How can you tell if ceiling joists are made of high-alumina cement?

What a joist is and where to find it

The floor slab (forjado in Spanish) is the horizontal structure between two storeys. In Spanish buildings from 1950 to 1970 it was often made with precast prestressed concrete joists laid in parallel, infill blocks between them (bovedillas) and a layer of concrete poured on site on top.

From inside a flat the joists are usually hidden under the plaster ceiling or a suspended ceiling. You can see them in basements, garages, storerooms or where a ceiling has been opened. If the floor was cast entirely on site, without precast joists, high-alumina cement is very unlikely.

What can you see in a joist without tests?

The type of floor and its condition: damp areas, cracks running parallel to the reinforcement, rust stains, exposed steel and degraded concrete. INTEMAC engineers, in Informes de la Construcción (CSIC, 1992), make that visual inspection the first step. What it does not reveal is the cement: a damp Portland cement joist rusts too.

A close visual inspection is always the first step. It shows what type of floor there is, locates damp areas (leaks, condensation, drainpipes) and looks for cracks running parallel to the steel, rust stains, exposed steel and degraded concrete. In the Madrid stadium studied in 1992, a characteristic change of colour in the joist concrete was also recorded.

All of that describes the condition of the floor, not its cement. A Portland-cement joist exposed to damp also rusts, and a high-alumina joist in a dry setting can be sound.

How the cement is confirmed

With samples taken from the joists themselves and tested in a laboratory: porosity, chemical analysis, X-ray diffraction or thermogravimetry. A study published in 1992 in Informes de la Construcción (CSIC) showed that a covermeter, calibrated against Portland cement concrete, gives a quick hint, although fly ash gives a similar reading.

Samples must be taken from the joists themselves and analysed: porosity, chemical analysis, X-ray diffraction or thermogravimetric analysis. Some tests cut out whole sections and are destructive. With the results, the professional works out how safe the floor is and, if needed, proposes repair or strengthening.

Quicker methods exist. A 1992 study showed that a covermeter, the device used to measure the concrete cover over the steel, can distinguish high-alumina concrete after calibration against Portland concrete, although fly-ash concrete gives a similar reading. It is a clue, not a substitute for the laboratory.

What mistakes are common when taking samples?

Sampling the wrong element and generalising from a single piece. INTEMAC cites, in Informes de la Construcción (CSIC, 1992), a Madrid building wrongly said to have high-alumina cement on the strength of samples from cast-in-place columns, where it was hardly used. Samples must come from the joists and cover both damp and dry areas.

Sampling the wrong element. INTEMAC cites a Madrid building wrongly said to contain high-alumina cement on the basis of samples from columns cast on site, where that cement was hardly ever used. Samples must come from the joists.

Generalising from one piece. A sample tells you about the joist it came from; that is why professionals first divide the building into damp and dry zones and spread the samples between them. Storeys added at a later stage of building may have a different floor system.

How HOUSINGFAX checks it

Check 03, “Is there a risk of aluminosis?”, combines the Catastro year of construction with whatever is on record about the building. If the official building inspection mentions damage to floors or joists, the report quotes it and adjusts what you should check. The result is a level in words, from very low to very high, never a percentage.

The free report shows that check with your building's real data and the key information blurred; the essential report costs €11 and the complete report €19. The report does not replace a technical inspection of the building: if the floor worries you, take the result to a professional.