Effect of Maintenance and Water–Cement Ratio on Foamed Concrete Shrinkage Cracking

Abstract

This is a study on how to reduce shrinkage and improve crack resistance of foamed concrete. By selecting different curing temperatures and humidity, six different curing conditions were analyzed. The shrinkage deformation and maximum crack width of foamed concrete blocks with water–cement ratios of 0.4 and 0.5, under six curing conditions, were measured by a comparator and optical microscope, and the cracking time was recorded. The effects of curing temperature, humidity and water–cement ratio on the shrinkage and crack resistance of the foamed concrete were analyzed by comparing the experimental results of each group. We studied the primary and secondary order of the three factors affecting the drying shrinkage of foamed concrete. The results show that: temperature is the primary factor that changes the drying shrinkage performance of foamed concrete, followed by the water-cement ratio, and finally humidity. The interaction of these three factors is not obvious. The shrinkage of foamed concrete increases with the increase in temperature; increasing the humidity of curing can control the water loss rate of foamed concrete and reduce shrinkage. Lower humidity and higher temperature will make cracks appear earlier; with an increase in the water–cement ratio, the initial cracking time is shortened and the cracking property of foamed concrete is improved

Full text

Citation: Li, C.; Li, X.; Li, S.; Guan,
D.; Xiao, C.; Xu, Y.; Soloveva, V.Y.;
Dalerjon, H.; Qin, P.; Liu, X. Effect of
Maintenance and Water–Cement
Ratio on Foamed Concrete Shrinkage
Cracking. Polymers 2022,14, 2703.
https://doi.org/10.3390/
polym14132703
Academic Editor: Xiaodong Ji
Received: 23 May 2022
Accepted: 28 June 2022
Published: 1 July 2022
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4.0/).
polymers
Article
Effect of Maintenance and Water–Cement Ratio on Foamed
Concrete Shrinkage Cracking
Chunbao Li 1, * , Xiaotian Li 1, Shen Li 2, Di Guan 2, Chang Xiao 1, Yanyan Xu 3, Valentina Y. Soloveva 4,
Hojiboev Dalerjon 5, Pengju Qin 6and Xiaohui Liu 7
1College of Pipeline and Civil Engineering, China University of Petroleum (East China),
Qingdao 266580, China; z21060136@s.upc.edu.cn (X.L.); z21060135@s.upc.edu.cn (C.X.)
2Construction Project Management Branch of China National Petroleum Pipeline Network Group Co., Ltd.,
Langfang 065001, China; lishen@pipechina.com.cn (S.L.); guandi@pipechina.com.cn (D.G.)
3Henan Huatai New Material Technology Corp., Ltd., Nanyang 473000, China; jennifer@gmail.com
4Emperor Alexander I ST Petersburg State Transport University, St. Petersburg 190031, Russia;
soloviova-pgups@mail.ru
5Mining-Metallurgical Institute of Tajikistan, Buston City 735730, Tajikistan; gmit_tajikistan@mail.ru
6College of Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China;
qinpengju@tyut.edu.cn
7Qingdao Urban Development Group Co., Ltd., Qingdao 266061, China; liuxiaohui2014upc@163.com
*Correspondence: 20070048@upc.edu.cn; Tel.: +86-532-8698-1820
Abstract:
This is a study on how to reduce shrinkage and improve crack resistance of foamed
concrete. By selecting different curing temperatures and humidity, six different curing conditions
were analyzed. The shrinkage deformation and maximum crack width of foamed concrete blocks
with water–cement ratios of 0.4 and 0.5, under six curing conditions, were measured by a comparator
and optical microscope, and the cracking time was recorded. The effects of curing temperature,
humidity and water–cement ratio on the shrinkage and crack resistance of the foamed concrete
were analyzed by comparing the experimental results of each group. We studied the primary and
secondary order of the three factors affecting the drying shrinkage of foamed concrete. The results
show that: temperature is the primary factor that changes the drying shrinkage performance of
foamed concrete, followed by the water–cement ratio, and finally humidity. The interaction of
these three factors is not obvious. The shrinkage of foamed concrete increases with the increase in
temperature; increasing the humidity of curing can control the water loss rate of foamed concrete
and reduce shrinkage. Lower humidity and higher temperature will make cracks appear earlier; with
an increase in the water–cement ratio, the initial cracking time is shortened and the cracking property
of foamed concrete is improved.
Keywords: foamed concrete; plastic shrinkage and cracking; concrete curing; water–cement ratio
1. Introduction
Traditional foamed concrete is defined as a lightweight, porous concrete material,
which is made by adding foam into slurry consisting of siliceous and calcareous com-
ponents, water and admixture, followed by proper curing in certain conditions. The
volume density of foamed concrete is 100–1600 kg/m
3
, the thermal conductivity is usually
0.05~0.46 W/(m·K)
, and the compressive strength is 0.2–30 MPa. The industry standard
“Technical Specification for Application of Foamed Concrete” (JGJ/T 341-2014) [
1
] defines foamed
concrete as a concrete where cement is the main cementitious material and bubbles are
added to the slurry made of aggregate, admixture and water. The lightweight porous
concrete with a closed cell structure is formed after mixing, stirring, pouring, molding,
and maintenance.
In recent years, China has continuously implemented reform of thermal insulation
materials and building energy conservation policies, putting forward new requirements for
Polymers 2022,14, 2703. https://doi.org/10.3390/polym14132703 https://www.mdpi.com/journal/polymers

