Zhichao Liu | University of Michigan (original) (raw)

Papers by Zhichao Liu

Deicer salt scaling is a result of excessive ice-growth in capillary pores. Ice-growth causes a s... more Deicer salt scaling is a result of excessive ice-growth in capillary pores. Ice-growth causes a substantial apparent pore volume increase in scaled materials. A conceptual scaling model is proposed. A mathematical description of sorptivity is derived from pore size distribution.

Freezing-thawing (F-T) exposure produces higher absorption in concrete than an isothermal conditi... more Freezing-thawing (F-T) exposure produces higher absorption in concrete than an isothermal condition. This work investigated the moisture absorption characteristics in different air-entrained concrete mixtures under two exposure conditions (water and a 3% salt solution) by simultaneously monitoring the weight change, mass loss and internal damage of 100 Â 100 Â 70 mm concrete blocks subjected to repetitive F-T cycles. The results indicate that permanent

Simultaneous measurement on surface scaling and moisture uptake is undertaken. HSC mixes are more... more Simultaneous measurement on surface scaling and moisture uptake is undertaken. HSC mixes are more prone to internal damage, albeit the high scaling resistance. A bi-linear pattern is noted for mass loss and moisture uptake of HSC mixes. Air-void characteristics is not a major factor in surface scaling of HSC mixes.

Key Engineering Materials, 2014

Salt frost scaling is the progressive deterioration of the concrete surface associated with freez... more Salt frost scaling is the progressive deterioration of the concrete surface associated with freezing and thawing in the presence of a salt solution. Its major mechanism has been confirmed to be cryogenic suction that promotes continuous ice growth in the surface region. In this paper, salt frost scaling of different air-entrained concretes is investigated, alone with the room-temperature sorptivity measurement. A clear correlation is found between the two properties, which is supported by a theoretical analysis on the sorptivity. These findings explain why HPC of low w/b ratio and associated low capillary porosity has much improved scaling resistance. Similar benefits are obtained for regular w/b ratio concrete containing high cementitious replacement level of slag cement.

The principal objective in this paper is to determine the internal viscoelastic modulus associate... more The principal objective in this paper is to determine the internal viscoelastic modulus associated with autogenous shrinkage of cementitious materials. This is accomplished by analyzing autogenous shrinkage of plain and reinforced paste and mortar specimens. The internal restraint from steel reinforcement creates uniaxial tensile stress condition increasing with shrinkage. Data analysis demonstrates a unique internal shrinkage modulus, which eliminates the need for stress relaxation modulus calculations. The internal modulus is in the range of 8000-9000 MPa, while the external composite modulus known as Young's modulus is 32 000-36 000 MPa for the low water-binder ratio (0.35) concrete.

Key Engineering Materials, 2014

High performance concrete (HPC) consisting of low water-binder (w/b) ratio and supplementary ceme... more High performance concrete (HPC) consisting of low water-binder (w/b) ratio and supplementary cementitious materials (SCM) is more prone to shrinkage cracking if subjected to external deformation restraint. The effectiveness of using lightweight fine aggregate (LWA) for autogenous shrinkage reduction is being studied along with implications on salt frost durability. HPC consisting of 0.33 w/b ratio and cementitious replacement level up to 50% by slag cement and natural sand replacement level by LWA of up to 50% is investigated. Results indicate that these concretes exhibit excellent salt frost resistance provided the HPC is sufficiently air entrained. The mitigation of autogenous shrinkage by LWA is analyzed by comparing the spacing of LWA particles in cement paste and the flow distance of retained moisture in LWA to the adjacent paste.

Salt frost property of air-entrained high strength concrete (HSC) was investigated by the CIF met... more Salt frost property of air-entrained high strength concrete (HSC) was investigated by the CIF method and good salt scaling resistance is found. This can be explained by the reduced capillary transport into concrete pores during both the room-temperature presaturation and F-T wet exposure and is supported by a good correlation between scaling rate and sorptivity. The coincidence of the transition point on the mass-loss and moisture uptake curves demonstrate the importance of initial moisture condition in concrete. Increased imperviousness of HSC to moisture ingress may render it more prone to long-term frost damage intrinsically, which is evidenced by much higher initial freezing strain in HSC from the length-change measurement on thin specimens at various degrees of saturation.

