This study aims to investigate the relationship between the heat of hydration and the strength development of cast-in-situ foamed concrete. First, indoor model tests are conducted to determine the effects of the casting density and the fly ash content on the hydration heat of foamed concrete in semiadiabatic conditions. Second, compression tests are carried out to evaluate the development of the compressive strength with the curing time under standard curing conditions and temperature matched curing conditions. Third, the hydration heat development of the foamed concrete is tested in four projects. The results showed that the peak temperature, the maximum temperature change rate, and the maximum temperature difference increased with the increase in the casting density at different positions in the foamed concrete. For the same casting density of the foamed concrete, the peak temperature, the maximum temperature change rate, and the maximum temperature difference decreased with the increase in the fly ash content. For the foamed concrete without the admixture, the early strength was significantly higher under temperature matched curing conditions than under standard curing conditions, but the temperature matched curing conditions had a clear inhibitory effect on the strength of the foamed concrete. The strengths during the early stage and the later stage were both improved under temperature matched curing conditions after adding the fly ash, and the greater the fly ash content, the larger the effect. The maximum temperature increments were higher in the indoor model test than in the field tests for the same casting density. Reasonable cooling measures and the addition of fly ash decreased the maximum temperature increments and increased the corresponding casting times.
Cast-in-situ foamed concrete is composed of cement, an admixture, and a proportion of stable tiny bubbles, and it is cast, molded, and cured at the construction site [
Most applications of foamed concrete are in large-volume concrete construction projects. The heat generated by the hydration affects the temperature field in the structure, resulting in three problems. First, the increase and decrease of the hydration heat lead to the expansion and contraction of the foamed concrete structures, which affects the structure itself and adjacent buildings. The maximum temperature is used as an indicator of this problem. Second, a temperature difference exists between the external and internal portions of the structure and the temperature stress results in cracks in the structure’s surface. This reduces the strength of the structure and affects its integrity and durability (Figure
Damage to foamed concrete structures caused by hydration heat.
Fly ash is a pozzolanic material and has been widely used as an admixture in concrete to address the problem of the hydration heat. Its application in concrete has been studied widely, but its application time in foamed concrete is short. Most studies on the addition of fly ash to foamed concrete have focused on the mechanical properties and durability [
The main goal of this study is to investigate the relationship between the hydration heat and the strength development of foamed concrete. First, six groups of indoor model tests with different casting densities and different fly ash dosages were conducted to study the effects of the casting density and fly ash content on the temperature profiles of the foamed concrete. Second, compression tests under two curing conditions (standard curing, temperature matched curing) were conducted to study the effects of the curing conditions on strength development. Finally, the changes in the temperatures of the foamed concrete were analyzed in four field tests.
The foamed concrete was comprised of ordinary Portland cement, water, and bubbles. The cement was Type I Portland cement conforming to GB 175-2007, the fly ash was Class F Type I conforming to GB/T 1596-2005, and the water was tap water. The bubbles were created using a synthetic foaming agent, which was highly eco-friendly, and its air bubbles were strong [
Table
Mix proportions and major parameters of the foamed concrete.
Theoretical casting density (kg/m3) | Water/binder | Water (kg/m3) | Cement (kg/m3) | Fly ash (kg/m3) | Fly ash/(cement + fly ash) | Air bubbles (l/m3) | Measured casting density (kg/m3) | Flow value (mm) | |
---|---|---|---|---|---|---|---|---|---|
1 | 400 | 0.75 | 169 | 225 | 0 | 0 | 763 | 406.7 | 165.4 |
2 | 700 | 0.62 | 267 | 430 | 0 | 0 | 612 | 708.4 | 168.8 |
3 | 1000 | 0.54 | 346 | 640 | 0 | 0 | 448 | 1004.7 | 171.9 |
4 | 700 | 0.62 | 267 | 386 | 43 | 10% | 609 | 707.3 | 172.3 |
5 | 700 | 0.62 | 267 | 301 | 129 | 30% | 604 | 705.1 | 175.7 |
6 | 700 | 0.62 | 267 | 215 | 215 | 50% | 598 | 707.9 | 179.1 |
Preparation of the foamed concrete slurry [
The layout of the Pt-100 thermal resistance thermometers in the indoor model test, which was conducted to determine the heat of hydration of the foamed concrete, is shown in Figure
Profile model of the test.
For this test, thirty identical samples (100 mm long × 100 mm wide × 100 mm high) were cast for each mix proportion. After the casting, half of the samples were cured under standard curing conditions, and the others were cured under temperature matched curing conditions. For the standard curing condition, the samples were cured in a standard curing room after they were unmolded until the test time. For the temperature matched curing conditions, the samples in the molds were placed in a constant temperature and humidity box. After the samples were unmolded, they were wrapped in bags cured in the constant temperature and humidity box (Figure
The constant temperature and humidity box.
Figure
Relationship between temperatures and casting time (pure cement). Note: 400-1 denotes the value of the test with a casting density of 400 kg/m3 in position 1.
The relationship between the temperatures change rate and the casting time for the three casting densities in position 1 is shown in Figure
Relationship between the temperature change rate and the casting time (pure cement; #1).
The relationship between the temperature difference and the casting time for the three casting densities is shown in Figure
Relationship between the temperature difference and the casting time (pure cement).
Figure
Relationship between temperature and casting time (containing fly ash). Note: 0%-1 denotes the value when the fly ash content is 0% in position 1.
