Early formwork removal from in-situ concrete slabs using sustainable concrete with CEM II/C cements

The further development of concrete through the use of CEM II/C cement instead of CEM II/B cement resulted, in the tests carried out with the parameters listed in Table 1, in a potential reduction in greenhouse gas emissions of approximately 12%, calculated using the GVTB GWP calculator [1]. This represents considerable potential for sustainable development in the construction of in-situ concrete slabs.

The aim of the research project discussed here is the systematic investigation of the effects on established work processes in building construction, as well as the changes resulting from innovative concrete compositions. This includes the striking of formwork from the concrete, which depends, amongst other things, on the hydration behaviour of the concrete type, particularly in terms of early strength. In some applications, there is a need to optimise the construction process by removing formwork from the in-situ concrete slab at an early stage. Therefore, a distinction is made below between two formwork striking methods: “conventional formwork striking” and “early formwork striking”. The decision as to when formwork may be removed lies with the site manager.

As a rule, the timing of formwork striking is usually determined – in the absence of continuous concrete monitoring – on the basis of industry-specific empirical values and structural specifications. A suitable verification using concrete monitoring is particularly recommended in the case of early formwork striking. In the case of conventional formwork striking the target strength is allowed to develop, after which the in-situ concrete slab is completely stripped of formwork. The slab deforms as a result of its own weight following complete stripping, and the reinforcement is activated.

The aim of the investigations was to analyse the potential effects of early formwork striking and different types of concrete on deflection and strength development. To this end, a test slab was produced three times in the series of tests carried out. In comparison with standard building slabs, the following deliberately unfavourable environmental conditions were selected in order to test the resilience of the method under realistic yet challenging conditions:

In order to analyse differences, the test slab was cast using two different concrete mixes under almost identical environmental conditions. The first test (V1) simulated the situation of early formwork striking using a CEM II/C concrete. The second test (V2) represented the comparative scenario of early formwork striking using a CEM II/B concrete. The third test (V3) simulated the situation of conventional formwork striking (target strength: 70% of the ultimate strength) for the in-situ concrete slab using a CEM II/C concrete. To ensure the concrete mixes were comparable, the concrete manufacturer supplied standard site mixes with comparable performance. In tests V1 and V3, the slab was constructed using a concrete mix containing CEM II/C cement (295 kg/m³). In test V2, a concrete mix containing CEM II/B (280 kg/m³) was used.

A measuring stand equipped with a laser measuring device was used to determine the slab deflection (Image 2). Deflection measurements were taken at seven points along the span before and after concreting and at subsequent stages. The results were then compared.

The test series included the concreting of the individual test slabs (VI, V2, V3) at a test site, as well as accompanying concrete tests both in the laboratory and on site. The deflection of the test slab was measured, as was the development of strength for the individual concrete mixes using test specimens. In addition to the standard classical compressive strength test on test specimens, the development of compressive strength was also determined using the maturity method as part of the research project.

During the slab concreting, tests in accordance with ÖNORM B 4710-3 were carried out both on the fresh concrete directly on site and on hardened concrete in the laboratory, as shown in Image 5 [4]. The following tests were conducted:

In the first two test structures (V1 and V2), early formwork striking began after just 1.5 days, when a compressive strength of at least 12 N/mm² had been achieved. To do this, the levelling layers were first removed and supported with floor props. The props for the first element were then lowered and the elements subsequently removed. The formwork-removed area was stabilised immediately afterwards with temporary props. The process was carried out in this order, element by element and row by row. This approach ensures that the in-situ concrete slab does not experience any significant deflection and the reinforcement is not activated. After approximately three days, once a compressive strength of 18.5 N/mm² had been achieved, the temporary shoring was completely removed, allowing the slab to bear its own dead load independently from that point onwards.

The conventional formwork striking for the slab in the third test (V3) took place once 70 per cent of the 28-day compressive strength had been achieved. For the strength class C25/30 used, this occurred after approximately five days at 21 N/mm². In this case, the entire floor area was completely stripped of formwork in one go. The in-situ concrete slab deflects under its own weight, and the reinforcement absorbs the tensile force. The reinforcement is thus activated. The in-situ concrete slab is then supported with temporary props.

The method investigated, involving the partial striking of the panel floor formwork and the immediate support of the fresh concrete slab with floor props without a drop head, had no negative effect on the deflection of the slab compared to conventional formwork striking. The measured difference of 1 mm to 2 mm between the three tests lies within the margin of error of the measurement technique used. In relation to the total deflection of over 70 mm under deliberately extreme environmental conditions, or to the usual deflections of 20 mm to 30 mm over a 7 m span, these deviations are to be classified as insignificant.

The tests demonstrated that optimizing the concrete mix design with regard to CO₂ reduction does not impose any limitations on the early striking of cast-in-place concrete slabs and is feasible in practice, depending of course on factors such as temperature and weather conditions. The positive results obtained from the tests using CEM II/C cement in concrete, which led to a reduction in CO₂ emissions, should be confirmed in further studies at lower temperatures and with different concrete mix compositions.

Alexander Kastner, TU Wien
Daniela Ehrenreich, Doka GmbH
Peter Aigenbauer Doka GmbH
Michael Steineder, Smart Minerals GmbH