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Bio-bricks: Biologically cemented sandstone bricks

D. Bernardi

aa

, J.T. DeJong

bb,,⇑⇑

_{, B.M. Montoya}

cc

_{, B.C. Martinez}

dd

a

a_{ENGEO Incorporated, Walnut Creek, CA, USA}_{ENGEO Incorporated, Walnut Creek, CA, USA} b

b_{Department of Civil and Environmental Engineering, University of California, Davis, CA, USA}_{Department of Civil and Environmental Engineering, University of California, Davis, CA, USA} c

c_{Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, NC, USA}_{Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, NC, USA} d

d_{Geosyntec Consultants, Oakland, CA, USA}_{Geosyntec Consultants, Oakland, CA, USA}

h i g h l i g h t s h i g h l i g h t s



A novel technique to manufacture bio-bricks using a biologically mediated natural cementation process is presented.A novel technique to manufacture bio-bricks using a biologically mediated natural cementation process is presented. 

 Results show that bio-brick Results show that bio-bricks s can have can have compcompressiressive strengths up to ve strengths up to 22 MPa.MPa. 

 P-wave velocity measurements show bio-brick stiffness to be relatively uniform and high. P-wave velocity measurements show bio-brick stiffness to be relatively uniform and high. 

 Bio-bricks are comparable to bricks prepared with the more conventional cement and hydraulic lime additives. Bio-bricks are comparable to bricks prepared with the more conventional cement and hydraulic lime additives.

a r a r t i t i c l c l e e i n i n f of o Article history: Article history: Received 24 September 2013 Received 24 September 2013

Received in revised form 9 January 2014 Received in revised form 9 January 2014 Accepted 11 January 2014

Accepted 11 January 2014 Available online 14 February 2014 Available online 14 February 2014

Keywords: Keywords: Bricks Bricks Calcite precipitation Calcite precipitation

Microbially induced calcite precipitation Microbially induced calcite precipitation Compress

Compression ion strengthstrength Stiffness

Stiffness

a b s t r a c a b s t r a c tt

The cementation of sand into sandstone through microbial activity is a novel technology with a wide The cementation of sand into sandstone through microbial activity is a novel technology with a wide range of possible applications. The cementation process involves the introduction of bacteria and range of possible applications. The cementation process involves the introduction of bacteria and nutri-ent

entss toto sansand,d, andand thrthrougoughh bacbacterterialial proprocescessesses calcalcitcitee preprecipcipitatitationbindsionbinds parparticticlesles togtogethether,er, ultultimaimateltelyy cre cre--ating a sandstone material. This technology could provide a new, more sustainable building material in ating a sandstone material. This technology could provide a new, more sustainable building material in the

the forformm ofof ‘‘b‘‘bio-io-bribrickscks’’.’’. ThiThiss pappaperer desdescricribesbes thethe tretreatmatmentent tectechnihniqueque asas welwelll asas resresultultss frofromm testestintingg afteafterr brick manufacturing. Bricks were tested to determine compression (p-wave) wave velocity, unconﬁned brick manufacturing. Bricks were tested to determine compression (p-wave) wave velocity, unconﬁned compression strength, and calcite concentration. P-wave velocity, stiffness, strength, and calcite content compression strength, and calcite concentration. P-wave velocity, stiffness, strength, and calcite content of bio-bricks all increase with further treatment of bacteria and cementation media. Results show that of bio-bricks all increase with further treatment of bacteria and cementation media. Results show that

bio-bio-brickbricks can s can have strenghave strengths ranging from 1ths ranging from 1 MPa to 2MPa to 2 MPa. Bio-bMPa. Bio-bricks are comparricks are comparable in terms of able in terms of stresstresss and stiffness to bricks prepared with the more conventional cement and hydraulic lime additives. and stiffness to bricks prepared with the more conventional cement and hydraulic lime additives.



1. Introduction 1. Introduction

The global use of resources and emphasis on sustainable The global use of resources and emphasis on sustainable infra-structure is a growing societal issue civil engineers must address structure is a growing societal issue civil engineers must address [21]

[21]. The international population is growing at an unprecedented. The international population is growing at an unprecedented rate, and in

rate, and in responresponse, civil infrasse, civil infrastructure mustructure must expand andt expand and be reha-be reha-bil

bilitateitatedd inin a sustaa sustainainableble manmanner. Thner. The demae demandnd onon natunaturalral resoresourceurcess is

is farfar greagreaterthanterthan thesupplyinthesupplyin botbothh devdeveloelopedandpedand devdeveloelopinpingg coucoun- n-tries

tries [1][1]. . SustainSustainable developable development must consider the ment must consider the energy andenergy and materia

materiall ﬂowﬂowss throughthrough thethe construcconstruction, tion, maintemaintenance, nance, dismadismantling,ntling, and material disposal related to a project

and material disposal related to a project [28] [28]. Meeting the societal. Meeting the societal deman

demands with ds with locally availablocally available le resourceresources s and minimal materialand minimal material and

and eneenergyrgy propromotmotee aa sustasustainainableble appapproaroachch toto devdeveloelopmpment.ent. Biol

