sensors Article

New System of Shrinkage Measurement through Cement Mortars Drying Carlos Morón 1, *, Pablo Saiz 2 , Daniel Ferrández 1 and Luisa García-Fuentevilla 1 1

2

*

Sensors and Actuators Group, Department of Building Technology, Madrid Technical University, 28040 Madrid, Spain; [email protected] (D.F.); [email protected] (L.G.-F.) Architectural Construction and Control Department, Madrid Technical University, 28040 Madrid, Spain; [email protected] Correspondence: [email protected]; Tel.: +34-91-336-7583; Fax: +34-91-336-7637

Academic Editor: Gonzalo Pajares Martinsanz Received: 18 January 2017; Accepted: 2 March 2017; Published: 6 March 2017

Abstract: Cement mortar is used as a conglomerate in the majority of construction work. There are multiple variants of cement according to the type of aggregate used in its fabrication. One of the major problems that occurs while working with this type of material is the excessive loss of moisture during cement hydration (setting and hardening), known as shrinkage, which provokes a great number of construction pathologies that are difficult to repair. In this way, the design of a new sensor able to measure the moisture loss of mortars at different age levels is useful to establish long-term predictions concerning mortar mass volume loss. The purpose of this research is the design and fabrication of a new capacitive sensor able to measure the moisture of mortars and to relate it with the shrinkage. Keywords: capacitive sensor; moisture; shrinkage; cement mortar; recycled aggregate

1. Introduction Massive exploitation of natural recourses in the Spanish construction sector increased in the years prior to the economic crisis with the exponential construction of new buildings, considerably affecting the environment. Under these circumstances, the current normative framework places emphasis on the incorporation of a new recycling politics, reducing in this way the use of natural raw material and allowing more sustainable development of the construction sector. Such is the case of the current Instruction on Structural Concrete EHE-08 that contains the specifications for recycled aggregates to be used in concretes in one of its annexes [1]. Construction and demolition waste (CDW) is produced during the construction and infrastructure life cycle, and in the rehabilitation and demolition processes [2]. CDW being correctly treated in recycling plants can be used as a material for the construction of new buildings, as is the case with ceramic and concrete recycled aggregates used as a raw material in mortars fabrication [3,4]. Nevertheless, the average recycling rate in Spain (15%) is much lower than the European average (50%), whereas some of the countries such as Denmark and Holland recycle more than 90% of their CDW [5]. In terms of the properties of recycled aggregates, there are many authors who studied their physical, chemical, and mechanical characteristics for their use in the construction, which are generally affected when the percentage of fine recycled aggregate replacement is more than 25% of aggregate content, producing mortars with lower density and higher water absorption [6]. In terms of the contribution of ceramic aggregates in mortar production, they did not significantly affect the mechanical behaviour of mortar when the percentage of ceramic recycled aggregate was not higher than 40% of the total volume of the natural aggregate; what is not true with other properties, Sensors 2017, 17, 522; doi:10.3390/s17030522

