properties of partition wall blocks prepared with high percentages of recycled clay brick after...

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Corresponding author: [email protected]

Original citation:

Xiao, Z, Ling, T.-C., Poon, C.-S., Kou, S.-C., Wang, Q, Huang, R. (2013) Properties of

partition wall blocks prepared with high percentages of recycled clay brick after

exposure to elevated temperatures. Construction and Building Materials; 49:56-61.

http://www.sciencedirect.com/science/article/pii/S0950061813007344

Properties of partition wall blocks prepared with high percentages of

recycled clay brick after exposure to elevated temperatures

Xiao, Z, Ling, T.-C., Poon, C.-S., Kou, S.-C., Wang, Q, Huang, R.

Research Highlights

• High temperature properties of concrete blocks made with clay brick aggregate

(CBA) were examined.

• At 300°C, the nature of CBA origin made the concrete blocks stronger/more stiff.

• At 800°C, a higher percentage of CBA used enabled a higher residual flexural

strength.

• At 800°C, CBA blocks retained about 48-91% of their original (20°C) compressive

strength.

Abstract

High temperature properties of partition wall concrete blocks prepared with recycled

clay brick aggregate derived from construction and demolition (C&D) waste streams

(e.g. collapsed masonry after an earthquake) were studied. For this purpose three series

of concrete block mixes were designed using coarse and fine clay brick aggregate to

replace recycled concrete aggregate and sand at ratios percentages of 25%, 50% 75%

and 100%. The residual density, mass loss, compressive and flexural strengths after

exposure to elevated temperatures of 300°C, 500°C and 800°C were determined. The

results demonstrated that selection of an appropriate replacement for both coarse and

fine clay brick aggregates can lead to better performance of the blocks at elevated

temperatures. It is expected therefore that there will be significant advantages in terms

of sustainability and fire safety by adopting this inherent fire-resistant material in

concrete blocks especially for low rise residential developments.

Keywords: Clay brick aggregate, recycled concrete aggregate, elevated temperatures,

concrete blocks, compressive strength, flexural strength

1. Introduction

A huge quantity of construction and demolition (C&D) waste, which constitutes a

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major portion of the total solid waste stream, is generated every day [1]. The C&D

waste includes, but is not limited to, demolished concrete, bricks, masonry, wood, steel,

plaster, etc. According to a study by Xiao et al. [2] who reported that in the 2008

Sichuan earthquake about 382 million tonnes of construction waste derived mainly

from collapsed buildings was generated. Most recently, a 6.6-magnitude earthquake hit

Ya’an city, Sichuan Province on 2013 April 20, killing at least 192 people and injuring

thousands [3]. It has been estimated that 90% of all houses in the Ya’an area collapsed

due to this earthquake [3], resulting in huge quantities of waste. The nature of waste

generated by earthquakes in China has created a great challenge in its disposal and its

impact on the environment has also drawn considerable attention.

For the past few decades, systematic management systems and methods of recycling

the C&D waste have been extensively discussed in the literature [4-7]. Among them, it

is commonly accepted that recycling and reuse of C&D waste as an alternative

aggregate in construction is one of the most economic and environmentally friendly

ways of managing the waste. In addition to the environmental benefits in reducing the

demand for land in which to dispose of the C&D waste, the recycled aggregates derived

from the waste can also help to conserve natural materials.

A large number of studies has focused on reusing recycled concrete aggregate (RCA) to

produce new concrete. The potential benefits and shortcomings of using RCA have

been well researched [8-12]. Partial replacements of natural aggregates by RCA (30%

or less) in concrete do not jeopardize the mechanical properties [9]. However, it is

generally agreed that the compressive strength of concrete gradually decreases as the

amount of RCA increases [10]. Furthermore, the high water absorption properties of

recycled fine aggregate significantly affect durability by inducing excessive shrinkage

problems in concrete [11,12]. The above mentioned drawbacks therefore restrict the use

of recycled fine aggregate in concrete and limit the amount of RCA that can be used for

structural concrete.

