Pore structure and chloride permeability of concrete containing

Pore structure and chloride permeability of concrete containing nano

-particles for pavement

Mao-hua Zhang ^a^,^b^,^, and Hui Li ^b

^a School of Civil Engineering, Northeast Forestry University, 150040 Harbin, China

^b School of Civil Engineering, Harbin Institute of Technology, 150090 Harbin, China

Received 21 March 2010;

revised 26 July 2010;

accepted 28 July 2010.

Available online 31 August 2010.

Abstract

Pore structure and chloride permeability of concrete containing nano-particles (TiO₂ and SiO₂) for pavement are experimentally studied and compared with that of plain concrete, concrete containing polypropylene (PP) fibers and concrete containing both nano-TiO₂ and PP fibers. The test results indicate that the addition of nano-particles refines the pore structure of concrete and enhances the resistance to chloride penetration of concrete. The refined extent of pore structure and the enhanced extent of the resistance to chloride penetration of concrete are increased with the decreasing content of nano-particles. The pore structure and the resistance to chloride penetration of concrete containing nano-TiO₂ are superior to that of concrete containing the same amount of nano-SiO₂. However, for the concrete containing PP fibers, the pore structure is coarsened and the resistance to chloride penetration is reduced. The larger the content of PP fibers, the coarser the pore structure of concrete, and the lower the resistance to chloride penetration. For the concrete containing both nano-TiO₂ and PP fibers, the pore structure is coarser and the resistance to chloride penetration is lower than that of concrete containing the same amount of PP fibers only. A hyperbolic relationship between chloride permeability and compressive strength of concrete is exhibited. There is an obvious linear relationship between chloride permeability and pore structure of concrete.

Keywords: Nano-particles; Pore structure; Chloride permeability; Polypropylene (PP) fibers; Pavement concrete

1. Introduction

Concrete durability has attracted a lot of attention from many researchers, because it has critical influence on the service life of concrete structure. Pavement is subjected to a harsh environment because of exposure in air and endures dynamic fatigue loads due to passing vehicles, and thus the durability of pavement concrete has received more attention. The durability properties of pavement concrete include many aspects such as permeability, impact resistance, abrasion resistance and frost resistance. In this study, the durability of pavement concrete is measured through permeability.

Permeability, which can be defined as the ease with which external media such as liquids, gases, various aggressive ions and other pollutants penetrate concrete [1] and [2], is considered to be one of the most important properties affecting concrete durability [3]. A lower permeability reduces the ingress and movement of fluid media in concrete and is therefore beneficial. Concrete with higher permeability allows faster penetration of fluid media, resulting in rapid deterioration of concrete.

Concrete is a heterogeneous and porous material, in which there are many pores with different sizes and shapes. It is well known that the pore structure of concrete strongly influences its physical properties. Many important properties, such as strength and permeability, are directly or indirectly related to the pore structure of concrete [4] and [5]. It is generally agreed that the pore structure of concrete is one of its most important characteristics and strongly affects both its durability and mechanical properties [6]. Therefore, study of the pore structure of concrete is essential to understand the nature of this complex material.

The permeability of concrete, which is strongly affected by the pore structure of concrete, is now accepted mainly to be a function of pore size distribution [4]. The permeability of concrete is intimately related to the pore connectivity, but the compressive strength of concrete is governed by the total porosity [5] and [7].

The permeability of concrete mostly lies on the pore structure and its development and change. There are three types of factors affecting the permeability of concrete [8]. The first one is the factors that influence the original pore structure of concrete such as water-to-binder ratio, mineral admixtures (such as silica fume, fly ash and blast furnace slag) and additive agents (such as water-reducing agent, air-entraining agent and expansive agent). The second one is the factors that affect the development of pore structure of concrete including the curing condition, age and the activity of binder. The third one is the penetration condition such as hydraulic gradient, penetration time and chemistry component of penetration media.

There are two types of arguments regarding the relationship between compressive strength and permeability of concrete. Some researchers consider that compressive strength is a crucial factor affecting the permeability of concrete, and the permeability of concrete decreases with increasing compressive strength. For instance, Mohr et al. [1] described the relationship between compressive strength and permeability of concrete by a power function, and a similar trend was found by Armaghani et al. [9]. While other researchers consider that there is no significant relationship between compressive strength and permeability of concrete, and the compressive strength of concrete cannot reflect its permeability, especially for high-performance concrete [7].

Many studies have shown that the addition of PP fibers is harmful to the impermeability of concrete. Toutanji et al. [10] and [11] found that the permeability of concrete containing PP fibers is enhanced, and this was attributed to that PP fibers increased the void content in concrete due to the lack of cohesiveness of cement matrix and poor dispersion of PP fibers. However, there are also many researchers who argued that the impermeability of concrete was significantly improved by the addition of PP fibers [12] and [13].

By the addition of mineral admixtures such as silica fume, the impermeability of concrete is increased to some extent. Toutanji [11] and Oh et al. [14] reported that the permeability of concrete containing silica fume was remarkably reduced because of the denser pore structure. Toutanji et al. [10] also indicated the addition of silica fume improved the dispersion of PP fibers in cement matrix, causing a marked reduction in the permeability of PP fiber-reinforced concrete.

Nano-materials have been considered as the most promising materials in 21 century by scientists. In recent years, much attention has been paid to the applications of nano-materials in civil engineering, because nano-particles possess many special properties such as huge specific surface area and high activity due to their small size.