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such as density and workability [7]. Nevertheless, some authors replaced 100% of natural aggregate with ceramic recycled aggregate, showing that it is possible to reach excellent behaviour with an adequate fabrication process of these mortars [8,9]. On the contrary, other authors observed that the contribution of ceramic recycled aggregates improves bond strength between them and cement paste due to their form, which is less rounded compared to natural aggregates [10]. On the other hand, the use of concrete recycled aggregate in cement mortars fabrication has negative consequences in terms of durability, as in general they are less strong and have a major pore content [11]. Due to this fact, the great majority of current studies fix the maximum percentages of concrete recycled aggregates in mortar mix to obtain reliable mixes with properties similar to those observed in natural aggregates [12,13]. One of the properties that is seen to be more affected in mortars fabricated with recycled aggregates is shrinkage, because the water absorption of this type of aggregate is much higher than that of natural aggregates [14]. This phenomenon of drying shrinkage is associated with bond strength loss in mortars, which may cause easier mortar paste detachment [15], caused by the volume reduction of mortars through the evaporation of water during its hydration [16]. For this reason, this phenomenon depends on various factors, such as drying ambient temperature, relative humidity of air, size and form of the specimen, the relation water–cement, and the relation cement–aggregate [17]. Thus, higher shrinkage is related to greater variation in the content of the specimen that can be measured using specific experimental equipment. Nevertheless, existing methodology of shrinkage measure through drying that was developed for cement mortars presents serious problems in its application to mortars fabricated with recycled aggregates, due to different hardening mechanisms. Nonetheless, because shrinkage is related to moisture variation, electronic equipment can be used to measure this variation of water content, which is more effective than traditional gravimetric methods based on the progressive loss of weight until reaching anhydrous state [18]. To this end, some authors used strain sensors based on the conductivity of two terminals to determine water content, obtaining good results in the measure of the moisture level of soils and its application in automatic irrigation systems [19]. Ultrasonic moisture meters based on different propagation speeds of waves that go through certain types of material were also used. In general, the application of ultrasounds in mortar and concrete is delegated to initial inspections, being a less precise measure of moisture content [20]. In this field of material moisture measure, capacitive sensors are very well accepted due to their easy fabrication, application, and low cost compared to other typologies. Some authors showed that application range of these sensors can be very wide, around 175–625 ppm [21], enabling their application in different materials; e.g., measurement of the moisture level and degradation of wood [22] and moisture content measurement in gases using high linearity in the capacitive sensors’ response [23]. This ideal/linear response is not always possible to obtain; such is the case with some strain sensors used to measure deformations and study material microstructure [24], whose response should be studied and improved to be interpreted. Due to its high sensibility, another application of the capacitive sensors is the monitoring of the water content in industrial fabrication lines [25]. Thus, the aim of this research is the design and fabrication of a new measuring equipment based on a capacitive sensor that allows a response to be obtained that can be related to moisture content of the specimens of mortar at different age levels, in order to extrapolate the results of shrinkage that these specimens experiment according to their volume loss. In such a way, the results of this research can improve the precision given by the current method of shrinkage measure through drying in mortars based on the measure of specimen length at different age levels. 2. Methodology This section describes both the process of fabrication of cement mortar specimens and the design of the sensor, as well as methods of moisture and shrinkage measure that were used.

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2.1. Fabrication of Mortar Specimens The codification that was used to identify different mortar mixes is the following: M-N-D, where M is a masonry mortar; N is a type of used aggregate that can be natural aggregate (NA), concrete recycled aggregate (Con RA) or ceramic recycled aggregate (Cer RA); D is a cement-aggregate in weight proportion, where X is used for 1:3 proportion and Y is used for 1:4 proportion. The dosage in weight used for different specimens can be observed in Table 1, where used quantities and ratios are indicated. Table 1. Mortars dosages.

Type

Cement (g)

Aggregate (g)

Water (g)

Additive (%)

M-NA-X M-Con RA-X M-Cer RA-X M-NA-Y M-Con RA-Y M-Cer RA-Y

450 450 450 337.5 337.5 337.5

1350 1350 1350 1350 1350 1350

261 306 306 202.5 300.4 300.4

11 11 11 11

Proportion (c/a) 2

(w/c) 3

1:3 1:3 1:3 1:4 1:4 1:4

0.58 0.68 0.68 0.60 0.89 0.89

Note: 1 1% of additive over the weight of cement; 2 Cement–aggregate dry proportion; 3 Water–cement proportion. Cer RA: ceramic recycled aggregate; Con RA: concrete recycled aggregate; M: masonry mortar; NA: natural aggregate; X: 1:3 cement to aggregate weight ratio; Y: 1:4 cement to aggregate weight ratio.

