Structure Strength Correction Value for Concrete’s Mix Proportion Strength Using Low-Quality Recycled Aggregate

Abstract

To develop a design method for concrete using low-quality recycled aggregates, an experimental study was conducted on applicability to examine the structural strength correction value (S value) and calculation of mix proportion strength of recycled aggregate concrete-Class M, which used recycled aggregate class L mixing with the normal aggregate. Cement used in the experiment was ordinary Portland cement (in this case, fly ash type II was used as a fine aggregate substitute), Portland blast-furnace slag cement type B, and low-heat Portland cement. As a result, the mix proportion strength of recycled aggregate concrete-Class M could be determined using the S value according to JASS 5 (2018) as normal-weight concrete.

1. Introduction

As regulated in the revision of JIS A 5022 (recycled aggregate concrete-Class M), recycled aggregate concrete-Class M (types 1 and 2) [1] can be manufactured using recycled aggregate class L mixing with the normal aggregate. In the future, it will be effective to expand the use of recycled aggregate class L due to its minute environmental impact. Therefore, it is necessary to discriminate between the applications of recycled aggregate concrete-Class M and -Class L.

A recycled aggregate comprises coarse aggregate (original aggregate) in the original concrete, mortar (adhered mortar), and cement paste (adhered paste) attached to the aggregate [2]. Because many adhered mortars and pastes are mixed in recycled aggregate class L, there is a large variation in quality. Therefore, data accumulation is necessary. In addition, for countermeasures against the alkali–silica reaction (ASR) of recycled aggregates [3] and improving long-term compressive strength, fly ash (FA) [4] and ground granulated blast-furnace slag (BFS) [5] are considered effective. It is necessary to consider the applications of these materials to ensure the required quality of structural concrete, especially the effect on reducing a decrease in compressive strength and an increase in drying shrinkage rate in concrete using recycled aggregate concrete-Class L [6].

In this study, we developed a design method for concrete using low-quality recycled aggregates. In concrete manufactured by mixing a low-quality recycled aggregate with the normal aggregate, using major cement and mineral admixture, the structural strength correction value (S value) was experimentally examined; consequently, the results of mix proportion strength calculations were shown.

2. S Value and Mix Proportion Strength

According to JASS 5 (2018) [1], for recycled aggregate concrete-Class M, which used recycled aggregate class L mixing with normal aggregate in a certain amount, the nominal and mix proportion strengths can be considered the same as those of normal concrete. However, there is no experimental study on the S value of concrete using recycled aggregate class L. In addition, the effect of FA and BFS on improving the quality of recycled aggregate concrete and the use of low-heat Portland cement (L) in mass concrete need to be examined.

The selection of cement types according to the required performance is shown in

Table 1. Selection of cement type according to required performance [2].

	Type of Cement	N	L	BB	N + FA 1
Required Performance
Measure against temperature cracks due to heat of hydration		-	○	-	-
Durability performance due to carbonation		○	○	-	-
Measure for suppressing ASR		-	-	○	○

¹ FA is used as a fine aggregate substitute.

[7]. The three types of cement are ordinary Portland cement (N), Portland BFS cement type B (BB), which is limited to underground structure, and L, which is used for preventing temperature cracking due to hydration heat in mass concrete. For N, FA type II (FAII) was used as a fine aggregate substitute with a mass of 20% of the total mass of N and FA, at least 80 kg/m³ to suppress the ASR [4].

Table 2. Structural strength correction value ₂₈S₉₁.

Type of Cement	Outline 1
N 2	Hot season	8 ≤ θ	0 ≤ θ < 8
BB	Hot season	13 ≤ θ	0 ≤ θ < 13
L	Hot season	14 ≤ θ	0 ≤ θ < 14
Structural strength correction value 28S91 (N/mm2)	6	3	6

¹ θ: temperature (°C) ² Including the case of (N + FA) in which FAII is used as a fine aggregate substitute.

shows the results of examining the S value when the three types of cement were used. In this study, the evaluation was performed mainly during the hot season, considering the application to mass concrete, such as the main building foundation (turbine construction) of a thermal power plant and the machine base foundation of a boiler.

2.1. Outline of Experiment

The main qualities of cement and mineral admixture are summarized in

Table 3. Main qualities of cement and mineral admixtures.