Biologicogical al procprocesseesses s have been have been harnharnesseessed d for for a a multmultituditude e of of engineering

engineering applicatioapplicationsns [9,10][9,10], DeJ, DeJonongg etet alal.. [11] [11]. . Bio-geocheBio-geochemicalmical

processes that induce mineral precipitation have been utilized for processes that induce mineral precipitation have been utilized for many applications, including improving the strength and stiffness many applications, including improving the strength and stiffness of soil

of soil [8,32,20] [8,32,20] as an alternative to traditional chemical grouting as an alternative to traditional chemical grouting whic

whichh cancan bebe envienvironmronmentaentallylly hazahazardourdouss [17][17].. MicMicrobirobiallyally induinducedced cal

calcitcitee preprecipcipitaitatiotionn (M(MICPICP)) cancan bebe useusedd forfor aa vavarierietyty ofof othotherer apapplpli- i-cations including

cations including environmeenvironmental remediationtal remediationn [13][13], improve, improvedd dura dura--bilit

bility y and and remremediaediation tion of of concconcreterete [25,7] [25,7], , calcalciucium m remremovoval al inin wastewater

wastewater [15] [15], and carbon sequestration, and carbon sequestration [26] [26].. Alth

Although various forms ough various forms of of MICP are MICP are avaiavailabllable e usinusing g diffdifferenerentt bacterial and precursors, the form of MICP treatment used for this bacterial and precursors, the form of MICP treatment used for this research utilized natural soil bacteria to metabolize urea, research utilized natural soil bacteria to metabolize urea, increas-ing

ing thethe pHpH ofof thethe poporere wawaterter,, prpromomotiotingng miminerneralal preprecipcipitaitatiotion.n. Ure Ure--olyt

olytic ic bactbacteria are eria are prevprevalenalent t in in natunatural ral soilsoils; s; they increasthey increase e thethe alkalinity of the soil by hydrolyzing the urea to produce ammonia alkalinity of the soil by hydrolyzing the urea to produce ammonia and carbon dioxide. This induces calcite precipitation primarily at and carbon dioxide. This induces calcite precipitation primarily at particle–particle contacts, which increases the strength and particle–particle contacts, which increases the strength and stiff-nes

nesss ofof thethe sansand.d. TheThe amamounountt ofof calcalcitcitee cemcemententatiationon isis propropoportiortionanall to

to the concethe concentratintrations of chemons of chemicalsicals suppsuppliedlied (e.g. urea(e.g. urea and calciand calcium)um) and

and thethe numnumbeberr ofof tretreatmatmenentsts peperforformrmeded.. ThThee reareactictionon netnetwoworkrk foforr the net urea hydrolysis reaction and formation of calcite is: the net urea hydrolysis reaction and formation of calcite is:

http://dx.doi.org/10.1016/j.conbuildmat.2014.01.019

0950-0618/

⇑

⇑ _{Corresponding author. Address: Department of Civil and Environmental Engi-}_{Corresponding author. Address: Department of Civil and Environmental}

Engi-neering, University of California, One Shields Ave., Davis, CA 95616, USA. Tel.: +1 neering, University of California, One Shields Ave., Davis, CA 95616, USA. Tel.: +1 530 754 8995.

530 754 8995.

E-mail address:

E-mail address: [email protected] [email protected] (J.T. DeJong). (J.T. DeJong).

Contents lists available at

Contents lists available at ScienceDirect ScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e :

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Current methods for brick manufacturing vary widely, but most methods include high energy processes of compression under high stresses and/or baking at high temperatures. The most common method of brick manufacturing is by ﬁring clay at high tempera-tures. Red clay bricks are typically placed in wood molds and dried in the sun for 2–3 days and then baked in the oven for 24 h at tem-peratures up to 1200_{C [6]. Engineering properties and physical}

characteristics differ between red clay bricks primarily due to the clay source andfiring temperature. For example, Lower Oxford Clay based bricks have a 28 day unfired strength of 3.5 MPa and a fired strength above 20 MPa [22] (and other references in Table 1). Alter-natives to red clay bricks include sand–lime bricks, which are man-ufactured using water, sand, and lime mixed together, compacted together at a pressure of 20 MPa and then autoclaved for up to 9 h at temperatures of up to 190 _{C (Fang et al. [12]). Another method}

uses clay in addition to lime, cement, and a manufacturing byprod-uct such as ground granulated blast furnace slag. The bricks are cured at room temperature, yielding strengths between 2.7 and 5 MPa [23]. Other methods of manufacturing earth-based building materials consists of adobe, cob, rammed earth, and compressed earth bricks [29,24]. The range of strengths and modulii for these and other bricks are summarized in Table 1.