The fabrication of all the mixes was carried out using the same technique and equipment according to the requirements of the standard UNE-EN 196-1 [26]. Cement mortar mixes were elaborated using a planetary mixer CIB-701 model of Ibertest brand with 3 litre capacity. Firstly, the aggregates, cement, and water were weighted separately using a scale of precision to 0.01 gram. Then, water and cement were poured into the bowl of the planetary mixer, while the aggregates were put into the hopper of the mixer. The time used for mortar mixes elaboration was fixed by the above-mentioned standard. In the mixes fabricated with recycled aggregates, it was necessary to use the plasticizer Glenium Sky 604, which is recommended for use in mortars by the BASF Company. This additive was incorporated in 1% over the weight of cement, complying with the recommendations of the manufacturer in order to obtain proper consistency. In this way, all the mixes elaborated in this work are of plastic consistency according to the values of 175 ± 10 mm established by the standard UNE-EN 1015-3 [27]. The dimension of the specimens was 25 × 25 × 287 mm. 2.2. Capacitive Sensor Design A capacitive sensor was developed to measure the moisture of mortar specimens. This sensor is connected to the auto-oscillatory circuit able to measure light variations in water loss of mortar specimens. The scheme of the used sensor can be observed in Figure 1. As can be seen in Figure 1, the sensor consists of two flat parallel sheets of cooper, whose surface dimensions are 25 × 247 mm adapting to the surface of samples without including their edges. The functioning principle of the sensor is based on the measure of capacitance of the condenser as the changes of moisture produced by water loss modify the permittivity of the mortar that acts as a dielectric according to Equation (1): A C = e· [ F ] (1) d where C is the capacitance of the condenser, e is the permittivity of the dielectric (in this case of wet sample of mortar), A is the surface of condenser sheets, and d is the distance between the sheets (in this case, it coincides with sample thickness).

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where C is the capacitance of the condenser, is the permittivity of the dielectric (in this case of wet 4 of 9 sample of mortar), is the surface of condenser sheets, and is the distance between the sheets (in this case, it coincides with sample thickness).

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Figure 1. Capacitive sensor scheme.

In such such aa way, way,ififthe thesurface surfaceand andseparation separationbetween betweensheets sheetsare are maintained fixed, variation In maintained fixed, thethe variation in in the capacitance of the sensor will depend only on the variation of the permittivity of a dielectric or the capacitance of the sensor will depend only on the variation of the permittivity of a dielectric or on on the moisture content of the sample. the moisture content of the sample. Moreover, the resonance frequency of the auto-oscillatory auto-oscillatory circuit directly related to the the Moreover, the resonance frequency of the circuit is is directly related to capacitance of of the the sensor sensor and and the the inductance inductance according according to to Equation Equation (2): (2): capacitance 1 = 1 [ ] (2) √ f = √ (2) [ Hz] LC where f is the resonance frequency, C is the capacitance determined by Equation (1), and L is the where f isof the frequency, C is the capacitance determined by Equation (1), and L is the induction theresonance reel. induction of the reel. In this way, it is possible to relate moisture loss of mortar samples after being mixed and their In thistoway, it is possiblevalue to relate moisture loss of mortar samples after being mixed and their shrinkage the capacitance of the used sensor. shrinkage to the capacitance value of the used sensor. 2.3. Method of Moisture and Shrinkage Measure 2.3. Method of Moisture and Shrinkage Measure After being prepared, the mortar samples were left in the laboratory under constant conditions After beingofprepared, mortar samples were left in the laboratory under of of temperature 20 ± 1 °Cthe and moisture of 60% ± 2%. Once setting began, theconstant process conditions of hydraulic ◦ C and moisture of 60% ± 2%. Once setting began, the process of hydraulic temperature of 20 ± 1 shrinkage was observed due to the evaporation of excess water in the mix that stays inside the shrinkage was observed due toEquation the evaporation of excess water in the mix that stays inside the sample sample without combination. (3) models this type of shrinkage: without combination. Equation (3) models this type of shrinkage:  (1 − ) ,  1 =2 ∙ 0.7 ∙ 1 − (3) , 1 (1 − η )   R H = 2R M · ·1 − 1 + 0.79  0.2 ∙ 1.024  (3) n s )2t D s 2 ( v 1 + 0.79 v t ·1.024 where is hydraulic shrinkage, is maximum shrinkage in about one year, is relative where R H in is hydraulic shrinkage,isRmaximum shrinkage about oneinyear, humidity humidity the atmosphere, particle size ofinaggregate mm,η is relative is a variable that M is maximum in the atmosphere, D is maximum particle size of aggregate in mm, n is a variable that depends on the depends on the holes left by aggregate, / is the free surface divided by the volume of mortar, and holes leftinbydays. aggregate, s/v is the free surface divided by the volume of mortar, and t is time in days. t is time The traditional measuring shrinkage in mortar samples is established by the standard traditionalmethod methodof of measuring shrinkage in mortar samples is established by the UNE 80-112-89 [28], and consists of the measurement of the length of samples using a lengtha standard UNE 80-112-89 [28], and consists of the measurement of the length of samples using comparator once samples are unmolded. Thus, shrinkage is expressed as a percentage ∆Lnd (%) length comparator once samples are unmolded. Thus, shrinkage is expressed as a percentage with respect to respect the initial length given by Equation ∆ (%) with to the initial length given by (4): Equation (4):

