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FIELD OF INVENTION

The present invention relates to a method of fabricating concrete building blocks. More specifically, the invention relates to a fabrication technique and an optimized mix design of concrete blocks for structure and infrastructure applications. BACKGROUND OF THE INVENTION

Since the discovery of concrete, it has been a hardcore material for construction industry. It is generally used as a binding medium that is formed from a mixture of a hydraulic cement and water. Portland cement is majorly used as hydraulic cement. The production process of Portland cement is a high temperature process that drives off carbon dioxide in huge amount. Because carbon dioxide is generated by both the cement production as well as by concrete blocks production plants, hence construction industry is currently one of the leading sources of current carbon dioxide atmospheric emissions. It is estimated that cement plants account for 5% of global emissions of carbon dioxide that is majorly responsible for the alarming problems such as global warming and ocean acidification.

Further, the construction industry is now experiencing a very competitive environment. All the industry players are striving to optimize the various aspects in the fabrication of concrete blocks ranging from transportation of raw materials, formulation of the concrete block, the manufacturing process and also the subsequent curing process in order to minimize production cost and yields maximum profit. Despite the various advanced optimization techniques being offered for fabricating concrete blocks, a need is still remaining particularly for producing a concrete building block which is low in carbon footprint and production embodied energy. In conventional compaction techniques, the fabrication of concrete blocks usually involves vibration and weighted-compaction forming technique, i.e. vibro-compaction technique or merely vibratory forming technique alone. On the other hand, concrete blocks are generally molded using zero-slump concrete mixtures in order to achieve a higher degree of early strength development and also the stiffness suitable for handling upon removal of mold (formwork). It is often difficult for the concrete block manufacturers to achieve high density mixes for attaining target strength of cement as it requires a higher content of cement. This is because the quality of hardened concrete block is very much dependent on the compacted density of the fresh concrete mix. The inefficiency of vibratory forming method also leaves the resultant concrete building block with a porous matrix phase due to the presence of a substantial amount of air voids within the concrete mass.

On the other hand, the substantial amount of air voids within the hardened concrete mix induces excessive degree of drying shrinkage. Consequently, micro-cracks are formed due to the high degree of drying and autogenous shrinkage experienced by the concrete block throughout its service duration. These factors have significantly reduced the mechanical strength and durability performance of concrete products formed by traditional molding method. The high cement content in the conventional concrete block has contributed to the significant carbon footprint of the material. Furthermore, high vibration amplitude and frequency is required to achieve the desired degree of compaction and density that may induce a mechanical damage to the block making machine which in turn increases the wear and tear of the machine and also increases the maintenance cost of the molding system as a whole. In the current industrial practice, freshly molded concrete blocks are subjected to either a low- pressure or high-pressure (autoclave] steam curing kiln to accelerate the rate of early age mechanical strength gain of the concrete blocks. For instance, low-pressure steam kilns currently in use at the concrete block production facilities operates at the atmospheric pressure, but with curing temperatures ranging from 45 to 85°C. This type of curing is accounted for the production of about 80% of concrete blocks manufactured worldwide. On the other hand, autoclave curing is usually implemented for the production of ready-to-use concrete blocks which are required to achieve a given mechanical strength in a short period of time. A typical autoclave kiln usually operates with temperature ranging from 150°C up to 190°C, coupled with internal pressure ranging from 80 to 185 psi. The aforementioned curing regime is generally sustained for duration between 6 hours and 24 hours. The primary disadvantage of the autoclave curing system is that the substantial amount of energy consumption (with a substantial energy carbon footprint of about 40kg CO2 eq. /m ³ concrete) incurs to sustain the required curing temperature and pressure. This increases the expense and adds embodied energy to the final concrete products.

Thus, it is desired to address the various shortfalls of the current production system of the precast concrete industries as aforementioned.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of fabricating concrete building blocks, the method comprising the steps of i) preparing zero-slump concrete mix, ii) hydraulically densifying the zero-slump concrete mix at a predetermined forming pressure to form concrete blocks without the aid of vibration forming, iii) pre-curing the concrete blocks to form pre-cured concrete blocks and iv curing the pre-cured concrete blocks.

