Optimization of rapid-repair material ratio and performance analysis based on orthogonal test

ABSTRACT

Through orthogonal experimental design, the influence of the water-binder ratio, binder-sand ratio, ultrafine mineral admixture content, and steel fiber content on the fluidity and basic mechanical properties of rapid-repair materials was studied. The optimal mixing ratio parameters were determined, and its swelling and shrinkage performance and microstructure were analyzed. The results showed that the water-binder ratio and steel fiber content were the most significant factors affecting the properties of rapid-repair materials. The steel fiber content in the rapid-repair material led to pull-out failure and significantly affected flexural strength but had a relatively small effect on compressive strength. With the determined optimal mix proportion, the rapid-repair material had good fluidity and high early strength, bonding strength, and volume stability. At age 3 h, the material’s flexural strength and compressive strength were 13.7 MPa and 27.8 MPa, respectively, and the adhesive strength was 3.65 MPa. The limited expansion rate of 56 d-age repair material was 326.5 × 10⁻⁶, which can better meet the requirements of repairing highway pavement and bridge expansion joints to reopen to traffic within 3 h of repair.

Keywords
Rapid-repair materials; Ultrafine mineral admixture; Flexural strength; Bonding strength

1. INTRODUCTION

By the end of 2021, the road maintenance distance in China accounted for more than 99.4% of the total road mileage, and the maintenance level and scale are increasing daily. The maintenance task is urgent and arduous, and the traditional road maintenance technology has been challenging to adapt to the new maintenance situation. The research and application of rapid-repair materials are of great significance to promote cost reduction and improved efficiency of highway maintenance to prolong the service life of highways and to promote the high-quality development of highway maintenance.

Various rapid hardening materials for concrete repair are reported in the literature: epoxy resins, polyester resins, polymer latex, polyvinyl acetate, different cement-based inorganic binders, calcium aluminate cement, alkali-activated cement and Portland cement, sulfoaluminate cement [¹,²,³,⁴,⁵,⁶,⁷,⁸,⁹]. This kind of material has the disadvantages of slow hardening and long strength rising periods, which cannot solve traffic jams caused by roadway restrictions or restrictions during the engineering maintenance and reinforcement period. Also, it has the defect of poor bonding performance of the repair structure interface [¹⁰, ¹¹]. XIAONI [¹²] added silica fume and polypropylene fiber to cement-concrete pavement-repair materials, improving the repair structure’s bonding performance and crack resistance. WHITING and NAGI [¹³] prepared a sulphoaluminate cement-based repair material with a 1-d compressive strength of 28 MPa. TAFRAOUI et al. [¹⁴] used a mixture of metakaolin as a mineral admixture, improving ordinary concrete’s properties. Deng YI [¹⁵] studied the influence of sand ratio and temperature on rapid-repair materials of concrete pavement. When the sand rate increased from 26% to 36%, the flexural and compressive strengths of the repaired materials first increased and then decreased and reached the maximum flexural and compressive strengths of 6 MPa and 38 MPa, respectively, when the sand rate was at 32%. The higher the temperature, the greater the early strength of the concrete, and the later strength becomes smaller instead. It is recommended that the quick repair material be applied when the ambient temperature is high. In recent years, magnesium phosphate cement (MPC) has received increasing attention as the repair material for cementitious structures thanks to its quick setting and hardening, low shrinkage, and good bonding ability [¹⁶,¹⁷,¹⁸]. The research on MPC repair performance mainly focuses on the repair of concrete materials, such as airport runways, bridges, and highways [¹⁹,²⁰,²¹,²²,²³,²⁴,²⁵]. Still, releasing ammonia gas in the reaction is a severe drawback in the fabrication process. The results show that excess potassium dihydrogen phosphate (KHPO) has a negative impact on overall properties. High quantities of KHPO lead to poor w ter resistance, which makes it quite challenging to use for larger-volume applications [²⁶, ²⁷]. HUANG [²⁸] and QING et al. [²⁹] used magnesium phosphate cement as a binder, mixed with self-made new additives, and prepared an ultra-early strength rapid-repair material that allowed reopening to traffic after 2 h of repair. CHEN et al. [³⁰] prepared composite repair materials by combining inorganic and organic materials. Compared to ordinary cement concrete, this repair material’s mechanical properties and durability are improved. Still, its early strength develops slowly, causing disruption of traffic operations for a long time. Because steel fiber has the best crack resistance and toughness, it can prevent concrete cracks from spreading and effectively improve repair materials’ flexural strength and toughness. Many steel wires at home and abroad are used for repairing concrete pavements and airport runways, and a good effect [³¹,³²,³³] has been achieved. However, to increase the setting and hardening rates and reduce the unusable time remaining challenges in rapid-repair materials, further research is needed to improve the early strength of concrete pavement-repair materials while ensuring the later strength.

