Sometimes, they return...

Concrete laying surfaces: years go by but the problems remain the same. Here are two examples of how NOT to do things.

There are days in which I really fail to understandwhy - even aftermore than two decades of seminars, meetings, andmore than ample technical presentation – I keep encountering the same problems again and again. Many people working in the construction business, who in my opinion are often unprepared, resolutely continue making the same mistakes, even if the correct technical information is copious and has beenwidely diffused throughout the nation: just consider how much money the leaders in the sector have invested in research and the distribution of technical documentation, videos, and brochures. As the Latin poets were fond of saying, perseverance in error is diabolic, butwhen facedwith certain problems, the entire body of technical knowledge seems to be no use at all to these people. Below, I describe situations that I examined in two different work environments in two geographically distant in which the final result was practically the same. In both cases, fortunately, the parquet installer assigned to the job proved to be sufficiently scrupulous, and raised his objections regarding the quality of the laying surface right from the start. These parquet installerswere undoubtedly dismissed as “fanatics”– as usual – but the final client forced to suffer the initial inconvenience, proved capable of solving the problemwithout increasing the damage. Let’s take a closer look at both cases in question. The first case: analyzing the screeds prior to laying My first technical inspection regarded the structures of a few self-leveling concrete screeds laid in a number of apartments for a total 600 m² surface area at a newbuilding construction site. Screed examination overview I analyzed the rapid hardening self-leveling screeds, which consisted of a floor slab and embedded underfloor heating systems. Around fourmonths had passed fromthe final casting of the screeds by the time I arrived. The Director of Works had informedme that their thickness varied from a minimum 3 to a maximum 8 cm. Measurements had been taken starting from points just over the heating systems. Metal reinforcementwebbing –without expansion joints – had been embedded inside the screed, especially near the “doors”, which are usually the areas of the room with the greatest variations in dimensions. For this type of screed, the surface area that should ideally be coveredwithout interruption is usually around 40 squaremeters, and so-called expansion joints are always created near door thresholds. My first visual inspection revealed the following anomalies in all the apartments with screeds already laid:

  1. whenever anyonewalked over them, some areas of the screed gave off a dry thump and flexed downward (relatively large areas of screed weremoving up and down);
  2. noticeable differences in levelwere also observable in these areas;
  3. the surfaces of the screedwere fairly “brittle”; in otherwords, the cementmixwas not compact, and so crumbly that it was immediately evident;
  4. noticeable cracks had formed near the thresholds of the doors, in particular.

The Project’s General Specifications included the laying of a solid wood floor by gluing in every part to be completed on site. The scruples of the conscientious parquet installer As luckwould have it, the parquet installer assigned to laying the floor was suspicious of these anomalies, even if the main contractor urged him to meet the deadline nonetheless. In order to get a better idea of its characteristics, I examined the technical data sheet of the rapid hardening self-leveling screed used.At the start of my technical inspection, I had tried to take a few samples of the screed without success due to the compactness of the cementmix, which was, in fact, so brittle that taking samples was impossible. The causes revealed Following other inspections and checks using the test instruments required to obtain a more complete picture of the laying of awood floor, themain and the contributing causes responsible for the various anomalies observed were identified as:

  1. “Screed areas that give off a dry thump and flex downward when walked on” This situation that was observed in all the rooms, especially in the areas nearwalls, can be ascribed to an “accidental” penetration of liquid cementmix beneath the heating panels. This infiltrationmight have been caused exclusively nearwallswhere the edges of the paneling had not been correctly “sealed”. This allowed the self-leveling cementmix to penetrate beneath the panels, in this way swelling up. During the moment of compression 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 photographic 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 container 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, itmust be specified that precisely due to their failed adhesion to the support floating screeds or screeds with sheets of PVC or other insulation 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 sideswith the creation 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 otherwords, the excessive quantity ofwater used for the concrete mix. The product’s data sheet refers to 4 liters per sack for the working of the freshmortar. The analysis of the cementmix as being overly brittle allows us to statewith reasonable certainty that the causemay be ascribed to quantities ofwater higher than the recommendedmaximumlimit. Consequently, superficial mechanical resistance decreases exponentially. After 28 days, an average compression resistance of approx. >5N/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 occur normally when the cement mix shrinks but rather authentic “cracks” caused by inhomogeneous shrinkage.

