Strength of parent rock made of artificial sand (unit, MPa).
This chapter presents different experimental results regarding the influences of normal-weight sand and lightweight sand (shale pottery (SP)) on different types of concrete. Because of the porosity of lightweight aggregates (LWAs), which can absorb and release water in concrete, the effect of concrete curing is better, and thus the properties of concrete are improved. On the other hand, because the lightweight coarse aggregate (LWCA) rises easily in all-lightweight concrete (ALWC) during pumping and vibration and the cost of ALWC is also higher, a method of replacing part of the lightweight aggregates in ALWC with normal-weight aggregates is used. These new types of concretes include sand lightweight concrete (SLWC), gravel lightweight concrete (GLWC), hybrid aggregate lightweight concrete (HALWC), and so on. This chapter mainly discusses the properties of lightweight aggregate concrete (LWAC), lightweight sand foamed concrete, lightweight sand mortar, and reinforced LWAC. The chapter also includes LWAC of high temperature, low temperature, durability, and uni- and multiaxial mechanical properties according to the results of our research group over recent decades. All of the experimental results show that the properties can meet Chinese National Code requirements.
- lightweight aggregate concrete
- foamed concrete
- reinforced concrete beam
- high temperature
- low temperature
- shale ceramsite
- shale pottery
There are notable differences between lightweight aggregates and normal-weight aggregates (NWA), so what are the different effects on different types of concrete?
Lightweight coarse aggregate has a softening effect; does lightweight aggregate concrete have this effect too?
Can the lightweight concrete be used under special environmental conditions such as negative temperature, elevated temperature, and chemical corrosion?
Are there any changes in the mechanical performance of lightweight concrete under complicated stress and in reinforced concrete?
Since 2005, it has been prohibited to mine river sand in most areas of China. The relevant laws, such as the
2. Technical requirements of normal-weight sand
NWFA can be distinguished from lightweight fine aggregate (LWFA) by the apparent density (
In order to standardise the artificial sand (AS), namely, manufactured sand (MS), the terms, definitions, classifications, specifications, technical requirements, test methods, inspection rules, and so on are stipulated in the Chinese national standards Sand for Construction (GB/T14684-2011) , Technical Specification for Application of Manufactured Sand Concrete (JGJ/T241–2011) , and Standard for Technical Requirements and Test Method of Sand and Crushed Stone (or Gravel) for Ordinary Concrete (JGJ52–2006) .
|Igneous rock||Metamorphic rock||Sedimentary rock|
The fineness module (
|Nominal diameter of sand and sieve hole||5 mm||2.5 mm||1.25 mm||630 μm||315 μm||160 μm|
|Mesh size (length of sieve hole)||4.75 mm||2.36 mm||1.18 mm||600μm||300μm||150 μm|
|Cumulative percentage retained (%)||Zone I||10–0||35 − 5||65–35||85–71||95–80||100–90|
|Zone II||10–0||25–0||50 – 10||70–41||92–70||100–90|
For concrete production, it is recommended that sand from Zone II be used. The sand ratio (sand-to-sand and coarse aggregate weight ratio,
3. Technical requirements of lightweight aggregate
Lightweight aggregate (LWA) includes artificial, natural, industrial waste slag, cinder, and spontaneous combustible coal gangue LWA . LWA is called super-lightweight coarse aggregate (SLWCA) when
|Bulk density grade (||Tube crushing strength (MPa)||Strength grade of LWAC|
According to the current LWA production technology and its use in actual engineering, the particle size distribution of LWA is shown in Table 4.