This invention is pointed out with particularity to the appended claims. Additional features and advantages of the system will become apparent to those skilled in the art by referring to the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made in detail to the exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawing. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

FIG. 1 illustrates a flow diagram representing a method of fabricating concrete building blocks;

FIG. 2 illustrates a flow diagram representing a method of producing c zero-slump concrete mix;

FIG. 3 demonstrates a table 1 of mix design for the fabrication of ECON concrete building block;

FIG. 4 demonstrates a table 2 of general properties of ECON concrete building blocks at 7 days of ambient temperature curing;

FIG. 5 demonstrates a table 3 of carbon footprint comparison between the carbon footprint of ECON and equivalent conventional concrete blocks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The main objective of present invention is to provide a detailed mix design, fabrication technique in the means of hydraulic densifi cation and also the subsequent curing regime used to fabricate eco-friendly ECON concrete building blocks. In an embodiment of the present invention, the ECON blocks fabricated consist of various Portland Cement (PC) content, hybrid supplementary binder comprising the combination of ground granulated blastfurnace slag (GGBS) and pulverized fuel ash (PFA), fine aggregate to binder ratio, coarse aggregate to binder ratio, water content, hydraulic forming pressure and curing regime. The mix design for the fabrication of ECON blocks is designed such that blocks with a wide range of compressive strength can be produced to cater for different performance grades and various construction applications. With the wide range of grade strength, the concrete block produced is suitable for various applications covering for use as structural/non-structural building block, paver block and soil retention structure block work.

An embodiment of the present invention depicts the fabrication and inventive steps to fabricate ECON concrete building blocks that include the preparation of raw materials, mixing process, forming and curing of ECON blocks. Generally, the present invention provides a method of fabricating concrete building blocks. The method consists of hydraulic densification of concrete without the aid of vibration forming. The method includes preparing zero-slump concrete mix; hydraulically densifying the zero-slump concrete mix at a predetermined forming pressure to form concrete blocks without the aid of vibration forming; pre-curing the concrete blocks to form pre-cured concrete blocks; and curing the pre-cured concrete blocks. An aspect of the present invention provides a method in which the zero-slump concrete mix is produced by following steps: preparing a dry mix by mixing a binder material, fine aggregates and coarse aggregates for a first amount of time; adding a predetermined amount of water to the dry mix; and mixing the mix for a second amount of time to produce zero-slump concrete mix.

The ECON concrete blocks comprise Portland Cement (PC) content, hybrid supplementary binder comprising the combination of ground granulated blastfurnace slag (GGBS) and pulverized fuel ash (PFA), fine aggregate to binder ratio, coarse aggregate to binder ratio, water content, hydraulic forming pressure and curing regime. The mix design for the fabrication of ECON blocks is designed such that blocks with a wide range of compressive strength can be produced to cater for different performance grades and various construction applications.

In an aspect of the present invention, a method is disclosed for fabricating the concrete building blocks in which the water to binder ratio is in range of 0.2 to 0.3, the fine aggregate to binder ratio is in range of 5.00 to 8.00, coarse aggregate to binder ratio is in the range of 2.00 to 5.00.

It is an object of the present invention to produce a concrete building block which is low in carbon footprint and low production embodied energy. This is achieved through the optimization of mix design, refinement of molding process and also improvement of the curing method which will be described in the following sections.

It is a further object of the present invention to cover the optimization of concrete mix design, fabrication technique and also the subsequent curing method to achieve the desired concrete blocks strength. Some or all of the aforementioned advantages of the invention are accrued by producing the concrete building blocks with comparatively low carbon footprint. With the aforementioned method, the concrete building blocks have the carbon footprint of approximately in range ofl63.46 CO2 Eq. Kg to 266.45 CO2 Eq. Kg comparison with the conventional concrete blocks while the conventional concrete blocks with normal or autoclave curing have the carbon footprint in range of 335 CO2 Eq. Kg to 380 CO2 Eq. Kg.

Further advantages of the present invention are accrued by producing a concrete block with minimal Portland Cement (PC) content, maximum low carbon footprint supplementary binder content and high fresh compacted density that only requires post fabrication curing at ambient temperature to achieve the desired engineering properties. Low carbon footprint supplementary binder comprises of GGBS and PFA combination with GGBS to PFA ratio in range of 0.25 to 4.00. The supplementary binder material phase can also be made up of 100% of pure GGBS or PFA.

Hence, the concrete blocks fabricated have similar or enhanced performance in comparison with the conventional concrete blocks available in the market.

It is also an object of the present invention to provide the optimization of concrete mix design, fabrication technique and also the subsequent curing method to achieve the desired concrete blocks strength. The enhancement in the manufacturing technique and the curing method is done such that the minimal modification to the existing production plant is required. This, in turn, keeps the production cost minimum. The present invention will now be described in detail in conjunction with the accompanying drawings which may be referred individually and in combination. Referring to FIGS. 1 and 2, in an embodiment of the present invention, a method 100 fabricating concrete building blocks is disclosed. The method 100 includes the step 102 for the preparing of zero-slump concrete mix. The preparation of the zero-slump concrete mix is briefly demonstrated below with FIG. 2.

Further, in step 104, densification of the zero-slump concrete mix with a predetermined forming pressure is done. The densification of the zero-slump concrete mix is done with or without the aid of vibration forming followed by pre-curing the concrete blocks, as shown in step 106. Further, in step 108, curing of the pre-cured concrete blocks is done to form low carbon footprint ECON concrete blocks.

