Maturity testing of 3D printing concrete with inert microfiller

The 3D Printing of cement composites is one of the fastest developing technologies of modern concrete. The 3D printing involves concretes with high amounts of microfillers. The study analyses the influence of curing conditions on the development of strength of concretes applicable in 3D printing. Ten mixes were tested in the study. In the studied cases cement constituted to 50% of the mass, while microfillers such as lime stone powder, kaolin, quartz powder and sand (up to 2 mm) constituted to the rest of the mass. Samples were cured for 7 days in exothermic conditions at 5°C, 20°C, 35°C. Standard mortar samples of 4x4x16 cm and cylinders with 46.5 mm and height of 35 mm that simulate the printed 3D path were made. The compressive strength was tested after 12h, 24h, 48h, 72h, 168 h and 28 days. Based on the acquired results the temperature development function was formulated and activation energy was determined. The results showed that the proposed method is useful in evaluation of printed concrete curing. It can be also used to determine the time of loading the wall which can speed up the process of constructing while maintaining degree of safety.


Introduction
A strong focus placed by various research facilities on 3D concrete printing has resulted in a development of many new research methods of this novel constructing system [1][2][3][4].However, there are still no established Standards defining requirements for this technology.The elements created by additive manufacturing show small cross-section, which causes them to adopt the external temperature rapidly.As a results the elements can be highly susceptible to changes in strength development caused by low or high temperatures.
The influence of temperature on the mechanical properties of ordinary concrete is well researched [5,6].The correlation between temperature of curing and mechanical properties is used daily in civil engineering (maturity method).This correlation allows to predict the strength of concrete, which results in shorter striking time or faster loading.Premature striking of the formwork was a cause of numerous structural failures and collapses [5].The 3D printing technology is issued with a similar problem of strength prediction in structures.Applying well known and widely used maturity method allows to determine the loading time (i.e. with upper floors) even in structures constructed by additive manufacturing.Currently only one research facility tries to address the issue [7].
The research was conducted on concretes with inert microfillers applicable in 3D printing due to their rheological properties and composition.For proposed mixes the apparent activation energy, coefficients of strength improvement for different temperatures, cross-over effect and equivalent time were determined.The study was conducted on standard samples and model samples similar to printed ones [4,8].

Maturity method
The maturity method is directed to determine mechanical properties of concrete based on the measurements of temperature during curing [5,9].The maturity method was firstly used in 1949 by Nurse [10] and MacIntosha [11].The studies were continued by Saul [12] in 1951.The determined function was based on the so-called maturity index (1).Initially used function did not reflect correctly the reality.As soon as in 1977 a research team have proposed an Arhennius equation for better projection of concrete curing.To concur the research a so called equivalent time was proposed (2).Many researchers (inter alia Carino [13], Kaszyńska [14]) proved that the equation (2) better reflects the maturity of concrete in structures than the Saul's formula (1).
The maturity method evolved with following years and have derived into different functions and methods for determining the mechanical properties of concrete [13,[15][16][17][18]. Today the most popular version is included in the American Standard ASTM C1074 [19], which was proposed by Carino [13].The method has a wide practical application [20].The aforementioned equation from ASTM C1074 is seen as (3). where: where

3D printingsize specimen problem
There is only a single study that tried to evaluate the applicability of maturity method for optimization of 3D printing [7].However, it is an overview article, that does not present any computations of coefficients found in (1)- (3).
Currently there are no Standards for applicability of the mixes for 3D printing.Many research facilities [4,[21][22][23][24] proposed own methods that require a printing device.The methods seem well-thought and clearly evaluate the applicability of the mix.However, they are costly and time consuming.
Several research facilities have proposed a framework for estimation of mix feasibility in 3D printing.A research team [8] proposed a test on a 60x35 mm samples in a specially designed test bench.The samples are loaded with several weights that simulate additional printed layers.A research team [4] proposed a cylinder stability test, which is conducted on a 60x40 mm samples with a 5.5 kg applied load.The second method also determines the deformation of the sample.The size of the deformation determines the readiness of the mix for printing.A research team [2] determines the usability of the mix based on a slump flow [21].Other teams [8,25] determine the usability with standard rheological tests conducted in a set period of time.
3 Materials and experimental procedure

Mix compositions
The mixes were made using 50% of CEM I 52.5 R (Rapid Portland Cement, according to European Standard) and 50% consisting of various microfillers including quartz powder (QP), limestone powder (LP), kaolin (K) and fine aggregate (FA).SP was added in percentage of cement mass to reach required consistency according to [21,26].Those 4 fillers were added in various combinations.Designed mixes are presented in Table 1.The water/cement ratio was set to 0.42.The mix consistency was regulated with a superplasticizer.
It should be noticed that these type of mixes are used in French research facilities [8,27] to 3D printing.The "LP/K" mix and its properties was presented in other studies [8] as a applicable 3D printing mix.The authors have confirmed the properties of the above mentioned mixes on their own.

