrete, used to move concrete laterally, or allowed to contact reinforcing steel for extended periods (brief incidental contact is unavoidable but re-vibrating the same location is harmless; using the vibrator as a rodding tool is not). Vibrator insertion points shall overlap to eliminate unvibrated zones.
Re-vibration of concrete in walls is permitted and often beneficial up to 45 to 60 minutes after placement, before initial set, provided the vibrator sinks freely into the concrete under its own weight. Re-vibration can close settlement cracks and voids that form as concrete bleeds.
No water shall be added to concrete at any time after the initial water was charged at the plant. Truck operators shall not add water without the Contractor superintendent's written authorization, and the Contractor superintendent shall not authorize water addition. Water addition voids the approved mix design, increases the w/cm, reduces strength, and increases shrinkage. Loads to which water was added shall be rejected. If additional workability is needed, the Contractor shall contact the concrete producer to adjust the admixture dosage and deliver a fresh load.
A cold joint forms when a layer of concrete has advanced beyond initial set before the next layer is placed against it. Cold joints create planes of weakness and potential leakage paths. Concrete shall be placed continuously within each element or lift, and the Contractor shall plan the placement rate to ensure that each layer is covered before initial set occurs, accounting for delivery time, transport distance, temperature, and admixture dosage. If placement is interrupted for any reason that risks a cold joint, the Contractor shall immediately notify the Engineer; the Engineer shall determine whether to proceed or to treat the interruption as a construction joint.
Construction joints occur where concrete placement is planned to stop and resume later. They are structural features that shall be located as shown on the drawings or in locations approved by the Engineer. Unauthorized construction joints shall not be created without prior written approval.
Before placing concrete against a hardened construction joint, the surface shall be cleaned of all laitance, loose aggregate, and debris, thoroughly wetted, and kept damp but free of standing water. Where a bonding agent is used, it shall be applied and the concrete placed within the manufacturer's open time. A key or roughened surface is the standard for monolithic structural joints; reliance on bonding agents alone is not recommended for shear-critical joints.
Isolation joints separate the slab from fixed elements — columns, walls, and equipment pads — to allow differential movement without restraint cracking. Isolation joints shall be full-depth pre-molded joint filler as shown on the drawings.
Contraction joints in slabs on grade provide stress-relief planes to control the location of shrinkage cracks. They shall be formed by saw cutting or by installed plastic or hardboard inserts as shown on the contract drawings. Saw-cut joints shall be cut within 4 to 12 hours after placement — the precise timing shall be established by the Contractor based on the concrete mix, ambient conditions, and concrete temperature to ensure the cut is made before uncontrolled cracking initiates but after the aggregate is no longer raveled by the saw. Early-entry saws allow earlier cutting when conventional wet saws would ravel.
Joint spacing for slabs on grade shall be as shown on the contract drawings. A common guideline is joint spacing (in feet) not exceeding 2 to 3 times the slab thickness (in inches). Oversized bays, high-shrinkage mixes, or rapid drying conditions require closer spacing.
Expansion joints accommodate thermal movement between adjacent structural bays or between the structure and surrounding paving. The joint width, filler type, and sealant shall be as shown on the contract drawings. Expansion joints shall be continuous through the full depth and thickness of the element and shall not be bridged by reinforcing bars or embeds unless specifically detailed as a structural sliding connection.
Rough form finish is the default for unexposed formed surfaces that will be concealed by backfill, other materials, or finishes. No work beyond form removal is required; tie holes shall be patched per Repair of Defects.
Smooth form finish is the standard for exposed formed surfaces visible in the completed building but not designated architectural concrete. Smooth form finish shall be achieved with plywood or steel forms that produce a uniform surface. Minor fins, seams, and form marks shall be ground flush. Tie holes shall be patched. Honeycombing, cold joints, or discoloration of more than minor extent shall be subject to rejection or repair as determined by the Engineer.
A screeded finish is used where the surface will receive topping, fill, or insulation and appearance is not important. The surface shall be struck off level with the screed, tamped to bring aggregate below the surface, and left without further work.
A floated finish is the standard for slabs that will receive floor coverings, coatings, or toppings. The surface shall be screeded, then floated with a bull float or darby to embed aggregate, remove ridges, and fill low spots. Power floating shall follow hand floating when the concrete has hardened sufficiently that the machine does not sink more than 1/8 inch under the weight of the operator.
