Page 28 - Parquet International december 2014

ON FIELD
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IN DEPTH ANALYSIS
The photos show some of the problems observed during inspection: differences in level, brittleness, and cracks
els. This infiltration might have been caused exclusively near walls where the edges of the paneling had not been correctly “sealed”. This allowed the self-leveling cement mix to penetrate beneath the panels, in this way swelling up. During the moment of com- pression caused by the passage of people, these portions sag and cause those dry “thumping” sounds and the downward flexure of corresponding portions of the screed. As may be seen in the photograph- ic documentation provided, it is clear that this penetration can go directly to the floor slab in the perimeter area (the panel does not reach the wall and no cement mix contain- er barriers exist).
2. “Differences in level”
Differences in level are a consequence of the infiltration described above, and are, in fact, observed exclusively near the perimetral walls. In this point however, it must be spec- ified that precisely due to their failed adhe- sion to the support floating screeds or screeds with sheets of PVC or other insula- tion material may undergo deformation caused by their shrinkage. Bearing in mind the content of the description provided in Point 4 below (very rapid surface drying), the screed may have assumed a concave shape (the screed rises up at the sides with the cre- ation of empty spaces below; see the figure above), which when subjected to loads (walking) breaks, or in other words, flexes even further downward.
3. “Poor surface compactness and brittleness of the cement mix”
These two problems share the same cause, or in other words, the excessive quantity of wa- ter used for the concrete mix. The product’s data sheet refers to 4 liters per sack for the working of the fresh mortar. The analysis of the cement mix as being overly brittle allows us to state with reasonable certainty that the cause may be ascribed to quantities of water higher than the recommended maximum lim- it. Consequently, superficial mechanical re- sistance decreases exponentially. After 28 days, an average compression resistance of approx. >5 N/mm2 should be obtained, if not the surface of the screed “crumbles easily”.
4. “Gaps and cracks”
These were not just the tiny fissures that oc- cur normally when the cement mix shrinks but rather authentic “cracks” caused by in- homogeneous shrinkage.
Gaps and cracks were due to both the pres- ence of too much water and the excessive ex- posure to ventilation of the surfaces in ques- tion while the cement mix was drying. Al- though producers’ recommendations in- clude providing the cement mix with pro- tection against draughts of air for at least the first 48 hours, in reality we can cut that time down to 24 hours, but in such case the sur- face must be provided with absolute pro- tection. At the worksite in question, the ex- ternal door and window frames had still not been installed even at the moment of our technical inspection, and as stated by the Di- rector of Works himself, the building’s ex- ternal openings were not even minimally protected by nylon tarpaulins or other ma- terial during the mortar casting phases. This led to the overexposure of the screed’s surfaces to excessive ventilation precisely during those crucial initial hours of cement mix hardening. In this regard, the “waves” in the screed’s surface provide the proof of this excessive “ventilation” during drying. The lack of joints of any kind and the evap- oration of the excess water led to the shrink- age observed in the cement mix producing these phenomena of “excessive shrinkage” that may be seen along the lateral walls in particular, as is clearly shown in the photos. As measured and verified, the entire screed execution process was clearly spoiled by the presence of evident anomalies ascribable to the working of the material.
During various discussions with the other people present during the inspections – with the exception of the Director of Works – another detail emerged that has a lot to show about certain problems involved. The company that formulated the cement mix for the screeds had rightly stated that it had no previous experience with this type of material, and for such reason had requested the presence of personnel from the product’s supplier.