|Mesh size (length of sieve hole)||4.75 mm||2.36 mm||1.18 mm||600μm||300μm||150 μm|
|Cumulative percentage retained (%)||0–10||0–35||20–60||30–80||65–90||75–100|
|Continuous grading (5–16 mm)||16.0 mm||9.50 mm||4.75 mm||2.36 mm|
|Cumulative retained percentage (%)||0–10||20–60||90–100||95–100|
|Continuous grading (5–10mm)||9.50 mm||4.75 mm||2.36 mm|
|Cumulative percentage retained (%)||0 − 15||90–100||95–100|
4. Lightweight aggregate concrete
All-lightweight aggregate concrete (ALWAC), also known as all-lightweight concrete (ALWC), is made from LWFA, LWCA, cement, water, and other admixtures. Although ALWC has many excellent properties, such as high specific strength (ratio of cubic compressive strength to dry apparent density,
The term ‘ALWAC’ was first mentioned in 1972 based on the available literature in the EI and SCI databases . If, on the basis of ALWAC, only a part of LWFA is replaced with normal-weight sand in the same volume ratio
Compared to ALWC and NWC, the abovementioned concrete can be uniformly named semi-lightweight concrete (semi-LWC), where the term ‘semi’ means the LWA is less than half, just half, or more than half in volume. But according to , the concrete is called LWAC when the dry apparent density of concrete (
In this study, the LWFA and LWCA are shale pottery (SP) and shale ceramsite (SC), while the NWFA and NWCA are MS or natural sand (NS, also known as river sand) and crushed stone (CS), respectively. Both LWCA and NWCA are crushed aggregates. The technical parameters of LWA and normal-weight aggregate (NWA) are listed in Tables 5 and 6, respectively.
The porosity of SC and SP is 51.3 and 23.9%, and the void ratio is 41.7 and 47.0% according to tests following GB/T17431.2-2010 , respectively. So the SC must be pre-wetted 24 hours (h) before production of concrete in order to prevent reabsorption of mixing water, because the water absorption rate (%,
The ALWC is more sensitive to mix design than NWC mainly because of the higher porosity, lower bulk density, and lower tube crushing strength (TCS). Although the interface between crushed angular LWA and mortar is very good, the TCS is significantly lower compared to the mortar, and the LWCA floats more easily. The internal structure of ALWC becomes non-uniform, and thus the strength of ALWC depends on the strength of the mortar.
Many experiments have shown that the strength of ALWC mainly depends on the mass of the maximum diameter of LWCA, the mass of cementitious material (cement, fly ash (FA), silica fume, and other admixtures with gelling capacity), the water-to-binder weight ratio (
Generally, the strength of NWC increases with curing time; however, the strength of ALWC decreases when the diameter of LWCA is larger than 20 mm. Because of that, the maximum diameter of LWCA should be 15 mm or smaller. On the other hand, the SC has a softening effect, the softening coefficient (
All of the following concretes are designed by pumping concrete; that is, the slump is 160–220 mm, mainly taking LC30, for example. The reference mixes and test results are shown in Table 8.
In Table 8, the cementitious material is PO42.5 Portland cement and Grade II fly ash. The water reducing rate of high performance water reducing agent is not less than 20% and added 1.6–2.0 wt% (by mass of cementitious material).
Because of the difference of LWA and NWA, the strength, elastic modulus, and dry apparent density are increased when LWFA or LWCA is replaced separately by normal-weight aggregates, but it is more complex when replaced at the same time.
Also, the ratio (
4.1 Autogenous shrinkage properties of LWACs
Because the hydration reaction of cement is an exothermic process, the amount of heat released leads to a temperature difference both inside and outside the concrete, and the temperature stress induces the appearance of cracks.
|SC||>16 mm (%)||16.0 mm (%)||9.50 mm (%)||4.75 mm (%)|
|GB/T17431.1–2010||≤ 5||≤ 10||20–60||85–100|
|Tube crushing strength (TCS) (MPa)||Mean||Over mean||GB/T17431.1–2010|
|SP||4.75 mm (%)||2.36 mm (%)||1.18 mm (%)||0.6 mm (%)||0.3 mm (%)||≤0.15 mm (%)|
|Gravel||>16 mm (%)||16.0 mm (%)||9.50 mm (%)||4.75 mm (%)||2.36 mm (%)|
|MS||4.75 mm (%)||2.36 mm (%)||1.18 mm (%)||0.6 mm (%)||0.3 mm (%)||≤0.15 mm (%)|
|GB/T 14684–2011||0–10||0–25||10− 50||41–70||70–92||80–94|
|NS||4.75 mm (%)||2.36 mm (%)||1.18 mm (%)||0.6 mm (%)||0.3 mm (%)||≤0.15 mm (%)|
|GB/T 14684–2011||0 − 10||0–25||10–50||41–70||70–92||90–100|
|Diameter range (mm)||0.5 h||1 h||2 h||4 h||6 h||8 h||12 h||24 h||32 h||48 h||72 h|
|Diameter range (mm)||7 d||14 d||28 d||60 d||90 d||120 d||180 d|
|Type of concrete|
|Maximum temperature (°C)||66.4||63.9||62.4||65.5|
|Calculated temperature (°C)||66.1||69.7||59.5||53.5|
Because the gas pressure in the Thermos bottle becomes higher as the hydration reaction proceeds, the Thermos glass liners burst at a certain time, as shown in Table 9. However, the length of time is shortest for HALWC. The reason may be the uniform distribution of normal-weight fine and coarse aggregates, which can improve the heat conduction rate and provide a better temperature distribution. On the contrary, the time period of the temperature rise is shorter for SC and SP compared to SLWC and GLWC, because of the higher porosity of LWA, which provides a better insulation performance. The difference between SLWC and GLWC may be that the distribution of NWFAs is more uniform than that of coarse aggregate, such as the sand particles touched in dot form can speed up heat conduction.