Polymers 2022,14, 2703 2 of 14
thermal insulation materials and energy-saving materials, such as light weight, thermal
insulation and high strength [
2
]. Foamed concrete is an ideal new material for heat preserva-
tion and energy conservation. It has comprehensive advantages over traditional materials,
such as being lightweight, having sound insulation, fire resistance, being waterproof, hav-
ing heat preservation and environmental protection [
3
]. Foamed concrete is widely used in
tunnel, highway and construction projects [
4
]. However, in practical applications, due to
the influence of the curing system, water–cement ratio and raw material properties [
5
–
8
],
foamed concrete is exposed to a series of problems, such as low strength and large amounts
of dry shrinkage. How to reduce the shrinkage of foamed concrete and improve its crack
resistance has become an important subject in foamed concrete engineering technology.
The China–Russia Eastern trunk line starts from the China–Russia border in Heihe
City, Heilongjiang Province, runs through Heilongjiang, Jilin, Inner Mongolia, Liaoning and
Heilongjiang, Beijing, Tianjin, Shandong, Jiangsu, Shanghai, and ends at Baihe End Station
in Shanghai, with a total length of 3334.6 km [
9
]. The design output is
380 ×108 m3/a,
the
design pressure is 12 MPa/10 MPa, and the pipe diameter is 1422 mm/1219 mm/101.6 mm.
According to the overall direction of the route, the China–Russia East Route pipeline
crosses the Yangtze River in Nantong Economic and Technological Development Zone and
Changshu Economic and Technological Development Zone in Jiangsu Province, via the
shield tunnel, which is the control project of the China–Russia East Route. The Yangtze
River shield tunnel is huge, with a horizontal length of 10,324 m. The horizontal length of
the shield tunnel crossing is 10226 m (north bank shaft center to south bank shaft center),
as shown in Figure 1. According to the preliminary design scheme of the project, there are
plans to fill the tunnel with foamed concrete after the installation of pipes, as shown in
Figure 2.
Polymers 2022, 14, x FOR PEER REVIEW 2 of 14
In recent years, China has continuously implemented reform of thermal insulation
materials and building energy conservation policies, putting forward new requirements
for thermal insulation materials and energy-saving materials, such as light weight, ther-
mal insulation and high strength [2]. Foamed concrete is an ideal new material for heat
preservation and energy conservation. It has comprehensive advantages over traditional
materials, such as being lightweight, having sound insulation, fire resistance, being wa-
terproof, having heat preservation and environmental protection [3]. Foamed concrete is
widely used in tunnel, highway and construction projects [4]. However, in practical ap-
plications, due to the influence of the curing system, water–cement ratio and raw material
properties [5–8], foamed concrete is exposed to a series of problems, such as low strength
and large amounts of dry shrinkage. How to reduce the shrinkage of foamed concrete and
improve its crack resistance has become an important subject in foamed concrete engi-
neering technology.
The China–Russia Eastern trunk line starts from the China–Russia border in Heihe
City, Heilongjiang Province, runs through Heilongjiang, Jilin, Inner Mongolia, Liaoning
and Heilongjiang, Beijing, Tianjin, Shandong, Jiangsu, Shanghai, and ends at Baihe End
Station in Shanghai, with a total length of 3334.6 km [9]. The design output is 380 × 108
m3/a, the design pressure is 12 MPa/10 MPa, and the pipe diameter is 1422 mm/1219
mm/101.6 mm. According to the overall direction of the route, the China–Russia East
Route pipeline crosses the Yangtze River in Nantong Economic and Technological Devel-
opment Zone and Changshu Economic and Technological Development Zone in Jiangsu
Province, via the shield tunnel, which is the control project of the China–Russia East
Route. The Yangtze River shield tunnel is huge, with a horizontal length of 10,324 m. The
horizontal length of the shield tunnel crossing is 10226 m (north bank shaft center to south
bank shaft center), as shown in Figure 1. According to the preliminary design scheme of
the project, there are plans to fill the tunnel with foamed concrete after the installation of
pipes, as shown in Figure 2.
Figure 1. The route of the East China–Russia pipeline.
Figure 1. The route of the East China–Russia pipeline.

Polymers 2022,14, 2703 3 of 14
Polymers 2022, 14, x FOR PEER REVIEW 3 of 14
(a) (b)
Figure 2. Schematic diagram of foamed concrete tunnel filling: (a) sectional structure diagram; (b)
diagram of the tunnel interior.
Domestic and foreign scholars have conducted much research on foamed concrete.
Deng J. et al. studied the influence of different lengths and different dosages of polyvinyl
alcohol fibers on the water absorption, strength and shrinkage of foamed concrete [10].
Some scholars have studied the influence of mix ratio on concrete [11,12]. Rong H. et al.
used yeast as a raw material to prepare microbial foaming agents and studied the influ-
ence of foaming technology on the performance and microstructure of foamed concrete
[13]. Mohamad Noridah et al. studied the effect of fiber on the properties of foamed con-
crete [14,15]. Othman Rokiah. et al. studied the relationship between foamed concrete den-
sity and compressive strength [16]. Abdullah Faisal Alshalif et al. studied the effect of
density and curing conditions on CO2 isolation in foamed concrete brick, and the effect of
bacterial self-healing and CO2 bio-sealing on the strength of bio foamed concrete brick
[17,18]. Yang S. et al. studied the preparation and properties of low-density foamed con-
crete [19]. Hou Li et al. studied the influence of foaming agent on foamed concrete [20].
Chen Y. et al. studied the application of foamed concrete in backfilling [21].
Combined with research from scholars at home and abroad, it has been found that,
in engineering applications, foamed concrete has many advantages when compared with
traditional concrete; however, there is a lack of systematic research on the factors affecting
the shrinkage and cracking of foamed concrete. In this paper, typical working conditions
were simulated within different scenarios, and the shrinkage and maximum crack value
of foamed concrete in these conditions were studied. This paper aims to systematically
summarize the influence of curing system and water–cement ratio on the cracking and
shrinkage of foamed concrete, and to provide theoretical guidance for the filling and con-
struction of foamed concrete.
At present, the problem of controlling the shrinkage cracking of foamed concrete is
mostly seen in the research of additives, material ratio, fibers, etc. Based on this, this study
formulates six curing systems, selects two water–cement ratios, and obtains the shrinkage
and cracking performance of foamed concrete under different curing conditions and wa-
ter–cement ratios, in order to systematically summarize the effects of temperature, hu-
midity and water–cement ratio on the shrinkage and cracking of foamed concrete, and
provide theoretical guidance for the filling and construction of foamed concrete.
Figure 2.
Schematic diagram of foamed concrete tunnel filling: (
a
) sectional structure diagram;
(b) diagram of the tunnel interior.
Domestic and foreign scholars have conducted much research on foamed concrete. Deng
J. et al. studied the influence of different lengths and different dosages of polyvinyl alcohol
fibers on the water absorption, strength and shrinkage of foamed concrete [
10
]. Some scholars
have studied the influence of mix ratio on concrete [
11
,
12
]. Rong H. et al. used yeast as a raw
material to prepare microbial foaming agents and studied the influence of foaming technology
on the performance and microstructure of foamed concrete [
13
]. Mohamad Noridah et al.
studied the effect of fiber on the properties of foamed concrete [
14
,
15
]. Othman Rokiah. et al.
studied the relationship between foamed concrete density and compressive strength [
16
].
Abdullah Faisal Alshalif et al. studied the effect of density and curing conditions on CO
2
isolation in foamed concrete brick, and the effect of bacterial self-healing and CO
2
bio-sealing
on the strength of bio foamed concrete brick [
17
,
18
]. Yang S. et al. studied the preparation and
properties of low-density foamed concrete [
19
]. Hou Li et al. studied the influence of foaming
agent on foamed concrete [
20
]. Chen Y. et al. studied the application of foamed concrete in
backfilling [21].
Combined with research from scholars at home and abroad, it has been found that,
in engineering applications, foamed concrete has many advantages when compared with
traditional concrete; however, there is a lack of systematic research on the factors affecting
the shrinkage and cracking of foamed concrete. In this paper, typical working conditions
were simulated within different scenarios, and the shrinkage and maximum crack value
of foamed concrete in these conditions were studied. This paper aims to systematically
summarize the influence of curing system and water–cement ratio on the cracking and
shrinkage of foamed concrete, and to provide theoretical guidance for the filling and
construction of foamed concrete.
At present, the problem of controlling the shrinkage cracking of foamed concrete
is mostly seen in the research of additives, material ratio, fibers, etc. Based on this, this
study formulates six curing systems, selects two water–cement ratios, and obtains the
shrinkage and cracking performance of foamed concrete under different curing conditions
and water–cement ratios, in order to systematically summarize the effects of temperature,
humidity and water–cement ratio on the shrinkage and cracking of foamed concrete, and
provide theoretical guidance for the filling and construction of foamed concrete.