Item		N	L	FAII 1	BB
Density (g/cm3)		3.15	3.24	2.24	3.04
Blaine fineness (cm2/g)		-	-	3400	-
Specific surface area (cm2/g)		3300	3300	-	3990
Moisture content (%)		-	-	0.5	-
MgO (%)		1.58	0.63	-	3.52
SO3 (%)		1.90	2.19	-	1.79
Cl− (%)		0.011	0.005	-	0.009
SiO2 (%)		-	-	≥45.0	-
Ignition loss (%)		2.06	0.70	1.40	1.60
Mortar flow ratio (%)		-	-	≥95	100
Activity index (%)	28 days	-	-	≥80	-
	91 days	-	-	≥90	-

¹ Manufactured in Hekinan.

, an outline of recycled aggregates is presented in

Table 4. Outline of recycled aggregate.

Type	Original Concrete	Manufacture Method
Recycled coarse aggregate class L: RLG1	Office building, reinforced concrete (RC) structure, about 45 years	Crushing and classifying original concrete at a demolition site
Recycled fine aggregate class L: RLS
Recycled coarse aggregate class L: RLG2	Steel chimney foundation and machine base foundation of a thermal power plant, RC structure, about 40 years

, and the qualities of aggregates are listed in Table 5.

2.1.1. Cement and Mineral Admixture

In this study, N was used as cement, which is in accordance with JIS R 5210 [8]. Mineral admixtures are FAII in accordance with JIS A 6201 [9] and BFS 4000 in accordance with JIS A 6206 [10].

2.1.2. Aggregates

For normal coarse aggregates, crushed limestone size 2005 (G₁) and crushed hard sandstone size 2005 (G₂) were used at Plant A, and crushed limestone size 2005 (G₃) was used at Plant B (where Plants A and B are two ready mixed concrete plants). For normal fine aggregate, a mixture of land sand and crushed sand in a mass ratio of 7:3 was used at Plant A (S₁), mountain sand (S₂) and crushed sand (S₃) were used at Plant B.

Two types of recycled coarse aggregates (RLG₁ and RLG₂) and one type of recycled fine aggregate (RLS) produced simultaneously with RLG₁ were manufactured from different original concrete and used in the experiment. For the recycled coarse aggregates, RLG₁ had a 6.60% absorption and content of materials finer than 75-μm sieve of 0.7%, whereas RLG₂ had a 6.22% absorption and content of materials finer than 75-μm sieve of 2.1%. Both of them are equivalent to recycled aggregate concrete-Class L in JIS A 5023 Annex A. The total amount of contained impurities was 1.03 and 0.03 mass% for RLG₁ and RLG₂, respectively. Moreover, RLS had a 15.0% absorption due to the effect of the adhesive mortar and cement paste. It did not satisfy the requirement of recycled aggregate concrete-Class L (≤13%); therefore, it was used as an equivalent product as recycled fine aggregate L (Appendix A). The total amount of contained impurities was 0.60 mass%. All aggregates were tested for alkali–silica reactivity and confirmed to be harmless.

2.1.3. Mix Proportion

Table 6 lists the mix proportions used in this study. The concrete in the experiment used N, BB, and L. Further, six types of recycled aggregate concrete L and two types of normal-weight concrete were produced at Plants A and B.

(1)

N

For concrete using N, FAII was used as a fine aggregate substitute with 20% of the total mass of N and FAII to suppress ASR [2]. The water–cement ratio (W/C) was set to two levels of 40% and 60%, and the replacement rate of recycled aggregates was 50% for the recycled coarse aggregates. Two types of recycled aggregate concrete-Class M were manufactured at Plant A. The chemical admixture was air-entraining and water-reducing (high-performance type). The target slump at the placement location was 18 ± 2.5 cm; therefore, 20 ± 2.5 cm was considered for slump loss. The target air content was 4.5 ± 1.5%.

Table 5. Qualities of aggregates.