|ALWC-MP50 + LP50||—||—||33.7||29.8||34.7||85.9|
|SLWC-MP50 + LP50||—||—||—||—||40.1||—|
|GLWC-MP50 + LP50||—||—||—||—||46.1||—|
|HALWC-MP50 + LP50||—||—||—||—||40.8||—|
Autogenous shrinkage of concrete mainly happens after the initial setting time, but chemical shrinkage, which includes three complete stages, also has a significant influence. According to the mechanism of concrete shrinkage, chemical shrinkage happens because the absolute volume of hydration products is smaller than that of water and binding material during the early stage. Autogenous shrinkage happens during the skeleton structure forming in a later stage, so the unhydrated cement particles react further. In fresh concrete, the volume can also cause shrinkage because of the setting of parts of particles. Drying shrinkage is caused by water loss. In short, the cracks in concrete are mainly caused by plastic shrinkage in the early stage.
In Figure 2, the curves are smoothed after the peak values because of higher fluctuation (shown by dashed lines). The higher porosity and rougher surfaces of SC and SP also result in larger specific surface areas (total area of material per unit mass, m2/g), which can absorb more cement particles and thus improve hydration conduction, so the autogenous shrinkage of ALWC is greater in a shorter time period and then becomes stable. Among SLWC, GLWC, and HALWC, the autogenous shrinkage is mainly determined by the amounts of LWA. Because the specific surface area of aggregates is different, the internal distribution of aggregates is uniform, and parts of cement particles are subsident, which can determine the internal temperature stress field, so the resistance capability of plastic deformation is different.
4.2 Durability properties of LWACs
The effects of different mineral admixtures on the durability of LWACs were studied. Table 10 shows the effects of substituting 75 wt% mineral powder (denoted as MP75; the activity index is 96% in 28 days) for fly ash and 50 wt% limestone powder (denoted as LP50), respectively. And both mineral powder and limestone powder are mixed in a ratio of 1:1, and total substitution for fly ash (marked MP75 + LP50) is based on Table 8 (with a slight difference). The test method is according to GB50082–2009 .
Mineral powder can enhance both the strength and durability of concrete. Although the added limestone powder only reduces the strength of concrete, the requirements can be met, and the cost can be reduced. Because fly ash has become a scarce resource in China, mineral and limestone powder can be an effective alternative in the ready-mixed concrete industry.
Generally, the effects of normal-weight aggregates and mineral admixtures on carbonation and electric flux are not obvious, but according to GB50082-2009 , when the electric flux is between 1000 and 2000 C, the grade of chloride iron penetration is low, so the concretes can meet the requirements of the code.
4.3 Softening properties of LWACs
Since SC has a softening effect according to Table 7, is there also an SC concrete softening effect? The test results (Table 11) show that the LWACs almost have no softening effects. A possible reason is that the LWAs are strengthened because of their absorption of cement particles and hydration; on the other hand, the main contribution to the strength comes from the cement mortar, which does not show softening.
|Type||28 d||60 d||90 d||120 d||180 d|
Table 11 shows that the concrete strength and elastic modulus increase under curing in water on different days. However, the HALWC falls slightly, and the elastic modulus is almost constant. Similarly, for any type of LWAC, all of the
The test results show the LWACs are without a softening effect, so they can be used in hydraulic structure engineering and underground engineering.