Polymers 2022,14, 2703 4 of 14
2. Materials and Methods
2.1. Materials and Specimen Preparation
2.1.1. Cement
P.O 42.5 grade ordinary Portland cement was produced by Huaxin Cement (Nantong,
China) Co., Ltd., and the physical properties of the cement are shown in Table 1.
Table 1. Cement physical performance indicators.
Label Initial Setting
Time/min
Final Setting
Time/min
Packing
Density/Kg/m3
The Compressive
Strength/Mpa Flexural Strength/MPa
3 Day 7 Day 28 Day 3 Day 7 Day 28 Day
Ordinary
Portland cement 70 242 1137 18.5 28 47 3.6 5.1 7.3
2.1.2. F2.1.2 Fly Ash
The fly ash, grade I, was sourced from Nanjing Huaneng Power Plant (Nanjing, China).
The chemical composition of fly ash is shown in Table 2.
Table 2. Chemical composition of fly ash.
Composition SO3MgO CaO Fe2O3Al2O3SiO2Ignition Loss
Content 0.66% 1.09% 3.53% 4.04% 30.37% 50.76% 3.32%
2.1.3. Foaming Agent
Foaming agent for HT foamed concrete is present in Figure 3.
Polymers 2022, 14, x FOR PEER REVIEW 4 of 14
2. Materials and Methods
2.1. Materials and Specimen Preparation
2.1.1. Cement
P.O 42.5 grade ordinary Portland cement was produced by Huaxin Cement (Nan-
tong, China) Co., Ltd., and the physical properties of the cement are shown in Table 1.
Table 1. Cement physical performance indicators.
Label Initial Setting
Time/min
Final Setting
Time/min
Packing Den-
sity/Kg/m3
The Compressive Strength/Mpa Flexural Strength/MPa
3 Day 7 Day 28 Day 3 Day 7 Day 28 Day
Ordinary Port-
land cement 70 242 1137 18.5 28 47 3.6 5.1 7.3
2.1.2. F2.1.2 Fly Ash
The fly ash, grade I, was sourced from Nanjing Huaneng Power Plant (Nanjing,
China). The chemical composition of fly ash is shown in Table 2.
Table 2. Chemical composition of fly ash.
Composition SO3 MgO CaO Fe2O3 Al2O3 SiO2 Ignition Loss
Content 0.66% 1.09% 3.53% 4.04% 30.37% 50.76% 3.32%
2.1.3. Foaming Agent
Foaming agent for HT foamed concrete is present in Figure 3.
Figure 3. Foaming agent for HT foamed concrete.
2.1.4. Water
Tap water is used, and, according to foamed concrete density grade A11, the compo-
sitions in 1 m3 foamed concrete are shown in Table 3.
Table 3. Foamed concrete base mix ratio.
Number Water-Ce-
ment Ratio The Fly Ash/kg Cement/kg Water/kg HTFC Mineral
Admixtures/kg
1 0.4 202 552 368 166
2 0.5 202 552 460 166
Figure 3. Foaming agent for HT foamed concrete.
2.1.4. Water
Tap water is used, and, according to foamed concrete density grade A11, the composi-
tions in 1 m3foamed concrete are shown in Table 3.

Polymers 2022,14, 2703 5 of 14
Table 3. Foamed concrete base mix ratio.
Number Water-Cement
Ratio
The Fly
Ash/kg Cement/kg Water/kg HTFC Mineral
Admixtures/kg
1 0.4 202 552 368 166
2 0.5 202 552 460 166
2.1.5. Admixtures
HTFC foamed concrete admixture is provided by Henan Huatai New Material Tech-
nology Co., LTD. (Henan, China).
2.1.6. Preparation of Foamed Concrete
The preparation of foamed concrete includes foam preparation and cement mortar
preparation and casting [
22
]. Foam was prepared by adding foaming agent into a foaming
machine. Fly ash, admixtures and cement slurry were stirred evenly with a blender at a low
speed for about 2 min, followed by adding water to prepare a homogenous slurry. Foam
made by the foaming equipment was then added into the slurry. The mixed slurry was
cast into a prepared mold, and a spatula was used to smooth the surface.
2.2. Design of Experiments
2.2.1. Designed Maintenance Regime
Given the possible environments that foamed concrete may encounter in this project,
six typical environmental conditions, including temperature and humidity, were selected
to simulate the actual conditions of the project: (1) temperature 25 ±1◦C and relative hu-
midity 75
±
5%; (2) temperature 25
±
1
◦
C and relative humidity 15
±
5%; (3) temperature
45 ±1◦C
and relative humidity 75
±
5%; (4) temperature 45
±
1
◦
C and relative humid-
ity 15
±
5%; (5) temperature 65
±
1
◦
C and relative humidity 75
±
5%; (6) temperature
65 ±1◦C and relative humidity 15 ±5%.
2.2.2. Drying Shrinkage Test
The drying shrinkage of each specimen was measured 1 day after demolding, and
continued for 28 days. The detailed test procedure was as follows:
(1)
Steel molds measuring 40
×
40
×
160 mm were cleaned and lubricated for easier de-
molding. In addition, due to the high fluidity of foamed concrete, a layer of petroleum
jelly was coated around each test mold to prevent the slurry from seeping out.
(2)
The foamed concrete slurry was cast in steel molds at the same temperature and
humidity as stated in the designed curing regime.
(3)
The specimens were demolded after 24 h and placed in a curing box, with constant
temperature and humidity control. The shrinkage of each specimen was measured
after curing for different times, according to the test scheme. The test instrument and
specimen are shown in Figure 4.
(4)
The drying shrinkage strain is determined as follows:
εst =
L0−Lt
Lt
×100% (1)
εst
—shrinkage strain of concrete at age t, where tis the initial measurement time of
drying shrinkage;
L0—initial length of prismatic specimen (mm);
Lt—the length of a prismatic specimen at time t(mm).