Item		Test Method	Normal Coarse Aggregate: 2005			Normal Fine Aggregate			Recycled Coarse Aggregate Class L: 2005		RFA 7
			Plant A		Plant B	Plant A	Plant B
			G1 1	G2 2	G3 3	S1 4	S2 5	S3 6	RLG1	RLG2	RLS
Density in oven-dry condition (g/cm3)		JIS A 1109 [11]	2.67	2.70	2.69	2.54	2.52	2.64	2.26	2.30	1.90
Absorption (%)		JIS A 1110 [12]	0.63	0.39	0.34	2.73	1.85	1.35	6.60	6.22	15.0
Fineness modulus (F.M.)		JIS A 1102 [13]	6.60 8	6.60 8	6.61 8	2.70 ± 0.2	2.10	2.64	6.56	6.52	3.63
Content of materials finer than 75-μm sieve (%)		JIS A 1103 [14]	≤3.0	≤3.0	0.6	≤3.0	1.6	3.4	0.7	2.1	3.0
Solid content in aggregate (%)		JIS A 1104 [15]	-	-	61.2	-	-	-	-	-	-
Solid content of particle shape (%)		JIS A 5005 [16]	≥56	≥56	59.6	-	-	60.1	59.1	60.5	57.8
Abrasion loss (%)		JIS A 1121 [17]	≤40	≤40	≤40	-	-	-	29.3	28.9	-
Soundness (%)		JIS A 1122 [18]	≤12	≤12	≤12	≤10	-	-	19.8	36.0	8.5
Chloride ion content (%)		JIS A 5002 [19]	-	-	-	-	0.001	-	0	0.001	0.004
ASR 9		JIS A 1146 [20]	-	-	H	-	H	H	H	H	H
		JIS A 1804 [21]	-	-	-	-	-	-	H	H	H
		ZKT-206 [22]	-	-	-	-	-	-	-	H	-
Amount of contained impurities (mass%) 10	A	JIS A 5023 [23]	-	-	-	-	-	-	0.660	0.007	0.401
	B								0.010	0.000	0.001
	C								0.000	0.003	0.008
	D								0.000	0.004	0.000
	E								0.145	0.005	0.119
	F								0.017	0.004	0.025
	Other								0.196	0.007	0.041
	Total								1.03	0.03	0.60

¹ G₁: Crushed limestone size 2005 from Akiyoshi, Yamaguchi Prefecture ² G₂: Crushed hard sandstone size 2005 from Ome, Tokyo. ³ G₃: Crushed limestone size 2005 from Torigata, Kochi Prefecture ⁴ S₁: Land sand from Kimitsu, Chiba Prefecture mixed with crushed sand from Kamiiso, Hokkaido with a ratio of 7:3 ⁵ S₂: Mountain sand from Ichihara City, Chiba Prefecture ⁶ S₃: Crushed sand from Torigata mountain in Kochi Prefecture ⁷ RFA: Recycled fine aggregate class L equivalent ⁸ For normal coarse aggregate Gmax = 20 mm ⁹ H: Harmless ¹⁰ Classification of A–F based on JIS A 5023.

Table 5. Qualities of aggregates.

Item		Test Method	Normal Coarse Aggregate: 2005			Normal Fine Aggregate			Recycled Coarse Aggregate Class L: 2005		RFA 7
			Plant A		Plant B	Plant A	Plant B
			G1 1	G2 2	G3 3	S1 4	S2 5	S3 6	RLG1	RLG2	RLS
Density in oven-dry condition (g/cm3)		JIS A 1109 [11]	2.67	2.70	2.69	2.54	2.52	2.64	2.26	2.30	1.90
Absorption (%)		JIS A 1110 [12]	0.63	0.39	0.34	2.73	1.85	1.35	6.60	6.22	15.0
Fineness modulus (F.M.)		JIS A 1102 [13]	6.60 8	6.60 8	6.61 8	2.70 ± 0.2	2.10	2.64	6.56	6.52	3.63
Content of materials finer than 75-μm sieve (%)		JIS A 1103 [14]	≤3.0	≤3.0	0.6	≤3.0	1.6	3.4	0.7	2.1	3.0
Solid content in aggregate (%)		JIS A 1104 [15]	-	-	61.2	-	-	-	-	-	-
Solid content of particle shape (%)		JIS A 5005 [16]	≥56	≥56	59.6	-	-	60.1	59.1	60.5	57.8
Abrasion loss (%)		JIS A 1121 [17]	≤40	≤40	≤40	-	-	-	29.3	28.9	-
Soundness (%)		JIS A 1122 [18]	≤12	≤12	≤12	≤10	-	-	19.8	36.0	8.5
Chloride ion content (%)		JIS A 5002 [19]	-	-	-	-	0.001	-	0	0.001	0.004
ASR 9		JIS A 1146 [20]	-	-	H	-	H	H	H	H	H
		JIS A 1804 [21]	-	-	-	-	-	-	H	H	H
		ZKT-206 [22]	-	-	-	-	-	-	-	H	-
Amount of contained impurities (mass%) 10	A	JIS A 5023 [23]	-	-	-	-	-	-	0.660	0.007	0.401
	B								0.010	0.000	0.001
	C								0.000	0.003	0.008
	D								0.000	0.004	0.000
	E								0.145	0.005	0.119
	F								0.017	0.004	0.025
	Other								0.196	0.007	0.041
	Total								1.03	0.03	0.60