Experimental procedure
After mixing, the concrete samples were placed in a climatic chamber and stored for 7 days in 3 different temperatures of 5°C, 20°C, 35°C.After the first stage the samples were stored in a laboratory conditions at 20°C ± 2°.Two types of samples were prepared including 40x40x160 mm standard samples and cylindrical samples with diameter of 46.5 mm and height of 35 mm that simulated a shape of printed layers.The analysis determined the cube/cylinder compressive strength ratio (fc.cube/fc,cylinder).Values of compressive strength were obtained at 28 day-age after curing at 20°C.Mean value of fc.cube/fc,cylinder is 1.74 with coefficient of variation of 3.93 %.Differences between cube and cylinder specimen are noticeable.Cylindrical specimens exhibit lower compressive strength.
The study focuses on 4 mixes (LP/K, QP/FA, QP/LP and LP/FA).Figure 2 and 3 present the strength development curves of selected mixes.

Apparent activation energy
Apparent activation energy (Ea) was established in accordance with A1.1.8.2 of ASTM C1074-11.Table 2 contains the values of k (rate constant) coefficient, T0 (datum temperature), Ea and R 2 (determination coefficient).The results show that Ea ranges from 34 to 44kJ/mol.The values of Ea between 34-44 kJ/mol are corresponding for this type of cement binder [28].

Equivalent time
For selected mixes an equivalent time was calculated based on the results in Table 2 and equation (4) (transformed formula (3)).Figure 4 shows equivalent time which is needed to reach 10%, 20%, 40%, 80% of ultimate compressive strength of concrete cured at 20°C (S20°/Su,20°).Figure 4 shows the results for selected mixes cured at different temperatures.The difference in equivalent time is important in the early stages of concrete curing when the concrete has not yet developed full strength.In such a case, prematurely loaded structure might collapse [5].During the first stage of curing up to S/Su=0.4 the development rate of strength is slower in low temperature (5°C) causing significant differences in te.For example for mix K/FA difference at S/Su=0.2 was 6.3h.This time seems short but in additive manufacturing even 6h is significant.In later stages (after S/Su=0.4) the differences are lowered, but are not so important considering relatively high concrete strength.Another issue occurring during curing is a cross-over effect, described below.
Studied mixes have exhibited a visible cross-over effect (Fig. 1, Fig. 2).Strength development rate in different temperatures changes during curing which was already studied elsewhere [29,30].Concretes with high amounts of cement or with rapid cements are strongly susceptible to the effect.Additionally, the higher temperature the more significant the differences in a final strength of concrete.In studied mixes the cross-over effect was visible between 30-90 hours.For mixes with fine aggregate (FA) and with kaolin (K) with different w/c ratio the effect has not occurred at all.

Conclusion
The conducted study has verified the composition of mixes with mineral additives suitable fo 3D printing.Highest compressive strength was obtained by QP, QP/FA, QP/LP and LP/FA concretes with addition of quartz and limestone powder, while the lowest by K concrete with kaolin.
The study focuses on the influence of temperature on the development of compressive strength.Two temperatures simulating conditions in spring-autumn (T=5°C) and summer (T=35°C) were taken into consideration.The influence of temperature was analyzed using maturity method.The study allowed to determine the values of activation energy for the mixes with mineral fillers and rapid Portland cement CEM I 52.5 R.This allowed to calculated the equivalent curing time and evaluation of compressive strength for different curing temperatures.Study has shown that in the case of 8 concretes the so-called crossover effect occurred.The phenomenon occurs as decline in compressive strength development rate after 3 days in concretes cured in higher temperatures.
The study has shown that compressive strength of cylindrical samples that simulate printed layers is significantly lower than compressive strength of standard cubical samples 10x10x10 cm.

Fig. 2 .
Fig. 2. Compressive strength development of LP/K and LP/FA cured in different temperatures.

Fig. 3 .
Fig. 3. Compressive strength development of QP/LP and K/FA cured in different temperatures.

Table 2 .
Parameters of maturity function.