Polymers 2022,14, 2703 6 of 14
Polymers 2022, 14, x FOR PEER REVIEW 5 of 14
2.1.5. Admixtures
HTFC foamed concrete admixture is provided by Henan Huatai New Material Tech-
nology Co., LTD. (Henan, China)
2.1.6. Preparation of Foamed Concrete
The preparation of foamed concrete includes foam preparation and cement mortar
preparation and casting [22]. Foam was prepared by adding foaming agent into a foaming
machine. Fly ash, admixtures and cement slurry were stirred evenly with a blender at a
low speed for about 2 min, followed by adding water to prepare a homogenous slurry.
Foam made by the foaming equipment was then added into the slurry. The mixed slurry
was cast into a prepared mold, and a spatula was used to smooth the surface.
2.2. Design of Experiments
2.2.1. Designed Maintenance Regime
Given the possible environments that foamed concrete may encounter in this project,
six typical environmental conditions, including temperature and humidity, were selected
to simulate the actual conditions of the project: (1) temperature 25 ± 1 °C and relative hu-
midity 75 ± 5%; (2) temperature 25 ± 1 °C and relative humidity 15 ± 5%; (3) temperature
45 ± 1 °C and relative humidity 75 ± 5%; (4) temperature 45 ± 1 °C and relative humidity
15 ± 5%; (5) temperature 65 ± 1 °C and relative humidity 75 ± 5%; (6) temperature 65 ± 1
°C and relative humidity 15 ± 5%.
2.2.2. Drying Shrinkage Test
The drying shrinkage of each specimen was measured 1 day after demolding, and
continued for 28 days. The detailed test procedure was as follows:
(1) Steel molds measuring 40 × 40 × 160 mm were cleaned and lubricated for easier
demolding. In addition, due to the high fluidity of foamed concrete, a layer of petro-
leum jelly was coated around each test mold to prevent the slurry from seeping out.
(2) The foamed concrete slurry was cast in steel molds at the same temperature and hu-
midity as stated in the designed curing regime.
(3) The specimens were demolded after 24 h and placed in a curing box, with constant
temperature and humidity control. The shrinkage of each specimen was measured
after curing for different times, according to the test scheme. The test instrument and
specimen are shown in Figure 4.
(a) (b)
Figure 4. Drying shrinkage test: (a) comparator; (b) drying shrinkage specimen.
Figure 4. Drying shrinkage test: (a) comparator; (b) drying shrinkage specimen.
2.2.3. Early Cracking
(1)
The mixed foamed concrete was cast into an elliptical mold (inner size
210 mm
×
90 mm
×
45 mm, outer size 250 mm
×
130 mm
×
45 mm) and placed in a
standard curing laboratory for curing.
(2) The specimens were cured for 18 h in a curing room at 25
±
1
◦
C with relative humidity
75 ±5% before demolding.
(3) When maintenance began, the strain gauge was pasted to collect data and observe the
initial time of crack emergence. When the first crack was found, artificial observations
were performed every 15 min for 12 h, then once a day afterwards.
(4) The crack width of the elliptical ring of foamed concrete was observed with a 100-fold
reading microscope, with a light source, and the development of the maximum crack
width was observed and recorded over time.
(5)
The crack width was recorded with a reading microscope. The crack width was the
average value of the crack width at 1/4, 1/2 and 3/4 of the height direction of the
elliptical ring sample. The time of crack initiation, and the number and width of
cracks in eadddch foamed concrete ellipse specimen were also observed and recorded.
The test specimen is shown in Figure 5.
Polymers 2022, 14, x FOR PEER REVIEW 6 of 14
(4) The drying shrinkage strain is determined as follows:
𝜀 =
𝐿−𝐿

𝐿
× 100% (1)
𝜀—shrinkage strain of concrete at age t, where t is the initial measurement time of
drying shrinkage;
𝐿—initial length of prismatic specimen (mm);
𝐿—the length of a prismatic specimen at time t (mm).
2.2.3. Early Cracking
(1) The mixed foamed concrete was cast into an elliptical mold (inner size 210 mm × 90
mm × 45 mm, outer size 250 mm × 130 mm × 45 mm) and placed in a standard curing
laboratory for curing.
(5) The specimens were cured for 18 h in a curing room at 25 ± 1 °C with relative humid-
ity 75 ± 5% before demolding.
(6) When maintenance began, the strain gauge was pasted to collect data and observe
the initial time of crack emergence. When the first crack was found, artificial obser-
vations were performed every 15 min for 12 h, then once a day afterwards.
(7) The crack width of the elliptical ring of foamed concrete was observed with a 100-
fold reading microscope, with a light source, and the development of the maximum
crack width was observed and recorded over time.
(8) The crack width was recorded with a reading microscope. The crack width was the
average value of the crack width at 1/4, 1/2 and 3/4 of the height direction of the el-
liptical ring sample. The time of crack initiation, and the number and width of cracks
in eadddch foamed concrete ellipse specimen were also observed and recorded. The
test specimen is shown in Figure 5.
(a) (b)
(c) (d)
Figure 5.
Apparatus for cracking test: (
a
) reading microscope; (
b
) oval mold; (
c
) casting the specimens;
(d) specimen forming.

