11 th INTERNA TlONAL BRICKlBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

STUDY ON SMALL HOLLOW CINDER CONCRETE LOAD-BEARING BLOCKS

Ning Cenan1

1; ABSTRACT

The study on small hollow cinder concrete load-bearing blocks inc1udes: (1 )Investigation of cinder block mixed ratio, (2) Investigation of block material characteristics, (3) Investigation ofblock mechanical properties, and (4)Investigation of complete technology such as experimental architectural design and construction, which established a new way of making light wall materiais by reasonably utilizing the local materiais and industrial waste cinder.

2. INTRODUCTION

The study on small cinder concrete load-bearing hollow blocks was the key scientific research item of the Science Commission of Anhui in 1980's, which took 3 years to have completed the subject with a large amount of manpower and material resources. The study covered a series of complete technology and a large amount of scientific experimental achievement, such as basic material characteristic of blocks, mechanical properties of blocks, experimental architectural design and construction. MU7.5 load­bearing cinder block has been developed, and a five-storey residential building built with these blocks has been used for ten years and in good condition. This subject passed the Provincial Appraisal in 1986, and the essays which passed the Appraisal inc1ude: the mixed ration plan and production process ofNo.75 small cinder concrete hollow blocks

KEYWORDS: Cinder concrete, smallload-bearing hollow blocks

1690 Wuwan Road, Bengbu Anhui 233017, China

Ning Cenan, senior engineer, second prize winner ofMMBI Scientific & Technological Achievement in 1984, and

fourth prize winner of Anhui Scientific & Technological Achievement in 1987.

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(BMZ-75)[I!; investigation bulletin of material characteristics of No.75 small cinder concrete hollow blocks(BMZ-75p!; and investigation bulletin of basic mechanical properties of small loading-bearing cinder block masonry envelope[3! . The complete summation is made as follows in accordance with the new specification[4! and the

application within ten years .

3. THE ,' INVESTIGATION OF CINDER BLOCK MIXED RA TIO AND PRODUCTION PROCESS

The raw material of cinder concrete load-bearing blocks is the mixture of both industrial cinder and cement. To produce cinder blocks with stable material characteristics and required strength, three issues on the production process must be investigated, which are: (1) basic material characteristics of cinder concrete, (2) mixed ratio cinder-to-cement; and (3)production technological processo

3.1 Basic material characteristics of cinder concrete

3.1. r Chemical composition of raw materiais

Cement - selected the No.425 cinder Portland cement made by Bengbu Cement Plant.

See Table I for its chemical composition. J

Table 1 Chemical composition ofNo.425 cinder Portland cement

The limit range of base-acid ratio in this cement composition shall be between 1.7 and 2.4. The actual value is:

___ C_a_O.....:(_ba_s_ic ___ C_o_m=-'P_ou_n_d...:..) ___ = 1.62 compoud)

(1)

The actual value of 1.62 is c10se to the lower limit. The content of harrnful components such as S03 and MgO in this cement has not exceeded the standard requirement, and its stability test is accepted, therefore, the cement can be used as the raw material to produce blocks.

Cinder - selected the industrial cinder from Bengbu Meat Processing Factory. See

Table 2 for its chemical composition.

Table 2 Chemical composition of industrial cinder

Chemical composition Si02 Alz03 Fe20 3 CaO MgO K2O+Na20 S03

Actual content % 48.04 28.01 2.52 3.34 0.73 1.58 0.36

Quality requirement >40 > 15 - - <5 <2.5 <4

Note: The baking loss of 18.68% is less than 20% and acceptable.

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The Content of harmful components such as MgO, K20+Na20, and S03 in this industrial f inder and the baking loss have not exceeded the standard requirement; and its stability test is accepted. Therefore, the cinder can be used as the raw material to produce blocks.

3.1.2 Mechanical property cinder concrete

The mechanical property of cinder concrete test block mixed with 1:5 weight ratio of cement-to-cinder:

Natural unit weight 17.5kN/m3

Dry unit weight 17.0 kN/m3

Cubic strehgth R=20.2 N/mm2

Axial cornpressive strength Ra=18.1 N/mm2

Ten!,ile strength RL=1.7 N/mm2

Modulusofe1asticity Eh=1.14 x 104 N/mm2

The mechanical property of this cinder concrete meets the requirement of NO.200 c1ass . 1.8 (17.0 ,r 19.0 kN/m3) unit weight light aggregate concrete. Only the Modulus of

elasticity is slightly lower. Therefore, it can be used as load-bearing material.

3.2 Mixed ratio of cinder-to-cement

Three types of mixed ratio of cinder-to-cement were selected to do multiple groups of strength test. See Table 3 for its detailed testing results .