4.4 Properties of LWACs after elevated temperature treatment
The appearance characteristics and strength of LWACs are shown in Tables 12 and 13 under different temperatures (
|300||Light grey||Fewer hairline cracks||0.06||0.04||0.05||0.04|
|400||Off-white||More hairline cracks||0.16||0.12||0.14||0.12|
|600||Brownness||Honeycomb cracks with surface wrapping||0.30||0.26||0.28||0.24|
Although the mass loss in different types of concretes shows no obvious difference after high-temperature treatment, the effect of the addition of NWAs alone on the strength and elastic modulus is higher and changes regularly; that is, added NWCA alone larger than added NWFA alone, but smaller when added at same time than added NWCA only.
In general, the residual values of elastic modulus are larger than those of strength, which means the anti-deformation capacity of concrete decreases with rises in temperature. On the other hand, the residual strengths of LWCAs after high-temperature treatment are very close to or even higher than that of NWC, which indicates that LWACs can be used for fire-resistant design. For example, the residual strength of axial compression is 95% at 200°C, 80–90% at 300°C, 70–75% at 400°C, 60–65% at 500°C, and around 50% at 600°C. The residual splitting tensile strength is around 90%, 75–80%, 60–65%, 50−55%, and around 45% at 200, 300, 400, 500, and 600°C, respectively.
4.5 Properties of LWACs cured at negative temperature
During the construction process used by the artificial freezing method, the ambient temperature in the working place is from −8 to −12°C in China. Concrete properties after curing at negative temperature are shown in Tables 14 and 15, where the specimen is wrapped with a layer of quilt after being poured and then put into a low-temperature test chamber.
|ALWC, ||(%)||GLWC, ||(%)|
|1 d||2 d||3 d||7 d||14 d||28 d||3 d||7 d||14 d||28 d|
|ALWC, ||(%)||GLWC, ||(%)|
|3 d||7d||14 d||28 d||3 d||7d||14 d||28 d|
The compressive strength and elastic modulus of the specimens cured at −5°C meet the basic requirements (up to 90%) both with and without anti-freezing agent, but at −10°C, they can satisfy the requirements when added anti-freezing agent.
Although the strength and elastic modulus of other groups fail the basic requirements, the hydration reaction does not stop but only diminishes. Compared with NWC, the strength of the specimen is higher, which shows that it helps promote the hydration reaction because of the heat preservation and inner curing effect of LWA. On the other hand, the strength of GLWC is slightly higher than that of ALWC under the same conditions. The reason is the same as the one mentioned above, namely, that the elastic modulus of gravel is larger than that of ceramsite.
Also, fibre can enhance the strength and elastic modulus of specimens cured at negative temperature, and the laws are also the same; that is, the performance of concrete is better with elastic modulus of fibre increasing.
To summarise, the LWACs can meet the requirements of freezing process construction.
4.6 Uniaxial stress-strain curves of LWACs
The uniaxial stress-strain curve of LWACs is similar to that of NWC, as shown in Figure 3, but the total strain of LWACs is significantly larger than that of NWC. The symbols of stress and strain obey the following rules: the plus sign ‘+’ denotes tension; the minus sign ‘–’ denotes compression.
where and .
|Strength grade||Ascending curve||Descending curve|
Although Figure 4 does not contain descending curves, the law of different LWACs is the same; that is, the ultimate stress decreases, and the ultimate strain increases with the temperature increase, which shows that the plastic deformation gets larger because the strength of the cement mortar matrix decreases. On the other hand, under the same temperature, the effects of a small quantity of NWA on the ultimate strain are not significant; only the effect on the ultimate stress is remarkable. At the same time, ALWC is similar to SLWC, and GLWC is similar to HALWC.
4.7 Multiaxial strength of LWACs
Under multiaxial compressive stresses, the ultimate compressive strength of concrete will increase significantly, and therefore the failure modes change. For example, ALWC undergoes the phenomenon of squeeze flow, and a plastic plateau appears in the stress-strain curve under the two larger lateral stresses. Tables 17 and 18 show the bi- and triaxial ultimate compressive strengths tested by a large real triaxial test system, respectively. To reduce the friction, a two-layer polythene film with lithium base oil smeared between the layers is used, which can guarantee that the strength under single stress action (
|Eq. (5) (MPa)|
|Eq. (6) (MPa)|
The smaller relative error indicates that the test data are reliable and the formulas are correct. On the other hand, the multiaxial ultimate strength increases with increasing lateral stress; the law is the same as for NWC, but the ratio of
The ultimate strength and elastic modulus of LWACs under traditional triaxial stresses are shown in Table 19.