− = Mnd − M1d∙ ·100 ∆ ∆L 100 [%] [%] nd = L0

(4) (4)

where is the base length of mould, is a sample measure with the comparator in n days, and where L0 is the base length of mould, Mnd is a sample measure with the comparator in n days, and is a measure of sample with the comparator the first day. M1d is a measure of sample with the comparator the first day.

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Thus, shrinkage shrinkageisisdetermined determinedbyby volume of mortar setting Onother the hand, other Thus, thethe volume lossloss of mortar onceonce setting starts.starts. On the hand, the moisture content can be determined through the process of weighting according to the moisture content can be determined through the process of weighting according to Equation (5): Equation (5): W −W M= = nd − 0∙·100 100 [[%] %] (5) (5) W0 where M isis moisture n, and and W0 is the moisture expressed expressed in percentage, Wnd isisthe the weight weight of of sample on day n, weight of of sample sample in in anhydrous anhydrous state. state. Finally, measured andand weighted, the samples are kept the laboratory under constant Finally,after afterbeing being measured weighted, the samples areinkept in the laboratory under temperature and humidity to avoid any possible on the measures of the sensor, obtaining constant temperature and humidity to avoid anyinfluence possible influence on the measures of the sensor, values of capacitance for each sample in order to in establish a correlation thebetween methodsthe of obtaining values of capacitance for each sample order to establish a between correlation moisture measurement and shrinkage measured during the period of time of 128 days. methods of moisture measurement and shrinkage measured during the period of time of 128 days. Figure manual shrinkage measuring machine based on volume loss described before Figure 22shows showsthe the manual shrinkage measuring machine based on volume loss described and the system based on the measure of capacitance of the sensor. before and the system based on the measure of capacitance of the sensor.

(a)

(b)

Figure 2. 2. Measuring Measuring equipment. equipment. (a) Manual device device of of shrinkage shrinkage measure; measure; (b) (b) Capacitive Capacitive sensor sensor and and Figure (a) Manual auto-oscillatory circuit for moisture loss measure. auto-oscillatory circuit for moisture loss measure.

3. Results and Discussion 3. Results and Discussion This part shows the results obtained from the measurement of three variables: moisture, This part shows the results obtained from the measurement of three variables: moisture, capacitance, and shrinkage, establishing a correlation between them for different dosages. capacitance, and shrinkage, establishing a correlation between them for different dosages. 3.1. Variation of Capacitance Capacitance with with Regard Regard to to the the Moisture Moisture 3.1. Variation of Firstly, the theresults resultsobtained obtained using sensor formeasurement the measurement of moisture insamples mortar Firstly, using the the sensor for the of moisture in mortar samples are shown. are shown. In Figures and4,4,aagood goodcorrelation correlationbetween between response of the capacitive sensor In Figures 33and thethe response in in pFpF of the capacitive sensor andand the the percentage of moisture in mortar samples can be observed. percentage of moisture in mortar samples can be observed. Figures 33 and and 44 show show that that the the variation variation of of the the capacity capacity of to moisture moisture Figures of the the condenser condenser with with regard regard to loss in mortar samples follows a parabolic behaviour according to the equations presented in the the loss in mortar samples follows a parabolic behaviour according to the equations presented in graphics, obtained by the process of adjusting through the method of least squares. The last measure graphics, obtained by the process of adjusting through the method of least squares. The last measure that corresponds corresponds to to 0% 0% moisture moisture was was carried carried out out by by taking taking the the samples samples to to anhydrous anhydrous state state through through that drying in a laboratory oven. On the other hand, it can be observed that during the first days the drying in a laboratory oven. On the other hand, it can be observed that during the first days the moisture loss is much more important, a lower density of points existing between the first and the last measures.