Table 3 Re1ations ofthe ratio cinder-to-cement Vs the strength of cinder concrete (Mpa)

Item 1:6 1:5 1:4 Remarks

Tes\iJ?g block (100 x 100 x 100mm) 18.1 20.2 24.7 Average value of 3 pcs

~Iock (390 x 190 x 190mm) 7.8 8.8 10.5 A verage value of 5 pcs r-----. Te~~ing block strength/Block strength 2.32 2.29 2.35 Average 2.32

lt can be seen from the results presented in Table 3 that while the amount of cement is increaseq by one leveI, the strength is increased by about 15%; and the ratio of testing block srrength to block strength is averaged 2.32, that is, the strength of the cinder concrltte is at least as 2.32 timés high as that of block.

Through the analyzing of the above tests, the mixed ratio (cement to cinder) of MU7.5 block is se1ected as 1 :5.

3.3 Production technological process

After the above tests, blocks were put into small batch production in order to make investigation into the material characteristics of load-bearing cinder blocks. For mid-

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period trial production. the movable forming machine was used to make blocks, and the cinder concrete was mixed mechanically and cured naturally. The blocks were delivered from factory after 28 days. The technological process is illustrated in Fig. I .

Cinder ~ Wheel grinding ~ Aging ~ Measurement . -L-

Cement ~ Measurement ~ Mixing ofmixed materiais ~ Vibration forming t -L-

Water ~ Measurement -----~ Test and delivery ~ Stack curing

Fig.1 Production technological process of cinder blocks

4. INVESTIGA TION OF THE MATERIAL CHARACTERISTIC OF BLOCKS

Investigation covers: (l)Basic material characteristics of Mu7.5 cinder blocks, (2) compressive strength and variation coefficient, and (3) breaking strength ofblocks.

4.1 Basic material characteristics ofMU7.5 cinder blocks

The investigation was performed under the condition of random sampling test, on the basis of the investigation of the material the material characteristics of cinder concrete and after small batch production of blocks.

4.1.1 The size, type, unit weight and water absorption rate ofblocks

The basic size of blocks is 390mm x 190mm x 190mm (length by width by height),

which. meets the specification Df small blocks[51. The gross area is 741 mm2 and its hollow rate with single row of holes K is 46%, which meet the requirement of hollow blocks. The natural unit weight is 0.13 kN per piece, and dry unit weight is 0.124 kN per piece, which belong to light blocks.

The water absorption rate of trus block is 8.45, less than 22%, meeting the requirement of standard[5J. Compared to the 16% water absorption rate of red brick, 16 - 22% of that of coai ash block and 2.4 - 8.8% of concrete block, the water absorption rate of this

cinder blocks is similar to the upper limit of concrete block, less than those of red brick and coai ash block. The dry shrinkage rate of coai ash block is about 0.07%, and that of concrete block is 0.023 - 0.042%. The dry shrinkage rate of cinder block is dose· to the

upper limit of concrete block.

The blocks are divided into two types according to their penetration resistant degree: water proof blocks and general blocks. The penetration resistance test of this block is acceptable, which can be used as water proof block.

4.1.2 The anti-freezing, softening coefficient and carbonizing coefficient of cinder

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blocks Freezing and melting test: take 10 sample randomly, tive of them undergo 15 cycles of freezing and melting, and the other tive is used for comparative testo The freezing and melting loss rate is:

Km=1.3%, less than 25%, acceptable (2)

Softening coefticient test: Take 10 samples randomly, tive ofthem is used for softening test, and the other tive is used for comparative testo The softening. coefticient is:

KFO.9, more than 0.75, acceptable (3)

Carbonizmg coefticient test: Take 10 samples randomly, tive of them is used for carbonizing test, and the other tive is used for comparative testo The carbonizing coefticient is:

K.,=O.92, more than 0.8, acceptable

4.2 Compressive strength and variation coefticient

4.2.1 Axial compressive strength of cinder blocks

(4)

In accordance with the requirement ofstandard[51, among the blocks ofbatch production, the average value ofaxial compressive strength of 5 sample~ randomly taken is not less than 7.5Mpa, and the minimum value of a single block is not less than 6.0 Mpa. The strength class oftht: block is assigned to Mu7.5. The strength ofthe 5 samples randomly taken and tested is shown in Table 4.

Table 4 The compressive strength fi (Mpa) test of cinder block

10.3 7.4 8.2 8.3 10.1

The average value fi of compressive strength of blocks is 8.8 Mpa, and the minimum strength of a siIlg1e block is 7.4 Mpa which is more than 6.0 Mpa. Therefore, this batch . ofblocks can be assigned to class MU7.5.