In comparison to wet-mixed conventional concrete, a number of studies [13-16] has

demonstrated that it is feasible to incorporate up to 100% RCA as coarse and fine

aggregates for the production of non-structural precast concrete blocks using a

dry-mixed approach. This is probably due to the use of a combination of vibration and

compaction forces in the dry-mixed production process that could improve the density

(packing) and the quality of the concrete products produced.

A preliminary study was conducted to investigate the influence of the incorporation of

crushed clay brick as a replacement for coarse RCA and natural sand on the properties

of dry-mixed masonry partition wall blocks [16]. It was found that the hardened density

and drying shrinkage of the blocks decreased with increase in the clay brick aggregate

content. The overall results suggested that the replacement percentage of coarse

aggregates by crushed clay brick should be controlled at less than 25%, while the

replacement of natural sand by fine crushed clay brick should be within 50-75%.

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As for building applications, masonry partition wall blocks prepared with recycled

aggregates (RCA and clay brick aggregates) may be affected by fire. The aim of the

present work is to study the properties of partition wall concrete blocks after being

subjected to elevated temperatures of 300°C, 500°C and 800°C. The residual high

temperature properties (compressive and flexural strengths) and the total mass loss of

concrete blocks prepared with coarse and fine crushed clay brick to replace recycled

concrete aggregate and natural river sand at different (25%, 50%, 75% and 100%)

ratios were examined.

2. Experimental details

2.1. Materials

The cementitious material used in this work was an ASTM type 1 ordinary Portland

cement (OPC) with a density of 3160 kg/m3. The recycled concrete aggregate (RCA)

used was obtained from a recycling facility located in Hong Kong. At the facility,

crushed concrete rubble (from demolition projects) was crushed and processed to

produce coarse recycled aggregate with particle sizes between 5 and 10 mm. Crushed

clay brick originally from red bricks were produced by crushing and sieving them into

two different particle sizes: 5-10 mm (coarse brick aggregate CBA) and <5 mm (fine

brick aggregate FBA). River sand with a maximum size of 2.36 mm was used as the

fine natural aggregate in this study. The physical properties of all coarse and fine

aggregates were tested according to BS 882 (1992) and ASTM C128 and the results are

presented in Table 1. The photographs and grading curves of these aggregates are

shown in Figs. 1 and 2.

2.2. Partition wall block mix proportions

A total of three series of partition wall block mixtures were designed (see Table 2) and

fabricated in the laboratory using a dry-mixed method described in our previous studies

[16,17]. The block specimens produced aimed to meet the requirements stipulated by

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the BS 6073 (1981) specification for partition wall blocks. All the mixtures were

proportioned with a fixed total aggregate/cement ratio of 11.5, and 65% of the total

aggregate was fine aggregates (<5 mm). In each of the mix series, five different mix

proportions were prepared.

In Series 1 and 2, the influence of using crushed clay brick for sand replacement (fine

aggregate) at 25%, 50%, 75% and 100% was examined. The coarse aggregates with

RCA/CBA ratios of 3 and 1 were fixed in Series 1 and 2 respectively. For Series 3, the

effect of using crushed clay brick as coarse aggregate was investigated with RCA being

replaced by CBA at 25%, 50%, 75% and 100% by weight, while a fixed sand/FBA ratio

of 1 was used (in fine aggregate). The details of the mix proportions are shown in Table

2.

Each concrete block was fabricated by applying a compaction stress of 25 N/mm2 on

the materials in steel moulds with internal dimensions of 200×100×60 mm. The

fabricated block specimens were then covered with a plastic sheet and left at room

temperature of 23±3 °C and 75±5% relative humidity (RH). After one day, they were

demoulded and further cured (covered by a hemp bag to maintain a RH of over 90%) at

room temperature of 23±3 °C for at least 28 days before testing.

2.3. Heating temperature and testing

The specimens were heated to 300°C, 500°C and 800°C separately. The heating rate

was set at 2.5°C per minute until the target temperature was reached and the respective

maximum temperature was maintained for 4 h to ensure uniform temperature

distribution throughout the specimens. The specimens were allowed to cool naturally to

room temperature prior to testing.