This paper summarizes a research program undertaken to develop a natural, bio-mediated process for the manufacturing of bio-bricks (Bernardi [4]). The materials, treatment methods, and measurement techniques are presented ﬁrst. Bricks produced using the novel technique are compared against cement and lime treated bricks. Results assessing the treatment uniformity within individual bricks, as measured and indicated by shear and com-pression wave velocity, are presented. The correlation between cementation level (precipitated calcite concentration) and wave velocities is then investigated. A comparison of brick strength between the three brick types is presented, followed by correla-tions between compressive strength and velocity measurements.

2. Materials and test methods 2.1. Soil

The sand used for productionof allbricks was silica rich #1masonrysand, quar-ried in Chico, California. Thissand was used because it is moderately graded, locally produced, and available in large quantities. Salient sand characteristics are pre-sented in Table 2.

2.2. Bacteria and growth conditions

The soil bacteriumutilized in this studywasSporosarcina paseurii(ATCC 11859). Cultures were grown in an Ammonium-Yeast Extract media (ATCC 1376) as described in Mortensen and DeJong [20] (0.13 M Tris Buffer, 10 g of (NH4)2SO4,

and 20 g of yeast extract per liter of deionized water). The bacteria were inoculated in the growth media and incubated aerobically in a 30C water bath shaken at 200 rpm for approximately 24 h. Bacteria were incubated until samples obtained an optical density near 1.0 using a spectrophotometer (600 nm wavelength). The

sand was inoculated with the bacteria by percolating the bacterial solution through the sand top-down, which was retained for 4 h in the soil before treatments with cementation media began.

2.3. Cementation media

A urea-calcium medium was used to drive calcite precipitation. The cementa-tion media consisted of urea (200 mM), calcium chloride (100 mM), and nutrient broth (0.5 g/L). The nutrient broth, which contains beef extract and peptone, was used to enable bacteria reproduction within the brick mold.

2.4. Brick mold

Three identical brick molds were fabricated from PVC plastic, with each mold containing five bricks with dimensions of 91 mm by 58 mm by 200 mm (similar dimensions as standard red clay bricks, Fig. 1, Bernardi [4]). The mold is assembled with screws andsilicone sealant with drain holes at the mold base to enablefluid to percolate through. The mold base enables saturation of the mold during bacterial treatment and relatively unobstructed flow during cementation treatment. Three plastic screens with different opening sizes (3.360 mm, 0.711 mm, and 0.178 mm) were placed at themoldbase to prevent soil lossduring treatment. The sand is then placed, three additional screens were placed on top of the sand, and a low confining stress (10 kPa) applied with a rubber band. Coarse gravel is placed on top to

pre-vent erosion of the sand when the treatment solution is added.

2.5. Preparation and treatment programs 2.5.1. MICP treatment method

The bacterial solution was added to the sand by percolation (i.e. unrestrained flushing of fluid from top to bottom). The treatment method implemented was selected in order to ensure bacteria attachment at particle contacts within the per-meable sandmatrix. Effluent consisting of the bacterial solutionwas cycled through the sand two additional times to improve bacteria attachment throughout the sam-ple, and during the second cycle the mold was sealed to create fully saturated con-ditions. Treatment media was added to the sand by percolation. Three brick molds were treated for different amounts of time. Since the bio-bricks were going to be compared to lime and cement treated bricks that were cured for up to 28 days, an equivalent treatment time was devised. Treatments ranged from 1 to 5 times per day, depending on permeability reduction from the treatment, so an average of 3 treatments per day was defined as equivalent to one day of curing of conven-tional bricks. The molds were treated at 7 days (21 treatments), 14 days (42 treat-ments), and 28 days (84 treatments). A 12 h retention was usually allowed overnight before treatment started the following day. All treatments contained the cementation media (Table 2). pH readings were made of the influent solution and the immediate effluent of each brick with the use of pH strips (displaying pH in the range of 6.5–9.0). Occasionally excess calcite precipitation on the injection face of the bricks reduced permeability sufficiently that the mold was partially dis-assembled and the screens cleaned. Once the required treatments were completed, two pore volumes of deionised water with50 mM sodium chloride were percolated through to rinse excess chemicals from the pore space. The mold was then disas-sembled and thebricks were oven dried overnight in a 77C oven.The brick dimen-sions were measured and the mold was weighed again to estimate changes in dry density and void ratio.