In an embodiment of the present invention, the zero-slump concrete mix is produced in accordance with the steps as depicted in FIG. 2 (step 102). In step 202, a dry mix is prepared by mixing a binder material, fine aggregates and coarse aggregates for a predetermined time. In further step 204, predetermined amount of water is added into the mix obtained from step 202. Further in step 206, the mix is mixed for a second amount of time to produce the zero-slump concrete material.

In a preferred embodiment of the present invention, the fabrication process of ECON concrete building blocks is initiated by dry mixing the binder material which is Portland cement, low carbon footprint supplementary binder (GGBS and PFA combination), fine aggregates and coarse aggregates for 3 minutes in epi-cyclic mixer. Next, measured mixing water are added into the mixtures and the mixing continues for another 2 minutes until uniformity is achieved and zero-slump concrete mixtures are produced. The range of mixing water added (0.20-0.30) is designed such that every mix design (with different PC, supplementary binder, fine and coarse aggregates content) can produce zero-slump concrete mixtures with minimal water content and adequate rheological properties for the forming process. A fixed amount of the freshly mixed concrete is then placed into the full-hydraulic operated block press for subsequent hydraulic densification. The maximum forming pressure of 7000 psi is prescribed for the hydraulic densification of the fresh concrete mix to achieve the minimum compression ratio of 1.48. Upon removal from full-hydraulic operated block press, the freshly formed ECON concrete blocks with the gross dimension of 290x140x110 mm are cured at ambient temperature of 25±2°C and relative humidity of 65±5% for 24 hours in order to achieve sufficient hardness for subsequent curing. Dimensional parameter of the block can be modified according to the suitability of final application. After 24 hours of pre-curing period, the ECON concrete blocks are then subjected to either moist/air/water curing. For moist curing, the pre-cured ECON concrete blocks are wrapped using a commercial cling film and stored at ambient temperature of 25±2°C and relative humidity of 65±5% for further curing. Air curing shall be performed as in accordance to the condition prescribed during the pre-curing stage. For the water curing regime, the ECON concrete blocks shall be submerged in lime-saturated water with temperature of 25±2°C for further curing.

In some embodiments, Portland Cement (PC) (EN197-1 Type I, II, III, IV and V) is used as the primary binder with or without the low carbon footprint supplementary binder in the ECON blocks fabrication. In an embodiment, crushed granite is used as the coarse aggregate and fine aggregates are locally sourced washed quartzitic natural river sand. The sand is dried and saturated before as constituent material in the fabrication of ECON blocks.

As depicted in FIG. 3, in an embodiment of the present invention, for fabricating the concrete building blocks, the water to binder ratio is in range of 0.2 to 0.3, the fine aggregate to binder ratio is in range of 5.00 to 8.00, and coarse aggregate to binder ratio is in the range of 2.00 to 5.00. Low carbon footprint supplementary binder comprises of GGBS and PFA combination with GGBS to PFA ratio in range of 0.25 to 4.00. The supplementary binder material phase can also be made up of 100% of pure GGBS or PFA. The low carbon footprint supplementary binder material can be used at up to 80% in terms of cement replacement level in order to minimize the total embodied carbon footprint of concrete produced.

Referring to FIG. 4, general properties of ECON concrete building blocks at 7 days of ambient temperature curing is shown in table 2. It is indicated that the compressive strength of ECON with 7 days of curing at ambient temperature is 10.9MPa to 40.8MPa and with 28 days of curing at ambient temperature is 16.5MPa to 60MPa. Similarly, the water absorption percentage for 7 days curing is in range of 1.20% to 7.40%. The water absorption percentage for 28 days curing is in range of 0.96% to 5.80%.

FIG. 5 depicts a table that indicates the carbon footprint comparison between ECON and equivalent conventional concrete blocks. The embodied CO2 amount with the mix elements and process such as constitute material (Portland cement, fine aggregate and Coarse Aggregate), fabrication process (concrete binding and vibration forming) and post fabrication curing (autoclave curing and ambient temperature curing) is shown in the table. The sum of carbon foot print is indicated for the ECON (i.e. 266.45 C0 ₂ Eq. Kg and 163.46 C0 ₂ Eq. Kg), for conventional concrete block in case of autoclave curing (i.e. 376.16 CO2 Eq. Kg) and for conventional concrete block in case of ambient curing (i.e. 336.16 CO2 Eq. Kg). Hence, it is clearly indicated that carbon footprint in case of ECON is least and highest in case of conventional concrete block with autoclave curing.

Described above are only preferred embodiments of the present invention, which are not intended to limit the present invention. Any modification, equivalent substitution and improvement made within the principle of the present invention shall be included in the scope of sought protection in the present invention.