Li et al. [15] and [16] investigated the improvement in compressive and flexural strengths, abrasion resistance, and flexural fatigue performance of concrete containing nano-particles. Li et al. [17] and [18] also studied the microstructure and self-sensing properties of mortar containing nano-particles. The results show good prospects of concrete (or mortar) containing nano-particles.

In cold area, deicing salt is often used to sprinkle on pavement after snowing and icing. Pavement concrete may fail due to the penetration of chloride, so the permeability measured by chloride ion is more consistent with the practical condition of pavement concrete. In this work, the pore structure and chloride permeability of concrete containing nano-particles (TiO₂ and SiO₂) for pavement is experimentally studied and compared with that of plain concrete, concrete containing PP fibers and concrete containing both nano-TiO₂ and PP fibers.

2. Experimental program

2.1. Materials and mixture proportions

The cement used is Portland cement (P.O42.5). The fine aggregate is natural river sand with a fineness modulus of 2.4. The coarse aggregate used is crushed diabase with a diameter of 5–30 mm. The properties of nano-particles (TiO₂ and SiO₂) are given in Table 1. The properties of modified PP fibers used are shown in Table 2. The water-reducing agent (UNF-5, one kind of β-naphthalene sulfonic acid and formaldehyde condensates) is employed to aid the dispersion of nano-particles in cement paste and achieve good workability of concrete. The defoamer (tributyl phosphate) is used to decrease the amount of air bubbles.

Table 1. Properties of

nano

-particles.

Item	Diameter (nm)	Specific surface area (m²/g)	Density (kg/m³)	Purity (%)	Phase
SiO₂	10 ± 5	640 ± 50	<120	99.9	–
TiO₂	15	240 ± 50	40–60	99.7	Anatase

Table 2. Properties of modified PP

fibers.

Elongation (%)	Titer (D)	Diameter (μm)	Length (mm)
40 ± 3	11 ± 0.5	84–92	15 ± 1

The water-to-binder (the sum of cement and nano-particles) ratio used for all mixtures is 0.42, and the sand ratio is 34%. The mixture proportions of concretes per cubic meter are given in Table 3. Here, PC denotes plain concrete. PPC6 and PPC9 denote the concrete containing PP fibers in the content of 0.6 and 0.9 kg/m³, respectively. NSC1 and NSC3 denote the concrete containing nano-SiO₂ in the amount of 1% and 3% by weight of binder, respectively. NTC1, NTC3 and NTC5 denote the concrete containing nano-TiO₂ in the amount of 1%, 3% and 5% by weight of binder, respectively. NTPC denotes the concrete containing both nano-TiO₂ in the amount of 1% by weight of binder and PP fibers in the content of 0.9 kg/m³.

Table 3. Mix proportions of concretes (unit: kg/m³).

Mixture type	Water	Cement	Sand	Coarse aggregate	PP fiber	Nano-SiO₂	Nano-TiO₂	UNF-5	Defoamer	Slump (mm)
PC	151	360	650	1260	–	–	–	5.4	–	50–60
PPC6	151	360	650	1260	0.6	–	–	5.4	–	30–40
PPC9	151	360	650	1260	0.9	–	–	5.4	–	20–30
NSC1	151	356.4	650	1260	–	3.6	–	5.4	0.216	20–30
NSC3	151	349.2	650	1260	–	10.8	–	7.2	0.288	10–20
NTC1	151	356.4	650	1260	–	–	3.6	5.4	0.216	20–30
NTC3	151	349.2	650	1260	–	–	10.8	7.2	0.288	20–30
NTC5	151	342	650	1260	–	–	18	7.2	0.288	10–20
NTPC	151	356.4	650	1260	0.9	–	3.6	7.2	0.288	–

2.2. Specimen preparation

To prepare the concrete containing nano-particles, water-reducing agent is firstly mixed into water in a mortar mixer, and then nano-particles are added and stirred at a high speed for 5 min. Defoamer is added as stirring. Cement, sand and coarse aggregate are mixed at a low speed for 2 min in a concrete centrifugal blender, and then the mixture of water, water-reducing agent, nano-particles and defoamer is slowly poured in and stirred at a low speed for another 2 min to achieve good workability.

To prepare plain concrete and the concrete containing PP fibers, water-reducing agent is firstly dissolved in water. After cement, sand, coarse aggregate and PP fibers (if used) are mixed uniformly in a concrete centrifugal blender, the mixture of water and water-reducing agent is poured in and stirred for several minutes.

Finally, the fresh concrete is poured into oiled molds to form cubes of size 100 × 100 × 100 mm to be used for compressive strength and permeability testing, prisms of size 100 × 100 × 400 mm for flexural strength and pore structure testing. After pouring, an external vibrator is used to facilitate compaction and reduce the amount of air bubbles. The specimens are de-molded at 24 h and then cured in a room at a temperature of 20 ± 3 °C and a relative humidity of 95% until the prescribed period.

2.3. Test methods

Both compressive and flexural tests are performed according to JTG E30-2005 (Test Methods of Cement and Concrete for Highway Engineering, China).

2.3.1. Pore structure measurement

There are many methods usually used to measure the pore structure, such as optics method, mercury intrusion porosimetry (MIP), helium flow and gas adsorption [19]. MIP technique is extensively used to characterize the pore structure in porous material due to its simplicity, quickness and wide measuring range of pore diameter [5] and [19]. MIP also provides the information about pore connectivity [19]. In this study, the pore structure of concrete is determined by using MIP.

