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Concrete Casting News from the Hill and Griffith Company

The Influence of Form Release Agent Application to the Quality of Concrete Surfaces

Posted by Hill and Griffith Company on Sep 19, 2019 11:00:54 AM

ACI 318-19 was published in response to new engineering practices and industry changes.

Excerpt from a technical paper by A. Klovas and M. Daukšys 2013 IOP Conf. Ser.: Mater. Sci. Eng. 47 012061.

Introduction

High quality surface finishes are a feature of self-compacting concrete (SCC), but by careful attention to mix design and job site workmanship, nice surfaces with the conventional concrete mixture could be achieved. The appearance of an element mainly depends on: the type of cement and addition used; the mix composition; the quality of the concrete mold and release agent; and the placing procedure. If compared self-compacting concrete with the conventional–the color is generally more uniform. Also, it is easier to avoid defects due to leakage spots at the location of mold joints. Blowholes, honeycombing and other blemishes can be found in all types of concrete but with more fluid concrete mixture it is possible to improve the surface finish. International Council for Building Research has provided guidelines how the concrete may be defined referring the surface quality:

  • ROUGH class – no special requirements for finishing
  • ORDINARY – surface finishing has a minor factor
  • ELABORATE – definite requirements for visual appearance
  • SPECIAL – highest standards for appearance
Concrete Form Release Agents Help Form Removal
photo credit: Bob Sawyer

Formworks are also very important factors for concrete surface quality. Scientist, J. Sousa Coutinho, has researched using two different formworks: controlled permeability (CP) and five-layer wood-based formworks. The results have shown that by using CP formworks, the pore diameter (nm) of concrete surface has decreased up to 50%, porosity – up to 45%, surface hardness (MPa) increased up to 70%, and blow-hole ratio has decreased up to 90 % compared with those concrete surfaces using five-layer wood-based formworks.
 
A number of studies determine how to achieve better consolidation resulting in fewer surface blemishes [7-15]. To minimize the size and number of bug holes and all other effects, the following practices should be followed:
  • Vibration period should be of sufficient duration
  • Vibrator insertions should be properly spaced and overlapped and the vibrator removed slowly
  • Each concrete layer should be consolidated from the bottom upward
  • Vibration periods should be increased on withdrawal when using impermeable forms that permit air trapped at the form surface to escape through joints as between
  • Inward sloping forms and other complex design details should be avoided
  • Vibrator should penetrate into the previous layer;

The main outcome of this research is to evaluate the usage of different form release agent applications on the formwork. In addition, this paper presents a technique which provides:

  1. A method how to evaluate the concrete surface quality using image analysis process;
  2. An evaluation of concrete surfaces quality by the following documents: CIB report no. 24 [4] and GOST 13015.0-83.
  3. A combined method how to evaluate and divide concrete surfaces into special categories provided by CIB report no. 24 GOST 13015.0-83 and “ImageJ” in respect to the area of blemishes.

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