A troweled finish is required for concrete floor slabs that will be exposed to foot traffic, vehicle traffic, or that will serve as a finish floor. After power floating, the surface shall be power troweled with progressively reduced blade angle until the surface is dense, smooth, and free of trowel marks. Troweling shall not be started until the surface has lost its sheen, and water or cement shall not be applied to the surface to aid troweling. Hard troweling while bleed water is still rising to the surface traps the water below a dense skin, creating dusting and delamination — the most common defect in interior concrete floor slabs.
Floor flatness (FF) and floor levelness (FL) shall be measured per ASTM E1155. F-numbers are a statistical measure and are not the same as traditional gap-under-straightedge tolerances. Where F-number requirements are specified, the measurement protocol (random traffic floor vs. defined traffic floor), the number of measurements, and the acceptance criteria shall be established before placement.
Exterior slabs, sidewalks, loading docks, ramps, and any slab subject to precipitation shall receive a light broom finish drawn perpendicular to the primary direction of foot or vehicle traffic. Broom texture shall be applied after floating, when the concrete has stiffened sufficiently to hold the texture without tearing. Heavy broom finish is appropriate for ramps and where additional slip resistance is required.
Curing is the maintenance of adequate moisture and temperature in freshly placed concrete to allow cement hydration to continue. Curing is not optional: properly cured concrete develops its design strength and durability; poorly cured concrete achieves only 50 to 70 percent of the strength of well-cured concrete and develops a highly permeable surface zone that is vulnerable to freeze-thaw damage, chemical attack, and abrasion. Curing shall begin immediately after finishing operations are complete and shall continue uninterrupted for the full required curing period. ACI 308R-16 is the governing reference for curing methods and practices.
Curing compound shall conform to ASTM C309, Type 1-D (dissipating) or Type 2, and shall be applied in two passes at 90 degrees to each other at the rate recommended by the manufacturer to provide a continuous, unbroken membrane. Curing compound shall not be applied to surfaces where adhesive bonding, paint, epoxy coatings, or additional concrete topping will be applied unless the compound is demonstrated to be compatible with or dissipates before the subsequent application. Where compatibility cannot be confirmed, wet curing methods shall be used.
Where wet curing is used, burlap shall be pre-soaked before application and shall be kept continuously wet during the curing period. The burlap shall be in direct contact with the concrete surface, not bridging across it; plastic sheeting placed over burlap retains moisture but does not substitute for the burlap. Burlap curing in hot and dry weather requires dedicated watering; drying of the burlap — even for a few hours — substantially diminishes curing effectiveness.
Mixes containing significant proportions of fly ash or slag cement develop strength more slowly than plain portland cement mixes and require longer curing. Reducing the curing period for SCM mixes because they appear strong enough is a common field error; the long-term durability benefits of SCMs depend on extended curing to develop the pozzolanic reaction.
When the ambient temperature is at or above 90°F, or when evaporation rate exceeds 0.20 lb/ft²/hr (calculate per ACI 305R-20 Figure 2.1.5 from temperature, relative humidity, and wind speed), the Contractor shall implement hot weather concreting procedures. These procedures shall include one or more of the following: cooling of mix water with ice, chilling of aggregates with water, reducing haul time and truck capacity to limit time from batching to placement, using a retarding admixture, placing in the cooler parts of the day, providing wind breaks to reduce evaporation, and applying an evaporation retardant (fog-mist or monomolecular film) to the slab surface during the period between screeding and final finishing. ACI 305R-20 provides detailed guidance on each measure and the evaporation rate nomograph.
Plastic shrinkage cracking is the most visible manifestation of inadequate hot weather concreting; cracks appearing within the first few hours after placement, before any loading, indicate rapid drying. Plastic shrinkage cracks do not recover on their own and typically extend through a significant fraction of the slab thickness. Prevention is the only effective strategy; re-troweling can temporarily close cracks that appear during finishing but does not eliminate the underlying microcrack.