¹ G₁: Crushed limestone size 2005 from Akiyoshi, Yamaguchi Prefecture ² G₂: Crushed hard sandstone size 2005 from Ome, Tokyo. ³ G₃: Crushed limestone size 2005 from Torigata, Kochi Prefecture ⁴ S₁: Land sand from Kimitsu, Chiba Prefecture mixed with crushed sand from Kamiiso, Hokkaido with a ratio of 7:3 ⁵ S₂: Mountain sand from Ichihara City, Chiba Prefecture ⁶ S₃: Crushed sand from Torigata mountain in Kochi Prefecture ⁷ RFA: Recycled fine aggregate class L equivalent ⁸ For normal coarse aggregate Gmax = 20 mm ⁹ H: Harmless ¹⁰ Classification of A–F based on JIS A 5023.

Table 6. Mix proportion.

Specimen 2	Mix Proportion 1								Unit Weight (kg/m3)
	Plant	CT 3	MA 4	RA 5 (%)		TS 6 (cm)	W/C (%)	s/a 7 (%)	W	C	Coarse Aggregate					Fine Aggregate					Ad 8	SP 9
											G1	G2	G3	RLG1	RLG2	S1	S2	S3	RLS	FA
				RLG	RLS
NFARLG150-40	A	N	FAII	50	0	20.0 ± 2.5	40	38.0	175	438	344	149	-	442	-	587	-	-	-	110	-	3.50
NFARLG150-60				50	0		60	45.6	175	292	336	144	-	427	-	778	-	-	-	73	-	1.90
BBRLG130RLS30-35		BB	-	30	30		35	35.0	198	566	490	211	-	266	-	365	-	-	131	-	14.15	-
BBRLG130RLS30-55				30	30		55	47.4	175	318	468	201	-	257	-	585	-	-	209	-	6.36	-
LG-40	B	L		0	0	17.0 ± 2.5	40	42.2	167	418	-	-	1029	-	-	-	429	297	-	-	4.18	-
LG-50				0	0		50	46.2	155	310	-	-	1023	-	-	-	501	348	-	-	3.10	-
LRLG250-40				50	0		40	40.7	176	440	-	-	516	-	464	-	403	281	-	-	4.40	-
LRLG250-60				50	0		60	47.3	158	263	-	-	510	-	459	-	522	364	-	-	2.63	-

¹ The target air volume in each specimen is 4.5 ± 1.5% ² In specimen name, types of cement, types of RLG, replacement ratio of RLG, RLS, and water-binder ratio are shown ³ CT: cement type ⁴ MA: mineral admixture ⁵ RA: replacement ratio of recycled aggregate ⁶ TS: Target slump, slump loss is considered as + 2.0 cm ⁷ s/a: Fine aggregate ratio ⁸ Plant A: Hydro complex/lignin derivative, AE water-reducing admixture, Plant B: Lignin sulfonate air-entraining and water-reducing admixture ⁹ Plant A: Poly-carboxylic acid copolymer high-performance air-entraining and water-reducing admixture.

Table 6. Mix proportion.

Specimen 2	Mix Proportion 1								Unit Weight (kg/m3)
	Plant	CT 3	MA 4	RA 5 (%)		TS 6 (cm)	W/C (%)	s/a 7 (%)	W	C	Coarse Aggregate					Fine Aggregate					Ad 8	SP 9
											G1	G2	G3	RLG1	RLG2	S1	S2	S3	RLS	FA
				RLG	RLS
NFARLG150-40	A	N	FAII	50	0	20.0 ± 2.5	40	38.0	175	438	344	149	-	442	-	587	-	-	-	110	-	3.50
NFARLG150-60				50	0		60	45.6	175	292	336	144	-	427	-	778	-	-	-	73	-	1.90
BBRLG130RLS30-35		BB	-	30	30		35	35.0	198	566	490	211	-	266	-	365	-	-	131	-	14.15	-
BBRLG130RLS30-55				30	30		55	47.4	175	318	468	201	-	257	-	585	-	-	209	-	6.36	-
LG-40	B	L		0	0	17.0 ± 2.5	40	42.2	167	418	-	-	1029	-	-	-	429	297	-	-	4.18	-
LG-50				0	0		50	46.2	155	310	-	-	1023	-	-	-	501	348	-	-	3.10	-
LRLG250-40				50	0		40	40.7	176	440	-	-	516	-	464	-	403	281	-	-	4.40	-
LRLG250-60				50	0		60	47.3	158	263	-	-	510	-	459	-	522	364	-	-	2.63	-