To prepare the samples for MIP measurement, the concrete specimens after the specified curing ages are first broken into smaller pieces, and then the cement paste fragments selected from the center of prisms are used to measure pore structure. The samples are immersed in acetone to stop hydration as fast as possible. Before mercury intrusion test, the samples are dried in an oven at about 105 °C to remove moisture in the pores until a constant weight is achieved.

MIP is based on the assumption that the nonwetting liquid mercury (the contact angle between mercury and solid is greater than 90°) will only intrude in the pores of porous material under pressure [5] and [19]. Each pore size is quantitatively determined from the relationship between volume of intruded mercury and applied pressure [5]. The relationship between pore diameter and applied pressure is generally described by Washburn equation as follows [5] and [19].

(1)D=-4γcosθ/Pwhere D is the pore diameter (nm), γ is the surface tension of mercury (dyne/cm), θ is the contact angle between mercury and solid (°), and P is the applied pressure (MPa).

The test apparatus used for pore structure measurement is AutoPore IV 9500 type. The density of mercury is 13.5335 g/mL. The surface tension of mercury is taken as 485 dynes/cm, and the contact angle selected is 130°. The maximum measuring pressure applied is 220 MPa (33000 psia), which means that the smallest pore diameter that can be measured reaches about 5 nm (on the assumption that all pores have cylindrical shape).

2.3.2. Permeability measurement

For permeability testing, conventional permeability test methods (e.g. water permeability test or gas permeability test) cannot be applied to high-strength and high-performance concrete because they are so dense and impermeable [14]. Permeability is one of the intrinsic properties of concrete and is only associated with the structural and chemical properties of concrete, not the media studied [20]. Therefore, taking into account the influence of deicing salt on pavement concrete, the rapid chloride permeability test method–NEL method [20] is adopted in this study, because the resistance to chloride penetration is one of the simplest measures to determine the durability of concrete [14].

The specimens for chloride permeability testing are obtained by sawing the cubic specimens of size 100 × 100 × 100 mm into two slices of about 50 mm thickness. The slices are washed thoroughly and then put in a vacuum box shown in Fig. 1a to expel all air from the internal voids. After the vacuum suction is maintained for 6 h at the vacuum level of −0.06 to 0.09 atm, saturated NaCl solution with a concentration of 4 mol/L is injected into the vacuum box and the slices are submerged in brine for 18 h so that all the voids are filled with brine. Finally, the slices are taken out and sandwiched between two copper electrodes shown in Fig. 1b under an 8 V low voltage. Partial conductivity of concrete is measured and the Nernst–Einstein equation is used to calculate the chloride diffusion coefficient [20].

(2)where D_i is the diffusivity of particle i (cm²/s); R is the gas constant (8.314 J/(mol K)); T is the absolute temperature (K); σ_i is the partial conductivity of particle i (S/cm); Z_i is the charge of particle i; F is the Faraday’s constant (96,500 Coul/mol); C_i is the concentration of particle i (mol/cm³).

Full-size image (20K)

Fig. 1.

Instrument used in the chloride permeability test.

If the partial conductivity σ_i and the concentration C_i have been determined, the diffusivity of particle i, D_i can be calculated from Eq. (2). The partial conductivity σ_i is

(3)σ_i=t_iσwhere σ is the conductivity of concrete; t_i is the transference number of particle i, which is defined as

(4)where Q_i and I_i is the electric quantity and current contribution of particle i to the total electric quantity Q and current I, respectively.

3. Test results and discussion

3.1. Compressive and flexural strengths

Table 4 shows the compressive and flexural strengths of concretes at 28 days. It can be seen that both the compressive and flexural strengths of concretes can be increased when nano-particles in a small amount are added; however, when nano-particles in a large amount are added, e.g. NSC3 and NTC5, the flexural strengths of concretes are lower than that of plain concrete although the compressive strengths are slightly increased.

Table 4. Compressive and flexural strengths of concretes.

Mixture type	Flexural strength			Compressive strength
	Value (MPa)	Variation coefficient (%)	Enhanced extent (%)	Value (MPa)	Variation coefficient (%)	Enhanced extent (%)
PC	5.46	6.29	0	59.08	3.16	0
PPC6	5.99	3.77	9.81	61.02	6.71	3.28
PPC9	6.60	5.51	20.87	63.29	3.44	7.12
NSC1	5.69	3.97	4.21	66.36	4.15	12.31
NSC3	5.36	6.78	−1.87	61.16	5.96	3.51
NTC1	6.02	4.20	10.28	69.73	5.47	18.03
NTC3	5.62	4.28	3.04	66.62	6.12	12.76
NTC5	5.28	6.00	−3.27	60.00	4.93	1.55
NTPC	4.85	7.79	−11.21	55.84	7.38	−5.48

The effectiveness of nano-TiO₂ in enhancing compressive and flexural strengths of concretes increases in the order: NTC5 < NTC3 < NTC1 (with the decrease on nano-TiO₂ content), and the similar results can be found for the concretes containing nano-SiO₂. That is to say, the enhanced extent of concrete strengths reduces with the increasing content of nano-particles. Furthermore, the compressive and flexural strengths of concretes containing nano-SiO₂ are smaller than that of concretes containing the same amount of nano-TiO₂.

The larger the content of PP fibers, the higher compressive and flexural strengths of concrete. When the content of PP fibers increases from 0.6 to 0.9 kg/m³, the compressive strength of concrete containing PP fibers enhances slightly, but the flexural strength improves significantly.

