Influence of the saturation state of recycled sand on mortars compressive strength

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International HISER Conference on Advances in Recycling and Management of Construction and Demolition Waste

21-23 June 2017, Delft University of Technology, Delft, The Netherlands

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Influence of the saturation state of recycled sand on mortars compressive strength

Yacoub A.1, Djerbi Tegguer A.2 and Fen-chong T.3

1, 2, 3

Université Paris-Est, MAST / FM2D / IFSTTAR, F-77447, Marne-la-vallée France, (+33) 01 81 66 80 00 ; E-mail: aiman.yacoub@ifsttar.fr

Abstract

This paper investigates the water content of pre-saturated recycled concrete aggregates (RCA) that has a diameter of 1-4 mm called recycled sand (RS), in order to determine the degree of saturation of the RS. It also puts forward the effects of the saturation state of the RS on the mechanical properties. For this matter, experiments were conducted in this research as: evaporation method to determine the water content of the RS and the compressive strength of mortars. These properties are tested for different saturation states and different saturation methods of the RS and for two types of curing conditions: laboratory curing conditions (LCC) and water immersion conditions (WIC). Results show that the standard method (NF EN 1097-6) largely underestimates the water absorption coefficient of RCA and that the saturation state of the RS and curing conditions affects the compressive strength of mortar.

Keywords: Recycled aggregates, saturation state, degree of saturation, compressive strength.

Introduction

The world business council for sustainable development stated that one cubic meter of concrete is consumed per year per person [1]. In order to conserve natural resources and for a better management of waste, construction and demolition waste are nowadays used as (RCA) in different construction fields.

However, the difference in water absorption coefficient (WAC) between the RCA and the natural one stand in front of completely replacing the natural aggregates by the RCA. Being unable to determine the WAC of the RCA causes an inaccuracy in the water/cement (W/C) ratio of concrete. This property affects directly the fresh and the hardened properties of the new concrete made of RCA.

Materials

The cement used for this study is CEM I 52.5 N CE CP2 NF. The RS (1-4 mm) used is provided by the French national project Recybeton. The RCA is a heterogeneous material and its content depends highly on its source and on the quantity of adherent cement [2]. The WAC should be determined: 6.86 % for the RS and 2.3% for the natural sand (NS).

Mixing procedure

Mortar specimens are fabricated according to the standard method NF EN 196-1 [3]. The natural sand is totally replaced by the RS. In addition to using the RS in a dry state, two pre-saturation methods are proposed in this study: the plastic bottle method and total immersion. Both methods consist of a pre-saturation time of 24 hours. In the case of pre-saturation in a

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plastic bottle, the RS is put in a plastic bottle with an imposed quantity water that corresponds to a percentage of its dry mass. The bottle is closed to avoid the evaporation of the water and agitated from time to time to ensure equal distribution of the water into the sand.

Testing procedures

The technique proposed in this paper to determine the quantity of water absorbed is by evaporation. The free water and the absorbed water do not evaporate at the same rate [4] and [6]. The transition point between these two phases is the SSD state of the sample. The transition point can be easily determined by drawing the derivative of the water content with respect to time.

A new method is developed in this article in order to determine the reference WAC of the RS denoted as RWAC.The specimen is saturated under vacuum for 24 hours [7]. The specimen undergoes a drying process by evaporation to determine the SSD state [8] and then dried in the oven at 80°C until constant mass. The RWAC and the density then are calculated using the same equations as in the standard method EN 1097-6 [9].

4 x 4 x 16 cm specimens were casted in order to determine the compressive strength. Specimens undergo two types of curing conditions: Laboratory curing conditions (LCC) and water immersion cure (WIC). The LCC is done at 20°C and a RH = 50%. After 28 days, the compressive strength of mortars is investigated.

Results and Discussion

WAC, RWAC and density. The WAC determined by the vacuum-based method is approximately 42.8% higher than the one determined by the standard method (NF EN 1097-6). These results prove that some pores in the RS need vacuum pressure to be accessible and that the RS is not fully saturated by total immersion, which is consistent with [10] that found that the 24h soaking at atmospheric pressure is not enough to saturate all pores.

Having the total porosity of the RS, a simple calculation was done in order to determine the volume fraction of water in the pores of the RS water-filled under vacuum. The result obtained shows that the volume of water in the RS is 99.7% of the pores volume. This indicates that the RS can be considered as fully saturated when saturating under vacuum.

Effects of the pre-saturation on the empty pores. The vacuum based method was found to completely fill the pores of the RS. Determining the degree of saturation (Vwater / Vpores) can

then be done for each pre-saturated specimen. Results are shown in figure 1 along with the compressive strength results. The degree of saturation increases with the increase of the initially added water. We notice that with a 25% initial added water, the degree of saturation is almost the same: the quantity of water needed to pre-saturate the RS can be optimized by the plastic bottle process, thus decreasing the waste water

Table 1. Physical propertis of RS.

Method WAC(%) Density (g/cm3)

NF EN 1097-6 6.86 2.44

Our method 9.8 2.66

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Compressive strength at 28 days. The compressive strengths of mortars fabricated are shown in figure 1. A decrease of 22.2% and 19.7% is observed when the NS is totally replaced by RS in the saturated (noted by S) and dry (noted by D) states respectively. This loss was expected because of the porosity and the poor properties of the RS. Using RS in the dry state lead to a better compressive strength [11] and [12]. This statement is compliant with the results obtained in this paper. Mortars cured in WIC develop compressive strengths higher than those cured in LCC; WIC provides a better hydration for the mortars. It is noticed that the difference in compressive strength between LCC and WIC is more important for mortars made of RS.

Using the plastic bottle as a pre-saturation method does not enhance the compressive strength of mortars. All mortars made of pre-saturated RS in a plastic bottle present compressive strength lower than the M-RS-D and M-RS-S. We also notice an increase in the compressive strength between 6.86% and 9% and then the values decreases with the increase of the pre-saturated water.

Conclusion

In conclusion, the standard method EN 1097-6 underestimates the WAC of the RS and the vacuum method fully saturates the RS. Using the plastic bottle saturation method, an important difference is noticed between the quantity of water imposed and the water absorbed after pre-saturation. For that matter, using the plastic bottle reduces water wastage.

The compressive strength of mortars made of dry sand is higher than the one made of saturated sand. Using the plastic bottle technique, the highest compressive strength value is obtained with a 9% of pre-saturation water but not as high as the mortar made of dry recycled sand 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 M-NS-D M-NS-S M-RS-D M-RS-S 6,86% 9% 12% 20% 25% R c (M P a ) V w a te r/ V p o re s (% ) WIC LCC Vwater/Vpores

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