Paving slabs with UHPC face concrete

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1 Paving slabs with UHPC face concrete Alexander Wetzel, Siemon Piotrowski, Bernhard Middendorf Institute of Structural Engineering, Dept. of Structural Materials and Construction Chemistry, University of Kassel, Germany Ultra-high performance concrete was optimized for the use as face concrete for the application of two layered concrete paving slabs. Untypically for UHPC the water/binder ratio was increased to 0.42 for the special requirements in case of concrete paving slab presses. During the production process the excessive water is pressed out of the face concrete into the core concrete. The resulting microstructure and the properties of the hardened face concrete after pressing equals ultra-high performance concrete. By intentional carbonation in an CO2-enriched environment the capillary porosity in the uppermost microstructure was decreased even more. The resulting concrete surface is free of efflorescence and shows a high abrasive resistance. Keywords: ultra-high performance concrete, microstructure, hydraulic press, porosity, carbonation 1 Introduction The surface appearance and durability of concrete paving slabs are of major interest. Therefore, the face concrete usualy has a higher quality compared to the core concrete, because the face concrete is visible and exposed to natural weathering conditions. Due to the higher quality of this face concrete the layer thickness is reduced to about 10 mm. The core concrete is a no-slump concrete with a low content of water. Although the face concrete is of higher quality, still efflorescence occurs at the surface of the concrete slabs. These efflorescence are white mineral precipitates, which are mainly calcium carbonates in case of cementitious building materials. These calcium carbononates evolve from reaction of Ca(OH) 2, which is formed during hydration of the cement, and CO 2 from the air. These efflorescence are visual impairments, which need to be avoided. Water repellents or sealings such as acrylates are applied onto the face concrete after hardening to prevent efflorescence. But, water repellents do not last very long and sealings have a low abrasive resistance and may delaminate due to osmotic pressure, followed by this efflorescence may evolve anyhow. As an alternative ultra-high performance concrete (UHPC) was applied as face concrete for two layered paving slabs. The efflorescence and the durability of the face concrete and therefore for the paving slab should be increased due to the low capillar porosity of UHPC. The high strength and durability of UHPC result beneath the use of fine reactive powder and the optimum packing density as one main issue from the very low water/cement or water/binder ratio. The water/binder ratio, which includes the silica fume beneath the cement, is in the range of 0.20 in case of UHPC. In order to improve the surface properties of concrete paving slabs an UHPC mixture was optimized for the use as face concrete of these two layered concrete paving slabs. Concrete paving slab production The upside down order of infilling is an important feature of the production of the two layered concrete paving slabs in an hydraulic press.to guarantee defect free striking of the slab directly after pressing, a flexible rubber plate is needed at the buttom of the formwork (Fig. 1-1). First the face concrete is filled in, followed by a vibration process to distribute the very fluent face concrete over the entire area (Fig. 1-2). Afterwards, the no-slump core concrete is filled in, followed by a vibration procedure to distribute it over the entire area of the formwork (Fig. 1-3). Subsequently, - 1 -

HiPerMat 2016: Ultra-High Performance Concrete and High Performance Construction Materials the entire material is pressed together with a pressure of about 15 MPa compacting the two layers and

2 HiPerMat 2016: Ultra-High Performance Concrete and High Performance Construction Materials the entire material is pressed together with a pressure of about 15 MPa compacting the two layers and pressing out the excessive water from the face concrete into the core concrete (Figs. 1-4 and 1-5). Immediatly after the pressing procedure the slab has a sufficient green strength and can be stored for hardening. A faultless automatic striking is possible due to the decompression of the flexible rubber plate and the resulting shear movements at the interface of rubberplate and face concrete (Fig. 1-6) Methods The water/binder ratio of UHPC is in the range of 0.2. Untypically this water/binder ratio was increased to 0.42 to meet the special requirements for the production of paving slabs with hydraulic presses. The flowability of the face concrete needs to be very high and the excessive water is needed in the core concrete because otherwise the water content would be to low. Due to the increase of the water/binder ratio the superplasticizer content could be reduced (Table 1). UHPC in fresh state normaly shows a certain stickiness due to the high content of superplasticizers. This reduced stickiness showed some advantages for a faultless striking after pressing. After the mix design was optimized in rheological behaviour, experiments were done using a press equipment (Fig. 2) Figure 1: Principle of hermetic press constructed to simulate the production of concrete paving slabs in an hermetic press on laboratory scale. In these experiments only small cylinders measuring about 5 cm in diameter were produced. After the UHPC mix was optimized on this scale, a press equipment for larger samples (20 x 20 x 4 cm 3 ) was used to face upscaling effects. In this setup, the water content of the face concrete was measured before and after pressing tested on the basis of DIN Based on the w/b-ratio resulting from these investigations, concrete cubes with this w/b-ratio were prepared for measuring the compressive strength. After the mix design was adapted to the requirement of the laboratory-scale equipment, trials were carried out on an industrial hermetic press producing concrete paving slabs with the dimension of 40 x 40 x 4 cm 3. To prevent agglomerations in the mix a compulsory mixer was used. The dosage of face concrete was reduced in case of UHPC face concrete