Gaps and cracks were due to both the presence of toomuchwater and the excessive exposure to ventilation of the surfaces in question while the cement mix was drying. Although producers’ recommendations include providing the cement mix with protection against draughts of air for at least the first 48 hours, in realitywe can cut that time down to 24 hours, but in such case the surface must be provided with absolute protection. At the worksite in question, the external door andwindowframes had still not been installed even at the moment of our technical inspection, and as stated by the Director of Works himself, the building’s external openings were not even minimally protected by nylon tarpaulins or other material 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 evaporation of the excesswater led to the shrinkage 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. Asmeasured and verified, the entire screed execution processwas 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 ofWorks – 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 experiencewith this type of material, and for such reason had requested the presence of personnel fromthe product’s supplier.
Unfortunately, as confirmed by the facts, the technical instructions provided during operations to themen assigned to actually laying the screedswere not sufficiently helpful.
If what I was told is true, to the extent that I could hardly believe my ears, the quantity ofwater used should absolutely have been controlled better, instead of advising the workers to addmore, as these technicians effectively did, without adequately monitoring the workplace subsequently and providing it with at least the minimum protection against gusts of wind.
At any rate, the damagewas done, and the question of how to recover the screeds for the purpose of containing the costs required for the recovery of the rest of thework, which could have been very high, given the presence of climate control systems with panels embedded into the cement mix was not an easy one to solve: the “cracks” had to be previously filled, and this required them to be widened in “V” shape by inserting iron bars and then filling the space createdwith the appropriate substances available in the market. The self-leveling screeds also had to be subjected to a restarting cycle of the radiating heating systemas specified by the standards in force in the sector (UNI EN 1264-1/2/3/4) and the entire cycle had to be brought to completion. This operationwas not only absolutely advisable but inmy opinion indispensable, also because the structural situation of the screed could have led to further shifting and crumbling of the cement mix in addition to additional cracks caused by shrinkage.

The second case

In the second case aswell, the purpose of the technical inspection conducted in an apartment complex located in a historical big city ™centerwas to evaluate the condition of the laying surfaces in traditional concrete cast expressly to underlie a solidwood parquet floor to be laid by total gluing.
Once again, the analysis wasmade by visual method using a few manual instruments over a total cemented surface area of around 85 m2 on one of the building’s floors. Aceramic floor had already been laid in the two bathrooms, while in the rest of the house (foyer, living room, kitchen, and three bedrooms), a solidwood floor in herringbone pattern - an element of notable prestige and quality - was scheduled to be laid.

Structural problems

Because thewood floorwould be laid by total gluing using a synthetic adhesive, the structural investigation of the laying surface was conducted by bearing in mind the contents of Standard a UNI 11371 dated September 2010 Wood and parquet flooring screeds – Properties and performance characteristic. According to the Director ofWorks, the screed had been cast approximately threemonths prior to the laying of the floor but an instrumental check revealed that a moisture content ofmore than themaximum permissible 2% remained.
Furthermore during destructive testing, aggregates of hygroscopic nature had emerged, in other words, lightening materials, presumably expanded clay; unfortunately however, the company that had laid the screed had not documented its composition. Investigation also revealed that no vapor barrier has been inserted, and that the cement aggregatemixwas absolutely brittle and inconsistent. What’smore, after the spreading of the concrete casting, the worker decided to also apply a layer of self-leveling agent over the surface of the screed that upon our inspection appeared to be clearly detached from the latter.
It was obviously impossible for us to learn theweight of themoisture content before the self-leveling agentwas0 applied, but seeing the detachment that occurred,moisture levelsmust undoubtedly have been high in the laying surface. Stress tests demonstrated the presence of extended areas of self-leveling agent that had already detached fromthe surface of the screed, most certainly due to the different hygroscopic shrinkage coefficients of these twomaterials,which definitely compromised the possibility of laying a glued parquet floor.
During inspection, the concrete screed laid in different rooms presented an evidently irregular and compromised structural situation. These problems leave little room for doubt. Furthermore, on thewhole, the screed did not even partially possess the performance characteristics summarized in Paragraph 4.2.2. Page 4 of UNI 11371:2010.
Lastly, the presence of hygroscopic material in the cement mix compromised the time required to reach a residual humidity value appropriate for parquet (2%), and the complete absence of a vapor barrier only made thingsworse . For this reason, in this case as well, as far as we were able to ascertain and measure, formed as itwas, the screedwas inadequate to underlying a glued wood floor, also because it presented defects that precluded its recovery even through the use of other products.

problems during inspection
The photos show some of the problems observed during inspection: differences in level, brittleness, and cracks.