¹ The target air volume in each specimen is 4.5 ± 1.5% ² In specimen name, types of cement, types of RLG, replacement ratio of RLG, RLS, and water-binder ratio are shown ³ CT: cement type ⁴ MA: mineral admixture ⁵ RA: replacement ratio of recycled aggregate ⁶ TS: Target slump, slump loss is considered as + 2.0 cm ⁷ s/a: Fine aggregate ratio ⁸ Plant A: Hydro complex/lignin derivative, AE water-reducing admixture, Plant B: Lignin sulfonate air-entraining and water-reducing admixture ⁹ Plant A: Poly-carboxylic acid copolymer high-performance air-entraining and water-reducing admixture.

(2)

BB

For concrete using BB, two levels of W/C were 35% and 55%, and the replacement ratio of recycled aggregate was 30% and 30% for the recycled coarse and fine aggregates, respectively. At Plant A, two types of recycled aggregate concrete-Class M were manufactured. An air-entraining and water-reducing admixture (high-performance type) was used as the chemical admixture. The target slump at the placement location was 18 ± 2.5 cm; therefore, 20 ± 2.5 cm was considered for slump loss. The target air content was 4.5 ± 1.5%.

(3)

L

For the concrete using L, two types of normal-weight concrete were manufactured at Plant B with two levels of W/C of 40% and 50% for the standard period. In addition, two types of recycled aggregate concrete-Class M were manufactured at Plant B, the W/C was 40% and 60%, and the replacement ratio of recycled aggregate was 50% for the recycled coarse aggregate. An air-entraining and water-reducing admixture (high-performance type) was used as the chemical admixture for these concretes. The target slump at the placing location was 15 ± 2.5 cm for all concretes; therefore, 17 ± 2.5 cm was considered for slump loss. The target air content was 4.5 ± 1.5%.

2.2. S Value in the Standard Period

In subsection 3.4 of JASS 5 (2003) [24], mix proportion strength at a controlled age of 28 days (F) was the larger of values calculated using Equations (1) and (2). Further, the value of L in this study was not specified.

F = F_q + T + 1.73σ

(1)

F = 0.85(F_q + T) + 3σ

(2)

• F: Mix proportion strength of concrete (N/mm²)
• F_q: Quality standard strength of concrete (N/mm²)
• T: Correction value of compressive strength due to the estimated average temperature from mixing to 28 days of structural concrete on 28th days (N/mm²)
• σ: Standard deviation of compressive strength of concrete (N/mm²)

F_q was calculated using Equation (3), the difference between the compressive strengths of structural concrete and standard cured specimen (ΔF), ΔF = 3 N/mm² was employed for this study (Appendix B).

F_q = F_c + ΔF

(3)

F_c: Design standard strength of concrete (N/mm²)

As shown in Table 2, one of the purposes of using L for recycled aggregate concrete-Class M is to reduce the thermal stress when mass concrete is used for a structure. Further, ΔF is employed for components that do not have a temperature record, and in case the temperature record is available, ΔF needs to be examined.

When applied to mass concrete, the temperature record of structural concrete is determined. Therefore, for the concrete using L, the validity of 3 N/mm² as the value of ΔF was confirmed based on experimental investigation.

Table 7. Calculation results of S value in the standard period in the case of L.

Specimen	Slump (cm)	Air Content (%)	Bulk Density (kg/m3)	Chloride Ion Content (kg/m3)	Placing Tempe- rature (°C)	Highest Tempe- rature (°C)	fm (N/mm2)		fnc (N/mm2)		mSn (N/mm2)
	JIS A 1101 [26]	JIS A 1128 [27]	JIS A 1116 [28]	JIS A 5308 [29]			28 Days	91 Days	28 Days	91 Days	m = 28 n = 28	m = 28 n = 91
LG-40	17.5	5.2	2334	0.020	21.4	43.9	43.1	68.1	45.9	53.7	−2.8	−10.6
LG-50	17.5	5.3	2327	-	24.1	35.6	32.6	47.9	33.2	43.1	−0.6	−10.5

shows the fresh condition properties for two types of concrete using L, and the compressive strength of cores obtained from the standard cured and block specimens (dimensions: 1.0 × 1.0 × 1.0 m³) (JIS A 1107 [25]). Further, Figure 1 shows the temperature records at the block specimen’s center.