It is not difficult to find that the comprehensive performance of rapid-repair materials is not only the superposition of the performance of several simple materials but also closely related to the dosage balance of various materials and their internal composition and structure. The synergy between solid waste’s physicochemical and mineral properties for waste recycling and cementitious material preparation is currently receiving attention and is of great significance for environmental protection [³⁴,³⁵,³⁶,³⁷,³⁸,³⁹]. Therefore, starting from the engineering practice, this paper takes sulphoaluminate cement as the primary cementing material and ultrafine mineral admixture as the auxiliary cementing material, adding steel fiber to prevent cracking and toughening. Based on the orthogonal test method [⁴⁰,⁴¹,⁴²], the effects of the water-binder ratio, cement-sand ratio, ultrafine mineral admixture content, and steel fiber content on the operational performance and mechanical properties of rapid-repair materials were studied, and the mixture ratio of rapid-repair materials was optimized. At the same time, the swelling and shrinkage performance and adhesive strength of rapid-repair materials at the best mixture ratio were tested, and the microstructure of the materials at different ages was analyzed by scanning electron microscopy (SEM). An application in practical engineering was demonstrated.

2. MATERIALS AND METHODS

2.1. Test material

Cement: Grade 42.5 fast-hardening sulphoaluminate produced by Guangxi Yunyan Special Cement Building Materials Co., Ltd. was used in this study, with the leading performance indices shown in Table 1.

Table 1.
Main physical properties of cement.

SPECIFIC SURFACE AREA/ (m2 kg−1)	COAGULATION TIME (min)		FLEXURAL STRENGTH (MPa)			COMPRESSIVE STRENGTH (MPa)
	INITIAL SET	FINAL SET	1 d	3 d	28 d	1 d	3 d	28 d
390	36	58	7.0	7.9	8.0	39	45	47

Machine-made sand: Limestone machine-made sand produced by a concrete factory in Guilin, Guangxi, with a density of 2693 kg/m, a fineness modulus of 3.32, a stone powder content of 11.2%, and a methylene blue value of 0.75.

Ultrafine mineral admixture: Steel ore slag, slag, and fly ash were ground and mixed according to the ratio of 3:5:2. The particle size distribution range was 2–10 μm, the average particle size was 3.64 μm, the 28-d total activity was 103%, and the expansion ratio was 100%. The main chemical compositions of raw materials are shown in Table 2.

Table 2.
Main chemical composition of ultrafine mineral admixture.

ULTRAFINE MINERAL ADMIXTURE	CHEMICAL COMPOSITION (%)
	Al2O3	CaO	Fe2O3	K2O	MgO	MnO	Na2O	P2O5	SiO2	TiO2
Slag	19.12	0.31	5.18	5.73	1.77	0.07	0.40	0.04	66.9	0.97
Ore slag	12.22	15.45	4.50	4.97	3.48	0.08	0.17	0.09	40.17	0.46
Fly ash	15.80	2.46	7.87	4.30	1.82	0.03	0.41	0.08	63.79	0.74

Steel fiber: Copper-plated steel fiber with a smooth surface was used. The specific parameters are shown in Table 3.

Table 3.
Main parameters of steel fiber.

DIAMETER (mm)	TENSILE STRENGTH (MPa)	LENGTH (mm)	DRAW RATIO
0.22	3000	13	60

Polycarboxylate superplasticizer was used as a water-reducing agent, with a solid content of 30% and a more than 40% water-reducing rate.