Polymers 2022,14, 2703 7 of 14
3. Results
3.1. Effect of Different Curing Regimes on Drying Shrinkage of Foamed Concrete
3.1.1. Drying Shrinkage Test Data of Foamed Concrete
The shrinkage data for foamed concrete during the curing regime were recorded using
a comparator, as listed in Table 4.
Table 4. Dry shrinkage of foamed concrete in different environments.
Number Water-Cement
Ratio
Maintenance Time before
Mold Removal
The Shrinkage of the Specimen (×10−6)
3 d 5 d 7 d 14 d 21 d 28 d
1
0.4 1 d 521 1014 1208 1391 1492 1588
3 d 394 933 1135 1321 1410 1480
0.5 1 d 547 1065 1225 1415 1556 1607
3 d 472 984 1193 1370 1484 1507
2
0.4 1 d 601 1075 1248 1450 1585 1633
3 d 580 990 1195 1387 1465 1580
0.5 1 d 679 1132 1350 1587 1788 1833
3 d 620 1078 1355 1468 1583 1654
3
0.4 1 d 624 1080 1325 1543 1680 1740
3 d 605 999 1200 1407 1545 1617
0.5 1 d 650 1195 1380 1576 1725 1794
3 d 590 1026 1214 1423 1550 1655
4
0.4 1 d 605 1125 1330 1510 1664 1712
3 d 609 1035 1270 1478 1624 1695
0.5 1 d 691 1205 1420 1630 1785 1860
3 d 658 1130 1325 1505 1695 1740
5
0.4 1 d 730 1316 1605 1745 1789 1807
3 d 720 1280 1410 1590 1695 1780
0.5 1 d 750 1378 1680 1793 1830 1850
3 d 727 1240 1420 1579 1680 1726
6
0.4 1 d 760 1446 1638 1685 1743 1778
3 d 705 1403 1587 1615 1874 1740
0.5 1 d 870 1524 1730 1784 1827 1896
3 d 820 1480 1670 1706 1801 1854
3.1.2. Variance Analysis of the Influence of Various Factors on the Drying Shrinkage of
Foamed Concrete
The test data were analyzed by multi-factor analysis of variance, with mathematical
statistical software SPSS (Statistical Product and Service Solutions) [
23
]; the results are
shown in Table 5.
Table 5. Analysis of the effect of various factors on the shrinkage of 28-day foamed concrete.
The Source of the Variance
The Sum of the
Deviations from
the Mean
Degree of
Freedom (df)
Average
Variance F Sig
Temperature 58,129.167 1 29,064.583 36.395 0.027
Humidity 28,856.33 1 8856.33 31.090 0.030
Water–Cement Ratio 28,227.000 1 28,227.000 35.346 0.027
Temperature×Humidity 9937.167 2 4960.583 6.2222 0.138
Humidity×Water–Cement Ratio 10,208.333 1 10,208.333 12.783 0.070
Temperature×Water–Cement Ratio 444.500 2 222.250 0.278 0.782
According to Table 5, among the three factors that affect the shrinkage of foamed
concrete, the sum of the mean deviation of temperature is 58,129.167, which is larger
than the other factors. Meanwhile, the corresponding F value is also the largest, and the
significance (Sig) is the smallest. It can be seen that temperature has the strongest influence

Polymers 2022,14, 2703 8 of 14
on the drying shrinkage of foamed concrete, followed by water–cement ratio, and finally
humidity. From the pairwise interaction analysis of these three factors, it can be seen
that the Sig value of humidity
×
water–cement ratio is greater than 0.05, so the pairwise
interaction of the three factors is not obvious.
As can be seen from Table 6, the estimated marginal mean value of shrinkage decreases
successively as the temperature increases from 25
◦
C to 65
◦
C, indicating that the shrinkage
of foamed concrete becomes higher as the temperature rises. It can be seen from Table 7
that 75% relative humidity has little influence on the shrinkage of foamed concrete. It
can be concluded from Table 8that the shrinkage of foamed concrete is slighter when the
water–cement ratio is 0.4. In summary:
(1)
When the humidity and water–cement ratio are the same, the 28-day shrinkage of
foamed concrete increases with the increase in temperature;
(2)
When the humidity and temperature are the same, the 28-day shrinkage of foamed
concrete decreases with the increase in water–cement ratio;
(3)
Temperature is the primary factor that affects the drying shrinkage performance of
foamed concrete, followed by water–cement ratio and humidity.
Table 6. Analysis of the effect of temperature on the shrinkage of 28-day foamed concrete.
Temperature Average Value Estimate the
Marginal Mean
95%Confidence Interval
Lower Limit Upper Limit
25 1665.3 14.1 1604.5 1726.1
45 1776.5 13.8 1715.7 1837.3
65 1832.7 13.7 1771.9 1893.5
Table 7. Analysis of the effect of humidity factors on the shrinkage of 28-day foamed concrete.
Humidity Average Value Estimated
Marginal Value
95% Confidence Interval
Lower Limit Upper Limit
15 1785.3 10.5 1735.7 1834.9
75 1735.0 11.5 1681.3 1780.7
Table 8.
Analysis of the effect of water–ash ratio factors on the shrinkage of 28-day foamed concrete.
Water-Cement
Ratio Average Value Estimated
Marginal Value
95% Confidence Interval
Lower Limit Upper Limit
0.4 1709.7 11.5 1660.0 1759.3
0.5 1806.7 10.5 1757.0 1856.3
3.1.3. Effect of Temperature on Drying Shrinkage of Foamed Concrete
Figure 6shows the drying shrinkage test results of foamed concrete in each group,
with a curing condition of 75% relative humidity.
Figure 7shows the drying shrinkage test results of foamed concrete in each group,
with a curing condition of 15% relative humidity.
As the temperature rises, the shrinkage of the foamed concrete continues to increase.
High temperature changes the drying shrinkage development process of foamed concrete.
The drying shrinkage rate of foamed concrete increases rapidly in the early stage, and it
then becomes stable afterward. In the initial stage, generally within the first 5 days, the
shrinkage rate of foamed concrete is the largest, and with the extension of curing age, the
shrinkage rate gradually slows down. When RH = 75%, the drying shrinkage of foamed
concrete with different water–cement ratios tends to be the same, while when RH = 15%,
the drying shrinkage is obviously different, due to different water–cement ratios. The
shrinkage of foamed concrete obviously decreases with the decrease in water–cement ratio