The relationship between the temperature change rate and the casting time of the foamed concrete with different fly ash contents in position 1 is shown in Figure
Relationship between temperature change rate and casting time (containing fly ash; #1).
Figure
Relationship between the temperature difference and the casting time (containing fly ash).
Figure
Relationship between compressive strength and curing time (pure cement). Note: 400 s denotes the value with the casting density of 400 kg/m3 under standard curing conditions. 400 m denotes the value with the casting density of 400 kg/m3 under temperature matched curing conditions.
The relationship between the compressive strength and the curing time with different fly ash contents is shown in Figure
Relationship between compressive strength and curing time (containing fly ash). Note: 0% s denotes the value of the test when the fly ash content is 0% under standard curing conditions. 0% s denotes the value of the test when the fly ash content is 0% under temperature matched curing conditions.
Studies have shown that there are more hydration products during the early stage and fewer hydration products during the later stage in the temperature matched conditions compared with the standard curing conditions for pure cement [
The SEM photographs of foamed concrete structures (28 d): (a) standard curing conditions (pure cement), (b) temperature-matched curing conditions (pure cement), (c) standard curing conditions (containing fly ash), and (d) temperature-matched curing conditions (containing fly ash).
In summary, when the casting density ranged from 400 kg/m3 to 1000 kg/m3 and no fly ash was used, the difference between the internal and external temperature of the cast-in-situ foamed concrete was significantly higher than 25°C, which does not meet the requirements of the standard for the construction of mass concrete. When the casting density was 700 kg/m3 and the fly ash content was 30%, the temperatures met the requirements of the standard for the construction of mass concrete. Therefore, it is necessary to consider field conditions and add a fly ash admixture or develop a system to contain moisture to meet the requirements.
The field tests consisted of four projects, and the details of the projects are shown in Table
Details of the projects.
Project | Design casting density (kg/m3) | Measured casting density (kg/m3) | Mix proportions (/m3) | Construction location | Weather |
---|---|---|---|---|---|
1 | 1000 | 1018.5, 1011.1 | C : F : W : A = 640 : 0:346 : 448 | Zigong city, Sichuan province | Wind speed: 1–3 |
Humidity: 78–80% | |||||
2 | 650 | 651.4, 656.7, 654.2 | C : F : W : A = 405 : 0:223 : 647 | Chengdu city, Sichuan province | Wind speed: 1–3 |
Humidity: 83–86% | |||||
3 | 670 | 657.8, 663.5, 675.6 | C : F : W : A = 420 : 0:231 : 634 | Huai’an city, Jiangsu province | Wind speed: 1–3 |
Humidity: 69–71% | |||||
4 | 700 | 702.1, 721.5, 694.8 | C : F : W : A = 308 : 132 : 246 : 602 | Guiyang city, Guizhou province | Wind speed: 0–4 |
Humidity: 61–81% |
Note: C : F : W : A = cement (kg) : fly ash (kg) : water (kg) : air bubbles (l).
Overview of the test sites: (a) project 1 (fill on the top of the arch bridge), (b) project 2 (backfill behind the wall), (c) project 3 (widening of expressway), and (d) project 4 (backfill of high-speed station).
Layout of the components in (a) project 1 and (b) projects 2, 3, and 4.
Figure
Relationship between temperatures and casting time: (a) project 1, (b) project 2, (c) project 3, and (d) project 4.
Compared with projects 2, 3, and the indoor model tests, the results of project 4 show that reasonable cooling measures and the addition of fly ash decrease the maximum temperature increments and increase the corresponding casting times. When it rains, the foamed concrete structure is affected because the permeability coefficient of the foamed concrete is large and water can penetrate into the foamed concrete [
Crack fractures caused by the shower in project 4.
The maximum temperature increments are higher for the indoor model test than the field test for the same casting density (Figure
Maximum temperature increments of the foamed concrete. Note: 700-10%-i denotes the value with a casting density of 700 kg/m3 with 10% content of fly ash and the indoor model test. 700-10%-f denotes the value with the casting density of 700 kg/m3 with 10% content of fly ash and the field test.
The following conclusions can be drawn based on the experimental and comparative results. For the foamed concrete with the casting densities of 400 kg/m3, 700 kg/m3, and 1000 kg/m3, the peak temperatures are 62.33°C, 81.03°C, and 94.27°C, respectively, and the maximum values of the temperature change rate are 7.5°C/h, 11.7°C/h, and 14.3°C/h in position 1. The peak temperature, the maximum temperature change rate, and the maximum temperature difference increase with an increase in the casting density at different positions in the foamed concrete. For the foamed concrete without an admixture, the strength increases significantly during the early stage and decreases during the later stage under temperature matched curing conditions. The strengths are improved for all curing times when the fly ash is added, and the effect increases with the increase in the fly ash content. Standard curing conditions and temperature matched conditions have an effect on the structure of composite cementitious materials mixed with fly ash at the later stage. The structure of foamed concrete hole wall is more compact under temperature matched conditions, increasing its compressive strength. Due to external factors occurring in the field tests, the maximum temperature increments are lower in the field tests than in the indoor model test for the same casting density. Reasonable cooling measures and the addition of the fly ash decrease the maximum temperature increments.
The data in this article allow researchers to verify the results, replicate the analysis, and conduct secondary analyses.
The authors declare that they have no conflicts of interest.
This work was supported by the Lanzhou Jiaotong University youth science foundation project (2019026), the Sichuan Science and Technology Support Project (2016JY0005), and the China Railway Science and Technology Research Plan Project (2015G002-K).