The mass of the concrete blocks before and after each heating cycle was determined

using an electronic digital balance with an accuracy of ±0.1 g. The mass loss values

were calculated according to Mloss = (Minitial - Mheated)/ Minitial, where Minitial and Mheated

are the initial mass (before heating) and the heated mass (after heating) respectively.

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The compressive strength was determined by using a universal testing machine with a

maximum capacity of 3000 kN. A loading rate of 450 kN/min was applied to the

nominal area of the block specimen. Prior to the loading test, the block was soft capped

with two pieces of plywood. Three samples were tested after heating to each

temperature and the average values are reported.

The flexural strength of the block specimens was determined by a three-point bending

test with a supporting span of 180 mm. For this test, a test machine with a maximum

load capacity of 50 kN was used and a displacement of 0.10 mm/min was set. Two

samples were tested for each group of specimens.

3. Results and discussion

3.1. Initial density and mass loss

The dry hardened densities of fabricated concrete blocks before being exposed to the

elevated temperatures are shown in Fig. 3. The density of the concrete blocks decreased

with increase in clay brick content. These results can be explained by the lower density

of the recycled brick aggregate as compared to both sand and RCA (see Table 1).

The mass loss of the heated specimens at 300°C, 500°C and 800°C are shown in Figs.

4-6 respectively. In general, the data show that the percentage of mass loss for Series 1

and 2 block specimens increased with increase of FBA content. It is clear that the

replacement of sand with clay brick with a much higher water absorption capacity

somewhat increased the mass loss. This was particularly noticeable at 300°C because

the mass loss at this temperature was mainly accounted for by the loss of the absorbed

water in the aggregates and the free water in the cement matrix. Beyond this

temperature, the effect of aggregate types on mass loss appeared to be less significant.

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On the contrary, the mass loss value of the block specimens in Series 3 decreased when

the CBA content in the mixtures increased. In this case, it seems that the water

absorption capacity of the aggregates was not the only reason affecting the mass loss of

the heated blocks. Loss of bound water from the dehydrated old-cement paste attached

to the RCA might have contributed to the additional mass loss as the temperature

reached 500°C and above.

3.2. Compressive strength

The effects of using coarse and fine clay brick for RCA and sand replacements on the

compressive strength at ambient temperature (20°C) have been previously discussed

[16]. Figs. 7-9 show the trend of compressive strength at the elevated temperatures for

Series 1-3 block specimens.

Regardless of the aggregate type, all the heated concrete blocks at 300°C and 500°C

exhibited higher compressive strength than that at 20°C. The increases in compressive

strengths of the concrete blocks with clay brick aggregates at these temperatures can be

attributed to a few reasons. First, the nature of the clay brick made them stronger/more

stiff under these temperatures as Nguyen et al. [18] reported that the compressive

strength of a pure clay brick is twice the original strength after exposure to 400°C.

Second, some of the unhydrated cement paste might undergo additional hydration at

these temperatures [19] as the high temperature could increase the degree of

crystallinity of originally formed hydrates [20] which helps to improve their cementing

ability and the concrete’s strength.

It can be seen that the influence of FBA content was very similar at 300°C and 500°C

(see Figs. 7 and 8), where the blocks with 25-50% and 50-75% of FBA content

displayed the highest strength in Series 1 and 2 respectively. In Series 3 (Fig. 9), an

increase in the CBA content resulted in a decrease in the compressive strength at

ambient temperature. However, above this temperature the CBA concrete blocks had

more or less similar residual strengths as compared to that of the reference mixture

(with 100% of RCA). This reveals that the coarse brick aggregate performed better at

high temperatures than the recycled concrete aggregate.