Lithium carbonate was used as a coagulant (Ron, AR, 99.5%, analytically pure reagent); tap water was used.

2.2. Experimental method

2.2.1. Orthogonal test level and parameter setting

Four factors and four levels (L16⁽⁴⁴⁾) were used for the orthogonal test. The effects of the water-binder ratio (A), cement-sand ratio (B), ultrafine mineral admixture content (C), and steel fiber content (D) on the fundamental properties of rapid-repair materials (Table 4) were investigated, and the optimal mix ratio was selected. The content of ultrafine mineral admixture and steel fiber was the mass fraction; the content of water reducer was 0.6% (based on the mass fraction of cementitious materials); and the content of coagulant was 0.04% (based on the mass fraction of cementitious materials).

Table 4.
Orthogonal test factors and levels.

LEVEL	FACTOR
	A: WATER-BINDER RATIO	B: CEMENT-SAND RATIO	C: ULTRAFINE MINERAL ADMIXTURE CONTENT (%)	D: STEEL FIBER CONTENT (%)
1	0.27	0.65	5	2
2	0.28	0.70	10	3
3	0.29	0.75	15	4
4	0.30	0.80	20	5

2.2.2. Specimen preparation

A standard cement mortar mixer was used for mixing. The mixer process was as follows: (1) A mixture of cement, ultrafine mineral admixture, machine-made sand, and coagulant was dried in a mortar mixer at low speed for 2 min. (2) A mixture of water and the water-reducing agent was added, stirred at low speed for 30 s, stopped stirring for 10 s, and stirred at high speed for 30 s. (3) Then steel fibers were added continuously at low speed. After the steel fibers were uniformly distributed, injection molding was performed, and the molding size was 40 × 40 × 160 mm. Plastic film covers were the test piece, the test piece was placed in a laboratory at 20°C for 2 h, and then the mold was removed. (4) The test block was put into a standard curing room with a temperature of 20 ± 2°C and humidity greater than 95% and cured to a specified age for the strength test.

2.2.3. Test methods

• (1)

  Mechanical properties were measured according to JTG 3420-2020 T0506-2005 “Test Method for Cement Mortar Strength” (ISO Method). The flexural and compressive strength at ages 3 h, 1 d, and 28 d were measured.

• (2)

  Expansion degree was measured according to JTG 3420-2020 T0507-2005 “Testing Method for Expansion of Cement Mortar” because the expansion of repair materials was too significant, and the jump table was not used. The test mold for the jump table test was placed on a smooth floor tile, the mixed repair materials were filled into the test mold, and the test mold was lifted vertically. After waiting for 10 s, the diameters of the bottom surface of the mixture in two vertical directions were measured with a ruler, and the average value was used to characterize the flowability of the rapid repair material.

• (3)

  For the repair material to have a good enough repair effect, the compatibility of the repair material with the substrate concrete needs to be tested [⁴³,⁴⁴,⁴⁵]. There are already more test methods for evaluating the repair material, including bonding strength, permeability, creep, etc. From the perspective of practical engineering applications, because the concrete panel plate is subjected to bending and tensile stresses at the bottom during use and is prone to fracture at the joint, the repair material studied in this test needs to have good bonding performance with the substrate concrete [⁴⁶,⁴⁷,⁴⁸,⁴⁹]. To test bonding performance, the new and old bonded specimens were formed in the mortar test mold, and the bonding strength was tested after curing to the specified age. The old specimens were ordinary mortar specimens after curing for 28 d. The bonding strength test diagram of the repaired structure is shown in Fig. 1.

• (4)

  Expansion and contraction performance was tested about “Concrete Expansion Agent” GB23429-2009.

• (5)

  Hydration samples of all test pieces in the orthogonal test group at the specified age were collected, and hydration was halted by absolute ethanol. Through orthogonal analysis, the hydration sample of the specimen with the best matching ratio was selected for gold spraying. The samples’ microstructure at 1 d and 28 d was analyzed by scanning electron microscopy (SEM).