If concrete is homogeneous and has no flaws, its compressive and flexural strengths should be increased synchronously. But in practice, the enhanced extent of compressive strength of concrete is greatly larger than that of flexural strength. This can be attributed to the presence of microcracks with different scales in concrete, and the effect of microcracks on flexural strength of concrete is greater than on compressive strength [21]. When the content of nano-particles is larger, e.g. NSC3 and NTC5, the slump of concrete becomes lower as shown in Table 3, and the number of microcracks in concrete increases, which results in the decrease of flexural strength of concrete. In addition, because the uniform dispersion of nano-particles in cement paste is difficult when the content of nano-particles is large, the weak zone (the conglomeration of nano-particles) in concrete increases [17], which also leads to the decrease of flexural strength of concrete. With the addition of PP fibers, the propagation of microcracks is inhibited and the scale of microcracks is reduced due to the crack-arresting effect, crack-thinning effect and crack-bridging effect of PP fibers [21] and [22], so the enhanced extent of flexural strength of concrete containing PP fibers is higher than that of compressive strength. The larger the content of PP fibers (when the content of PP fibers increases from 0.6 to 0.9 kg/m³), the stronger these effects (crack-arresting effect, crack-thinning effect and crack-bridging effect).

The compressive and flexural strengths of concretes containing nano-SiO₂ are lower than that of concretes containing the same amount of nano-TiO₂, which is primarily attributed to the fact that the particle diameter (shown in Table 1) of nano-SiO₂ is smaller than that of nano-TiO₂, and the specific surface area (shown in Table 1) of nano-SiO₂ is much larger than that of nano-TiO₂, so that it is more difficult to uniformly disperse than nano-TiO₂ in cement matrix.

For the concrete containing both nano-TiO₂ (1%) and PP fibers (0.9 kg/m³), its compressive and flexural strengths are both decreased, which indicates that the hybrid addition of nano-TiO₂ and PP fibers has a negative effect on the compressive and flexural strength of concrete. When nano-TiO₂ (1%) and PP fibers (0.9 kg/m³) are respectively added into concrete, the slump of fresh concretes is lower than that of plain concrete; when the concrete contains both nano-TiO₂ and PP fibers, its slump becomes more lower, which makes the density of hardened concrete decreased and the void content in concrete increases, so the compressive and flexural strengths are reduced.

3.2. Pore structure

The test results of MIP in this study include the pore structure parameters such as total specific pore volume, most probable pore diameter, pore size distribution, porosity, average diameter, and median diameter (volume).

In terms of the different effect of pore size on concrete performance, the pore in concrete is classified as harmless pore (<20 nm), less-harmful pore (20–50 nm), harmful pore (50–200 nm) and more-harmful pore (>200 nm) [8]. In order to analyze and compare conveniently, the pore system of concrete is divided into four ranges according to this sort method in this study.

3.2.1. Total specific pore volume and most probable pore diameter of concrete

The integral and differential curves of pore size distribution of concrete can be drawn from the data of MIP, and the two curves can show the pore network of concrete.

Fig. 2 shows the integral curves of pore size distribution of concretes. The peak value of integral curve denotes the total specific pore volume of concrete, which is the total volume of mercury intruded into the sample with a mass of 1 g. The larger the total specific pore volume, the coarser the pore structure.

Full-size image (18K)

Fig. 2.

Integral curves of pore size distribution of concretes.

Fig. 3 presents the differential curves of pore size distribution of concretes. The diameter corresponding to the peak value of differential curve is regarded as the most probable pore diameter, which is the diameter whose size occupies the largest proportion of pores compared with other pore diameters. The smaller the most probable pore diameter, the finer the pore structure.

Full-size image (20K)

Fig. 3.

Differential curves of pore size distribution of concretes.

The total specific pore volumes and most probable pore diameters of various concretes are analyzed and compared, and the results are given in Table 5.

Table 5. Total specific pore volumes and most probable pore diameters of concretes.

Mixture type	Total specific pore volume		Most probable pore diameter
	Value (mL/g)	Reduced extent (%)	Value (nm)	Reduced extent (%)
PC	0.0534	0	47	0
PPC6	0.0557	−4.31	51	−8.51
PPC9	0.0578	−8.24	54	−14.89
NSC1	0.0501	6.18	40	14.89
NSC3	0.0515	3.56	44	6.38
NTC1	0.0470	11.99	32	31.91
NTC3	0.0496	7.12	39	17.02
NTC5	0.0510	4.49	43	8.51
NTPC	0.0592	−10.86	57	−21.28

It can be seen from [Fig. 2] and [Fig. 3], and Table 5 that with the addition of nano-particles, the total specific pore volumes of concretes decrease, and the most probable pore diameters of concretes shift to smaller pores and fall in the range of less-harmful pore, which indicates that the addition of nano-particles refines the pore structure of concretes.

The effectiveness of nano-TiO₂ in reducing the total specific pore volumes and most probable pore diameters of concretes increases in the order: NTC5 < NTC3 < NTC1, and the similar results can be observed for the concretes containing nano-SiO₂. The pore structure of NTC1 is refined most significantly, and the total specific pore volume and most probable pore diameter decrease by 12% and 31.91%, respectively.

With the increasing content of nano-particles, the reduced extent of total specific pore volume and most probable pore diameter decreases, and the refinement on pore structure of concretes is weakening. The pore structure of concretes containing nano-TiO₂ is finer than that of concretes containing the same amount of nano-SiO₂. The total specific pore volume and most probable pore diameter of NSC1 are close to those of NTC3, and the total specific pore volume and most probable pore diameter of NSC3 are close to those of NTC5.

