Subject matter

The delamination of the surface layer of concrete floors during or after the surface finishing actions (floating, troweling, polishing) is no new phenomenon. However increased numbers of cases have been observed these last years.

In currently encountered cases, two main causes for delamination have been observed: an excess of entrapped or entrained air in the concrete and late bleeding. An excess of air has been observed in concrete compositions containing certain superplasticizers. Finishing operations change the shape of the initially spherical voids into numerous elongated, discontinuous and stringy voids, the coalescence of which can ultimately lead to delamination. In case of premature finishing of the concrete surface (while the concrete is still bleeding), bleeding water continues to rise in the underlying concrete until it becomes trapped beneath the layer of dense surface paste. The trapped water or air can thus create a plane of weakness near the concrete surface. Charges induced by drying shrinkage or the usage of the floor can ultimately lead to delamination.

In concrete slabs there exists the notion of the “window of finishability”. This is the period of time during which the finishing operations should take place. Premature finishing may lead to a wavy and cracked surface or the entrapment of bleeding water with blistering and delamination as a result. When the finishing operations are carried out too late, the specified flatness and surface quality may not be achieved.

In literature, three main concrete properties are considered to have an important influence on the delamination of the surface layer, namely the air content, the bleeding and the hardening properties (∼ window of finishability). Moreover, these parameters are interconnected at some levels. Factors that can affect these three parameters are:

- The concrete composition: type and content of sand, aggregate gradation, Dmax, water reducing admixtures (type, quantity, moment of adding,...), W/C ratio, cement (type, quantity, fineness, alkalicontent,...), mineral additions,...

- The environmental conditions: wind, temperature (of the air, ground, concrete), humidity,...

- The mixing and placing of the concrete: transportation period, compaction, mixing time,...

In a concrete mix with excessive fines (cement, fine sand), the air is held within the concrete, which results in a “sticky” concrete mix. This type of mixture is placed more difficultly, is sealed more easily by the finishing operations of the fresh concrete and has a tendency to dry faster at its surface (i.e. ‘crusting’, resulting in a water bed effect). Overuse of vibration during compaction may leave the surface with excessive fines, also inviting crusting. Finishers could assume that this dry surface signals that the finishing operations may start, but when they try this it leads to a wavy or cracked surface. However, when they do wait until the underlying concrete is stiff enough, the surface layer has usually become too hard to process correctly.

All finishing operations will tend to move entrapped air towards the surface. At later stages, the trowel blades are tilted to increase the pressure. Air and water just under the dense surface are forced ahead of the blades until enough is concentrated (usually near a large aggregate) and a blister is formed.

A slow setting increases the window of finishability, but this can have several disadvantages. Besides the increased risk of damage by rain and plastic shrinkage, crusting is more likely to occur, which makes it difficult for the finishers to time the finishing operations.

These last few years, this type of damage appeared in elevated frequencies on an international level, while the level of expertise of the contractors and the concrete plants hasn’t apparently changed.

Some questions concerning the influence of vibration on air release in industrial floors exist: large air voids could be released but the small voids could be difficulty released with vibration depending on the viscosity of the concrete.

Recommendations concerning concrete composition and execution of industrial floors are needed to avoid delamination and other degradations.

Terms of reference

The committee will comprise a selected membership of organisations and research centres world-wide that are active in concrete technology. It will include universities, research institutes, material suppliers, contractors, engineering consultants, building owners, public agencies,... The members will also be recruited from participants in relevant international symposiums and workshops.

The state-of-the-art-report (STAR) on “Delamination of trowelled concrete industrial floors” will be based on the following input:

- Previous state-of-the-art reports related to this phenomenon (issued by ACI,...)

- Collection of data from relevant work, as published in international scientific journals

- The committee will represent a majority of ongoing research projects in this field. The committee will gather, as far as possible, the results of relevant parts of these projects

Estimated duration of the work is 3 years. The new Committee could start its activities in the fall of 2012.

Detailed working programme

The committee work will be carried out with close links to on-going research projects. The main activities are as follows:

- Establishing of consensus on strategy and limitation of work

- Collection and discussion of published data and experiences

- Identification and discussion of on-going projects regarding the degradation of concrete floors (including hard-trowelled), with a focus on delamination

- Defining a table of contents of the STAR on “Delamination of concrete industrial floors”, and identifying authors

- First draft of the different chapters of the STAR, by different authors

- Discussion of the first draft

- Planning of symposium

- Second draft of the STAR and formulation of recommendations

- Final version of the STAR and recommendations, in ready to print format

- Planning of symposium