Figure 1.
Schematic of bonding strength test principle.

3. RESULTS AND DISCUSSIONS

According to the orthogonal test factors and level table (Table 4), the flexural strength, compressive strength, and expansion degree of rapid-repair materials at 3 h, 1 d, and 28 d with different mix ratios were measured. The test data are shown in Table 5.

Table 5.
Mixing ratio of rapid-repair materials and test results.

TEST NUMBER	FACTOR				FLEXURAL STRENGTH (MPa)			COMPRESSIVE STRENGTH (MPa)			EXPANSION DEGREE (mm)
	A	B	C	D	3 h	1 d	28 d	3 h	1 d	28 d
1	1	1	1	1	6.7	11.8	13.7	21.5	76.9	104.2	285
2	1	2	2	2	7.7	13.4	16.6	22.3	78.5	113.8	280
3	1	3	3	3	9.9	15.1	19.8	24.8	79.2	111.2	275
4	1	4	4	4	12.7	16.4	25.6	24.1	82.3	109.4	272
5	2	1	2	3	13.7	18.4	26.7	27.8	89.1	121.7	285
6	2	2	1	4	12.7	16.4	25.8	25.4	86.4	115.8	280
7	2	3	4	1	7.7	10.8	17.9	20.5	82.5	112.3	305
8	2	4	3	2	8.1	11.5	18.7	22.4	84.3	109.1	300
9	3	1	3	4	7.8	14.3	23.1	13.0	84.8	113.1	285
10	3	2	4	3	6.4	13.2	19.1	9.9	79.4	107.5	295
11	3	3	1	2	5.2	12.1	17.8	13.2	81.3	112.3	300
12	3	4	2	1	4.6	9.7	15.2	10.7	79.5	115.2	305
13	4	1	4	2	5.2	6.8	15.1	10.4	76.6	104.7	300
14	4	2	3	1	4.7	6.7	13.4	9.9	77.6	106.7	304
15	4	3	2	4	5.7	12.7	16.9	12.2	75.8	113.2	295
16	4	4	1	3	5.9	10.9	14.7	12.6	82.6	109.0	295

3.1. Expansion analysis of rapid-repair materials

Fig. 2a shows the range of expansion degrees. The relationship between the expansion degree and the level of each influencing factor is shown in Fig. 2b. It can be seen from Fig. 2a that the order of influence of various factors on expansion is A (water-binder ratio) > D (steel fiber content) > B (binder-sand ratio) > C (ultrafine mineral admixture content). The water-binder ratio and steel fiber content have the most significant influence on expansion.

Figure 2.
(a) Range of expansion degree; (b) Relationship between expansion degree and various influencing factors.

As the water-binder ratio increases, the expansion degree of the rapid-repair-material mixture gradually increases (Fig. 2b), mainly because the increase in free water in the mixture plays a role in lubrication and reduces the friction between materials. With the increase in steel fiber content, the expansion degree of the rapid-repair-material mixture gradually decreases, mainly because randomly distributed steel fibers form a network structure in the mixture. This constrains the movement of other materials, and steel fibers adhere to a large amount of free water due to surface tension, thus further reducing the expansion degree of the mixture.

3.2. Analysis of mechanical test results of rapid-repair materials

Rapid-repair materials’ flexural and compressive strength with curing ages of 3 h, 1 d, and 28 d were tested. When using a range for analysis, the influence of the range R on the strength index can be judged. The greater the R, the more significant the impact of the factor level on the strength index. Meanwhile, the relationship between different factor levels and the strength index was analyzed.

3.2.1. Analysis of mechanical test results of rapid-repair materials

• (1)

  Three-hour flexural strength analysis shows that the order of factors influencing the 3-h flexural strength of rapid-repair materials is A (water-binder ratio) > D (steel fiber content) > B (binder-sand ratio) > C (ultrafine mineral admixture content) (Fig. 3a). The optimal combination is A₂-B₁-C₂-D₄ (Fig. 3b).