By the addition of PP fibers, the total specific pore volumes of concretes increase, and the most probable pore diameters of concretes shift to larger pores and fall in the range of harmful pore, which shows that the addition of PP fibers coarsens the pore structure of concretes. Moreover, with the increasing content of PP fibers, the total specific pore volume and most probable pore diameter of concrete increase and the pore structure of concrete becomes coarser.

For the concrete containing both nano-TiO₂ and PP fibers, the total specific pore volume also increases, and the most probable pore diameter also shifts to larger pore and falls in the range of harmful pore. The enhanced extent of total specific pore volume and most probable pore diameter of NTPC is obviously larger than that of concrete containing the same amount of PP fibers only, which indicates that the hybrid addition of nano-TiO₂ and PP fibers makes the pore structure of concrete coarser.

Table 6 gives the porosities, average diameters and median diameters (volume) of various concretes. The regularity of porosity is similar to that of total specific pore volume. The regularity of average diameter and median diameter (volume) is similar to that of most probable pore diameter. Therefore, the details are not given herein.

Table 6. Porosities, average diameters and median diameters (volume) of concretes.

Mixture type	Porosity		Average diameter		Median diameter (volume)
	Value (%)	Reduced extent (%)	Value (nm)	Reduced extent (%)	Value (nm)	Reduced extent (%)
PC	11.10	0	41.7	0	57.1	0
PPC6	11.49	−3.55	43.1	−3.36	59.4	−4.03
PPC9	11.90	−7.24	44.3	−6.24	62.8	−9.98
NSC1	10.33	6.93	37.8	9.35	49.8	12.78
NSC3	10.80	2.66	40.1	3.84	54.6	4.38
NTC1	9.22	16.89	35.6	14.63	45.8	19.79
NTC3	10.23	7.85	37.4	10.31	49.4	13.49
NTC5	10.71	3.53	39.9	4.32	54.2	5.08
NTPC	12.52	−12.82	51.2	−22.78	66.3	−16.11

3.2.2. Pore size distribution of concrete

The pore size distribution of concretes is shown in Table 7. Fig. 4 shows the percent of specific pore volume of various grade pore size accounting for total specific pore volume. It can be seen that by the addition of nano-particles, the amounts of harmless and less-harmful pores in concretes increase, and the amounts of harmful and more-harmful pores decrease, which shows that the density of concretes is increased and the pore structure is refined.

Table 7. Pore size distribution of concretes.

Mixture type	Pore size distribution (mL/g)				Total specific pore volume (mL/g)
	Harmless pores (<20 nm)	Less-harmful pores (20–50 nm)	Harmful pores (50–200 nm)	More-harmful pores (>200 nm)
PC	0.0072	0.0163	0.0188	0.0111	0.0534
PPC6	0.0071	0.0162	0.0200	0.0124	0.0557
PPC9	0.0070	0.0160	0.0211	0.0137	0.0578
NSC1	0.0075	0.0179	0.0157	0.0092	0.0503
NSC3	0.0075	0.0166	0.0171	0.0104	0.0516
NTC1	0.0073	0.0183	0.0136	0.0079	0.0471
NTC3	0.0075	0.0178	0.0156	0.0089	0.0498
NTC5	0.0074	0.0168	0.0168	0.0100	0.0510
NTPC	0.0071	0.0155	0.0220	0.0147	0.0593

Full-size image (29K)

Fig. 4.

Percent of specific pore volume of various grade pore size accounting for total specific pore volume.

The effectiveness of nano-TiO₂ in refining the pore structure of concretes increases in the order: NTC5 < NTC3 < NTC1, and the similar results can be found for the concretes containing nano-SiO₂. The harmless and less-harmful pores in NTC1 increase by the largest extent, while its harmful and more-harmful pores decrease by the largest extent, which indicates that the pore structure of NTC1 is most significantly refined. With the increasing content of nano-particles, the enhanced extent of harmless and less-harmful pores and the reduced extent of harmful and more-harmful pores in concretes are both decreased, and the refinement on pore structure of concretes is weakening.

The pore size distribution of NSC1 and NTC3 is close to each other. The harmless and less-harmful pores in NTC3 are both more than that in NSC1; though the harmful pores in NTC3 are slightly more than that in NSC1, its more-harmful pores are less than that in NSC1. Therefore, the pore structure of NTC3 is slightly finer than that of NSC1. The pore size distribution of NSC3 and NTC5 is also close to each other. Although the harmless pores in NTC5 are less than that in NSC3, its less-harmful pores are more than that in NSC3, and the harmful and more-harmful pores in NTC5 are both less than that in NSC3. Therefore, the pore structure of NTC5 is slightly finer than that of NSC3.

With the addition of PP fibers, the amounts of harmless and less-harmful pores in concretes decrease, and the amounts of harmful and more-harmful pores increase, which shows that the density of concretes is reduced and the pore structure is coarsened. With the increasing content of PP fibers, the reduced extent of harmless and less-harmful pores and the enhanced extent of harmful and more-harmful pores in concretes are both increased, and the pore structure of concrete becomes coarser and coarser.

For the concrete containing both nano-TiO₂ and PP fibers, the harmless and less-harmful pores decrease by the largest extent, while its harmful and more-harmful pores increase by the largest extent. The pore structure of NTPC is obviously coarser than that of concrete containing the same amount of PP fibers only, which indicates that the hybrid addition of nano-TiO₂ and PP fibers makes the pore structure of concrete further coarser.

