Pre-wetted Lightweight Aggregate Research Articles

Combined Effects of Internal Curing, SCMs, and Expansive Additives on Concrete Shrinkage

Abstract Internal curing (IC) using prewetted lightweight aggregate (LWA) has often been used to improve the cracking performance of concretes with low water-to-binder (w/b) ratios (<0.42) when autogenous shrinkage and self-desiccation are of concern. Shrinkage-compensating admixtures (SCAs) are also used to improve the cracking performance of concrete. A modified version of ASTM C157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete, in which length-change measurements begin 5 1/2 ± 1/2 h after casting concrete, was used to evaluate the effects of IC obtained through the use of intermediate-sized prewetted LWA, partial replacements of cement with the supplementary cementitious materials slag cement and silica fume (SCMs); incorporation of calcium oxide–based and magnesium oxide–based SCAs; and combinations of IC, SCMs, and SCAs using eleven concrete mixtures with moderate w/b ratios (0.45). The results show that the modified ASTM C157 method helps to capture the early-age behavior of concrete mixtures. IC provided by partial replacement of total aggregate with intermediate-sized prewetted LWA is effective in reducing drying shrinkage in concrete made with moderate w/b ratios. Partial replacements of cement with slag cement and silica fume induce increased first-day expansion and reduce shrinkage. A further increase in first-day expansion and a reduction in shrinkage is obtained when IC is used in conjunction with slag cement and silica fume. The SCAs evaluated in this study reduce the tendency to develop shrinkage strain. The calcium oxide–based SCA induces the more rapid expansion of greater magnitude, while the magnesium oxide–based SCA expands more gradually. When the calcium oxide–based SCA is incorporated in a mixture containing SCMs or SCMs and IC, expansion is further increased. The same observation cannot be made for mixtures incorporating SCMs with the magnesium oxide–based SCA.

Self-compacting lightweight aggregate concrete in Vietnam

Self-Compacting Lightweight Aggregate Concrete (SCLC) is a new building material in Vietnam. The lightweight aggregates (keramsite) produced from expanded clay or slate is used for the SCLC. If dry Keramsite is used in making the concrete mixture, it will partly absorb water from the cement paste, which causes the slump loss of concrete mixture, increasing shrinkage of cement matrix in the concrete. With regard to Self-Compacting Structural Lightweight Aggregate Concrete (SCLC), the cement content used is significant, the ratio of Water to Binder is rather low, and the structure of cement paste is very dense. This does not only increase the autogenous shrinkage of concrete but also have influence on the effectiveness of external curing. Taking advantage of the special structure of Lightweight aggregate as Keramsite, internal curing can be performed in order to improve the quality of SCLC by wetting Keramsite before mixing with suitable water absorption.This paper presents some research results about SCLC, the incorporation of internal curing with other measures such as the use of dispersal fiber reinforcement, viscous stabilizing admixtures in order to restrain the segregation level of lightweight aggregates, reducing shrinkage and improving mechanical-physical properties of SCLC in the hot-humid climate condition of Vietnam. The research results show that, from selected materials, lightweight concretes with unit weight of 1600 - 1800 kg/m3, compressive strength of 30 - 35 MPa, good workability, capable of self-compacting can be manufactured. The use of pre-wetted lightweight aggregate with suitable humidity for producing SCLC will result in better effectiveness than the use of dry lightweight aggregate, as in reducing restraining shrinkage, improving the strength for SCLC.