Cold weather concreting provisions apply when the air temperature has fallen below 40°F or is expected to fall below 40°F during the protection period, in accordance with ACI 306R-16. Fresh concrete must be protected from freezing until it achieves a compressive strength of at least 500 psi; concrete that freezes before reaching 500 psi may be permanently damaged regardless of subsequent warming. Minimum concrete temperature at delivery shall be increased per ACI 306R-16 Table 3.1 based on section thickness, with thinner sections requiring higher delivery temperatures because they lose heat faster.
Carbon monoxide from combustion heaters is a serious safety hazard in enclosed forms and enclosures. The Contractor shall provide adequate ventilation wherever combustion heaters are used and shall monitor CO levels. Forced-air propane heaters that exhaust to the outside are preferred over open-flame heaters inside the enclosure.
The rate of cooling after the protection period ends is as important as temperature during curing. Concrete shall not be cooled more than 5°F per hour in any surface layer, and the differential between the core and the surface of mass elements shall not exceed 35°F. Rapid surface cooling after removal of insulation causes thermal gradient cracking that is not related to freeze damage and can occur even after the concrete has reached design strength.
Dimensional tolerances for all cast-in-place concrete work shall comply with ACI 117-10 (Reapproved 2015). Tolerances define the acceptable deviation from specified dimension, alignment, and location; work outside tolerance shall be evaluated by the Engineer for structural acceptability and remedial action shall be taken as directed.
Embedded items, sleeves, anchor bolts, and openings shall be located within the tolerances shown on the structural drawings or, where not shown, within ±1/2 in. of the design location in plan. Anchor bolt groups for column base plates shall be set using a template secured to the formwork to maintain the pattern tolerance required by the structural steel fabricator; anchor bolt misalignment is a frequent cause of costly steel column erection delays.
Field testing shall be performed by the Owner's testing agency on concrete delivered to the project. The Contractor shall notify the testing agency of each pour date and time with a minimum 24-hour advance notice. The testing agency shall sample and test concrete at the frequency below.
Each strength test set shall consist of a minimum of four cylinders: two tested at 7 days for early strength indication and two tested at 28 days for acceptance. Additional cylinders may be made and field-cured for stripping decisions. Cylinders for acceptance testing shall be standard-cured (moist-cured at 73 ± 3°F) in accordance with ASTM C31/C31M.
At each sampling, the testing agency shall measure and record:
The strength of a concrete mix is considered satisfactory when the following two conditions are met, in accordance with ACI CODE-318-25 Section 26.12.3:
Both conditions must be satisfied simultaneously. Failure of Condition 1 alone indicates that the mix average is trending low and the Contractor shall investigate and adjust; failure of Condition 2 alone indicates an outlier or a specific batch problem. When both conditions are failed, the concrete is presumed defective and core testing per ACI 318-25 Section 26.12.4 shall be required.
A 7-day cylinder strength of approximately 65 to 75 percent of the 28-day strength is typical for Type I/II portland cement. A 7-day result significantly below this range is an early warning that the 28-day result may fall short of f'c; the Contractor shall not wait until 28-day results are received before investigating a trend that is already visible at 7 days.
Any batch of concrete in which fresh test results exceed the limits below shall be rejected and returned to the plant, and no portion of the rejected batch shall be placed in the work:
The testing agency shall immediately notify the Contractor and Engineer when a rejection criterion is triggered. The Contractor shall provide the replacement load without interrupting the pour where possible; if interruption causes a cold-joint risk, the Engineer shall determine how to proceed.
Where 28-day cylinder results are low or structural loads must be applied before 28-day results are available, in-place strength estimation may be used as a supplemental measure. Methods include rebound hammer per ASTM C805/C805M, penetration resistance per ASTM C803/C803M, or maturity method per ASTM C1074/C1074M. These methods are supplemental and correlational, not substitutes for core testing or standard cylinders for acceptance decisions. Their results shall be correlated to standard cylinder data for the specific mix.
Where compressive strength results indicate that an element may contain defective concrete, the Engineer may require extraction and testing of cores per ASTM C42/C42M. Cores shall not be extracted before 28 days unless early stripping or loading requires immediate evaluation. Concrete in an area represented by a core is considered structurally adequate if the average of three cores in that area is at least 85 percent of f'c and no single core is below 75 percent of f'c, in accordance with ACI CODE-318-25 Section 26.12.4.3. Core locations and testing procedures shall be determined by the Engineer.