3.2.3. Discussion

The mechanism that nano-particles refine the pore structure of concrete can be interpreted as follows. Supposed that nano-particles are uniformly dispersed in concrete and each particle is contained in a cube pattern, the distance between nano-particles can be determined. After hydration begins, hydrate products diffuse and envelop nano-particles as kernel. If the content of nano-particles and the distance between them are appropriate, the crystallization will be controlled in a suitable state through restricting the growth of Ca(OH)₂ crystal by nano-particles. Moreover, nano-particles located in cement paste as kernel can further promote cement hydration due to their high activity. This makes the cement matrix more homogeneous and compact. Consequently, the pore structure of concrete is refined evidently such as the concrete containing nano-TiO₂ in the amount of 1% by weight of binder.

With the increasing content of nano-particles, the refinement on pore structure of concrete is weakening. This can be attributed to that the distance between nano-particles decreases with the increasing content of nano-particles, and Ca(OH)₂ crystal cannot grow up enough due to limited space and the crystal quantity is decreased, which causes the ratio of crystal to C–S–H gel to become small and the shrinkage and creep of cement matrix to increase [23], thus the pore structure of cement matrix is coarser relatively.

The pore structure of concretes containing nano-SiO₂ is coarser than that of concretes containing the same amount of nano-TiO₂, which is mostly attributed to the fact that the particle diameter of nano-SiO₂ is smaller than that of nano-TiO₂, and the specific surface area of nano-SiO₂ is much larger than that of nano-TiO₂, so that the water demand of concrete containing nano-SiO₂ is more than that of concrete containing the same amount of nano-TiO₂. Consequently, when the mixture proportion of concrete is the same, nano-SiO₂ is more difficult to uniformly disperse than nano-TiO₂ in cement matrix, and is unable to exert its advantages.

The pore structure of concrete containing PP fibers is coarser than that of plain concrete, which can be attributed to the fact that the addition of PP fibers increases the interface in concrete and results in the increase of void content (pore volume) in concrete [10].

The pore structure of concrete containing both nano-TiO₂ and PP fibers is the coarsest. When nano-TiO₂ and PP fibers are respectively added into concrete, the slump of fresh concrete is lower than that of plain concrete; when nano-TiO₂ and PP fibers are both added into concrete, the slump of fresh concrete further becomes lower. Because both nano-TiO₂ and PP fibers need water to envelop, and there is insufficient water used for the hydration of cement, therefore the pore structure of hardened concrete is coarse.

To sum up, the addition of nano-particles refines the pore structure of concrete. On the one hand, nano-particles can act as a filler to enhance the density of concrete, which causes the porosity of concrete to reduce significantly. On the other hand, nano-particles can not only act as an activator to accelerate cement hydration due to their high activity, but also act as a kernel in cement paste which makes the size of Ca(OH)₂ crystal smaller and the tropism of Ca(OH)₂ crystal more stochastic.

3.3. Chloride permeability

3.3.1. Test results of chloride permeability

The test results for rapid chloride permeability of concretes at 28 days are presented in Table 8. It can be seen that the addition of nano-particles enhances the resistance to chloride penetration of concretes. The resistance to chloride penetration of concretes containing nano-TiO₂ is higher than that of concretes containing the same amount of nano-SiO₂.

Table 8. Test results for chloride permeability of concretes.

Mixture type	Chloride diffusion coefficient (10⁻¹³ m²/s)	Variation coefficient (%)	Reduced extent (%)
PC	1.9356	5.82	0
PPC6	2.1477	7.45	−10.96
PPC9	2.3198	7.78	−19.85
NSC1	1.5864	4.90	18.04
NSC3	1.7326	2.11	10.49
NTC1	1.3355	1.26	31.00
NTC3	1.5554	2.21	19.64
NTC5	1.7083	2.07	11.74
NTPC	2.4393	6.48	−26.02

The effectiveness of nano-TiO₂ in enhancing the resistance to chloride penetration of concretes increases in the order: NTC5 < NTC3 < NTC1. The resistance to chloride penetration of NTC1 increases by 31%. Even for NTC5, the resistance to chloride penetration also increases by 11.74%. The similar results can be observed for the concretes containing nano-SiO₂, which indicates that the enhanced extent of the resistance to chloride penetration of concretes reduces with the increasing content of nano-particles.

However, the resistance to chloride penetration of concretes containing PP fibers decreases. The larger the content of PP fibers, the lower the resistance to chloride penetration of concrete. The similar conclusion can be found in the literatures [10] and [11].

For the concrete containing both nano-TiO₂ and PP fibers, the resistance to chloride penetration is reduced significantly, and even lower than that of concrete containing the same amount of PP fibers only.

The enhancement or reduction of the resistance to chloride penetration is essentially due to the improvement or degeneration of pore structure of concretes. By the addition of nano-particles, the pore structure of concretes is refined, so the resistance to chloride penetration is enhanced. With different content of nano-particles, the change trend of the resistance to chloride penetration of concrete is entirely consistent with that of its pore structure. The pore structure of concretes containing nano-TiO₂ is finer than that of concretes containing the same amount of nano-SiO₂, so the resistance to chloride penetration of concretes containing nano-TiO₂ is higher than that of concretes containing the same amount of nano-SiO₂. The pore structure of concretes containing PP fibers is coarsened, so the resistance to chloride penetration is reduced. With the increasing content of PP fibers, the pore structure of concrete is coarser and coarser, so the resistance to chloride penetration becomes lower and lower. For the concrete containing both nano-TiO₂ and PP fibers, the pore structure is further coarser, so the resistance to chloride penetration decreases significantly.