Calculation of shrinkage stress in concrete structures with impact of internal curing

This paper presents recent advances in the calculation of shrinkage stress in concrete structures, especially focus on the effect of internal curing. In the modeling, an integrative model for autogenous and drying shrinkage predictions of concrete under drying environments is introduced first. Second, a model taking both cement hydration and moisture diffusion into account synchronously is used to calculate the distribution of internal relative humidity in concrete. Using the above two models, along with the shrinkage stress model for concrete pavement under nonlinear strain along the slab thickness, the development of shrinkage stress in concrete pavements since casting made by low, middle and high strength concrete respectively is calculated. In particular, the effects of internal curing with pre-wetted lightweight aggregates on the progress of shrinkage stress are investigated and discussed. Based on the present study, the following conclusions can be drawn: (1) the progress of shrinkage stress in concrete pavements obeys two-stage mode since concrete casting, a small stress stage (close to zero) in the moisture saturated period and a gradual increasing stage with the relative humidity gradually decreasing in the concrete; (2) the closer to the drying face, the higher the shrinkage stresses; (3) the shrinkage stress is greater in high strength slab than that in low or middle strength slab; (4) internal curing with pre-wetted lightweight aggregate can greatly reduce shrinkage stress in concrete pavement, either in low strength slab or high strength slab.

Chloride Ingress in Internally Cured Concrete under Complex Solution

AbstractInternal curing (IC) technology using prewetted lightweight aggregates (LWAs) as additives has been proven to be an effective means for mitigating both autogenous shrinkage and early-age cr...

Internal curing of pervious concrete using lightweight aggregates

Pervious concrete is an important stormwater management technique for the urban environment. Surface raveling is the single most common distress caused primarily by plastic drying and paste shrinkage. Internal curing is one mechanism to provide additional moisture to elevate internal humidity and reduce shrinkage and is most commonly provided through prewetted lightweight aggregates (PLWA). This study evaluated how fine PLWA influence properties when used to completely replace the small portion of conventional fine aggregate present in pervious concrete. Samples were placed at a fixed void content to not confounded results with changes in density. Strength, degree of hydration, shrinkage, and freeze–thaw testing all showed substantial improvements over the control mixture, strongly suggesting that internal curing become routine for pervious concrete.

Experimental investigation on the compressive strength and shrinkage of concrete with pre-wetted lightweight aggregates

Experimental studies are presented on the influence of three types of pre-wetted lightweight aggregates (PLWA), through their volume fractions and moisture content, on the compressive strength and shrinkage of concrete and expansive concrete. Via integrated thermal analysis, the hydration degree during 7d–28d was found to be increased by the internal curing. The use of PLWA increased the shrinkage at 240d due to the significant increase in the shrinkage deformation during the age of 28d–90d caused by internal curing. The results also suggest that incorporation of 10-Mas. % expansive agent is useful to diminish the shrinkage, especially by its combined use with PLWA. Predictive models for the compressive strength and shrinkage of concrete containing PLWA were discussed and a unified model predicting the compressive strength and shrinkage of concrete containing PLWA was suggested.

Effect of Internal Curing on Characteristics of Self-Compacting Concrete by Using Fine and Coarse Lightweight Aggregates

AbstractInternally cured concrete made with prewetted lightweight aggregates (LWAs) has a risk of segregation because of floating LWAs in the process of vibration. Such risk can be reduced by apply...

Influence of Curing Conditions on Properties of Normal and High Strength Concrete with and without Pre-Wetted Lightweight Aggregates

Influence of Curing Conditions on Properties of Normal and High Strength Concrete with and without Pre-Wetted Lightweight Aggregates

Superabsorbent biopolymers for mitigating autogenous shrinkage in cement-based materials