3 Paving slabs with UHPC face concrete resulting in about the half of the layer thickness compared to the reference measuring about 5 mm. After the pressing procedure the concrete paving slabs were weather stored in storage racks without preventing drying or they were sealed with plastic foils to ensure good curing conditions. After 28 days the slabs were sampled for microstructural investigations (light microscopy, mercury intrusion porosimetry) and durability tests (CDF, abrasive resistance). For the porosity measurements the uppermost layer was sampled by a precision low speed saw followed by a drying process over 24 h at 40 C before the measurements were conducted. Intentional carbonation for avoiding efflorescence Different strategies were followed in order to reduce efflorescence of the paving slab surface. The frist two rely on the principles of ultra-high performance concrete: the enhanced packing density goes hand in hand with the reduction of the porosity in general and especially of the capillary porosity, which is important to avoid the water ingress causing efflorescence. The Figure 2: Equipment for hermetic press experiments producing cylinders (Ø=5cm) water ingress followed by leaching of Ca(OH) 2 and precipitation of salts, mainly calcium carbonate, is inhibited due to the decrease of capillary porosity. The second advantage of UHPC is the reduction of calcium hydroxide due to the consumption by the pozzolanic reaction with silica fume. Thus, the soluble and efflorescence driving calcium hydroxide is reduced. For further reduction of the Ca(OH) 2 the UHPC was intentionally carbonated by curing at CO 2 - enriched environment while the relative humidity is kept high. The high humidity prevents recently striked concrete from drying. Thus, the CO 2 -enriched environment leads to carbonation within the capillary pores [2, 3], where residual calcium hydroxide is consumed. Therefore, a CO 2 -reactor was constructed making a curing at variable rel. humidities and CO 2 concentrations possible (Fig. 3). The mass flow controller regulate the CO 2 (Fig. 3-1), the water saturated synthetic air (Fig. 3-2) and the dry synthetic air (Fig. 3-3). The synthetic air is watersaturated by conveying it through a gas washing bottle (Fig. 3-5). The samples with sizes up to 20 x 20 x 4 cm 3 were placed in the reactor chamber (Fig. 3-6) and the gas mix is distributed in the chamber by a fan (Fig. 3-8). 3 Results and discussions An upscale on sample sizes of 20 x 20 x 4 cm3 was done after the ultra-high performance concrete was optimized for the requirements of the hydraulic press and a low inclination to efflorescence was investigated by drop tests. Before casting the fresh UHPC has an water/binder ratio of After the pressing process a w/b-ratio of 0.24 was estimated for the UHPC face concrete after the water content was measured on the basis of DIN Cylinders (Ø=15cm) of this UHPC formulation (Table 1) with the estimated w/b-ratios of 0.42 and 0.24 were produced stored under water and the compressive strength was measured after 28 days. The sample with the water/binder ratio of 0.42 shows a strength of 87 MPa while the sample with the w/b ratio of 0.24 had a compressive strength of 136 MPa. If a higher curing temperature is applied the strength can be increased to 182 MPa

HiPerMat 2016: Ultra-High Performance Concrete and High Performance Construction Materials Figure 3: CO2-reactor for intentional carbonation(a-overview, b-gas washing bottle, c-reactor): (1-3) mass

cohesion failure within the core concrete with tensile strength of 1.8 MPa. This test indicates a good bonding of the two layers.

40 g/m 2 far below the critical value (1500 g/m 2 ). The abrasive resistance was measured according to DIN V 18500 and the loss of volume was 10 cm 3 / 50 cm 2.