Polymers 2022,14, 2703 9 of 14
in a dry environment. At the same time, with the increase in water–cement ratio, the total
shrinkage deformation of foamed concrete increases at 28 days.
Polymers 2022, 14, x FOR PEER REVIEW 9 of 14
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
2000
25 ℃，RH=75%
25 ℃，RH=15%
65 ℃，RH=75%
65 ℃，RH=15%
error bar
Time/d
Drying shrinkage/10
-6
W/C=0.4
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
2000
25 ℃，RH=75%
25 ℃，RH=15%
65 ℃，RH=75%
65 ℃，RH=15%
error bar
Time/d
Drying shrinkage/10
-6
W/C=0.5
(a) (b)
Figure 6. Drying shrinkage of foamed concrete at different curing temperatures: (a) 0.4 water–ce-
ment ratio dry shrinkage of foamed concrete; (b) 0.5 water–cement ratio dry shrinkage of foamed
concrete.
Figure 7 shows the drying shrinkage test results of foamed concrete in each group,
with a curing condition of 15% relative humidity.
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
25 ℃
45 ℃
65 ℃
error bar
RH=15% W/C=0.4
Time/d
Drying shrinkage/10
-6
0 5 10 15 20 25 30
600
800
1000
1200
1400
1600
1800
2000
25 ℃
45 ℃
65 ℃
error bar
Time/d
Drying shrinkage/10
-6
RH=15% W/C=0.5
(a) (b)
Figure 7. Drying shrinkage of foamed concrete under different curing temperatures (RH = 15%): (a)
0.4 water–cement ratio dry shrinkage of foamed concrete; (b) 0.5 water–cement ratio dry shrinkage
of foamed concrete.
As the temperature rises, the shrinkage of the foamed concrete continues to increase.
High temperature changes the drying shrinkage development process of foamed concrete.
The drying shrinkage rate of foamed concrete increases rapidly in the early stage, and it
then becomes stable afterward. In the initial stage, generally within the first 5 days, the
shrinkage rate of foamed concrete is the largest, and with the extension of curing age, the
shrinkage rate gradually slows down. When RH = 75%, the drying shrinkage of foamed
concrete with different water–cement ratios tends to be the same, while when RH = 15%,
the drying shrinkage is obviously different, due to different water–cement ratios. The
shrinkage of foamed concrete obviously decreases with the decrease in water–cement ra-
tio in a dry environment. At the same time, with the increase in water–cement ratio, the
total shrinkage deformation of foamed concrete increases at 28 days.
Figure 6.
Drying shrinkage of foamed concrete at different curing temperatures: (
a
) 0.4 water–cement
ratio dry shrinkage of foamed concrete; (
b
) 0.5 water–cement ratio dry shrinkage of foamed concrete.
Polymers 2022, 14, x FOR PEER REVIEW 9 of 14
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
2000
25 ℃，RH=75%
25 ℃，RH=15%
65 ℃，RH=75%
65 ℃，RH=15%
error bar
Time/d
Drying shrinkage/10
-6
W/C=0.4
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
2000
25 ℃，RH=75%
25 ℃，RH=15%
65 ℃，RH=75%
65 ℃，RH=15%
error bar
Time/d
Drying shrinkage/10
-6
W/C=0.5
(a) (b)
Figure 6. Drying shrinkage of foamed concrete at different curing temperatures: (a) 0.4 water–ce-
ment ratio dry shrinkage of foamed concrete; (b) 0.5 water–cement ratio dry shrinkage of foamed
concrete.
Figure 7 shows the drying shrinkage test results of foamed concrete in each group,
with a curing condition of 15% relative humidity.
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
25 ℃
45 ℃
65 ℃
error bar
RH=15% W/C=0.4
Time/d
Drying shrinkage/10
-6
0 5 10 15 20 25 30
600
800
1000
1200
1400
1600
1800
2000
25 ℃
45 ℃
65 ℃
error bar
Time/d
Drying shrinkage/10
-6
RH=15% W/C=0.5
(a) (b)
Figure 7. Drying shrinkage of foamed concrete under different curing temperatures (RH = 15%): (a)
0.4 water–cement ratio dry shrinkage of foamed concrete; (b) 0.5 water–cement ratio dry shrinkage
of foamed concrete.
As the temperature rises, the shrinkage of the foamed concrete continues to increase.
High temperature changes the drying shrinkage development process of foamed concrete.
The drying shrinkage rate of foamed concrete increases rapidly in the early stage, and it
then becomes stable afterward. In the initial stage, generally within the first 5 days, the
shrinkage rate of foamed concrete is the largest, and with the extension of curing age, the
shrinkage rate gradually slows down. When RH = 75%, the drying shrinkage of foamed
concrete with different water–cement ratios tends to be the same, while when RH = 15%,
the drying shrinkage is obviously different, due to different water–cement ratios. The
shrinkage of foamed concrete obviously decreases with the decrease in water–cement ra-
tio in a dry environment. At the same time, with the increase in water–cement ratio, the
total shrinkage deformation of foamed concrete increases at 28 days.
Figure 7.
Drying shrinkage of foamed concrete under different curing temperatures (RH = 15%):
(
a
) 0.4 water–cement ratio dry shrinkage of foamed concrete; (
b
) 0.5 water–cement ratio dry shrinkage
of foamed concrete.
3.1.4. Effect of Humidity on Drying Shrinkage of Foamed Concrete
Figure 8shows the drying shrinkage test results of foamed concrete in different
humidity at 25 ◦C and 65 ◦C.

Polymers 2022,14, 2703 10 of 14
Polymers 2022, 14, x FOR PEER REVIEW 10 of 14
3.1.4. Effect of Humidity on Drying Shrinkage of Foamed Concrete
Figure 8 shows the drying shrinkage test results of foamed concrete in different hu-
midity at 25 °C and 65 °C.
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
2000
25 ℃，RH=75%
25 ℃，RH=15%
65 ℃，RH=75%
65 ℃，RH=15%
error bar
W/C=0.4
Time/d
Drying shrinkage/10
-6
0 5 10 15 20 25 30
400
600
800
1000
1200
1400
1600
1800
2000
25 ℃，RH=75%
25 ℃，RH=15%
65 ℃，RH=75%
65 ℃，RH=15%
error bar
Time/d
Drying shrinkage/10
-6
W/C=0.5
(a) (b)
Figure 8. Drying shrinkage of foamed concrete under different curing humidity: (a) 0.4 water–ce-
ment ratio dry shrinkage of foamed concrete; (b) 0.5 water–cement ratio dry shrinkage of foamed
concrete.
As can be seen in Figure 7, the shrinkage of foamed concrete is significantly greater
with low humidity. Increasing the humidity in the curing environment can control the
loss rate of water in foamed concrete, thus reducing the drying shrinkage value and effec-
tively improving the volume stability of foamed concrete. As the temperature rises, the
difference in drying shrinkage under humidity of 15% and 75% is reduced. When the tem-
perature is 25 °C, the shrinkage difference between RH15% and RH75% is 96 × 10−6. When
the temperature is 65 °C, the shrinkage difference of foamed concrete under different hu-
midity is 66 × 10−6.
3.2. Influence of Different Curing Regimes on Early Cracking of Foamed Concrete
3.2.1. Early Cracking Test Data of Foamed Concrete
Table 9 shows the initial cracking time of foamed concrete in six different curing sys-
tems.
Table 9. Initial cracking time under different curing environments (unit in the table: h).
Water-Cement Ratio
0.4 0.5
Number
1 70.8 68.3
2 45.3 41.7
3 48.2 46.5
4 16.4 11.3
5 39.8 35.7
6 8.3 7.2
Figure 8.
Drying shrinkage of foamed concrete under different curing humidity: (
a
) 0.4 water–cement
ratio dry shrinkage of foamed concrete; (
b
) 0.5 water–cement ratio dry shrinkage of foamed concrete.
As can be seen in Figure 7, the shrinkage of foamed concrete is significantly greater
with low humidity. Increasing the humidity in the curing environment can control the loss
rate of water in foamed concrete, thus reducing the drying shrinkage value and effectively
improving the volume stability of foamed concrete. As the temperature rises, the difference
in drying shrinkage under humidity of 15% and 75% is reduced. When the temperature
is 25
◦
C, the shrinkage difference between RH15% and RH75% is 96
×
10
−6
. When the
temperature is 65
◦
C, the shrinkage difference of foamed concrete under different humidity
is 66 ×10−6.
3.2. Influence of Different Curing Regimes on Early Cracking of Foamed Concrete
3.2.1. Early Cracking Test Data of Foamed Concrete
Table 9shows the initial cracking time of foamed concrete in six different curing systems.
Table 9. Initial cracking time under different curing environments (unit in the table: h).
Number
Water-Cement Ratio 0.4 0.5
1 70.8 68.3
2 45.3 41.7
3 48.2 46.5
4 16.4 11.3
5 39.8 35.7
6 8.3 7.2
3.2.2. Effect of Temperature and Humidity on Early Cracking of Foamed Concrete
Figure 9reflects the influence of different curing conditions on the initial cracking time
of foamed concrete. It can be seen from the figure that the initial cracking time of foamed
concrete decreases with the increase in temperature. When the temperature is 25
◦
C, the
maximum initial cracking time of foamed concrete is 70.8 h. The cracking sensitivity of
foamed concrete increases as temperature increases. Combined with the drying shrinkage
analysis of foamed concrete, it can be seen that with the increase in temperature, the
early hydration speed of foamed concrete increases, and the shrinkage increases, which
accelerates the appearance of cracks. Humidity has a great influence on the initial cracking
time of foamed concrete. In the same temperature conditions, the difference in initial
cracking time caused by different humidity is up to 32 h. The initial cracking time of
foamed concrete decreases with the decrease in humidity.