Availability of water plays an essential role in the hydration of ordinary portland cement. At low water-to-cement (w/c) ratios, for example, hydration processes can rapidly deplete water. The increased demand for more water exacerbates surface tension-induced stresses within fine capillary pores, which causes shrinkage of the cement paste. This phenomenon, termed autogenous shrinkage, is often prevented with sufficient curing, namely by keeping the surface of the concrete continuously wet. However, such conventional curing methods are not sufficient for ultra-high-performance concretes, which are produced with very low w/c ratios. In these applications, autogenous shrinkage is mitigated via the use of internal curing approaches, either by inclusion of prewetted lightweight aggregates or superabsorbent polymers (SAPs). SAPs are ultra-hydrophilic polymer networks capable of absorbing 100,000% of their dry weight in water. A variety of acrylic-based monomers are typically employed in the preparation of SAPs, as are a number of aggressive solvents and time- and energy-intensive polymerizations. This paper presents recent experimental efforts on synthesizing and characterizing superabsorbent polymers from biorenewable resources and principles of green chemistry. In this work, the chemical synthesis and physical swelling of superabsorbent biopolymers are investigated in (a) water and (b) synthetic concrete pore solutions. Results demonstrate that biobased SAPs that absorb in excess of 20,000% their weight in water can be synthesized using ambient-condition polymerizations and green solvents, thus offering a potential biobased solution to successfully mitigating autogenous shrinkage in ordinary portland cement paste and mortars.

Prediction model for relative humidity of early-age internally cured concrete with pre-wetted lightweight aggregates

The decrease of internal relative humidity (IRH) in high-performance concrete (HPC) induces autogenous shrinkage, which is fundamental to evaluate the cracking resistance. Internal curing (IC) with pre-wetted lightweight aggregates (LWAs) is used to mitigate the shrinkage of HPC, and prediction of shrinkage of HPC requires knowledge of the IRH variations. Although investigations on IRH in concrete have been conducted, the prediction model for IRH in internally cured concrete with LWAs considering IC remains lacking. Experimental research and analysis on IRH in early-age concrete with pre-wetted LWAs were conducted. Test results show that: (1) the average value of IRH decrease rate in internally cured concrete with pre-wetted LWAs decreased nonlinearly with the increase of IC water-to-cement (w/c) ratio; (2) the prediction models for critical time and working w/c ratio were proposed; (3) a prediction model for the IRH in early-age internally cured concrete was proposed considering the working w/c ratio, critical time, and age. These models showed the good accuracy of the experimental results.

Early-age tensile creep and cracking potential of concrete internally cured with pre-wetted lightweight aggregate

High-performance concrete (HPC) is widely used in practice due to its potential long-term benefits, such as high strength and low permeability. However, high self-desiccation and high temperature rise occur due to the low water-to-cement (w/c) ratio of HPC, both of which would increase the cracking potential of concrete at early age. Although the creep and cracking potential of early-age HPC have been investigated, studies on the tensile creep and cracking potential of internally cured concrete with pre-wetted lightweight aggregates (LWAs) at early age under adiabatic condition at various w/c ratios remain lacking. In present study, the tensile creep and cracking potential of concrete at early age were experimentally investigated under adiabatic condition using the temperature stress test machine. Test results and corresponding analysis showed that: (1) a model for predicting the compressive strength of concrete was presented in consideration of the influence of pre-wetted LWAs; (2) the adding of pre-wetted LWAs reduced the autogenous shrinkage of concrete with different basic w/c ratios; (3) the basic tensile creep/shrinkage and absolute value of basic tensile creep of internally cured concrete were lower than that of normal concrete when the basic w/c ratios were the same; (4) the basic tensile creep/shrinkage and absolute value of specific basic tensile creep of normal and internally cured concrete at the age of cracking both increased with the decrease of basic w/c ratio; (5) the cracking potential of normal and internally cured concrete both increased with the decrease of basic w/c ratio and the adding of pre-wetted LWAs reduced the cracking potential.

IMPROVING OF LIGHTWEIGHT SELF- CURING CONCRETE PROPERTIES

This study investigates the effect of self-curing agents on the mechanical properties of concrete. The self-curing agent is prewetted lightweight aggregate crushed over burnt clay bricks (COBCB). The COBCB has been used for replacement coarse aggregates in a ratios of 20%, 50% and 100 %. The mechanical properties were evaluated while the concrete specimens were subjected to different curing methods; conventional water curing, air curing regime (in the laboratory environment with 25°C) and chemical curing. The results show that, the use of self-curing agents COBCB in concrete with replacement ratio 20% by volume of coarse aggregates is effectively improved the mechanical properties in all cases of curing.