2.5.2. Lime treatment method

Lime bricks were prepared by combining dry sand with varied volumes of hydraulic lime. The evaluated percents of hydraulic lime to sand by volume were 20%, 25%, 30%, 40%, and 50% for each set of ﬁve bricks (these correspond to percent-ages by weight of about 10.1%, 12.7%, 15%, 20.9%, and 26.7%). These mixtures brack-et the strengthsthatwere expected from the bio-bricks (2 MPa) andwereselected

in part frommanufacturer recommendations. The limeused was fromthe manufac-turer St. Austier and is a natural hydraulic lime (NHL5) with approximately 20–30% clay included as the silica source. The lime and the sandwere measured, dry mixed, and then water was added until proper workability was achieved (250 mL of

water per brick). The sand–lime mixture was placed in approximately 2.54 cm lifts and tamped 50 times using the steel overburden stress tamper. The overburden stress was then applied the same way as the bio-brick treatment. The three brick molds were used to make batches of bricks to be tested at different curing times. The bricks set for 2 days and then cured for 7, 14, and 28 days in a constant humid-ity chamber (95% humidity,13.3C). After curing was complete, the brick molds

were disassembled and bricks dried for up to 2 days in a 77C oven before testing.

Table 1

Strength of other bricks and materials.

Bricks Strength (Mpa) Elastic modulus (MPa) Autoclaved bricks 20 –

Red clay bricks >20 – Compressed earth block 0.7–3.1 200 Rammed earth 0.75–1.5 72–102 Adobe 1.2–1.8 100–300 Sandstone 70 45,000 Limestone 10–70 – [12,14,5,18,30,3,27,16] . Table 2 Sand characteristics.

Material D50 (mm) C u C c Gs emin emax Mineralogy

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2.5.3. Cement treatment method

Cement bricks were prepared by combining dry sand to various volumes of type II/V cement according to ASTM C150. The cement was added in quantities of 5%, 10%, 15%, 20%, and 25% by volume for each respective brick in a batch (corresponding to percents by weight of about 3.7%, 7.4%, 11.2%, 15.3%, and 19.3%). The cement amount selected was to bracket the expected strengths of the bio-bricks. The cement and sand were measured and manually mixed together dry and then water was added until a proper consistency for workability was achieved (250 mL per brick). The mixture was then placed in approximately

2.54 cm lifts and tamped 50 times using the overburden stress tamper. An overburden stress was applied. The bricks set for one day and then cured for 7, 14, and 28 days in a constant humidity chamber (95% humidity,13.3C). After

curing was completed, the bricks were dried for up to 2 days in a 77C oven before testing.

2.6. Measurement methods 2.6.1. Compression wave velocity

Seismic wave measurements are effective in monitoring the incremental cementation that occurs with MICP [8,9,31]. As cementation occurs the contacts between particles become increasingly stiff, which results in an increase in bulk stiffness and faster transmission of seismic waves. Compression seismic waves (p-waves) were used post-treatment to assess cementation uniformity along the brick length for the bio-bricks, hydraulic lime, and cement bricks. The equipment, settings, and techniques used are speciﬁed by Weil et al. [31]. A pair of 0.5 MHz Panametrics V101-RB ultrasonic transducers were powered by an HP 33120A Func-tion Generator and received by a Fluke PM3384oscilloscope. The transmitted signal was a +10 V, 60 kHz square wave and the received signal was subjected to a 1 kHz high pass ﬁlter to remove background noise and a 30 dB gain. Five equidistant loca-tions along the brick lengths were measured. All of the p-wave measurements were performed across the brick width (91 mm). Vacuum grease was used to provide

coupling between the transducer faces and brick surfaces.

2.6.2. Unconﬁned compressive strength

Unconﬁned compression testing wasperformed on all bricks in accordance with ASTM C67-07a [2]. Tests were performed on a GeoTac Sigma-1 Triax machine with a 4.5 Mg (10 kip) load frame. Bricks were cut in thirds instead of halves, as recom-mended in the ASTM standard, to better assess brick spatial variability. All bricks were cut with a wet masonry saw to minimize disturbance. The dimensions of both faces of each brick section undergoing loading were taken, as well as the average depth of the brick. All surfaces of the testing apparatus were cleaned and the spec-imenswere placed without sulfur or gypsum capping because surfacesof thebricks were ﬂat, smooth, and parallel. The load was seated on the specimen and testing initiated. The test was strain controlled with a rate of 2% strain/min (0.045 cm/

min). Tests were performed until failure and the maximum stress was recorded [2].