4 HiPerMat 2016: Ultra-High Performance Concrete and High Performance Construction Materials Figure 3: CO2-reactor for intentional carbonation(a-overview, b-gas washing bottle, c-reactor): (1-3) mass flow controller, (4) operating PC, (5) gas washing bottle, (6) reactor chamber, (7) CO2-sensor (output) and (8) fan The adhesion strength was measured according to DIN , the mode of failure was cohesion failure within the core concrete with tensile strength of 1.8 MPa. This test indicates a good bonding of the two layers. The freeze-thaw resistance was measured according to DIN CEN/TS , the amount of scaling per unit surface area due to 28 cycles of freezing and thawing in the presence of deicing salt was with 40 g/m 2 far below the critical value (1500 g/m 2 ). The abrasive resistance was measured according to DIN V and the loss of volume was 10 cm 3 / 50 cm 2. These two values indicate the very high durability of the face concrete and of the paving slabs. a b UHPC face concrete Figure 4: Casting of UHPC as face concrete in a paving slab machine Microscopic images on the cross sections of reference face concrete slab and UHPC face concrete slab are compared in Fig. 5. In these thin sections the pores are filled with blue coloured epoxy resin. The layer thickness of the UHPC face concrete could be reduced to about 5 mm compared to about 10 mm for the reference. It is obvious that the core concrete shows a much higher porosity compared to both face concretes. Furthermore, enrichments of pores directly at the interface of core and face concrete can be seen. This observation was done on both, reference and UHPC face concrete. Comparing the porosity of the reference and UHPC face concrete (see binary pictures in lower part of Fig. 5) a lower porosity of the UHPC face concrete - 4 -

5 Paving slabs with UHPC face concrete is clear. As described above, the uppermost layer of 2 mm thickness was sampled by a precision saw in order to measure the porosity by mercury intrusion porosimetry. The air pores ( µm) the capillary pores ( µm) and the gel pores (<0.03 µm) are given for both, the reference and the UHPC face concrete (Fig. 5). The capillary pores are reduced to about 1 Vol.-% fitting to the durability properties measured on the hardened concrete, such as the good freeze-thaw resistance. The tendency for efflorescence was reduced due to the low capillar porosity, too. In the industriell trials curing and hardening in atmospheric environment were compared. For curing the paving slabs were sealed with plastic vapour diffusion resistant foils to avoid fast drying. In case of normal drying the porosity was higher with about 4.4 Vol.-% for the capillary porsosity. Figure 5: Cross section of reference sample and UHPC-face-concrete sample in light micrcopic pictures (top) and binary pictures showing the pore distribution (black) and the porosity with Vol.-% values determined via MIP. Intentional carbonation in a CO 2 -reactor was conducted on UHPC-samples of the same formulation. A drop test on samples stored at standard conditions (20 C, 65% rel. hum.) and samples stored in CO 2 -enriched environment was conducted for testing the efflorescence potential according to Kresse [4]. Already after 10 hours at an environment with 1 % CO 2 and an humidity > 60 % rel. hum. a sufficient reduction of efflorescence was achieved (Fig. 6). This is probably due to a reduction of capillary porosity by carbonation of residual Ca(OH) 2 within the pores, which was guaranted by the increased humidity within the carbonation reactor. This reduction of capillary pore content was proven by mercurcy intrusion porosimetry, an increase of calcite could also be measured by x-ray powder diffraction analysis

HiPerMat 2016: Ultra-High Performance Concrete and High Performance Construction Materials Figure 6: Drop tests from 1 day until 28 days for samples cured under standard conditions compared to

6 HiPerMat 2016: Ultra-High Performance Concrete and High Performance Construction Materials Figure 6: Drop tests from 1 day until 28 days for samples cured under standard conditions compared to storage under enriched CO2 environment. 4 Conclusions The application of UHPC as face concrete for paving slabs was successful. By increasing the w/b-ratio the UHPC was applicable for the press production of paving slabs and the properties of the hardened face concrete are comparable to UHPC cast the usual way. In these studies on one hand the UHPC was optimized for the production of paving slabs and on the other hand the efflorescence of the UHPC was reduced by intentional carbonation. This intentional carbonation is quite promising as a curing system for concrete in general but also for UHPC which is already almost free of capillary pores. 5 Ackknowledgements We thank our project partners of FC Nüdling GmbH for their cooperartion. We greatfully acknowledge the research Fund AiF-ZIM which financially supports the project no. KF HF2. References [1] Fehling E, Schmidt M, et al. Entwicklung, Dauerhaftigkeit und Berechnung Ultra-Hochfester Betone (UHPC), kassel university press, Kassel, 2005 [2] M. Schwotzer, T. Scherer, A. Gerdes. Protective or damage promoting effect of calcium carbonate layers on the surface of cement based materials in aqueous environments. Cement and Concrete Research (40), 2010, pp [3] M. Seidemann, A. Müller, H.-M. Ludwig, Gezielte Karbonatisierung - Verbesserte Qualität von Betonrezyklaten durch Bindung von Kohlendioxid, Farbe und Lack 118 (3), 2012, pp [4] P. Kresse. Ausblühungen und ihre Vermeidung Efflorescence and its Prevention. Betonwerk+Fertigteile (10), 1991, pp

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