the percentage of moisture in mortar samples can be observed. Figures 3 and 4 show that the variation of the capacity of the condenser with regard to moisture loss in mortar samples follows a parabolic behaviour according to the equations presented in the graphics, obtained by the process of adjusting through the method of least squares. The last measure Sensors 2017, 17, 522 6 of 9 that corresponds to 0% moisture was carried out by taking the samples to anhydrous state through drying in a laboratory oven. On the other hand, it can be observed that during the first days the moisture is much more important, a lower density of points existing between the first and of the Sensors 2017,loss 17, 522 9 moisture loss is much more important, a lower density of points existing between the first and 6the last measures. last measures. Moreover, aa great great difference difference in in the the response response of of the the sensor sensor between between the the first first and and the the second second Moreover, Moreover, a great difference in the response of the sensor between the first and the second measure measure can be observed. This is caused by the strong exothermal reaction produced in the mortars measure can be observed. This is caused by the strong exothermal reaction produced in the mortars can be observed. This is caused by the strong exothermal reaction produced in the mortars after setting, after setting, setting, where where aa great great part part of of mix mix water water combines combines with with cement cement that that conforms conforms mortar, mortar, releasing releasing after where a great part of mix water combines with cement that conforms mortar, releasing a great quantity great quantity quantity of of energy energy that that facilitates facilitates the the evaporation evaporation of of water water that that stays stays inside inside the the sample sample aa great of energy that facilitates the evaporation of water that stays inside the sample without combination. without combination. In contrast, the variation of moisture from this moment onwards is less less without combination. In contrast, the variation of moisture from this moment onwards is In contrast, the variation of moisture from this moment onwards is less pronounced, in accordance pronounced, in in accordance accordance with with the the theoretical theoretical predictions. predictions. pronounced, with the theoretical predictions.

Mortar Dosage Dosage 1:3 1:3 Mortar 154 154

0.2878x ++ 134.66 134.66 0.0627x22 ++ 0.2878x yy == 0.0627x R² = 0.939 R² = 0.939 1.1166x ++ 133.48 133.48 0.0413x2 ++ 1.1166x yy==0.0413x2 R²==0.9701 0.9701 R²

152 152

Capacitance Capacitance (pF) (pF)

150 150 148 148 146 146

0.0324x22 ++ 2.1048x 2.1048x ++ 133.67 133.67 yy == 0.0324x R² == 0.9119 0.9119 R²

144 144 142 142

NA 1:3 1:3 NA

140 140 138 138

RA Concrete Concrete 1:3 1:3 RA

136 136

RA Ceramic Ceramic 1:3 1:3 RA

134 134 132 132 00

11

22

33

44

55

66

77 88 99 Moisture% Moisture%

10 10

11 11

12 12

13 13

14 14

15 15

Figure 3. Relation Moisture–Capacitance for mortars with 1:3 dosage. Figure 3. 3. Relation Relation Moisture–Capacitance Moisture–Capacitance for for mortars mortars with with 1:3 1:3 dosage. dosage. Figure

Mortar Dosage 1:4 152 y = 0.0464x2 + 1.3637x + 132.78 R² = 0.9871

150

Capacitance (pF)

148 146

y = 0.0685x2 + 2.924x + 133.08 R² = 0.924

144

y = 0.043x2 + 0.6867x + 134.19 R² = 0.9519

142 140

NA 1:4

138 136

RA Concrete 1:4

134 132

RA Ceramic 1:4

130 0

1

2

3

4

5

6

7 8 Moisture%

9

10

11

12

Figure 4. Relation Moisture–Capacitance for mortars with 1:4 dosage. Figure 4. Relation Moisture–Capacitance for mortars with 1:4 dosage.