Defects discovered after form removal shall be reported immediately to the Engineer. No repair shall be undertaken without the Engineer's review and written direction. Not all defects require repair, and not all defects can be structurally repaired; concealing defects behind coatings or unauthorized patching before Engineer review is grounds for rejection of the element.
Minor surface blemishes include small bug holes (air voids less than 1/4 in. diameter), small form-joint ridges, and surface discoloration from form release agent. Minor blemishes in exposed-to-view surfaces that are cosmetically objectionable may be filled with a grout mix of portland cement and fine sand at approximately 1:1.5 ratio, with water added to a stiff consistency, after thoroughly pre-wetting the surface. Dry-packing of form-oil stains or excessively smoothed (burned) areas is not effective; these areas shall be mechanically abraded to open the surface before patching.
Honeycombing is the term for areas of incomplete consolidation where coarse aggregate shows on the surface due to mortar not filling the voids between aggregates. Honeycombing ranges from minor surface bugholes to deep voids that expose or undermine reinforcing steel. The Engineer shall determine whether honeycombing is cosmetic, repairable, or structural.
All repair areas shall be saw-cut or chipped to sound concrete with square or undercutting edges; feathered edges shall not be used. The repair surface shall be cleaned of all loose material, pre-wetted, and the repair material applied in a manner that ensures intimate contact with the substrate. Repair materials shall match the concrete's color, texture, and performance properties as closely as practical. Repaired areas shall be cured not less than 7 days.
Form tie break-backs shall be cleaned of all oil, loose concrete, and debris, recessed at least 3/4 in. from the surface, and patched with dry-pack cement mortar. Dry-pack mortar shall be placed in layers not exceeding 3/8 in. and consolidated by rodding before each layer is added. The patch surface shall match the surrounding concrete texture.
Cold joints in structural elements are presumed to represent a plane of reduced shear transfer and potential leakage. The Engineer shall evaluate cold joints against the structural demand at that location. Cold joints in slabs on grade may be satisfactory for non-structural slabs; cold joints in shear walls, columns, or beams shall be treated as requiring investigation. Where cold joints occur in below-grade walls, waterproofing continuity shall be verified per Below Grade Waterproofing.
Surface cracks less than 0.010 in. (10 mil) width in non-water-retaining elements are generally not structurally significant but shall be documented. Cracks wider than 0.010 in. in structural elements, or any cracks in elements designated to retain liquid, shall be reported to the Engineer for evaluation and direction. Epoxy injection per ACI 224.1R is the standard repair for structural cracks that must be re-united to restore monolithic behavior.
Concrete surfaces shall be protected from damage by subsequent construction operations, foot traffic before adequate strength, vehicle traffic, materials storage, chemical spills, and frost. Temporary protection coverings shall be maintained until the work can sustain imposed loads without damage. Impact damage, oil or paint contamination, and rust staining from stored steel shall not be accepted as normal construction damage and shall be repaired at the Contractor's expense.
No traffic shall be permitted on concrete floor slabs until the concrete has achieved sufficient strength to support the load without cracking, spalling, or surface damage. As a minimum:
Concrete surfaces shall be protected from oil, acid, and other chemical spills until a permanent floor coating, sealer, or topping is in place. Concrete floors intended to be left bare shall receive a penetrating sealer within 30 days of completion to protect against oil staining and surface dusting.
The Contractor shall warrant concrete work against defects in materials and workmanship, including defects that manifest as premature strength failure, excessive cracking attributable to construction practice, surface scaling from freeze-thaw damage within the specified design exposure class, and delamination of troweled surfaces, for the project warranty period.
Warranty claims arising from structural loading, unanticipated chemical exposure, or conditions not described in this specification are not covered under the Contractor's warranty unless the Contractor's work failed to meet the requirements of this standard. The Engineer shall determine the cause of any defect before warranty claims are pursued.
Concrete mix designs that were approved by the Engineer and placed in accordance with this specification, but that produce 28-day strengths meeting the acceptance criteria, are not subject to warranty defect claims on the basis of strength alone. Where service conditions later produce distress not anticipated by the specified exposure category, the Engineer shall determine the cause and recommend remediation; responsibility for mix redesign in changed conditions is a design matter, not a construction defect.