4.l.2 Variation coefficient ofthe compressive strength fi ofblocks

Since a cinder block is composed of inhomogenious materiais, the quality of its product has a certain discretion. In addition, the variation in production process operation wiIl cause the product quality to vary within a certain range. The determination of variation coefficient may indicate the product -quality status objectively. The testing method is to take 50 samples randomly from the matrix to determine the variation coefficient, see formats (6), (6) and (7) for detailed testing results.

Averagevalue f, =;Lf,+ ... +fn =8.38MPa (5)

Standard difference S = ~L(f, - fY /50-1 = 1.49MPa (6)

Variation coefficient w = (S~) x 100% = 17.78% (7)

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The compressive strength ofblocks is 11.1 Mpa maximum, and 5.3 Mpa Minimum. The Grubs rule of probability statistic principIe was used to do rough·difference rejection test, and testing believable leveI a was selected as 0.05, alI the testing results of 50 data were normal. ludged with vertical block method and X2 testing method, the whole follows normal distribution. (The operation is omitted here.)

The testing result of believable range for average compressive strength of blocks is : 7 .96MPa::;fl~8.8Mpa (8)

That is, the 6 believable range meets the strength requirement of c1ass MU7.5.

But the above probability statistical analysis, the probability distribution of the matrix compressive strength of this testing piece folIows normal distribution, and the believable range meets the requirement of standard[Sl .

4.3 Breaking strength ofblocks

Generally, the breaking strength of blocks is not used directly to judge the engineering quality, which mainly shows the block strength and the capability adapting to the deforming of walls. The breaking to compressing ratio is also an important index in material mechanics, but the specific control index for .the breaking strength has not been presented in standard[Sl.

Three groups of samples were taken in this experimental to do breaking strength test, and another group was taken to do compressive test for comparison. The breaking strength of cinder blocks is 1.51 Mpa maximum, 0.81Mpa minimum and 1.33 Mpa average, and the breaking to compressing ratio is 0.126.

5. INVESTIGA TION OF MECHANICAL PROPERTIES OF BLOCKS

The performing of investigation of mechanical properties of blocks meets N1U7.5 specification, which mainly inc1udes the testing of: (1) The average axial compressive strength of masonry envelope and its variation coefficient, (2)the axial compressive modulus of elasticity of masonry, and (3) shearing strength of masonry envelope.

5.1 The average axial compressive strength of masonry envelope and its variation coefficient

Standard masonry envelope is used. The size of the envelope is 590mm x 190mm x

590mm (L x B x H), the envelope containing two horizontal joints, and the testing

method follows the Code. The testing results are shown in Table 5.

6.2

Table 5 The testing value ofaxial compressive strength (Mpa) of cinder block masonry envelope

3.7 5.0 5.5 5.1 Average value is 5.1

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ihe average strength fi of the block used by this sample is 9.6 Mpa, and the average compressive strength 6 ofmortar is 9.2 Mpa.

The average compressive strength fm of masonry envelope is proportional to the block strength fi and the mortar strength f2. As per the statistical formula in the Code for masonry envelope structure design[61, the calculation formula of small concrete block is as follows:

fm =0.46f/9(l +0. 07fv k2 (Mpa)

where, Whileh=O, k2=0.8 In other case, k2=1.0

(9)

The compressive strength fm of masonry envelope ca1culated from formula (9) is 5.8 Mpa, which is about 10% more than the testing value. While making design, the attention shall be paid and appropriate structural measures shall be taken, such as adding columns with reinforced concrete to strengthen.

The variation coefficient testing results ofaxial compressive strength of masonry envelope were solved through 50 compressive strength values of masonry envelopes, and the average compressive strength values of masonry envelopes, and the average compressive strength value fm is 5.07 Mpa, standard difference is 0.76 Mpa and variation coefficient /) is 15.1 %. The maximum value is 6.5 Mpa and minimum value is 3.2 Mpa, which are normal values after the testing results are rejected through rough difference. The overall distribution follows the normal distribution.

5.2 The axial compressive modulus of elasticity of masonry envelope

Take three kinds of M2.5, M5 and MIO mortar masonry envelopes to measure the modulus of elasticity. See Table 6 for more information.

Table 6 The modulus of elasticity of small MU7.5 cinder block masonry envelope (Mpa)

M2.5 M5 MIO Actual I Theoretical Actua1 I Theoretical actual I Theoretical

3.2 x 10 I 1.9 x 10 3.4 x 10 I 2.6 x 10 4.6 x 10 I 3.7 x 10

The theoretical value in Table 6 is computed with the modulus of elasticity of small reinforced concrete block in the Code for the structural design of masonry envelope[6]. Ali the actual values are bigger than the theoretical values, therefore, the values in Table 6 can be used directly.

5.3 Shearing strength of masonry envelope

Take three kinds of M2.5, M5 and MIO mortar masonry envelopes to do the shearing

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strength testo See table 7 for more information.