Influence of prewetted lightweight aggregates on the behavior and cracking potential of internally cured concrete at an early age

High-performance concrete (HPC) is widely used in practice. The water-to-cement ratio of HPC is low, and self-desiccation and high temperature rise occur. Internal curing (IC) with prewetted lightweight aggregates (LWAs) is applied to enhance the early-age behavior of concrete structures in terms of temperature, shrinkage, creep deformation, and stress, thus resulting in low cracking potential. Although tests on the effect of prewetted LWAs on the cracking potential of concrete under semi-adiabatic or isothermal conditions have been conducted, studies on the influence of the amount of prewetted LWAs on the cracking potential of internally cured concrete under adiabatic conditions remain lacking. In this study, the cracking potential of internally cured concrete with prewetted LWAs was experimentally investigated with a temperature stress testing machine. The effect of prewetted LWAs in reducing the cracking potential in HPC was investigated with different amounts of LWAs (0%, 10%, 30%, and 50%) in four pairs of large prismatic HPC specimens tested simultaneously under free and fully restrained shrinkage. Test results showed that (1) both temperature rise and drop of concrete occurred as prewetted LWAs increased; (2) the autogenous shrinkage and cracking stress of concrete decreased with the increase in the amount of prewetted LWAs; (3) the tensile stress rate of concrete with prewetted LWAs decreased; (4) the ratio of cracking stress to the tensile strength of concrete increased with the increase in the amount of prewetted LWAs; (5) the cracking age of concrete with the LWAs increased; (6) the specific tensile creep of concrete with prewetted LWAs decreased; and (7) the cracking potential of concrete with prewetted LWAs was reduced, as obtained from the integrated criterion. Internally cured concrete with prewetted clay LWAs is more robust for construction at early ages.

Effect of prewetting degree of ceramsite on the early-age autogenous shrinkage of lightweight aggregate concrete

Lightweight aggregate (LWA) can provide internal curing to lightweight aggregate concrete (LWAC) due to the water absorption and desorption capability of ceramsite. Hence, the addition of ceramsite has a strong effect on the early age autogenous shrinkage of LWAC. In this paper, the early-age autogenous shrinkage of LWAC with different prewetting degree LWA was quantitatively studied when the same net water to cement ratio (NWC) or total water to cement ratio (TWC) was used. During the liquid phase and skeleton-formational phase, the positive effect of the 24h prewetted ceramsite on the early-age autogeneous shrinkage of LWAC is obvious. During the hardening phase, the LWACs show small expansion strains. Therefore the early age autogeneous shrinkage of LWAC decreases with the increase of prewetting degree of ceramsite for the same NWC or TWC.

Plastic shrinkage of mortars with shrinkage reducing admixture and lightweight aggregates studied by neutron tomography

Water transport in fresh, highly permeable concrete and rapid water evaporation from the concrete surface during the first few hours after placement are the key parameters influencing plastic shrinkage cracking. In this work, neutron tomography was used to determine both the water loss from the concrete surface due to evaporation and the redistribution of fluid that occurs in fresh mortars exposed to external drying. In addition to the reference mortar with a water to cement ratio (w/c) of 0.30, a mortar with the addition of pre-wetted lightweight aggregates (LWA) and a mortar with a shrinkage reducing admixture (SRA) were tested. The addition of SRA reduced the evaporation rate from the mortar at the initial stages of drying and reduced the total water loss. The pre-wetted LWA released a large part of the absorbed water as a consequence of capillary pressure developing in the fresh mortar due to evaporation.