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It can be seen from the above figures and Table 3 that all the compressive strengths

obtained at 800°C were lower than the initial strength (at ambient temperature). Such

results were not unexpected due to the decomposition of the new calcium silicate

hydrates (CSH) of the cement paste and the old cement mortar adhering to the RCA. In

comparison, Series 1 concrete blocks with a higher RCA/CBA ratio experienced a

slightly higher reduction of strength as compared to the concrete blocks in Series 2. It is

worth noticing that at this temperature, the Series 1-3 concrete blocks maintained

48-73%, 62-75% and 48-91% of their respective initial strengths, depending on the

content of clay brick aggregate used. This shows that the higher the clay brick

aggregate content is, the higher the strength that can be retained. This points to a

potential application of the blocks containing clay brick in high temperature conditions

(compared to normal concrete only 10-30% of its original strength can be retained after

exposure to high temperature at 800°C [21-23]).

3.3. Flexural strength

Figs. 10-12 show the effects of high temperature on flexural strength. It can be clearly

seen that the content and type of aggregate used had a similar influence at 20°C and

300°C. This is particularly noticeable when the replacement level of sand by clay brick

was up to 100%. For instance, the flexural strength of S1-100, S2-100 and S3-100

mixes at 300°C were enhanced by 27%, 24% and 15% respectively when compared to

their original strength obtained at the ambient temperature. The improvement in

flexural strength can be partly accounted for by the better bonding behaviour of the clay

brick aggregates at this temperature. This is in agreement with the finding by Lea and

Straddling [24], who indicated that the higher gain in flexural strength of concrete at

temperatures up to 300°C was mainly attributed to the increase of bond strength

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between the aggregate and the paste.

Unlike compressive strength, at the temperature of 500°C the flexural strength suffered

higher losses ranging from 20-54%, regardless of the content and type of aggregate

used. Apart from the dehydroxylation of calcium hydroxide (CH) occurring at

300-500°C [25,26], it seems that the microcracking which occurred at this temperature

affected flexural strength (further weakening the bond and the interfacial transition

zone in the cement matrix) more than it did the compressive strength. This is because

generally the flexural strength is more sensitive to the presence of such cracks than

compressive strength.

Beyond 500°C, a continuous drop in the flexural strength was observed and only a

small part of the initial strength was left at 800°C. This is particularly obvious for

control mixes, as S1-0, S2-0 and S3-0 only retained 8%, 10% and 12% of their initial

strength respectively. The decrease in flexural strength at this temperature is known to

be affected mainly by the dehydration of CSH gels as well as the transformation of

quartz (silica sand) occurring early at 570°C [27]. This also explains why the control

(reference blocks containing 100% of sand as fine aggregates) had the highest loss in

strength than those blocks made with clay brick aggregate (see Table 4).

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4. Conclusion

This study focused on the behaviour of partition wall concrete blocks after exposure to

elevated temperatures. The residual density, mass loss, and evolution of compressive

and flexural strengths of the block specimens prepared with recycled concrete and brick

aggregate were determined after heating cycles up to 300°C, 500°C and 800°C.

At 300°C, all the concrete blocks show better compressive and flexural strengths than

that at room temperature due to the natures of the clay brick making the blocks

stronger/more stiff. A decrease in both the compressive and flexural strengths was

observed between 300°C and 500°C which was mainly attributed to the

dehydroxylation of calcium hydration and the development of microcracks. The

microcracking at this temperature seemed to have a more detrimental effect on the

flexural strength than on compressive strength. Up to 800°C, the dehydration of CSH

gels caused a greater degree of strength loss. Only a small part (8-41%) of the initial

flexural strength was retained in all the concrete blocks. However the blocks were still

able to retain about 48-91% of their original compressive strength.

As far as compressive strength at elevated temperatures is concerned, the optimum

replacement level of sand by fine clay brick aggregate were determined to be 25-50%

for Series 1 and 50-75% for Series 2. As for Series 3, the incorporation of coarse clay

brick aggregate seemed to offset the detrimental effect observed at ambient temperature.

Also, it was found that the use of clay brick aggregate had more favourable effects on

flexural strength than on compressive strength. At all the maximum temperatures tested,

particularly at 800°C, a higher percentage of brick aggregate used enabled a higher

residual flexural strength.

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Acknowledgements

The authors wish to acknowledge the State Key Laboratory of Geohazard Prevention

and Geoenvironment Protection (SKLGP2012K002) and The Hong Kong Polytechnic

University for funding support.

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