2.6.3. Calcite measurements

Calcite concentration measurements were performed following the methods described in Mortensen and DeJong [20] and Martinez et al. [19]. Samples were obtained from the middle of each brick section after compression testing. The dry weight of the sample was taken, then acid washed with 5 M HCl, and dry weights obtained again after acid washing to determine the amount of cementation that occurred.The sand used does contain a smallpercentage of fines whichis lost when undergoing this treatment, so the calcite test was also performed with un-treated sand to determine the average loss of fines and any calcite existing in the soil. This was determined to be approximately 0.88% of the sample, so the values presented are adjusted for the loss in fines.

3. Results and discussion

3.1. Small-strain properties and uniformity

The compression wave (p-wave) velocities along the length of the brick are plotted in Fig. 2 with the ranges, average, and coefﬁcient of variability presented in Table 3. P-wave velocity for

Overburden Stress Rip Rap

Top Filter Screens

Brick Divider

Bottom Filter Screens and Drain Holes

Fig. 1. Image of assembled brick mold (left image with one side removed for display purposes).

7 Day 14 Day 28 Day 0 1000 2000 3000 4000 P-wave Velocity (m/s) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) Brick 11 Brick 12 Brick 13 Brick 14 Brick 15 0 1000 2000 3000 4000 P-wave Velocity (m/s) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) Brick 6 Brick 7 Brick 8 Brick 9 Brick 10 0 1000 2000 3000 4000 P-wave Velocity (m/s) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) Brick 1 Brick 2 Brick 3 Brick 4 Brick 5

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Fig. 2. P-wave velocities of bio-bricks at (a) 7 day, (b) 14 day, and (c) 28 day treatment.

Table 3

Minimum, maximum, average, and coefﬁcient of variation for p-waves of bio-bricks.

Treatment time (Days) Average V p (m/s) Min V p (m/s) Max V p (m/s) COV (%) 7 1560.8 1383.6 2591.5 6.0 14 2269.9 2008.0 2591.5 6.9 28 3062.7 2723.4 3576.5 6.6

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bio-bricks exceeds 3500 m/s with an average coefﬁcient of varia-tion within a single brick of 6.5%. The trend of the p-wave velocity for the 7 day bio-brick indicates uniformity within the center of the

brick (range of 177 m/s), but the top portion exhibits the highest compression wave velocity with a slight decrease toward the brick base. The trend of p-wave for the 14 day bio-brick batch have a range of 262 m/s and is slightly higher at the top of the brick than at the bottom of the brick, indicating more cementation occurring in the top of the brick and gradually decreasing throughout the brick length (Fig. 6b). The p-wave velocities for the 28 day bio-brick batch have a range of 339 m/s. The top of the bio-brick possesses a high p-wave velocity with a decrease toward the upper third to the brick center. P-wave velocity then increases again towards the brick base, which results in a p-wave velocity being compara-ble to the top of the brick. The biased cementation at the brick top is most likely due to a majority of the bacteria being present at the top of the brick (since bacteria were initially ﬂushed top-down).

P-wave velocities of lime and cement bricks for the range of treatment levels and time periods showed consistent spatial uni-formity and increasing velocity as expected. Representative results

0 1000 2000 3000 4000 P-wave Velocity (m/s) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) 5% Cement 10% Cement 15% Cement 20% Cement 25% Cement 0 1000 2000 3000 4000 P-wave Velocity (m/s) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) 20% Lime 25% Lime 30% Lime 40% Lime 50% Lime

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Fig. 3. P-wavevelocitiesof lime and cementtreated bricksafter28 daycuring time.

0 2 4 6 Strain (%) 0 250 500 750 1000 S t r e s s ( k P a ) 0 2 4 6 Strain (%) 0 50 100 150 200 S t r e s s ( k P a ) 0 2 4 6 Strain (%) 0 1000 2000 S t r e s s ( k P a ) 28 Day 14 Day 7 Day

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Fig. 4. Representative stress strain curves of bio bricks tested in unconﬁned compression.

Table 4

Minimum, maximum, average, and coefﬁcient of variation for strength values of bio-bricks. Treatment time (Days) Average strength (kPa) Min strength (kPa) Max strength (kPa) COV (%) 7 69.1 43.3 121.7 37.0 14 441.3 115.5 934.7 48.7 28 1645.4 910.9 2286.6 26.4 Table 5

Minimum, maximum, average, and coefﬁcient of variation for E 50 modulus values of bio-bricks. Treatment time (Days) Average E 50 (kPa) Min E 50 (kPa) Max E 50 (kPa) COV (%) 7 5782.4 3140.6 13868.5 49.2 14 15019.5 4525.0 29964.7 47.4 28 83915.5 27433.8 131398.3 36.5 0 2 4 6 Strain (%) 0 1000 2000 S t r e s s ( k P a ) 0 2 4 6 Strain (%) 0 1000 2000 S t r e s s ( k P a ) 40% Lime 20% Cement

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Fig. 5. 28 Day stress strain curves of 40% line and 20% cement bricks tested in unconﬁned compression.