3.3.2. Relationship between chloride permeability and compressive strength of concrete

At present, there is no agreement about the effect of compressive strength on the permeability of concrete as mentioned in Section 1. By nonlinear curve fitting on the data of compressive strengths and chloride permeability of concretes, the relationship between chloride diffusion coefficients and compressive strengths of concretes, except the concretes containing PP fibers only, is shown in Fig. 5.

Full-size image (14K)

Fig. 5.

Relationship between chloride diffusion coefficients and compressive strengths of concretes.

As mentioned in Section 1, the permeability of concrete is intimately related to the pore connectivity, but the compressive strength of concrete is governed by the total porosity [5] and [7]. Because PP fibers do not participate in hydration and the addition of PP fibers increases the interface in concrete, the resistance to chloride penetration of concretes containing PP fibers is reduced, but the compressive strengths are slightly enhanced due to the crack-arresting effect, crack-thinning effect and crack-bridging effect of PP fibers [21] and [22]. There is no significant relationship between chloride permeability and compressive strength of concretes containing PP fibers.

Fig. 5 indicates that the chloride diffusion coefficient of concrete decreases with increasing compressive strength. It can be observed that the curve approaches hyperbola basically, which confirms that the compressive strength is an important factor affecting the resistance to chloride penetration of concrete. The relationship between chloride diffusion coefficients and compressive strengths can be expressed by

(5)where C_dc is the chloride diffusion coefficient of concrete (10⁻¹³ cm²/s); f_cu is the compressive strength (MPa); θ₁ and θ₂ are constants that can be obtained by curve fitting technique and given in Table 9.

Table 9. Regression coefficients in Eq. (5).

θ₁	θ₂	Correlation coefficient r
2.0031	−88.1728	0.9624

Fig. 5 shows that the enhancement in compressive strength leads to higher and higher resistance to chloride penetration. From durability view, continued increase in compressive strength is beneficial. Therefore, the compressive strength can be used as a roughly comparative or predictive indicator for the resistance to chloride penetration of pavement concrete.

3.3.3. Relationship between chloride permeability and pore structure of concrete

To investigate the relationship between chloride permeability and pore structure of concrete, the regression analysis of data obtained by MIP and NEL is shown in Fig. 6.

Full-size image (57K)

Fig. 6.

Relationship between chloride diffusion coefficients and pore structure parameters of concretes.

It is observed from Fig. 6 that there is reasonably close correlation between chloride diffusion coefficients and pore structure parameters of concretes. The result of regression analysis can be written as follows

(6)C_dc=α·P_ps-βwhere C_dc is the chloride diffusion coefficient (10⁻¹³ cm²/s); P_ps is the pore structure parameter; α and β are constants and presented in Table 10.

Table 10. Regression coefficients in Eq. (6).

Parameter of pore structure	α	β	Correlation coefficient r
Total specific pore volume	92.3371	3.0141	0.9991
Most probable pore diameter	0.0469	0.2567	0.9907
Porosity	0.3746	2.2293	0.9803
Average diameter	0.0758	1.2624	0.9449
Median diameter (volume)	0.0556	1.2216	0.9892

It can be seen from Table 10 that there is significant linear relationship between chloride diffusion coefficients and all pore structure parameters of concretes, and the correlation coefficients are basically larger than 0.95, which indicates that the resistance to chloride penetration of concretes is strongly influenced by the pore structure. The finer the pore structure of concrete, the higher the resistance to chloride penetration of concrete. The total specific pore volume seems to be the parameter with the closest relation to chloride permeability.

4. Conclusions

The following conclusions are drawn from this study:

(1) The pore structure of concretes containing

nano

-particles is refined, and the refined extent is enhanced with the decreasing content of

nano

-particles. The pore structure of concretes containing

nano

-TiO₂ is finer than that of concretes containing the same amount of

nano

-SiO₂. The pore structure of concretes containing PP

fibers

is coarsened, and becomes coarser and coarser with the increasing amount of PP

fibers.

For the concrete containing both

nano

-TiO₂ and PP

fibers,

the pore structure is also coarsened and further becomes coarser than that of concrete containing the same amount of PP

fibers

only.

(2) The addition of nano-particles enhances the resistance to chloride penetration of concretes, and the enhanced extent is increased with the decreasing content of nano-particles. The resistance to chloride penetration of concretes containing nano-TiO₂ is higher than that of concretes containing the same amount of nano-SiO₂. The addition of PP fibers reduces the resistance to chloride penetration of concretes. The larger the content of PP fibers, the lower the resistance to chloride penetration of concrete. For the concrete containing both nano-TiO₂ and PP fibers, the resistance to chloride penetration is also decreased and even lower than that of concrete containing the same amount of PP fibers only.

(3) Except the concretes containing PP fibers only, the relationship between chloride diffusion coefficients and compressive strengths of concretes approaches hyperbola basically, which indicates that the resistance to chloride penetration of concrete increases with increasing compressive strength. There is a significant linear relationship between chloride permeability and pore structure of concretes. The finer the pore structure of concrete, the higher the resistance to chloride penetration of concrete.

Acknowledgements

The careful review and comments of Dr. Jinchi Lu, at the University of California, San Diego, is greatly appreciated. This study is financially supported by NSFC with Grant No. 50908067, Postdoctoral Foundation of China and the Fundamental Research Funds for the Central Universities (DL09BB29).