  It can be seen from Fig. 3b that the water-binder ratio has the most obvious influence on the flexural strength of rapid-repair materials. As the water-binder ratio increases, the flexural strength of rapid-repair materials first increases and then decreases. When the water-binder ratio is level 2 (0.28), the flexural strength reaches the maximum of 10.55 MPa. To some extent, the water ratio affects the mixture’s workability and the cementitious material’s hydration degree. When the water-binder ratio is less than 0.28, on the one hand, the low fluidity of the mixture affects the compactness of the matrix; on the other hand, when the water-binder ratio is small, the hydration of cement is insufficient, which affects the number of hydration products of cementitious materials. When the water-binder ratio is greater than 0.28, the free water content in the matrix increases, and capillary water and pore water are gradually converted into bound water, which makes the internal pores of the matrix increase, and the strength decreases macroscopically. As the content of steel fiber increases, the flexural strength of rapid-repair materials gradually increases, but the effect is not obvious before the rapid-repair materials crack. When the steel fiber cracks, it prevents and restricts crack development [⁵⁰]. Currently, the anti-damage mechanism involves pulling out the steel fiber from the matrix.

• (2)

  One-day flexural strength analysis shows that the order of factors influencing the 1-d flexural strength of the rapid-repair material is D (steel fiber content) > A (water-binder ratio) > C (ultrafine mineral admixture content) > B (binder-sand ratio) (Fig. 4a). The optimal combination is A₂-B₁-C₂-D₄ (Fig. 4b).

  As the curing age increases, the influence of steel fiber content on flexural strength exceeds the water-binder ratio. The flexural strength increases with the increase in steel fiber content, and the number of steel fibers drawn out of the matrix increases, thus increasing the flexural strength. The influence mechanism of the water-binder ratio on the flexural strength of rapid-repair materials is the same as in (1). At the same time, the other two factors have less influence on the 1-d flexural strength.

• (3)

  Twenty-eight-day flexural strength analysis shows that the order of factors influencing the 28-d flexural strength of rapid-repair materials is D (steel fiber content) > A (water-binder ratio) > B (binder-sand ratio) > C (ultrafine mineral admixture content) (Fig. 5a). It can be seen from Fig. 5b that the optimal combination is A₂-B₁-C₄-D₄. The influence mechanism of various factors on the 28-d flexural strength of rapid-repair materials is the same as in (1).

Figure 3.
(a) Range of 3-h flexural strength; (b) Relationship between flexural strength and various factors at 3 h.

Figure 4.
(a) Range of 1-d flexural strength; (b) Relationship between flexural strength and various factors at 1 d.

Figure 5
(a) Range of 28-d flexural strength; (b) The relationship between flexural strength and various factors at 28 d.

3.2.2. Analysis of compressive strength

3.3. Determination of the best mixture ratio

The comprehensive balance method was used to analyze the influence of various factors on the performance indices of rapid-repair materials [⁵¹] to obtain the best mix ratio of rapid-repair materials.

The influence of A (water-binder ratio) on each performance index was determined. The effects on the flexural strength of A at 3 h, 1 d, and 28 d ranked first, second, and second, respectively, which are essential factors. The flexural strength of A at 3 h, 1 d, and 28 d was the highest at level 2 (0.28), so A₂ was recommended. The influence on the compressive strength of A at 3 h, 1 d, and 28 d ranked first, first, and second, respectively, and the compressive strength was the highest at level 2 (0.28), so A₂ was preferable. According to the comprehensive analysis above, factor A was A₂. (Fig. 6a–8b).

Figure: 6
(a) Range of 3-h compressive strength; (b) Relationship between compressive strength and various factors at 3 h.

Figure: 7
(a) Range of 1-d compressive strength; (b) Relationship between compressive strength and various factors at 1 d.

Figure: 8
(a) Range of 28-d compressive strength; (b) Relationship between compressive strength and various factors.