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after 28 days of curing for all of the lime and cement treated bricks are shown in Fig. 3 (with further details in Bernardi [4]). An increase in curing time resulted in very little change in p-wave velocities; on average p-wave velocities increased by 250 m/s for both lime and cement bricks. The average coefﬁcient of variability within a single brick for the lime bricks was 12.8% while the cement bricks were 15.8%.

3.2. Compressive strength and secant modulus

Stress strain curves of bio-bricks were obtained from compres-sion testing on ﬁve bricks at each treatment interval, with repre-sentative stress strain curves presented in Fig. 4. The peak strength of the stress strain curve and the modulus (computed as a secant elastic modulus based on the strain required to mobilize

Table 6

Constants for statistical ﬁts of strength and modulus plotted with p-wave velocity.

Admixture X -axis Y -axis A B C R2

MICP P-wave Strength 0 0.0016 2.45 0.75

P-wave E 50 423 0.0018 5.61 0.38

P-wave Calcite content 5 0.0004 1.55 0.83

Calcite content Strength 5265 0.0232 8.49 0.75

Calcite content E 50 6527 0.2052 8.1 0.38

Lime P-wave Strength 140 0.0010 4.69 0.65

P-wave E 50 16,010 0.0006 9.54 0.34

P-wave Percent mass 50 0.0003 3.84 0.81

Percent mass Strength 0 0.1038 3.84 0.75

Percent mass E 50 0 0.0957 7.78 0.70

Cement P-wave Strength 70 0.0011 4.14 0.83

P-wave E 50 1570 0.0014 7.00 0.63

P-wave Percent mass 9 0.0004 2.05 0.97

Percent mass Strength 400 0.1144 5.90 0.87

Percent mass E 50 24,345 0.0978 10 0.53

Lime Bricks Bio-Bricks Cement Bricks 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 250 500 750 1000 S t r e n g t h ( k P a ) _{7 Day} 14 Day 28 Day R 2_{= 0.65} 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 25000 50000 75000 100000 E 5 0 M o d u l u s ( k P a ) 7 Day 14 Day 28 Day R 2 =_0.34 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 1000 2000 3000 4000 S t r e n g t h ( k P a ) _{7 Day} 14 Day 28 Day R 2_{= 0.83} 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 50000 100000 150000 200000 250000 E 5 0 M o d u l u s ( k P a ) 7 Day 14 Day 28 Day R 2_{= 0.63} 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 1000 2000 3000 S t r e n g t h ( k P a ) 7 Day 14 Day 28 Day R 2 _{= 0.76} 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 50000 100000 150000 200000 E 5 0 M o d u l u s ( k P a ) 7 Day 14 Day 28 Day R 2 _{= 0.38}

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(b)

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50% of the peak strength, E 50) was computed for each brick section

tested. The average and range of strengths and modulii as well as the coefﬁcient of variation are presented in Tables 4 and 5, respec-tively. The 7-day treatments of bio-bricks have a relatively low strength and modulus exhibiting a ductile failure (Fig. 4a). After 14 days of treatment the bio-bricks increase in both strength and modulus with a transition between ductile and brittle behavior that can be seen between the top and middle/bottom segments (Fig. 4b). Upon further treatment to 28 days, the strength and mod-ulus continue to increase throughout the brick with the top of the brick being stronger than both the middle and bottom. The bottom of the brick becomes stronger than the middle portion of the brick at this treatment level, which coincides with the p-wave data (Fig. 3c). All brick segments exhibit brittle failure after 28 days of treatment (Fig. 4c). Overall, strengths and modulii both increase with continued treatment. The average strength increases from 69 kPa to 1600 kPa, while the average modulus increases from 5782 kPa to over 83,900 kPa from 7 to 28 days of treatment.

Representative stress strain curves for 40% lime and 20% cement bricks cured at 28 days are plotted in Fig. 5 for comparison. Lime and cement bricks show a general trend of the top and middle brick segments having a higher strength and modulus than the bottom brick segment. Lime bricks exhibit ductile failure exclu-sively, while cement exhibits ductile failure at a low admixture percent (10% and lower) while changing to brittle failure above 10% concentration. Strength and modulii for lime and cement sys-tematically increased with increase in admixture (Bernardi [4]). Strength and modulii for lime bricks remain low relative to the strength gain of both bio-bricks and cement bricks. The lime bricks tested at 50% lime content had a maximum strength of 0.98 MPa and a maximum secant modulus of 544 MPa, which is less than that claimed by the manufacturer (compressive strength of 2.2 MPa, elastic tangent modulus of 10,800 MPa). The source of the difference was investigated with additional tests, but could not be identiﬁed. Compression strengths and modulii for cement bricks tend to increase more rapidly with increasing percent of admixture.