Actual

0.10

M2.5

Table 7 The shearing strength test of small MU7.5 cinder block masonry envelope (Mpa)

M5

I Theoretical Actual I Theoretical Actual

I 0.11 0.13 I 0.15 0.19

MIO

1 Theoretical

I 0.21

The theoretical value in Table 7 is computed according to the statistical formula of shearing strengthof small reinforced concrete block masonry envelope in [6] . Refer to formula (10) for more information.

/v.m=O.069h (lO)

Where,h represents the mortar strength.

It can be seen from Table 7 that ali the actua1 values of the shearing strength of small hollow cinder block masonry envelope are about 10% less than the specified values. While making design, attention shall be paid. The better way is to add reinforced concrete columns as appropriate in order to increase the shearing strength of masonry envelope.

6. EXPERIMENTAL ARCHITECTURAL DESIGN AND CONSTRUCTION

Afier the completion of the above three experimental research, the representation of the scientific research achievement was to have built up a 4-storey apartrnent of small hollow cinder block. The plot area is 44.4m by 8.6m, the floor area is 1909.0m2

, and the floor c1earance is 2.8m. The cinder block is load-bearing through transverse walls and

__________ ~--f>~ ________ __

.~- ~" '---j'----j

'V !f."

'Sr

_.U ____ -'v .....

,.,. --=-

Figure 2 Plan and section of a small hollow cinder block building

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the pre-stressed reinforced concrete floor slabs prefabricated. See Figure 2 for more information.

The experimental building is located at the central field of Bengbu, where the geographic location and climate conditions are suitable for the construction of block buildings. The earthquake intensity in this region is a 7 degree seismic area, The architectural pattem is simple, both the longitudinal walIs are aligned at most, and the opening rate in the longitudinal walIs is reduced. The reinforced concrete colurnns are poured at alI the connections ofthe longitudinal and transverse walIs and five reinforced

. concrete colurnns are also built at the walI comer of staircase cantilever platform to increase local load-bearing area. Ring beams are set on each longitudinal and transverse walIs of every floor. At that time, the structure design was made as reference to "The Regulations for Design and Construction of SmalI HolIow Concrete Block Building "

(JGJ14-82), and the original design was checked to conform with the requirement ofthe new regulations after the New Regulation[4] has been issued. The building was put in use in 1986, which has lasted 10 years and resulted in good effect.

6. CONCLUSION

It can be conc1uded as folIows through the above complete investigation into the smalI holIow MU7.5 load-bearing cinder blocks and the application of the experimental building:

(1) The production process of small holIow load-bearing cinder blocks must strictly folIow the process requirement presented in this research reporto The application of raw materiais must be tested.

(2) The strength of cinder blocks is proportional to the cement grade and its amo:nt. The basic mechanical properties of smalI holIow cinder blocks produced with the recipe presented in the report are as folIows: Average compressive strength: 8.8Mpa Average breaking strength: 1.33 Mpa, Breaking-to-compressing ratio is 0.126

The probability distribution of the compressive strength of block matrix folIows the normal distribution, which reaches MU7.5 block standard.

The natural unit weight of smalI holIow cinder block is 0.13 kN per piece, the water absorption rate is 8.4%, dry shrinkage rate is 0.042%, the strength loss rate Km after freezing and melting is 1.3%, softening coefficient Kr is 0.9, and carbonizing Kc is 0.92, which meet the requirement of standard!51.

(3) The mechanical properties ofMU7.5 cinder block masonry envelope are as folIows : The average compressive strength of the envelope is used by multiplying the specified value!41 by factor 0.9;

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The average shearing strength of the envelope is used by multiplying the specified value[41 by factor 0.9;

The average compressive modulus of elasticity adopts the specified value[41.

(4) Through the accumulation of scientific research achievement in recent ten years, China has issued and implemented the new Technical Regulations for the Small Hollow Concrete Block Buildings (JGJ/T14-95). In the new regulations , th<: static and seismic design and its construction for the small hollow light aggregate concrete. block building was added, which provides conditions to further develop light blocks, thus to facilitate the application and development of the new material.

7. REFERENCE

[1 ]Kui Zhenping, Shentu Xiamin, Investigation of mixing ratio plan and production process ofNo.75 small hollow cinder concrete blocks, Oct. 1985. [2] Ning Cenan, report on the experimental study of material characteristics of NO.75 small hollow cinder concrete blocks, Oct. 1985. [3]Chen Dongming, Report on experimental study of basic mechanical properties of smallload-bearing cinder block masonry envelope, Aug. 1985. [4]Architectural technical regulations for small hollow concrete blocks, JGJIT14-95. [5]Small hollow light aggregate concrete blocks, GB 15229-94. [6]Code for the masonry envelope structure design, GBJ3-~8 .

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