The influence of B (cement-sand ratio) on each performance index showed that the effect on the flexural strength of B at 3 h, 1 d, and 28 d ranked third, fourth, and third, respectively, which was not the main influence factor. The flexural strength of B at 3 h, 1 d, and 28 d was the highest at level 1 (0.65), so B₁ was preferable. The influence of B on the compressive strength at 3 h, 1 d, and 28 d ranked fourth, third, and fourth respectively. With the increase in age, the influence of B on the flexural strength of rapid-repair materials gradually decreased. The compressive strength of level 1 (0.65) was the largest at 3 h and 1 d, and B was the secondary factor at 28 d. The maximum compressive strength at level 3 (0.75) was 112.25 MPa compared to level 1 (0.65). According to the comprehensive analysis above, factor B was B₁.(Fig. 6a–8b).

The influence of C (the content of ultrafine mineral admixture) on various performance indices showed that the effect on the flexural strength of C at 3 h, 1 d, and 28 d ranked fourth, third, and fourth, respectively, which were unimportant factors. The flexural strength of C at 3 h and 1 d at level 2 (10%) level was the largest, and that of C at 28 d was the least important factor. The flexural strength of C at 3 h at level 4 (20%) was 19.43 MPa, and the extreme difference was 1.43 MPa, so it was preferable to use C₂. The influence on the compressive strength of C at 3 h, 1 d, and 28 d ranked third, fourth, and first, respectively. The influence factors of C at 3 h and 1 d were insignificant, but the influence factors of C at 28 d were significant. The compressive strength of level 2(10%) was the highest at 3 h; level 1 (5%) compressive strength was the highest at 1 d, and level 2 (10%) was the highest at 28 d, so factor C was C₂.(Fig. 6a–8b).

Effects of D (steel fiber content) on various performance indices were analyzed. The influences on the flexural strength of D at 3 h, 1 d, and 28 d ranked second, first, and first, respectively, which were important factors. The flexural strength increased with the increase in steel fiber content, and the flexural strength of each age reached its maximum at level 4 (5%). The influence of D on the compressive strength at 3 h, 1 d, and 28 d ranked second, second, and third, respectively. D was an essential factor at 3 h and 1 d, and the compressive strength of level 3 (4%) was the largest. At 28 d, D was the secondary factor, and the maximum compressive strength of the fourth level was 112.88 MPa, which was only 0.53 MPa stronger than that of level 3 (4%). The influence of steel fiber content on compressive strength was negligible. It can be seen from Fig. 3 that the content of steel fiber greatly influenced fluidity; therefore, D₃ was preferable from an economic and workability perspective. According to the above analysis, factor D was D₃.(Figure 6a–8b).

Through the abovementioned influence analysis, we concluded that the best blending ratio of rapid-repair materials was A₂B₁C₂D₃. From the perspective of water-cement ratio, sand rate, and ultrafine mineral admixture, a lower water-cement ratio, reasonable sand rate, and appropriate amount of ultrafine mineral admixture can reduce the porosity of the repair material and make the particles accumulate more uniformly, thus improving the strength [⁵²,⁵³,⁵⁴,⁵⁵,⁵⁶,⁵⁷,⁵⁸]. It can also be seen that the steel fiber dosing has a significant effect on the performance of the repair material, with a significant increase in flexural strength and toughness. This is due to the role of fibers that control the widths of the different cracks by blocking the continuous development of the diagonal cracks [⁵⁹,⁶⁰,⁶¹,⁶²,⁶³].

3.3.1. Bonding performance

The bonding property is an essential index of rapid-repair materials. When repairing, it is necessary to ensure that external action does not damage the interface between new and old structures. Therefore, the interface of repaired structures needs good bonding properties. According to JTG D40-2011 [⁶⁴], the bonding strength of repaired structures should reach 2.75 MPa. Figure 8a shows a schematic of the bonding strength test, and the bonding strength of repaired structures with the best mix ratio at 3 h, 6 h, 1 d, 3 d, 7 d, and 28 d was tested, as shown in Fig. 9b.

Figure 9.
(a) Schematic of bonding strength test; (b) Optimal mixing ratio bonding strength.

It can be seen that the early strength of the repair material developed rapidly, and after 3 hours, the strength already exceeded 3 MPa, meeting the requirements of JTG D40-2011. During the aging period from 1 d to 3 d, the strength increased and eventually exceeded 5 MPa. For cementitious repair materials, hydration, and microstructure play an important role in the interface’s mechanical properties, and the concrete matrix’s water content affects the bond strength of the interface. This phenomenon is attributed to the changes in cement hydration and microstructure development in the repair system due to the water exchange between the repair material and the concrete matrix [⁶⁵,⁶⁶,⁶⁷,⁶⁸,⁶⁹].

