Concrete structural systems

Reinforced concrete is the most common structural material for mid-rise and high-rise buildings in Canada and much of the United States, and for parking structures, podiums and foundations almost everywhere. It is strong in compression, can be cast into any shape, is naturally fire-resistant and is built largely on site, which gives constructors great control over its quality and great responsibility for it. This lesson covers how reinforced concrete works, the common floor systems, post-tensioning, and the factor that matters most to constructors: how concrete gains strength with time and what that means for stripping forms and loading young slabs.

CON-110 covered concrete materials, mix design, placing and curing. This lesson focuses on structural behavior and its construction consequences.

How reinforced concrete works

Concrete has high compressive strength but low tensile strength, roughly a tenth of its compressive strength. Under bending, the tension side of an unreinforced beam cracks almost immediately. Steel reinforcing bars are placed where tension develops so that, once the concrete cracks, the bars carry the tension and the concrete carries the compression. The two materials work together because concrete bonds to the deformed bars and both expand by similar amounts with temperature.

Reinforcing does three jobs: flexural (bending) reinforcement in the tension zones, shear reinforcement (stirrups or ties) near supports, and minimum or temperature and shrinkage reinforcement to control cracking. Columns have longitudinal bars and ties or spirals that confine the concrete and stop bars from buckling.

Concrete cover, the thickness of concrete between the bars and the surface, protects bars from corrosion and fire. Too little cover leads to corrosion and spalling; too much reduces the effective depth and therefore the strength. Bar supports (chairs and bolsters) hold bars at the right height, and cover is inspected before the pour.

In the United States, cover and detailing follow ACI 318 and ACI 301, with bars to ASTM A615 or A706 (typically Grade 60, sized #3, #4, #5 and so on, where the number is the diameter in eighths of an inch). Typical minimum cover is 1½ in for interior beams and columns and more for exposure to weather or soil, such as 2 in for formed surfaces exposed to weather and 3 in for concrete cast against earth.

Concrete floor systems

Several floor systems are common, each with different spans, depths, formwork and construction implications.

Flat plates dominate residential towers in Canadian cities and many US markets because they give the thinnest floor-to-floor height and the fastest, most repetitive formwork. Their weak point is punching shear: the slab can fail by a column punching through it, in a cone-shaped failure, if the slab is too thin, the concrete too weak, or holes are cut close to the column. Shear studs or stirrups around columns, drop panels or thicker slabs are used to prevent it. For constructors, the consequence is that coring and sleeves near columns, early loading and missing shear reinforcement are serious matters.

Post-tensioned concrete

Post-tensioning (PT) puts steel strands, called tendons, through the slab in ducts or sheaths, then stresses them with hydraulic jacks after the concrete has gained enough strength. The tendons are draped, high over supports and low at mid-span, so that when tensioned they push up against the gravity loads. PT allows thinner, longer-span slabs with less cracking and deflection, and is common in parking structures and high-rise residential and office buildings.

Most building PT uses unbonded single-strand tendons coated in grease and plastic sheathing, anchored at each end. Bonded PT uses multi-strand tendons in ducts that are grouted after stressing, more common in bridges and transfer structures.

PT raises special construction issues:

• Tendon profiles must be placed to the specified heights; small errors change the uplift.
• Concrete must reach the specified strength, often 20 to 25 MPa (3,000 to 3,500 psi), before stressing, confirmed by field-cured cylinders.
• Stressing must follow the specified sequence, and elongations are measured and compared with calculated values.
• Formwork and shores must not be removed until tendons are stressed, because the slab relies on PT for its strength.
• Once stressed, tendons store large forces. Coring, cutting or drilling a PT slab without scanning and engineering approval can cut a tendon, which can whip out with deadly force and also seriously weaken the slab.

How concrete gains strength

Concrete gains strength as cement hydrates, rapidly in the first few days and then more slowly. The specified strength (f'c) is usually measured on standard cylinders at 28 days, though some high-strength mixes specify 56 days. Strength at earlier ages depends on cement type, mix design, admixtures and above all temperature. Cold slows hydration dramatically; heat speeds it up.

Because strength depends so much on temperature, formwork specifications usually set stripping and loading criteria as a strength to be confirmed by testing, not just a number of days. Testing is done with field-cured cylinders kept beside the slab, or by the maturity method.

The maturity method estimates in-place strength from the temperature history of the concrete. Sensors cast into the slab record temperature, and a calibration curve for the specific mix converts the accumulated temperature-time factor into an estimated strength. It is standardized in ASTM C1074 and widely used in both countries to decide when to strip, stress or load.

Stripping and loading young concrete

Removing formwork too soon is one of the most common causes of concrete collapses during construction. When forms and shores are removed, the slab must carry its own weight and anything placed on it, often while also supporting the floors above through reshores. A slab at 50 percent of its strength has roughly half its capacity, and also deflects more because its stiffness is lower.

In the United States, OSHA 1926.703(e) requires that forms and shores not be removed until the employer determines that the concrete has gained sufficient strength to support its own weight and superimposed loads, based on plans and specifications or on tests that show the concrete has reached the required strength. ACI 347R and ACI 301 give further guidance.

Practical controls include a written stripping and reshoring procedure from the formwork engineer, strength confirmation by testing before each strip, sign-off by the engineer or designated person, and clear site rules against loading new floors until approved.

Precast and other concrete systems

Not all concrete is cast in place. Precast concrete members, such as hollow-core planks, double tees, beams, columns, stairs and wall panels, are made in a plant and erected on site. Precast saves formwork and time and gives high quality, but it shifts the critical issues to connections, bearing lengths, erection stability and grouting. Hollow-core planks need adequate bearing on their supports, typically at least 75 to 100 mm (3 to 4 in), and their cores must not be cut through without the supplier's approval. Lesson 8 returns to precast and tilt-up.

Composite construction combines steel and concrete, for example a concrete slab on steel deck connected to steel beams by shear studs so that the slab and beam act together. Lesson 6 covers it.