Polymers 2022,14, 2703 11 of 14
Polymers 2022, 14, x FOR PEER REVIEW 11 of 14
3.2.2. Effect of Temperature and Humidity on Early Cracking of Foamed Concrete
Figure 9 reflects the influence of different curing conditions on the initial cracking
time of foamed concrete. It can be seen from the figure that the initial cracking time of
foamed concrete decreases with the increase in temperature. When the temperature is 25
°C, the maximum initial cracking time of foamed concrete is 70.8 h. The cracking sensitiv-
ity of foamed concrete increases as temperature increases. Combined with the drying
shrinkage analysis of foamed concrete, it can be seen that with the increase in temperature,
the early hydration speed of foamed concrete increases, and the shrinkage increases,
which accelerates the appearance of cracks. Humidity has a great influence on the initial
cracking time of foamed concrete. In the same temperature conditions, the difference in
initial cracking time caused by different humidity is up to 32 h. The initial cracking time
of foamed concrete decreases with the decrease in humidity.
20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
RH=15%
RH=75%
error bar
W/C=0.4
temperature/℃
Maximum crack width/mm
20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
RH=15%
RH=75%
error bar
temperature/℃
Maximum crack width/mm
W/C=0.5
(a) (b)
Figure 9. Initial cracking time of foamed concrete under different curing conditions: (a) 0.4 water–
cement ratio initial cracking time of foamed concrete; (b) 0.5 water–cement ratio initial cracking time
of foamed concrete.
3.2.3. Effect of Water–Cement Ratio on Early Cracking of Foamed Concrete
Figure 10 shows the early cracking of foamed concrete when the water–cement ratios
are 0.4 and 0.5.
25 45 65
0
10
20
30
40
50
60
70
80
temperature/℃
W/C=0.4
W/C=0.5
Initial
cracking
time/h
RH=75±5%
25 45 65
0
10
20
30
40
50
temperature/℃
W/C=0.4
W/C=0.5
Initial cracking time/h
RH=15±5%
(a) (b)
Figure 10. Initial cracking time of foamed concrete with different water–cement ratios: (a) RH = 75%
initial cracking time of foamed concrete; (b) RH = 15% initial cracking time of foamed concrete.
As shown in Figure 10, different water–cement ratios lead to changes in initial crack-
ing times. The initial cracking time of a specimen with a water–cement ratio of 0.4 is
Figure 9.
Initial cracking time of foamed concrete under different curing conditions:
(
a
) 0.4 water–cement ratio initial cracking time of foamed concrete; (
b
) 0.5 water–cement ratio
initial cracking time of foamed concrete.
3.2.3. Effect of Water–Cement Ratio on Early Cracking of Foamed Concrete
Figure 10 shows the early cracking of foamed concrete when the water–cement ratios
are 0.4 and 0.5.
Polymers 2022, 14, x FOR PEER REVIEW 11 of 14
3.2.2. Effect of Temperature and Humidity on Early Cracking of Foamed Concrete
Figure 9 reflects the influence of different curing conditions on the initial cracking
time of foamed concrete. It can be seen from the figure that the initial cracking time of
foamed concrete decreases with the increase in temperature. When the temperature is 25
°C, the maximum initial cracking time of foamed concrete is 70.8 h. The cracking sensitiv-
ity of foamed concrete increases as temperature increases. Combined with the drying
shrinkage analysis of foamed concrete, it can be seen that with the increase in temperature,
the early hydration speed of foamed concrete increases, and the shrinkage increases,
which accelerates the appearance of cracks. Humidity has a great influence on the initial
cracking time of foamed concrete. In the same temperature conditions, the difference in
initial cracking time caused by different humidity is up to 32 h. The initial cracking time
of foamed concrete decreases with the decrease in humidity.
20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
RH=15%
RH=75%
error bar
W/C=0.4
temperature/℃
Maximum crack width/mm
20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
RH=15%
RH=75%
error bar
temperature/℃
Maximum crack width/mm
W/C=0.5
(a) (b)
Figure 9. Initial cracking time of foamed concrete under different curing conditions: (a) 0.4 water–
cement ratio initial cracking time of foamed concrete; (b) 0.5 water–cement ratio initial cracking time
of foamed concrete.
3.2.3. Effect of Water–Cement Ratio on Early Cracking of Foamed Concrete
Figure 10 shows the early cracking of foamed concrete when the water–cement ratios
are 0.4 and 0.5.
25 45 65
0
10
20
30
40
50
60
70
80
temperature/℃
W/C=0.4
W/C=0.5
Initial
cracking
time/h
RH=75±5%
25 45 65
0
10
20
30
40
50
temperature/℃
W/C=0.4
W/C=0.5
Initial cracking time/h
RH=15±5%
(a) (b)
Figure 10. Initial cracking time of foamed concrete with different water–cement ratios: (a) RH = 75%
initial cracking time of foamed concrete; (b) RH = 15% initial cracking time of foamed concrete.
As shown in Figure 10, different water–cement ratios lead to changes in initial crack-
ing times. The initial cracking time of a specimen with a water–cement ratio of 0.4 is
Figure 10.
Initial cracking time of foamed concrete with different water–cement ratios: (
a
) RH = 75%
initial cracking time of foamed concrete; (b) RH = 15% initial cracking time of foamed concrete.
As shown in Figure 10, different water–cement ratios lead to changes in initial cracking
times. The initial cracking time of a specimen with a water–cement ratio of 0.4 is longer.
When the water–cement ratio increases to 0.5, the initial cracking time is shortened by 1–5 h.
This shows that under the same environmental conditions, the water–cement ratio has a
greater influence on the cracking of foamed concrete, i.e., with the increase in water–cement
ratio, the cracking of foamed concrete increases.
3.2.4. Effect of Temperature and Humidity on Maximum Crack Width of Foamed Concrete
As shown in Figure 11, with the increase in temperature, the maximum crack width of
foamed concrete presents an increasing trend. At relative humidity of 75%, the maximum
crack width increases linearly from 1.1 mm to 2.7 mm when the temperature increases from
25 to 65
◦
C. The crack width expands further when the ambient humidity is lowered to
15%. At the temperature of 65
◦
C and relative humidity of 15%, the maximum crack width
reaches 3.3 mm.