Predicting temperature effects on rheology and hydration of cementitious slurries considered for ... more Predicting temperature effects on rheology and hydration of cementitious slurries considered for oil-well cement applications is important as temperature can vary significantly in a bore-well. During the plastic stage the fluid properties (yield and plastic viscosity) are investigated using a rheometer, while isothermal calorimetry is used to evaluate setting characteristics and early-age (0-7 days) heat of hydration. In this paper, experimental results at 3 different temperatures show that a 50/50 blend of Type I portland cement and slag cement at a 0.45 water-cementitious ratio has similar rheological and hydration properties as the reference system (Type I portland cement paste). The rheological properties were found to follow a two-parameter Bingham model, and temperature effects can be accounted for by an Arrhenius model. The effect of temperature on hydration rate can be predicted by a maturity function, which also is based on the Arrhenius rate model, where the apparent activation energy is a measure of temperature sensitivity.

Due to the resource-intensive process of conducting salt frost scaling test there is a strong int... more Due to the resource-intensive process of conducting salt frost scaling test there is a strong interest in performance-based criteria tests that are indicative of frost resistance as these tests, although not as a replacement for scaling measurement, are intended to establish material level improvements. Previous studies have shown cryogenic capillary suction is a major liquid transport mechanism during salt/F-T exposure. The intrinsic suction rate of concrete surface region is mainly controlled by the paste pore structure. In this paper, Sellevold's model on concrete sorptivity is extended to incorporate the pore size distribution by aggregating capillary pores into clusters of straight circular tubes of different sizes. This refined methodology shows the strong dependency of sorptivity on the pore volume and its size distribution and holds the potential to model pore shape and continuity effects. This model elucidates the effective measures to mitigate salt frost scaling: reduction in waterbinder (w/b) ratio or partial replacement of Portland cement by slag cement.

The air-void system in both fresh and hardened states for different concrete mixes was measured a... more The air-void system in both fresh and hardened states for different concrete mixes was measured and characterised using the methods presented in ASTM C231 and ASTM C457 respectively. The mix variables included two water/ binder (w/b) ratios (0·33 and 0·45), one replacement level of Portland cement with slag cement (50%) and a range of air contents achieved through the use of different air-entraining agents. Consistent results were obtained on duplicate specimens using the two testing procedures. The cumulative size distribution of the air voids estimated from chord length measurement was satisfactorily represented by an S-shaped function from which the differential distribution can be derived. The importance of air-void fineness on the spacing was clearly illustrated by the small variation in Powers' spacing factor when large air bubbles were included.

The interaction between a salt solution and ice formation in capillary pores might provide new in... more The interaction between a salt solution and ice formation in capillary pores might provide new insight into the salt frost scaling process. To accomplish this small prismatic specimens (10 mm × 10 mm × 90 mm) were cut from air-entrained concrete, dried at 50°C until constant weight, then immersed in sodium chloride (NaCl) solutions of different concentrations (0%, 3%, 9% and 12%) until they reached full capillary saturation. They were then subjected to a freeze-thaw (F-T) cycle in a high resolution low temperature dilatometer (LTD) with the surface temperature and uniaxial length-change continuously monitored. Internal saturation of capillary pores with increasing salt solutions is found to have a profound effect on the F-T response including the temperature rise and instant dilation associated with ice nucleation. It is concluded that salt ions retard ice formation and the effect of ice-growth on pore expansion during an F-T cycle. The presence of salt ions in the concrete pores and surface liquid has a counter-balancing effect on specimen length-change associated with ice-growth, which may provide one possible explanation for the pessimum salt concentration effect. The extent of surface scaling is also affected by the intrinsic capillary transport property (i.e., sorptivity) of the porous cementitious binder. Salt scaling is exacerbated in concretes with increasing sorptivity.

Key Engineering Materials, 2014