S value is calculated using the compressive strength of standard curing and core specimens in a certain age using Equation (4).

_mS_n = f_m − f_nc

(4)

• f_m: Compressive strength of standard curing specimen on mth day (N/mm²)
• f_nc: Compressive strength of core specimen on nth day (N/mm²)

The concept of the S value is that when _mS_n < 0, then _mS_n is considered 0. Therefore, according to Table 7, when the control age of the standard curing specimen strength in the case of L was 28 days and the control age of structural concrete was 91 days, ₂₈S₉₁ was negative. Thus, ₂₈S₉₁ = 0 N/mm² (ΔF = 0 N/mm²). Meanwhile, when the control age of structural concrete was 28 days, the maximum value of _mS_n was −0.6 N/mm². In this study, for safety evaluation, ₂₈S₂₈ in the case of L was considered to be 3 N/mm² (ΔF = 3 N/mm²), as indicated in JASS 5 (2003). Because ₂₈S₉₁ in the standard period of JASS 5 (2018) was 3 N/mm², this value can be used in safety evaluation.

The S value in the case of N (FAII is used as a fine aggregate substitute) and BB in the standard period was set based on the relationship between the average temperature up to the age of 28 days and the structural strength correction value ₂₈S₉₁ in Explanatory figure 5.3 of JASS 5 (2018) [1]. Alternatively, for the difference in the measured strength of standard cured specimen at 28 days and the structural concrete at 91 days, the concrete using N, FA cement type B (FB), and BB had the upper limit of 3 N/mm². Therefore, in all cases, the value of 3 N/mm² was adopted for ₂₈S₉₁.

2.3. S Value in Hot Season

The simple adiabatic curing method shown in Figure 2 was performed as the temperature-controlled curing for structure during a hot period. S value was calculated from the difference in compressive strength between the standard and simple adiabatic cured specimens.

Table 8. Qualities of fresh concrete.

Specimen	Slump (cm)	Air Content (%)	Bulk Density (kg/m3)	Chloride Ion Content (kg/m3)
	JIS A 1101	JIS A 1128	JIS A 1116	JIS A 5308
NFARLG150-40	21.5	4.2	2264	-
NFARLG150-60	22.5	5.6	2219	-
BBRLG130RLS30-35	18.0	3.4	2271	-
BBRLG130RLS30-55	20.5	5.6	2193	-
LRLG250-40	17.5	4.2	2290	0.025
LRLG250-60	18.0	4.8	2266	-

shows the properties of fresh conditions for the various types of concrete in the experiment, and

Table 9. Results of simple adiabatic curing method.

Specimen	Placing Temperature (°C)	Highest Temperature (°C)	fm (N/mm2)		fn (N/mm2)			mSn (N/mm2)
			28 Days	91 Days	28 Days	81 Days	91 Days	m = 28 n = 28	m = 28 n = 81	m = 28 n = 91
NFARLG150-40	33.3	73.2	44.8	49.7	36.3	39.3	40.5	8.5	5.5	4.3
NFARLG150-60	33.4	59.5	28.2	34.2	24.5	26.8	27.2	3.7	1.4	1.0
BBRLG130RLS30-35	32.5	76.5	47.5	57.8	39.5	43.6	44.8	8.0	3.9	2.7
BBRLG130RLS30-55	32.0	55.9	29.9	37.0	26.2	29.9	29.4	3.7	0	0.5
LRLG250-40	33.4	54.1	35.3	49.1	32.8	40.9	41.6	2.5	−5.6	−6.3
LRLG250-60	33.2	45.5	22.1	36.3	22.7	29.4	30.1	−0.6	−7.3	−8.0

shows the results of the simple adiabatic curing method. Further, Figure 3 shows the temperature records of specimens by cement type. The target temperature at the time after mixing was 35 °C [1], which was regulated for concrete in a hot period, and the experiment was implemented at room temperature of 25 °C.