Polymers 2022,14, 2703 12 of 14
Polymers 2022, 14, x FOR PEER REVIEW 12 of 14
longer. When the water–cement ratio increases to 0.5, the initial cracking time is shortened
by 1–5 h. This shows that under the same environmental conditions, the water–cement
ratio has a greater influence on the cracking of foamed concrete, i.e., with the increase in
water–cement ratio, the cracking of foamed concrete increases.
3.2.4. Effect of Temperature and Humidity on Maximum Crack Width of Foamed
Concrete
As shown in Figure 11, with the increase in temperature, the maximum crack width
of foamed concrete presents an increasing trend. At relative humidity of 75%, the maxi-
mum crack width increases linearly from 1.1 mm to 2.7 mm when the temperature in-
creases from 25 to 65 °C. The crack width expands further when the ambient humidity is
lowered to 15%. At the temperature of 65 °C and relative humidity of 15%, the maximum
crack width reaches 3.3 mm.
20 30 40 50 60 70
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Maximum crack width/mm
temperature/℃
RH=15%
RH=75%
W/C=0.4
Figure 11. Maximum crack width of foamed concrete under different curing environments.
4. Discussion
In the case study of the China–Russian Eastern Gas Pipeline, the shrinkage and crack-
ing of foamed concrete was studied under typical environments, where temperature, hu-
midity and water–cement ratio were representative of project scenarios. The results show
that the decrease in humidity and the increase in temperature will reduce the shrinkage
and cracking of foamed concrete. During the test, it was found that the range of water–
cement ratio 0.4–0.5 can meet the shrinkage and cracking requirements of foamed con-
crete. In simulated conditions, the maximum crack width of foamed concrete was 3.3 mm,
which meets the requirements of the “Technical Regulations for Filling Lightweight Foamed
Soil Rail Transit” (CECS453-2016) [24]. The width of the unstressed penetration crack
should be less than 5 mm. Through the analysis of variance, the interaction between the
three factors was studied, and the results show that: in the two-by-two interaction analysis
between the three factors, the least significant is the Sig value of humidity × water–cement
ratio greater than 0.05; the pairwise interaction of the three factors is, therefore, not obvi-
ous. In addition, the results show that a combination of temperatures of 25 ± 1 °C and RH
75 ± 5% has the best effect on inhibiting the shrinkage and cracking of foamed concrete.
This provides a reference for the maintenance of large-volume foamed concrete, but there
are many other factors [25] that need to be considered in order to obtain the best perfor-
mance, such as apparent density [26] and proportions of concrete mix. Therefore, a large
number of relevant tests are needed.
Figure 11. Maximum crack width of foamed concrete under different curing environments.
4. Discussion
In the case study of the China–Russian Eastern Gas Pipeline, the shrinkage and
cracking of foamed concrete was studied under typical environments, where temperature,
humidity and water–cement ratio were representative of project scenarios. The results show
that the decrease in humidity and the increase in temperature will reduce the shrinkage and
cracking of foamed concrete. During the test, it was found that the range of water–cement
ratio 0.4–0.5 can meet the shrinkage and cracking requirements of foamed concrete. In
simulated conditions, the maximum crack width of foamed concrete was 3.3 mm, which
meets the requirements of the “Technical Regulations for Filling Lightweight Foamed Soil Rail
Transit” (CECS453-2016) [
24
]. The width of the unstressed penetration crack should be less
than 5 mm. Through the analysis of variance, the interaction between the three factors
was studied, and the results show that: in the two-by-two interaction analysis between the
three factors, the least significant is the Sig value of humidity
×
water–cement ratio greater
than 0.05; the pairwise interaction of the three factors is, therefore, not obvious. In addition,
the results show that a combination of temperatures of 25
±
1
◦
C and RH
75 ±5%
has
the best effect on inhibiting the shrinkage and cracking of foamed concrete. This provides
a reference for the maintenance of large-volume foamed concrete, but there are many
other factors [
25
] that need to be considered in order to obtain the best performance, such
as apparent density [
26
] and proportions of concrete mix. Therefore, a large number of
relevant tests are needed.
Author Contributions:
Conceptualization, C.L., X.L. (Xiaotian Li), S.L., D.G., C.X., Y.X., V.Y.S.,
H.D., P.Q. and X.L. (Xiaohui Liu); Funding acquisition, P.Q.; Investigation, C.L., V.Y.S. and H.D.;
Supervision, S.L., D.G., C.X., V.Y.S., H.D., P.Q. and X.L. (Xiaohui Liu); Writing—original draft, C.L.;
Writing—review & editing, C.L. and X.L. (Xiaotian Li). All authors have read and agreed to the
published version of the manuscript.
Funding:
This research was funded by the National Natural Science Foundation of China, grant
number 42177138.Funder: P.Q.
Data Availability Statement:
The data used to support the findings of this study are available from
the corresponding author upon request.

Polymers 2022,14, 2703 13 of 14
Acknowledgments:
We kindly thank Research Center of Civil Engineering, China University of
Petroleum (East China) for providing laboratory and experimental devices.
Conflicts of Interest:
The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this paper.
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