13

14

15

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3.2. Variation of Shrinkage with Regard to the Moisture 3.2. Variation of Shrinkage with Regard to the Moisture Furthermore, a relation between the moisture loss of samples and manually measured Furthermore, a relation between the moisture loss of samples and manually measured shrinkage shrinkage was established. The obtained results are shown in Figures 5 and 6. was established. The obtained results are shown in Figures 5 and 6. As can be observed in Figures 5 and 6, the moisture loss produces a shortening in the initial As can be observed in Figures 5 and 6, the moisture loss produces a shortening in the initial length of the sample as age increases, following the exponential behaviour observed in the graphics. length of the sample as age increases, following the exponential behaviour observed in the graphics. This shrinkage related to mortar drying is more pronounced in recycled aggregates compared to This shrinkage related to mortar drying is more pronounced in recycled aggregates compared to natural aggregates, in accordance with the values of capacity measured with the sensor in Figures 3 natural aggregates, in accordance with the values of capacity measured with the sensor in Figures 3 and 4. In such a way it is possible to establish a relation between mortar shrinkage values that were and 4. In such a way it is possible to establish a relation between mortar shrinkage values that were expected with the measures obtained using the capacitive sensor, as both measures depend on expected with the measures obtained using the capacitive sensor, as both measures depend on samples’ samples’ moisture loss. moisture loss.

Mortar Dosage 1:3

1.4

Shrinkage (mm/m)

1.2 1 0.8 0.6 0.4

y = 21.042e - 6.598x R² = 0.9585

NA 1:3

y = 6.761e -1.407x R² = 0.8887

RA Concrete 1:3

y = 5.2447e - 2.068x R² = 0.9874

RA Ceramic

0.2 0 0

1

2

3

4

5

6

7 8 9 Moisture%

10

11

12

13

14

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Figure 5. Relation Moisture–Shrinkage mortars with dosage. Figure 5. Relation Moisture–Shrinkage for for mortars with 1:3 1:3 dosage.

Shrinkage (mm/m) Shrinkage (mm/m)

1.2 1.2 1

0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0

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MortarDosage Dosage1:4 1:4 Mortar

1.4 1.4

1

15

y = 10.415e - 6.639x y = 10.415e - 6.639x R² = 0,9206 R² = 0.9206

NA 1:4 NA 1:4

y = 4.6196e - 1.523x y = 4.6196e - 1.523x R² = 0.9133 R² = 0.9133

RA Concrete 1:4 RA Concrete 1:4

y = 5.0682e - 2.384x y = 5.0682e - 2.384x R² = 0.9817 R² = 0.9817

RA Ceramic RA Ceramic

0 0

0

1

1

2

2

3

3

4

4

5

5

6

6

7 8 9 10 11 12 13 14 15 7 8 9 10 11 12 13 14 15 Moisture% Moisture%

Figure 6. Relation Moisture–Shrinkage for mortars with 1:4 dosage. Figure Figure6.6.Relation RelationMoisture–Shrinkage Moisture–Shrinkagefor formortars mortarswith with1:4 1:4dosage. dosage.

4. Conclusions A system that allows the moisture content of samples to be obtained was developed. For the case of moisture loss in cement mortars, the capacitance follows a parabolic behaviour regarding moisture content, these values being more pronounced when the quantity of water inside the sample increases. Moreover, an exponential relation between the moisture content and drying shrinkage was

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4. Conclusions A system that allows the moisture content of samples to be obtained was developed. For the case of moisture loss in cement mortars, the capacitance follows a parabolic behaviour regarding moisture content, these values being more pronounced when the quantity of water inside the sample increases. Moreover, an exponential relation between the moisture content and drying shrinkage was observed. A direct relation between the response in capacity of the sensor and a sample volume loss was obtained comparing both measures, thus validating this method as an alternative measurement method for drying shrinkage in cement mortars. Acknowledgments: This research would not have been possible without the collaboration of technical architect Alejandro Payán, who made the measures to move forward with his effort and perseverance and without the support of Technical University of Madrid. Author Contributions: C.M., P.S., D.F. and L.G.-F. conceived, designed and performed the experiments. Additionally, C.M., P.S., D.F. and L.G.-F. analyzed the data and participated in writing the paper. Conflicts of Interest: The authors declare no conflicts of interest.

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New System of Shrinkage Measurement through Cement Mortars Drying.

Cement mortar is used as a conglomerate in the majority of construction work. There are multiple variants of cement according to the type of aggregate...
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