Handling ready-mixed concrete on site

In beams and columns with heavy reinforcement, most concrete defects appear when the mix has to pass through the cage: the aggregate locks between bars, the vibrator head cannot reach to consolidate, or the formwork gives way under a mix more fluid than the one assumed in its design. The manual sets out how ready-mixed concrete is handled on site to control those situations, from defining the order to stripping the formwork, and how the technical supervisor checks it. It is written for the site engineer, the pouring foreman, the lab technician and the supplier's representative.

The order is defined from the clear spacing measured in the assembled cage. NSR-10 C.3.3.2 limits the nominal maximum aggregate size to 3/4 of the minimum clear spacing between bars and to 1/5 of the narrowest form dimension. With No. 8 bars in beams the design clear spacing is 25.4 mm (C.7.6.1) and the allowable size is 0.75 × 25.4 = 19.05 mm, so 3/4" aggregate passes with only 0.05 mm to spare. The actual spacing is computed inside the stirrups, s = [b − 2(r + dₑ) − n·d_b]/(n − 1), and gives the vibrator head that fits: in a 300 mm beam with four No. 8 bars, 40 mm cover and a 9.5 mm stirrup there are 33.1 mm of clear space, enough for heads up to 23 mm.

The manual meets the formwork calculator at the concrete pressure. The ACI 347 equations assume slumps up to 175 mm and internal vibration down to 1.2 m; the high-workability mixes that congestion calls for reach 200 mm, with an upper end of 200 to 225 mm, and fall outside that assumption. For them the manual proposes checking columns with full hydrostatic pressure, p = γ·h with γ = 24 kN/m³, which gives 72 kPa for a 3.0 m pour height and 96 kPa for 4.0 m. The calculator makes that switch on its own when the slump exceeds 175 mm or when there is self-consolidating concrete, external vibration or pumping from the base.

For stripping, the commentary to NSR-10 C.6.2 allows side forms to be removed after 12 hours of accumulated curing with normal cements. With mixes containing superplasticizer and retarder, whose data sheet states a final set of 9 ± 1 h, the manual sets 24 hours as a minimum plus confirmation with field-cured cylinders. Beam soffits and shores have no fixed number of days: they depend on the shoring and reshoring procedure that C.6.2.2.1 requires from the contractor and on the strength shown by analysis and testing.

Each requirement carries a source letter: N for NSR-10 requirements, T for referenced technical standards (NTC, ASTM, ACI), F for supplier and guide recommendations, and D for criteria proposed by DIADICA. The latter are gathered in chapter 14 so that the engineer in charge of each project can confirm or change them before the first pour.