Internal Relative Humidity and Drying Shrinkage of Lightweight Aggregate Concrete

In this paper, the influence of water cement ratio, prewetting degree of lightweight aggregate, flyash content and air content on the internal relative humidity (IRH) and drying shrinkage of lightweight aggregate concrete (LAC) were studied using a relative humidity sensor and drying shrinkage apparatus. Results showed that the IRH of concrete decreased rapidly at early age and slowed down at late age, and prewetting lightweight aggregate could compensate the loss of IRH. The effect of compensation could be enhanced with the increasing of prewetting degree of lightweight aggregate, and the addition of flyash retarded the decrease of IRH at early age. The drying shrinkage of LAC increased quickly at early stage and it slowed down to the even after 60 days or longer age. Furthermore, the drying shrinkage of LAC increased with increasing of water cement ratio. The effect of air content on the drying shrinkage of LAC was limited at early stage. However, at the late stage it grew at different level with increasing of air content. By raising the prewetting degree of lightweight aggregate, mixing with proper dosage of flyash, the shrinkage rate of LAC can be reduced markedly. Key words: lightweight aggregate concrete, internal relative humidity, drying shrinkage, prewetting degree, flyash.

Cracking Tendency of Lightweight Aggregate Bridge Deck Concrete

Early-age cracking in bridge decks is a severe problem that may reduce the functional life of the structure. In this project, the effect of using lightweight aggregate on the cracking tendency of bridge deck concrete was evaluated using testing frames that restrain movement due to volume change effects from placement to cracking. Expanded shale, clay, and slate lightweight coarse and fine aggregates were used to produce internal curing, sand-lightweight, and all-lightweight concretes to compare their behavior relative to a normalweight concrete in bridge deck applications. Specimens were tested under temperature conditions that simulate summer and fall placement scenarios. Increasing the amount of pre-wetted lightweight aggregate in the concrete systematically decreased the density, modulus of elasticity, and coefficient of thermal expansion of the concrete. When compared to a normalweight concrete, the use of lightweight aggregates in concrete effectively delays the occurrence of early-age cracking in bridge deck applications.

Using a Centrifuge to Determine Moisture Properties of Lightweight Fine Aggregate for Use in Internal Curing

Abstract Internally cured (IC) concrete is frequently produced using pre-wetted lightweight aggregate (LWA). One important aspect associated with preparing quality IC concrete is being able to determine the absorbed moisture and surface moisture associated with the LWA which enables aggregate moisture corrections to be made for the concrete mixture. ASTM 1761-13b specifies the use of the “paper towel method” to define a surface dry state for pre-wetted LWA; however, this can be time consuming and hard to perform at the batching plant. This paper proposes an alternative method for obtaining a surface dry state of the aggregate based on spinning the pre-wetted LWA in a centrifuge. It is shown that using a centrifuge (with bowl radius of 11.4 cm) for 3 min at 2000 rpm permits a surface dry state to be obtained consistently which enables surface and absorbed moisture to be determined accurately and rapidly. The moisture values obtained using the centrifuge correspond well with those obtained using the paper towel method.

Influence of Slag Aggregate Production on Its Potential for Use in Internal Curing

Internal curing is effective at reducing shrinkage and early-age cracking in cementitious systems with low water-to-cementitious materials ratios. In the United States, internal curing is typically accomplished using prewetted lightweight aggregate made by expanding slate, clay, or shale. This research focused on the use of porous slag aggregate, a byproduct of the iron and steel industry, for the internal curing of concrete. Five aggregates were evaluated for use in internal curing. The aggregates were produced from different manufacturing processes. Expanded, pelletized, and air-cooled slag aggregates were chosen for advanced testing. The research began by measuring the absorption and desorption properties of the aggregates. Laboratory testing of concrete mixtures containing select aggregates was performed to evaluate mechanical and durability properties. Full-scale testing was carried out with concrete produced at a ready-mix plant. A conventional department of transportation bridge deck mixture was compared with a similar concrete that was internally cured with prewetted expanded slag aggregate. Internally cured concrete made with expanded slag aggregate was shown to reduce shrinkage cracking with similar or improved overall mechanical and durability properties when compared with the conventional mixture.

Special Section on Advances in Internally Cured Concrete

Special Section on Advances in Internally Cured Concrete