When the concrete substrate is very dry and porous, the substrate will absorb a large amount of water from the repair material. The remaining water may not be sufficient for cement hydration, which can damage the mechanical properties of the repair material and the interface and may result in low bond strength. Therefore, during the construction of damaged pavement repair, the concrete substrate should be kept in a moisture-saturated state, the moisture in the repair material will not be absorbed, and the w/c ratio of the repair material should be kept constant [⁷⁰,⁷¹,⁷²,⁷³,⁷⁴,⁷⁵,⁷⁶].

3.3.2. Expansion and contraction performance

To prevent cracking and improve durability, materials with good toughness must be chosen, and the causes of cracking must be determined. In engineering and technical circles, it is generally believed that 80% of the cracking of structures is caused by volume deformation, and the expansion and contraction of rapid-repair materials are one of the leading causes of deformation. The typical value for autogenous shrinkage of UHPC is larger than 800 με, which has a high cracking potential [⁷⁷,⁷⁸,⁷⁹,⁸⁰,⁸¹], so improving the volume stability of rapid-repair materials is particularly important. Cement-based materials have significant chemical shrinkage and self-shrinkage in the early stage. The autogenous shrinkage strains of the concrete with a high w/c ratio developed rapidly, even at earlier ages [⁸²,⁸³,⁸⁴]. The limited expansion and contraction rate can reflect the microstructure of steel fiber and cement-based materials in a macroscopic way. To a certain extent, it can concurrently characterize the shrinkage compensation performance of ultrafine mineral admixture for a rapid-repair material system. From the root, there are four specific ways to control the shrinkage of the repair material: (1) control the hydration rate of the cementitious material; (2) increase the internal humidity of the repair material; (3) through the auxiliary restraint volume change (4) through the internal force compensation; it can be seen that the mineral admixture is added in this study to replace part of the cement, thus reducing the hydration rate [⁸⁵,⁸⁶,⁸⁷]. Therefore, based on the best mix ratio, this experiment studies rapid-repair materials’ expansion and contraction properties with ultrafine mineral admixtures of 0% and A₂B₁C₂D₃. Each data point was the average test result of three samples, and the shape and test of the samples are shown in Fig. 10a. After molding for 4 h, demolding and curing were performed to the specified age. Figure 9b shows the expansion and contraction performance test results.

Figure 10.
(a) Specimen forming and testing; (b) Rapid-repair material swelling and shrinkage performance.

According to the test results, the rapid-repair material shrank continuously as it aged. In general, ultrafine mineral admixtures compensate for the shrinkage of the rapid-repair material matrix. After 56 d, the shrinkage gradually stabilized. The shrinkage rates without adding mineral admixture and with 10% mineral admixture specimens were 428.3 × 10⁻⁶ and 326.5 × 10⁻⁶, respectively. The shrinkage rate was small, which ensured the dimensional stability of the matrix and the excellent working performance of steel fiber and cementitious materials.

The causes of compensating shrinkage were analyzed. We found that (1) the ultrafine mineral admixture contained fly ash, and the “ball” effect of fly ash made the matrix denser and the filling effect better [⁸⁸]. (2) The substitution of ultrafine mineral admixture for cement reduced the matrix’s hydration heat and the matrix’s temperature-shrinkage rate. (3) Ultrafine mineral admixtures exerted their pozzolanic properties, and the secondary crystallization made the microstructure of the matrix more compact. (4) At a higher water-cement ratio, the internal porosity of the repair material increases, and the intra-pore connectivity is more potent, thus accelerating the migration of internal water to the surface, and evaporation accelerates the drying shrinkage of the material. The material prepared in this paper uses a lower water-cement ratio to control the repair material’s drying shrinkage effectively [⁸⁹,⁹⁰,⁹¹,⁹²,⁹³,⁹⁴,⁹⁵].