2.3.1. Calculation Method of S Value

The S value was calculated from the compressive strength (JIS A 1108 [30]) of standard and simple adiabatic cured specimens in a specified age based on JASS 5 (2018) as follows [1]:

_mS_n = f_m − f_n

(5)

• f_n: Compressive strength of simple adiabatic cured specimens on nth days (N/mm²)

2.3.2. Calculation Results of S Value

(1)

N

For the case FAII was used as a fine aggregate substitute in N specimens, the compressive strength of standard cured specimen NFARLG₁50-40 with recycled coarse aggregate at a 50% replacement ratio and 40% W/C was 44.8 and 49.7 N/mm² at 28 and 91 days, respectively. Meanwhile, for NFARLG₁50-60 with a 60% W/C, it was 28.2 and 34.2 N/mm² at 28 and 91 days, respectively; the difference was about 5–6 N/mm². For the simple adiabatic cured specimens, the placing temperature was 33 °C, the maximum temperature of NFARLG₁50-40 was 73.2 °C, and the compressive strength was 36.3 and 40.5 N/mm² at 28 and 91 days, respectively; the increase in compressive strength was ~4 N/mm². ₂₈S₂₈ = 8.5 N/mm², which exceeded 6 N/mm², but ₂₈S₈₁ = 5.5 N/mm² and ₂₈S₉₁ = 4.3 N/mm². Moreover, for NFARLG₁50-60, the maximum temperature was 59.5 °C, and the compressive strength of the simple adiabatic cured specimen was 24.5 and 27.2 N/mm² on 28th and 91st days, respectively; the increase in strength after 28 days was about 3 N/mm². ₂₈S₂₈ = 3.7 N/mm², and ₂₈S₉₁ = 1.0 N/mm², which are both lower than 6 N/mm².

(2)

BB

The compressive strength of the standard cured specimen BBRLG₁30RS30-35 using BB with a 35% W/C and replacement ratio was 30% of recycled coarse aggregate and 30% of recycled fine aggregate was 47.5 and 57.8 N/mm² on 28th and 91st days, respectively. For BBRLG₁30RS30-55 with a 55% W/C, the compressive strength was 29.9 and 37.0 N/mm² on 28th and 91st days, respectively; the difference in strength was about 7–10 N/mm². For the simple adiabatic cured specimen, the placing temperature was about 32 °C, the maximum temperature was 76.5 °C for BBRLG₁30RLS30-35, and the compressive strength was 39.5 and 44.8 N/mm² on 28th and 91st days, respectively; the increase in compressive strength after 28 days was about 5 N/mm². ₂₈S₂₈ = 8.0 N/mm², which exceeded 6 N/mm², but ₂₈S₈₁ = 3.9 N/mm² and ₂₈S₉₁ = 2.7 N/mm². Meanwhile, the maximum temperature of BBRLG₁30RLS30-55 was 55.9 °C, the compressive strength was 26.2 and 29.4 N/mm² on 28th and 91st days, respectively; the increase in strength from 28 days to 91 days compared with the case of 35% W/C was quite similar, about 3 N/mm². ₂₈S₂₈ = 3.7 N/mm² and ₂₈S₉₁ = 0.5 N/mm², which are both lower than 6 N/mm².

(3)

L

The compressive strength of the standard cured specimen LRLG₂50-40 with a 40% W/C was 35.3 and 49.1 N/mm² on 28th and 91st days, respectively. For LRLG₂50-60 with a 60% W/C, the strength was 22.1 and 36.3 N/mm² at 28 and 91 days, respectively; the difference was about 14.0 N/mm². For the simple adiabatic cured specimen, the placing temperature was about 33 °C, the maximum temperature was 54.1 °C for LRLG₂50-40, and the compressive strength was 32.8 and 41.6 N/mm² on 28th and 91st days, respectively; the increase in compressive strength from 28 to 91 days was about 9 N/mm². ₂₈S₂₈ = 2.5 N/mm² and ₂₈S₉₁ = −6.3 N/mm², which were both lower than 6 N/mm². Meanwhile, for the LRLG₂50-60 specimen, the maximum temperature was 45.5 °C, the compressive strength was 22.7 and 30.1 N/mm² on 28th and 91st days, respectively; the strength increase from 28 to 91 days was about 7 N/mm². ₂₈S₂₈ = −0.6 N/mm² and ₂₈S₉₁ = −8.0 N/mm², which were both lower than 6 N/mm².

Based on these results, the S value for all concrete specimens with m = 28 and n = 91 was within 6 N/mm² during the hot season, as mentioned in JASS 5 (2018) [1]. Therefore, the value of 6 N/mm² can be used for ₂₈S₉₁ in the hot season. In the cold season, the value of 6 N/mm² was also employed for ₂₈S₉₁.