|−2 MPa||−4 MPa||−6 MPa||−8 MPa||−10 MPa|
All of the strength and elastic modulus values increase with increasing confining pressure. Under the same temperature and confining pressure, the effect on NWA is highest when using the lowest amount of hybrid aggregate, such as gravel, NS, and LWS. However, when the temperature is above 300°C, the strength of HALWC is smaller than those of SLWC and GLWC. At the same time, below 300 °C, the strength increases, except for HALWC, whose strength increases at temperatures below 200°C. The relationship between ultimate compressive strength and confining pressure can be expressed by Mohr-Coulomb theory as shown in Eq. (7).
The absolute values of relative error are all smaller than 5%.
5. Properties of lightweight sand foamed concrete
In China, traditional foamed concrete generally consists of cement, NS, water, foam agent, and so on [13, 14]. Because the densities of cement and NS are significantly higher than the density of water, these particles sink easily and therefore cause the foamed concrete to crack. According to the properties, the bulk density of LWA is smaller than that of water, and thus the foamed concrete consists of LWA foamed by physical foaming, which can be called all-lightweight foamed concrete (ALWFC). It does not crack and also makes a higher-strength-grade concrete (up to LFC 30; LFC is the code name of strength grade of mortar). Because of these properties, it can be widely used in non-structure and structure concrete and pumped but not vibrated. The LWAs are SC and SP in foamed concrete in this study, and the mixes are shown in Table 20.
Although there are countless air pores in ALWFC, most of these pores are discontinuous, so ALWFC has better durability. According to the test results, the carbonation depth generally does not exceed 5 mm in 56 days, and the resistance performance with regard to chloride ion permeability, that is, the electric flux after 6 hours, is smaller than 1000 C. On the other hand, the ALWFC also has better fire resistance, sound insulation, and sound absorption capabilities.
6. Properties of reinforced ALWC
|PSB||8||316||434||2.1 × 105|
|10||329||457||2.0 × 105|
|12||335||482||2.0 × 105|
|CRB||16||342||527||2.0 × 105|
|18||362||576||2.0 × 105|
|22||396||612||2.0 × 105|
|No.||Cracking load and maximum width of crack||Service load and maximum width of crack||Failure load and maximum width of crack|
|No.||Yield load and deflection||Ultimate load and deflection|
The test results according to GB50152-2012  are as follows.
According to [16, 17], during the beam flexural test, the maximum crack width should not exceed 0.3 mm under service loads, and the deflection should not exceed
For the shear beam, because there is no warning before the occurrence of diagonal cracks, the diagonal cracks occur in the shear span section when the load is 20% of the ultimate load and then rapidly expand to the length of 100–150 mm. The initial width of the diagonal crack is generally 0.05 mm in the reinforced NWC beam compared to 0.03 mm in this study. At the same time, the maximum width of cracks and the deflection under service loads are 0.29 mm and 10.05 mm, respectively, thus meeting the code requirements.
The theoretical and test values of ultimate strength for normal and diagonal sections are shown in Table 25. All the test values slightly exceed the theoretical values. Compared to a reinforced NWC beam with the same stiffness, the width and height of the cross-section need to be increased by 18%, respectively. On the other hand, if the section remains unchanged, according to the numerical simulation results for a seven-storey residential building, the total weight of the building is reduced by around 14.8%, and the inter-storey displacement angle is increased by around 27.5% under earthquake loading. This is because ALWC has a bigger ratio of cubic compressive strength to dry apparent density, a smaller elastic modulus, and a larger anti-deformation capacity.
7. Properties of lightweight sand mortar
7.1 General performance of mortar
Because of the rough surface and higher porosity, SP can absorb cement particles and water, which leads to poor mixture workability, so an admixture of cellulose ether, emulsion powder, and so on must be added to meet the code requirements for the mortar consistency and delamination degree of mortar [18, 19, 20]. The mix proportions for different strength grades and the test results are shown in Table 26.