Starting all over again

In this case, the laying of the parquet floor onsite was possible only by completely dismantling the existing laying screed and redoing it all over again, naturally after first checking the thicknesses remaining after removing the screed and choosing a product suited to its new formation, ideally a quick hardeningmaterial capable of shortening the execution times that had already been extended far too long. As may be seen in the photos, it is simply amazing that such brittle and inconsistent screeds could be laid, without even going into the question of the absence of the vapor barrier, a function that appears to be entirely unknown tomost people when instead it is just another simple product that can be very useful in the success of their day to day work.

photos second inspection
These photos were taken during the second inspection.

The paradox of saving

A wooden sports floor in an indoor sports stadium, under-floor radiant panel heating and parquet blocks that move under the baskets were just some of the problems to be overcome in a new, particularly intricate job

The case we present here can be seen as an example of the paradox of sav- ing, i.e. a good parquet fitter should never “save” when it comes to the materials that dictate the final success of his work. It would be a bit like a cowboy builder looking to save money by “forgetting” to add cement to the screed mix, leading later to not only technical problems, but also extra costs for both the customer and the builder himself. Unfortunately, as various inspections and surveys revealed, the case in hand is the outcome of having forgotten something that led to problems further down the line and which took a lot of money to set right.

The problems on the “games court”

The object inspected was an indoor sports floor, a large wooden surface installed inside a sports hall that hosts regular events at both a national and international level. The experts had been asked to consider both the supply and the installation of the wooden flooring for sports use, with special attention to how the wooden flooring had been fitted, as this was used for both basketball and volleyball championship matches. The floor consisted of a double framework of wooden joists (called “tozzetti”) with an overlying double framework of decking planks (the lower planks parallel to the longer side of the court, the upper ones perpendicular to the lower ones), a nylon canvas and finally the wooden parquet boards. There was also a quite rare feature for a sports hall: the wooden flooring had been laid on top of a floor heating system. The complaint concerned the “gaps that exist” between the extreme peripheral parts (heads or ends) of the boards, particularly evident in those areas close to the baskets at either end of the court. Indeed, the owners complained that the individual boards in the wooden floor “widened” at the ends, often during games of both basketball and volleyball. The situation was made worse by the non-alignment of the lines marking the court areas, which led to many disputes during the course of games. Protests were so animated that the owners saw fit to take the case to court. Over the course of several inspections, it was also found that the parquet boards with gaps at the ends were never the same and never in the same place.
How could this be possible? A defect cannot just appear and disappear or rather move randomly across the surface of a floor. The only common element was that the gaps between the boards were generally found at either end of the court, near the baskets, i.e. the areas most subject to stress and pressure, where the athletes concentrated their efforts.
We were asked to investigate the matter in order to understand the origin of the defect and, of course, solve it.

In search of a cause

The owners had started to raise objections as early as the final testing of the newly laid floor. Their first few letters had, in fact, pointed out that “the parquet sports floor has problems regarding gaps appearing between the boards both lengthwise and transversely.” This problem had also been reported by the various sports clubs using the sports hall.
These reports were accompanied by photos showing the areas of the court near the baskets, clearly showing that the position of the gaps between the boards changed each time. Meanwhile, the floor fitters rebutted with the claim that the gaps were due to the excessive temperature of the radiant panels in the floor heating system, but after an audit by a technical expert appointed by the owners, it emerged the temperatures posed no risk to the stability of the floor:

  • Parquet surface temperature = max 25.6 °C and min 25.2 °C.
  • Parquet temperature where covered by the PVC platform = 26.9 °C
  • Air temperature under the floor = 23.4 °C
Other abnormalities then emerged, which, although not actually rendering the floor unusable, did affect the aesthetics: the ends of numerous boards were virtually right up against each other, meaning there was no offset distance between them. The presence of shrinkage or cracks along the longitudinal side of each boards was also noticed, which was in turn composed of individual layers of parquet, although there were no particular anomalies in the size of the boards.
In essence, the shrinkage was minimal and in line with the physiological behaviour expected of wooden flooring on a heated substrate.
However, the flooring had been raised above the heated surface by inserting wooden blocks (“sleepers”) and loadbearing joists upon which the parquet rested. In other words, joists (double oscillation) had been placed on these wooden blocks, followed by a second decking layer (the counter floor) and finally the final wooden flooring. Furthermore, two polyethylene sheets had also been laid: the first under the heating system pipes between the bottom surface and the base of sleepers; the second between the counter floor and the individual parquet boards making up the sports floor. Another important factor was that the wood used had all the necessary certification, namely DIN V 18032-2:2001-04 dated 28-02-2005 by DIN CERTCO in Berlin. After taking into account the findings of these technical investigations, it became clear that the work had been carried out without taking into account the manufacturer’s technical requirements, thus compromising the warranty of the product itself, since it could no longer respond to the basic requirements for which it had been approved. Moreover, it also emerged that the installation instructions, which indicated the size of the nails to be used as well as other obvious technical matters, had been ignored. In fact, the movement of the end boards and their continuous change in position made one think that the problem lay in the anchoring method rather than being connected to the heating system. It therefore became necessary to proceed with an invasive investigation in order to verify the presence or absence of the anchor nails and their size, given that these were required in the technical documents accompanying the product.

“Site” survey

After removing at least three of the parquet boards, it was found that the first and the third boards had no “fixing nails”, while the second had 4. Furthermore, the following floor structure was observed: 1) polyethylene sheet laid under the parquet surface and on top of the counter floor; 2) the counter floor consisting of wooden elements (spruce) 90/95 mm wide with a variable space (distance from centre) between the various strips, ranging from 50 mm to about 55 mm; 3) the initial joists consisting of 2 single wooden elements with flexible coupling, positioned about 450 mm apart; 4) wooden sleepers with nominal dimensions of 93 mm wide and 80 mm high (the official measurements were 100/ 120 mm wide and 60/80 mm high); 5) polyethylene sheet laid directly on the raw concrete floor with the heating system pipes clearly visible on top of this. With regard to the temperature of the parquet flooring and that in the “air gap” created by inserting the wooden sleepers, the investigations found no negative or unexpected values, including the dynamic balancing of hygroscopic wood as foreseen by UNI EN 1264-1/2/3/4. However, when it came to the question of how the parquet sports floor had been “nailed”, it was found during the invasive investigation that the wooden elements had been nailed in an approximate manner, not in line with the technical documentation received (in particular, the specifications in the installation instructions drawn up by the manufacturer and attached to the DIN V 18032-2:2001-04 certification dated 28-02-2005). For example, there were no nails in two of the three boards and the “nails” used were inappropriate for the intended use of the sports floor. Indeed, nails of the wrong size had been used, incapable of withstanding the envisaged stress: 1.2 x 40 mm long with a 1.8mm head instead of the recommended 2.2 mm x 40 mm long nails with a 7mm head. Only after the invasive investigation was it possible to detect the lack of nails in the two of the boards examined. Whilst it is certainly true that such a small number of boards cannot be considered representative of the entire floor, one can assume that the practice to simply “nail down the boards from time to time” had been repeated right across the surface given the observed dynamics of the way the blocks “had grown apart” several times and in a “random” manner. Without prejudice to these findings, there are currently no sector-specific regulations regarding the installation of wooden parts to be nailed down giving the precise dimensions of the nails to be used (nor were there any at the time of the technical inspection), yet contrary to what is usually the case, the fact that there were technical documents with specific installation instructions from the manufacturer means that precise elements do, in fact, exist that allow one to assess the causal link.

All suspicions therefore fall on the “NAILING”

From the findings of the technical inspections, one can safely state that not enough importance was paid to how the boards should have been nailed down when the sports floor was being laid, a crucial oversight given the high levels of stress and strain (attrition) that such floors are subject to. Moreover, after reading the manufacturer’s installation instructions, provided with the certification from DIN CERTCO of Berlin (DIN V 18032-2-2001-04 dated 28-02-2005, the whole bundle having been given to the customer), it is evident that the people responsible for fitting the floor did not follow some of the basic technical instructions. The invasive investigation has clearly afforded us the chance to discover some technical deficiencies that form the probable cause of the problem.
We have been able to verify the following:

  1. The fitters did not nail down the floor – a stage required by and foreseen by the manufacturer and clearly indicated in its technical instructions provided with the DIN certification – in a manner compliant with the mandatory technical instructions.
  2. There are currently no sector-specific regulations regarding how such wooden parts should be nailed down (nor were there any before or at the time that the sports floor was laid). The only exception is the European technical specification UNI/CEN/TS 15717 dated October 2008 General Guidelines for Floor Laying, which, with regard to the laying of wooden floors requiring nailing, reads: “the type of screws/nails must be selected in accordance with the manufacturer’s specifications.”
    In fact, as can be seen in the technical instructions attached to the product certification documents, the manufacturer did provide specifications in this sense, i.e. 2.2x38mm T-nails should have been used to fix the parquet boards to the counter floor. Furthermore, one can assume that the fitters used the same non conforming nails to join the two load-bearing levels (the double oscillation joists and the counter floor) instead of the 2.2x32mm Tnails specified by the manufacturer.
  3. The gaps between the parquet boards were caused by these boards slipping at the moment of impact (attrition) by the rubber-soled shoes worn by players during basketball games.
  4. The complaint that the gaps between the boards “change their position”, as proven by the fact that certain boards open and then close after manual traction stress, confirms the fact that the type of nailing used, where present, and found to be non conforming with the official technical specifications, was insufficient to “withstand tension in the boards as a result of the surface friction.”
    The heat coming from under the floor cannot be considered to cause the detachment of the boards (gaps between boards), despite the fact that the heat in the air-gap under the wooden floor is propagated solely by natural convection.
  5. The gaps, in varying positions, only occurred at the extreme ends (heads) of the boards and were of differing dimensions; these then “disappeared” if stressed, i.e. the boards were made to slide back and so close the gap.
    The fact that these gaps regularly occurred can be further confirmed by the presence of “grouting substances” used to close the gaps and found at the ends of the boards.
    There is no information about when the grouting substance was first applied, although certain photos lead one to presume with a fair amount of certainty that the gaps started to appear between boards from the very time the court lines were first drawn on the floor.
  6. The constant heat, as is the case with all heated supports where a wooden floor comes into direct contact with a hot surface, creates shrinkage in the wooden elements in a longitudinal direction only, to a varying degree depending on the timber used and the direction of its grain. For instance, wooden elements obtained from tangential-cut elements will shrink at least three times as much as radial-cut elements.
  7. In the case in hand, we have only found localised gaps (in the playing areas) of varying dimensions at the front end of individual boards; no similar gaps or other deformations caused by shrinkage have been found across the whole surface of the floor. If the under-floor heating system had been the cause, we would have expected similar problems across the entire surface of the wooden floor, not just in the playing areas, i.e. under the baskets.


To sum up, after several months of meetings, inspections and discussions, we have reached the conclusion that “nails were missing”.
The nailing of the floor was done in a discontinuous manner, using nails that did not comply with the manufacturer’s installation instructions and this saving in nails led to a whole series of incredible problems that required the intervention of many experts.
Was it really worth it?
We hope that this case will serve as a lesson when facing similar jobs in the future. The old adage “the cheapest is the dearest” has been proved true once again.

Pavimentazione di una palestra

Salve, sono un posatore e devo stilare un preventivo per un'eventuale posa di un pavimento in legno o in laminato all'interno di una palestra dove si praticherà soprattutto fitness. Il problema per il quale scrivo e per cui chiedo consiglio ai vostri esperti è relativo a quale dei due materiali costituisca la soluzione più adatta e comunque quali vantaggi presentano il legno e il laminato in questa situazione. Datemi tutti i consigli che ritenete utili allo scopo. Grazie per l'assistenza tecnica e cordiali saluti. [G. D. V.]

Egregio G. D. V. riguardo la sua richiesta in merito al dilemma laminato o legno per la palestra, direi che stante le attività che vi si andranno a svolgere, e certamente la superficie elevata in questione, opterei senza indugio per il legno. Il laminato, ottimo materiale, ma per altre situazioni, necessita di una serie di accorgimenti tecnici che difficilmente potrebbero accorarsi con le necessità di una palestra dedicata al fitness; il tutto ovviamente al fine di dare al suo lavoro un minimo di garanzia nel tempo.
Opterei quindi per una pavimentazione di legno, costituita da doghe stratificate posizionate su di una struttura portante, sempre in legno, e questo perché le pavimentazioni in legno posizionate nelle palestre, debbono sempre avere anche una adeguata flessibilità strutturale. Premesso tutto ciò, e per fornirle maggiori dettagli, tenga ben presenti le dimensioni del locale e la destinazione d’uso ovvero se è una palestra professionale o è privata in quanto vi sono normative in merito e che andrebbero, nel primo caso, applicate.

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