Strengths and modulii of bio, lime, and cement bricks are plot-ted versus p-wave velocity in Fig. 6. A three constant single expo-nential growth equation as shown in the plots was ﬁtted to each data set:

y ¼ A þ eðBxþC Þ ð1_Þ

The constants for each ﬁt are presented in Table 6. Constant A is chosen to anchor a p-wave velocity between 150 and 300 m/s based off of values from previous research on dry sand with 100 kPa of conﬁnement. The R2_{values for strengths versus p-wave are above}

0.65 while the R2 for modulus versus p-wave are higher than 0.34. The plots of modulus generally have a higher data spread, resulting in lower R2. In all cases, brick strength and modulus gen-erally increase with an increase in p-wave velocity.

Bio-bricks range in strengths from 1.0 to 2.2 MPa, lime reaches as high as 1.0 MPa, and cement has a maximum strength of 2.5 MPa. Other brick materials made of earth and sustainable material such as adobe and rammed earth (Table 1) have strengths ranging from 0.7 MPa to 3.1 MPa. A more conventional building material such as ﬁred red clay brick has strengths above 20 MPa, much higher than bio-bricks. The p-wave velocities of bio-bricks are comparable to sandstone. Natural sandstone and limestone have an unconﬁned compressive strength of up to 70 MPa, which is much higher than any material tested herein. A direct compari-son of the tangent secant modulus reported herein cannot be directly compared to the elastic modulus values in Table 1 as these values are for small strain elastic modulus. For example, the mate-rials in Table 1 can have an elastic modulii up to 300,000 MPa

while the tangent secant modulus, E 50, of bio-bricks only reach

up to 130 MPa.

3.3. Relation to calcite/admixture concentrations

Results showing percent of calcite along the length of the bio-bricks are presented in Fig. 7. Trends of calcite concentration along the length of the bricks seem to be counterintuitive at ﬁrst, indi-cated a smaller amount of calcite at the top of the brick where the higher strength occurs (Fig. 7a). The 14 day bio-bricks are slightly more varied than the 7 day, with a couple of bricks con-taining higher calcite at the top of the brick and gradually decreas-ing along the brick. Bricks 9 and 10 of Fig. 7b are exceptions containing a low calcite percentage at the top relative to what is expected to occur. For the 28 day batch in Fig. 7c the calcite con-tent variability is fairly narrow at the top of the brick, widens in the middle, and then narrows again towards the bottom of the brick. This data is consistent with the p-wave and strength data collected for the 28 day bio-bricks where the values of all these properties are higher in the top and bottom portions of the brick than in the center. The trends of calcite, particularly observed in the 7 and 14 day data, with increasing calcite along the brick length that are not congruent with the p-wave data are attributed to a measurement error associated with the calcite measurement technique. As mentioned previously, 0.88% of the sample consists of ﬁne soil particles that could be washed out during the calcite

7 Day 14 Day 28 Day 0 5 10 15 20 Calcite Content (%) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) Brick 1 Brick 2 Brick 3 Brick 4 Brick 5 0 5 10 15 20 Calcite Content (%) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) Brick 6 Brick 7 Brick 8 Brick 9 Brick 10 0 5 10 15 20 Calcite Content (%) 20 15 10 5 0 D i s t a n c e F r o m T o p o f B r i c k ( c m ) Brick 11 Brick 12 Brick 13 Brick 14 Brick 15

(a)

(b)

(c)

Fig. 7. Percent calcite for bio-brick for (a) 7 day, (b) 14 day, and (c) 28 day treatment times.

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measurement. These same fine particles could have first migrated downwards inside the bricks during treatment, resulting in higher fines content near the brick base than at the top. When the calcite measurement is then performed and the average value of 0.88% subtracted to correct for the average fines content this likely results in an overcorrection for fines near the specimen top (result-ing in too low of a calcite estimate) and an under correction near the base (resulting in too high of a calcite estimate). Unfortunately the experimental program did not allow this issue to be explored further.