Compared with the code [18, 19], the dry apparent density of lightweight sand mortar is smaller than 1900 kg/m3, and the heat conduction coefficient is smaller than 0.8–1.0 W/(m·K). Because of the porosity of SP, the dry apparent density and heat conduction coefficient are smaller than those of normal-weight sand mortar, so SP has better thermal insulation performance.
7.2 Durability of mortar
Taking carbonisation, chloride iron penetration, and sulphate attack, for example, the test results are shown in Table 27.
|Strength grade||Mass loss (g)||Corrosion resistance coefficient (%)|
The durability of mortar is enhanced with increases in the strength grade. Especially until 30 cycles, the mass increases. Analogously, the sulphate resistance coefficient is also enhanced until 15 cycles. The reason for this is the porosity and water absorption capacity of SP, which can strengthen the internal curing capacity and thus promote the hydration reaction.
7.3 Fire-resistant performance of mortar
The test results for cubic compressive strength, tensile bond strength, and heat conduction coefficient after elevation of the temperature are shown in Table 28, where the average values of mass loss are 9, 19, 39, 53, and 65 g after 100, 200, 300, 400, and 500°C, respectively.
|LM 5.0||LM 7.5||LM 10||LM 15||LM 20|
The behaviour of the mortar is similar to that of concrete after the elevation of temperature, and the residual strengths after high-temperature treatment are almost 75% at 500 °C and can therefore meet the fire protection design requirements. Below 300°C, the strength and heat conduction coefficient increase; however, at temperatures above 300°C, the strength and heat conduction coefficient decrease, and all of the parameters increase with increases in the strength grade.
ALWC has a number of advantages and disadvantages. Using NS (or MS) and crushing stone to replace a part of LWAs alone or at the same time in equal volume ratio, the new concrete types can be called semi-lightweight concrete (semi-LWC), which includes SLWC, GLWC, HALWC, and so on. Semi-LWC can not only reduce the cost of ALWC but also increase the properties of ALWC, such as workability, strength, durability, anti-deformation, fire resistance, and so on. Especially, moderate amounts of mineral powder and limestone powder can significantly increase the strength and durability.
All types of the concrete can meet the Chinese National Code requirements as well as have a smaller heat conduction coefficient and higher ratio of cubic compressive strength to dry apparent density than NWC. However, the effect of NWAs on semi-LWC is different. Gravel aggregates are bigger than sand aggregates, so the effect is more complex when added simultaneously. At the same time, the multiaxial strength increases with increasing lateral pressure, and the ratio of biaxial compressive strength to uniaxial compressive strength is slightly larger compared to NWC. This is because the NWA has better thermal conductivity and a small quantity of NWAs can help to reduce autogenous shrinkage of ALWC. On the other hand, the axial compressive strength of ALWC is close to cubic compressive strength, which shows that the ALWC has a self-lubricated antifriction effect because of the SC. On the contrary, if the diameter of SC is too large, its TCS is too small. The strength of ALWC decreases with increasing curing age, so the diameter of SC should not exceed 15 mm in general.
Although the SC has a softening effect, the LWACs do not, so they can be used in hydraulic structure engineering. Even the stiffness of the reinforced ALWC is smaller than that of NWC; because of the smaller modulus of elasticity and apparent density, the reinforced ALWC has a better bending and shear properties. Moreover, the maximum width of crack in ALWC is smaller than that in NWC, so buildings made of ALWC can have better anti-seismic properties.
The highest strength grade of foamed concrete made of SC and SP can be up to LFC 30, so it can be used in non-structure and structure construction, and it has better performance in terms of thermal resistance, sound absorption, insulation, fire resistance, and so on. On the other hand, mortar made of SP can also be used in plastering mortar and masonry mortar and has the abovementioned excellent characteristics.
At high temperature, the performances of LWACs and mortar decrease with increasing temperature but can be increased with increasing lateral pressure. In negative-temperature curing within the range of −15°C, the LWACs can meet the construction requirements of the artificial freezing method.
Finally, it should be pointed out that the descending curve of the stress-strain curve of LWACs cannot be measured easily, especially after elevation of the temperature. Even so, the formulas provided in this paper can meet the demands of experimental precision under both uni- and multiaxial stress states.
This work was financially supported by the National Natural Science Foundation of China (41172317; 51774112; 51474188). This chapter was polished by Proof-Reading-Service.com Ltd.