References

[1] P. Mohr, W. Hansen, E. Jensen and I. Pane, Transport properties of concrete pavements with excellent long-term in-service performance, Cem Concr Res 30 (2000), pp. 1903–1910. Article | PDF (284 K) | View Record in Scopus | Cited By in Scopus (5)

[2] L. Basheer, J. Kropp and D.J. Cleland, Assessment of the durability of concrete from its permeation properties: a review, Constr Build Mater 15 (2001), pp. 93–103. Article | PDF (332 K) | View Record in Scopus | Cited By in Scopus (44)

[3] M.G. Alexander and B.J. Magee, Durability performance of concrete containing condensed silica fume, Cem Concr Res 29 (1999), pp. 917–922. Article | PDF (103 K) | View Record in Scopus | Cited By in Scopus (26)

[4] L. Bágel and V. Živica, Relationship between pore structure and permeability of hardened cement mortars: on the choice of effective pore structure parameter, Cem Concr Res 27 (8) (1997), pp. 1225–1235. Abstract | Article | PDF (691 K) | View Record in Scopus | Cited By in Scopus (18)

[5] K. Tanaka and K. Kurumisawa, Development of technique for observing pores in hardened cement paste, Cem Concr Res 32 (2002), pp. 1435–1441. Article | PDF (499 K) | View Record in Scopus | Cited By in Scopus (14)

[6] C.S. Peon, Y.L. Wong and L. Lam, The influence of different curing conditions on the pore structure and related properties of fly-ash cement pastes and mortars, Constr Build Mater 11 (7–8) (1997), pp. 383–393.

[7] T.J. Zhao, J.Q. Zhu and N.Q. Feng, Correlation between strength and permeability of concrete, Ind Constr 27 (5) (1997), pp. 14–17 (in Chinese).

[8] Z.W. Wu and H.Z. Lian, High performance concrete, Railway Press of China, Beijing (1999) p. 43 (in Chinese).

[9] J. Armaghani, T. Larsen and D. Romano, Aspects of concrete strength and durability, Transport Res Rec 1335 (1992), pp. 63–69.

[10] H. Toutanji, S. McNeil and Z. Bayasi, Chloride permeability and impact resistance of polypropylene-fiber-reinforced silica fume concrete, Cem Concr Res 28 (7) (1998), pp. 961–968. Article | PDF (611 K) | View Record in Scopus | Cited By in Scopus (21)

[11] H.A. Toutanji, Properties of polypropylene fiber reinforced silica fume expansive-cement concrete, Constr Build Mater 13 (1999), pp. 171–177. Article | PDF (152 K) | View Record in Scopus | Cited By in Scopus (9)

[12] G. Vondran and T. Webster, The relationship of polypropylene fiber reinforced concrete to permeability, ACI (SP-108) (1987), pp. 85–97.

[13] W.H. Huang, Properties of cement-fly ash grout admixture with bentonite, silica fume or organic fiber, Cem Concr Res 27 (3) (1997), pp. 395–406. Abstract | Article | PDF (809 K) | View Record in Scopus | Cited By in Scopus (15)

[14] B.H. Oh, S.W. Cha, B.S. Jang and S.Y. Jang, Development of high-performance concrete having high resistance to chloride penetration, Nucl Eng Des 212 (2002), pp. 221–231. Article | PDF (179 K) | View Record in Scopus | Cited By in Scopus (28)

[15] H. Li, M.H. Zhang and J.P. Ou, Abrasion resistance of concrete containing nano-particles for pavement, Wear 260 (11–12) (2006), pp. 1262–1266. Article | PDF (153 K) | View Record in Scopus | Cited By in Scopus (27)

[16] H. Li, M.H. Zhang and J.P. Ou, Flexural fatigue performance of concrete containing nano-particles for pavement, Int J Fatigue 29 (2007), pp. 1292–1301. Article | PDF (205 K) | View Record in Scopus | Cited By in Scopus (17)

[17] H. Li, H.G. Xiao, J. Yuan and J.P. Ou, Microstructure of cement mortar with nano-particles, Composites Part B 35 (2004), pp. 185–189. Article | PDF (335 K) | View Record in Scopus | Cited By in Scopus (57)

[18] H. Li, H.G. Xiao and J.P. Ou, A study on mechanical and pressure-sensitive properties of cement mortar with nanophase materials, Cem Concr Res 34 (2004), pp. 435–438. Article | PDF (176 K) | View Record in Scopus | Cited By in Scopus (42)

[19] A.B. Abell, K.L. Willis and D.A. Lange, Mercury intrusion porosimetry and image analysis of cement-based materials, J Colloid Interf Sci 211 (1999), pp. 39–44. Abstract | PDF (765 K) | View Record in Scopus | Cited By in Scopus (38)

[20] X.Y. Lu, Application of the Nernst–Einstein equation to concrete, Cem Concr Res 27 (2) (1997), pp. 293–302. Abstract | Article | PDF (740 K) | View Record in Scopus | Cited By in Scopus (38)

[21] S.F. Chen, D.L. Zhang and J. Zhang et al., The study of the road performance of the polypropylene fiber concrete, Northeast Highway 24 (2) (2001), pp. 23–25 (in Chinese).

[22] M.K. Lee and B.I.G. Barr, An overview of the fatigue behavior of plain and fibre reinforced concrete, Cem Concr Compos 26 (2004), pp. 299–305. Article | PDF (287 K) | View Record in Scopus | Cited By in Scopus (28)

[23] Q. Ye, Z.N. Zhang and D.Y. Kong et al., Influence of nano-SiO₂ addition on properties of hardened cement paste as compared with silica fume, Constr Build Mater 21 (2007), pp. 539–545.

Corresponding author at: School of Civil Engineering, Northeast Forestry University, 150040 Harbin, China. Tel./fax: +86 451 86282027.

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