The relationship between calcite content, p-wave velocity, strength, and modulus are presented in Fig. 8. A trendline using Eq. (1) above was ﬁt to all three relationships. In general, p-wave velocity, strength, and modulus increase with calcite content with

R2values of 0.83, 0.75, and 0.38, respectively. The p-wave velocity was again anchored to the range of 150–300 m/s as explained pre-viously. The plot of calcite content versus strength was best ﬁt when anchored to a calcite content of approximately 3.4%, which was then used for the graph of modulus. Similar trends were observed between the lime and cement admixture concentration versus p-wave velocity, strength, and modulus (Bernardi [4]). Like the bio-bricks, plots of percent of mass of admixture with p-wave velocity and strength yield a higher R2 _{values than when plotted}

with modulii.

3.4. Void ratio and dry density

The average ﬁnal void ratio and ﬁnal dry density after treatment to the designated time increments are presented versus p-wave velocity in Fig. 9. The data plotted was calculated using the

dimensions and mass of the full dry bio-bricks after treatment and the average p-wave velocities for an entire brick. The average initial void ratio for each bio-bricks was 0.51, and the initial dry density was 1.72 Mg/m3. As evident, the void ratio following MICP treatment decreases to about 0.33, a value signiﬁcantly less than the minimum void ratio for the untreated sand. The total dry den-sity of the bricks increases with treatment up to a value of about 1.9 Mg/m3. The relatively small increase in density compared to the reduction in void ratio is attributed to the low dry density of precipitated calcite relative to that of the silica sand particles.

4. Conclusions

The novel MICP bio-brick manufacturing technique developed herein can produce compressive strengths that exceed2.0 MPa. Re-sults from 7, 14, and 28 day treatment schemes indicate that the strength and stiffness is scalable, enabling customization to project and site speciﬁc requirements. The strengths obtained after these different treatment durations followed trends similar to sand-based bricks treated with lime (27% by weight) and cement (12–18% by weight), though treatment with cement increased strength more consistently with respect to concentration of admixture.

The stress–strain behavior became more brittle with increasing cementation level, eventually exhibiting behavior after 28 day treatment consistent with that for lime and cement admixture bricks. Secant elastic modulus corresponding to 50% mobilized strength, capturing the stiffness upon initial loading, increased from 5300 kPa after 7 days of treatment up to 83,900 kPa after 28 days of treatment. The secant elastic modulus also increased with cement admixture concentration and to a lesser extent when lime was used.

Compression wave, or p-wave, velocity is effective in mapping the extent and uniformity of calcite cementation along the length of the bricks, with p-wave values after 28 days of treatment exceeding 3500 m/s and being comparable to sandstone and lime-stone rock. Variability within individual bio-bricks averaged about 6.5% in terms of coefﬁcient of variation (COV), indicating high spec-imen uniformity. In comparison, lime and cement treated bricks had average COV values of 11.1% and 6.4%, respectively. P-wave

0 5 10 15 20 Calcite Content (%) 0 50000 100000 150000 E 5 0 M o d u l u s ( k P a ) 7 Day 14 Day 28 Day R 2 _{= 0.38} 0 5 10 15 20 Calcite Content (%) 0 1000 2000 3000 S t r e n g t h ( % ) 7 Day 14 Day 28 Day R 2 _{= 0.75} 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 5 10 15 20 C a l c i t e C o n t e n t ( % ) 7 Day 14 Day 28 Day R 2 _{= 0.83}

(a)

(b)

(c)

Fig. 8. Calcite content plotted with (a) p-wave velocity, (b) strength, and (c) E 50

modulus. 0 1000 2000 3000 4000 P-wave Velocity (m/s) 0 0.2 0.4 0.6 0.8 1 V o i d R a t i o 7 Day 14 Day 28 Day e min e max 0 1000 2000 3000 4000 P-wave Velocity (m/s) 1.25 1.5 1.75 2 2.25 D r y D e n s i t y ( M g / m 3 ) 7 Day 14 Day 28 Day d max d min

(a)

(b)

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velocity also correlated directly with admixture concentration, secant elastic modulus, and strength for MICP, lime, and cement additives. Correlations developed generally followed exponential growth trends, with R2values greater than 0.6. For the bio-bricks speciﬁcally, estimates of calcite, secant elastic modulus and strength could be predicted with an accuracy of about ±30%.

Overall this research has demonstrated the strong potential of manufacturing sand-based bricks using a natural microbial pro-cess. This is attractive given its simplicity, low embodied energy in materials, and scalability. Further research is needed to obtain a more accurate estimate on cost, durability, and net carbon foot-print of MICP brick production relative to more conventional methods.

Acknowledgements

Funding provided by the United States National Science Foun-dation (#072746). Any opinions, ﬁndings, and conclusions or rec-ommendations expressed in this material are those of the writer(s) and do not necessarily reﬂect the views of the National Science Foundation.

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