EvapMargin

The ACI 305 nomograph

The ACI 305 nomograph reads how hard the air pulls water from a fresh concrete surface. It gives that as an evaporation rate, in kilograms per square metre per hour. This guide says what the rate means for a print, how to read the figure, and how far to trust the number.

Background

A printed wall has no formwork. Every fresh layer stands open on its top and both flanks, and the mortar does not bleed. So the air takes water from the surface faster than it does from a cast section. The layer below holds the drying layer while it shrinks, and that is the condition in which a crack opens.

Figure in preparation

Figure 01.1Where the water leaves a printed wall. The wall in section, with the open face, the water that leaves it, and the layer below that holds the drying layer.

How to read it

The nomograph is four panels, read clockwise from the top left. One construction line threads through all four, from the ambient temperature to the evaporation rate. Two carriers join the panels: the air moisture level and the moisture deficit. Each panel also draws both thresholds in its own input, so it says how far that input can move.

Figure in preparation

Figure 02.1The four panels and the two carriers. The panels as boxes and the carriers as arrows, in the order the construction line meets them, so the loop closes on the evaporation axis.

What to measure

The tool reads four inputs. Each is a value measured at the print, not a value assumed for the hall. A hall can carry 3 to 5 °C between floor and ceiling. So a sensor on the wall reads a different air from the fresh layer. The surface temperature is the input the equation is most sensitive to.

Table in preparation

Table 03.1The four inputs. Each input with its unit, its range, where to take the reading, and the proxy the tool accepts.

The model

The evaporation rate is the ACI 305 value, computed exactly. No factor is added for printing, so the chart stays the ACI 305 nomograph. Menzel (1954) measured the relationship at an open water surface, and Uno (1998) fitted the closed form this tool uses. Three assumptions travel with it, and each one fails at a printed face.

Figure in preparation

Figure 04.1From four inputs to the evaporation rate. Each step of the equation as one box with its formula, from the two temperatures and the humidity to the rate.

Accuracy and limits

The number is honest about the air. It is not a measured rate for a printed face, because the equation assumes a film of free water on a flat slab. Read it as how hard the air pulls, and compare prints by it. What would settle the printed case is calibration data from vertical printed faces, and nobody holds that data, Vertico included.

Figure in preparation

Figure 05.1Where the method is reliable. The evaporation axis with both thresholds, and the range over which the published accuracy holds.

Sources

Every figure on this page comes from one of these sources. Several are quoted as the named papers report them. The originals are named here so that a reader can check each one.

  • ACI 305R, Guide to Hot Weather Concreting. American Concrete Institute. Source of the nomograph and the 1.0 kg/m²/h limit.
  • ACI 308R-01, Guide to Curing Concrete. American Concrete Institute. Accuracy of the nomograph: within ±25 % up to about 1 kg/m²/h.
  • Menzel, C. A. (1954). Causes and prevention of crack development in plastic concrete. Portland Cement Association. The equation behind the nomograph.
  • Uno, P. J. (1998). Plastic shrinkage cracking and evaporation formulas. ACI Materials Journal 95(4): 365–375. The closed-form equation used here, and the origin of the 1.0 limit.
  • Al-Fadhala, M. and Hover, K. C. (2001). Rapid evaporation from freshly cast concrete and the Gulf environment. Construction and Building Materials 15(1): 1–7. The nomograph is reliable up to about 0.5 kg/m²/h.
  • Moelich, G. M., Kruger, J. and Combrinck, R. (2020). Plastic shrinkage cracking in 3D printed concrete. Composites Part B 200: 108313. And (2022), the plastic-shrinkage crack-risk model, Cement and Concrete Composites.
  • Markin, S., Combrinck, R. and Mechtcherine, V. (2023). Cement and Concrete Research, and preprint SSRN 4511047. Plastic shrinkage of printed concrete with and without formwork, and the effect of filament thickness.
  • Deysel, Boshoff and Smit (2023). Implementing capillary pressure control measures to prevent plastic shrinkage cracking in concrete. Construction and Building Materials. The cracking evidence that brackets the 0.25 line.
  • Slowik, V., Schmidt, M. and Fritzsch, R. (2008). Capillary pressure in fresh cement-based materials and identification of the air entry value. Cement and Concrete Composites 30(7): 557–565.