3.3.3. Expansion and contraction performance

As shown in Fig. 11a, the hydration products of the hardened paste with the best mix ratio at 1 d age are mainly rod-shaped ettringite and gelled C-S-H. The hydration reaction of fast-hardening sulphoaluminate mainly produces these hydration products, and the whole structure is compact under the filling effect of ultrafine mineral admixture spherical beads. As can be seen from Fig. 11b, with the extension of hydration age, at 28 d age, the rod-shaped ettringite crystals are wrapped by scaly substances, and the spherical microbeads of ultrafine mineral admixture are entirely surrounded by scaly hydration products and fused into a whole. This indicates that the active ingredients in ultrafine mineral admixture undergo secondary pozzolanic reaction in an alkaline environment, the structure is denser, and the pores are either eliminated or reduced in size. Many thick rod-shaped ettringite structures crisscross and interweave with other hydration products to make the paste structure more stable and firm. The paste’s integrity, compactness, and mechanical properties are further improved in the later stage [⁹⁶].

Figure 11.
(a) Scanning electron microscopy images of the best mixture ratio at 1 d; (b) SEM images of the best mixture ratio at 28 d age.

3.3.4. Engineering demonstration application

• (1)

  The best mix ratio of rapid-repair materials was demonstrated to repair cement-concrete pavement pits with the K1705 + 000∼K1706 of Guilin-Yongfu, G322 line (Fig. 12). First, the repair site was pitted, and the rapid-repair material mixture was prepared according to the laboratory preparation process. Finally, the material was applied to repair, plaster, and maintain.

• (2)

  For the performance test, after curing for 1 h, the curing film was removed from the repair area. First, the repaired area was rolled over by SUVs and other minibusses. The test results showed that the repaired structure was in good condition. The repair material had completely hardened and formed strength, which can meet the opening requirements for minibusses. After curing for 2 h, the repaired structure remained intact after rolling with multiple trailers under dynamic load conditions. Furthermore, the surface of the repaired part was not crushed, and the joint was not cracked. Therefore, the repaired effect is excellent.

Figure 12.
Engineering demonstration application.

The main performance characteristics of rapid-repair materials in the application process are rapid hardening but slow setting. Furthermore, the rapid-repair material has good workability in construction and is easy for quick leveling and plaster repair. Additionally, the rapid-repair material has high early strength and good bonding performance with old concrete; therefore, the repaired road can be reopened to traffic within 3 h of repair.

4. CONCLUSIONS

Here, the L₁₆(4⁴) orthogonal test scheme was designed by four factors: water-binder ratio, cement-sand ratio, ultrafine mineral admixture content, and steel fiber content. The rapid-repair material’s expansion degree and basic mechanical properties were optimized and analyzed, and the best mix ratio was A₂B₃C₂D₂.

The water-binder ratio and steel fiber content were the most significant factors affecting the expansion of rapid-repair materials. The expansion of rapid-repair materials gradually increased with the increase in the water-binder ratio. With the increase in steel fiber content, the expansion of rapid-repair content gradually decreased.

When the water-binder ratio was 0.28, the performance of the rapid-repair material was excellent. Steel fiber in rapid-repair materials led to pull-out failure. Steel fiber content significantly influenced flexural strength but had a relatively small influence on compressive strength.

Ultrafine mineral admixtures had the most significant influence on the 28-d compressive strength. Through SEM, we found that the secondary volcanic reaction of ultrafine mineral admixtures made the microstructure of rapid-repair materials denser.

The rapid-repair material had high early strength and stable volume, with the flexural strength and compressive strength of 13.7 MPa and 27.8 MPa at 3 h, respectively, the bonding strength of the repaired structure of 3.65 MPa at 3 h, and the limited expansion rate of 56-d repair material of 326.5 × 10⁻⁶. The demonstration application proved that the prepared rapid-repair material could be used to repair concrete pavement and to be ready for traffic within 3 h of repair.

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Publication Dates

• Publication in this collection
  12 May 2023
• Date of issue
  2023

History

• Received
  08 Mar 2023
• Accepted
  29 Mar 2023