2.4. Mix Proportion Strength

The mix proportion strength should satisfy Equations (6) and (7) based on JASS 5 (2018) [1]. The calculation of F_q is based on Equation (9) (Appendix B). Further, the concrete using L was applied to actual structures [2]; an outline of the applied structure is shown in

Table 10. Outline of the applied structure.

Item		Outline
Certificated by MLIT 1		MCON-2090
Applied structure		Foundation of the main building of thermal power plant (Turbine building)	Machine base foundation in thermal power plant 2
Structure type		RC structure (upper frame: steel structure)	RC structure
Location		Coastal area in Kanagawa Prefecture
Design standard strength: Fc		21 N/mm2
Recycled aggregate concrete-Class M1	Amount	About 8000 m3	About 3000 m3
	Use for	Mass concrete
	Cement	L
	Replacement ratio	Recycled coarse aggregate: 50%

¹ Minister of Land, Infrastructure, Transport and Tourism ² Foundation of HRSG (Heat recovery steam generator for gas turbine), transformer, and air intake chamber.

, and examples of mix proportion strength are shown in

Table 11. Example of mix proportion strength calculation.

Cement Type	Range of θ (°C)	Fc (N/mm2)	28S91 (N/mm2)	Fm = Fc + 28S91 (N/mm2)	σ (N/mm2)				F (N/mm2)
					Plant 1	Plant 2	Plant 3	Set Value	28 Days	81 Days	91 Days
N + FA	14 ≤ θ2	21	3	24	2.0	2.5	2.0	3.0	F ≥ 29.2	F ≥ 29.4	29.4
BB	13 ≤ θ
L 1	14 ≤ θ
N + FA	0 ≤ θ < 8 2		6	27	2.2	3.0	2.5		F ≥ 32.2	F ≥ 32.0	32.2
BB	0 ≤ θ < 14
L1	0 ≤ θ < 14

¹ Mixing date of structure 14 ≤ θ: March 30–May 18, 0 ≤ θ < 14: December 24–March 27 ² FA is used as a fine aggregate substitute.

.

F ≥ F_m + 1.73σ

(6)

F ≥ 0.85F_m + 3σ

(7)

• F: Mix proportion strength of concrete (N/mm²)
• F_m: Mix proportion control strength of concrete (N/mm²)
• σ: Standard deviation of compressive strength of concrete (N/mm²)

F_m = F_q + _mS_n

(8)

• F_q: Quality standard strength of concrete (N/mm²)
• _mS_n: Structural strength correction value to be derived from the difference between the compressive strength of standard cured specimen on m^th day and the compressive strength of structural concrete on n^th day (N/mm²), m = 28, n = 91

F_q = F_c

(9)

• F_c: Design standard strength of concrete (N/mm²)

3. Conclusions

For developing a design method for concrete using low-quality recycled aggregates, the structural strength correction value and calculation of mix proportion strength of concrete using recycled aggregate class L were examined within a certain range of recycled aggregate replacement ratio. As a result, the following conclusions have been drawn.

(1)

From the difference in compressive strength between a structural concrete and standard cured specimen, for the concrete using L with 40% and 50% W/C, ₂₈S₂₈ can be considered as 3 N/mm² (ΔF = 3 N/mm²), which was indicated in JASS 5 (2003) [24]. Further, because ₂₈S₉₁ in the standard period of JASS 5 (2018) is 3 N/mm², this value can be employed for safety evaluation. In addition, the value can be applied for the concrete using N (with FAII used as a fine aggregate substitute) and BB.

(2)

The S value of the recycled aggregate concrete-Class M using N (with FAII used as a fine aggregate substitute), BB, and L with W/C in the range of 35–60%, m = 28, n = 91 during the hot season can be employed as 6 N/mm². Further, in the cold season, ₂₈S₉₁ can also be employed as 6 N/mm².

(3)

The structural strength correction value shown in JASS 5 (2018), which are ₂₈S₉₁ = 3 N/mm² and ₂₈S₉₁ = 6 N/mm² can be applied to concrete using low-quality recycled aggregate based on the condition of temperature and cement types. Further, L can be employed in a structure for actual mass concrete.

From the above conclusions, within the scope of this study, the mix proportion strength of recycled aggregate concrete-Class M can be determined based on the structural strength correction value indicated in JASS 5 (2018) [1].