Early age concrete--properties and performance

Aci Materials Journal

The very high mechanical strength and enhanced durability of ultra highperformance concrete (UHPC) make it a strong contender for several concrete applications. However, UHPC has a very low water-to-cement ratio, which increases its tendency to undergo early-age shrinkage cracking with a risk of decreasing its long-term durability. To reduce the magnitude of early-age shrinkage and cracking potential, several mitigation strategies have been proposed including the use of shrinkage reducing admixtures, internal curing methods (e.g. superabsorbent polymers), expansive cements and extended moist curing durations. To appropriately utilize these strategies, it is important to have a complete understanding of the driving forces behind early-age volume change and how these shrinkage mitigation methods work from a materials science perspective to reduce shrinkage under field like conditions. This dissertation initially uses a first-principles approach to understand the interrelation mechanisms between different shrinkage types under simulated field conditions and the role of different shrinkage mitigations methods. The ultimate goal of the dissertation is to achieve lower early-age shrinkage and cracking risk concrete along with reducing its environmental and economic impact. As a result, a novel environmentally friendly shrinkage reducing technique based on using partially hydrated cementitious materials (PHCM) from waste concrete is proposed. The PHCM principle, mechanisms and efficiency were evaluated compared to other mitigation methods.

Materials and Structures, 2011

This experimental study investigated the effects of drying conditions on the autogenous shrinkage of ultra-high performance concrete (UHPC) at early-ages. UHPC specimens were exposed to different temperatures, namely, 10, 20 and 40°C under a relative humidity (RH) ranging from 40 to 80%. The effects of using a shrinkage-reducing admixture (SRA) and a superabsorbent polymer (SAP) as shrinkage mitigation methods were also investigated. The results show that autogenous and drying shrinkage are dependent phenomena. Assuming the validity of the conventional superposition principle between drying and autogenous shrinkage led to overestimating the actual autogenous shrinkage under drying conditions; the level of overestimation increased with decreasing RH. Both SRA and SAP were very effective in reducing autogenous shrinkage under sealed conditions. However, SRA was efficient in reducing drying shrinkage under drying conditions, while SAP was found to increase drying shrinkage. Generally, results indicate that adequate curing is essential for reducing shrinkage in UHPC even when different shrinkage mitigation methods are applied.

Materials and Structures, 2009

Moist curing improves the properties of concrete. However, shrinkages at early ages are found to increase with increased curing. The reason for this phenomenon is studied with four binders and two types of curing. The binders are comprised of Portland cement/slag blends with 0, 35, 50 and 65% of slag. Initial moist curing times of 1 and 7 days were studied. The samples were then exposed to standard drying conditions (23°C and 50% RH). During drying, the moisture losses in 7-day cured concretes were about 50% less than in 1-day cured concretes; however, the early age shrinkages were significantly higher in 7-day cured concrete. Pore size distribution tests and analyses showed that the pore radius where meniscus forms during drying is smaller in 7-day cured concrete due to finer pores, as compared to 1-day cured concrete. Further, good correlation can be seen between the meniscus radius and shrinkage, regardless of the binder and curing types. This provides the explanation for the increased early age shrinkage with increased curing. Further, this study demonstrates that the capillary tensile force is the governing mechanism for early age shrinkage.

Lecture notes in civil engineering, 2019

High Performance Concrete (HPC), particularly high strength concrete mixes (60-100MPa) containing high cementitious content and low w/b ratios (0.40-0.25) is used for some precast elements. Ultra High Performance Concrete (UHPC) up to 200MPa containing very high cementitious content and very low w/b ratios (0.25-0.17) is also used for precast and prestressed elements in buildings and bridges. Shrinkage characteristics of HPC and UHPC differ considerably from those of conventional concrete. Due to their high cementitious content and low w/b ratios, the drying shrinkage component is significantly smaller when compared with the autogenous shrinkage component. HPC and UHPC elements, when steam-cured, undergo majority of their shrinkage within a few days of casting. In this paper, mechanisms of autogenous shrinkage of HPC and UHPC at early ages are compared under different curing conditions. Contribution of autogenous shrinkage and drying shrinkage components on the total shrinkage is discussed. At very early ages, plastic shrinkage and plastic settlement cracking and factors affecting them for HPC and UHPC are also outlined.

Civil Engineering Journal

Concrete is indeed one of the most consumed construction materials all over the world. In spite of that, its behavior towards absolute volume change is still faced with uncertainties in terms of chemical and physical reactions at different stages of its life span, starting from the early time of hydration process, which depends on various factors including water/cement ratio, concrete proportioning and surrounding environmental conditions. This interest in understanding and defining the different types of shrinkage and the factors impacting each one is driven by the importance of these volumetric variations in determining the concrete permeability, which ultimately controls its durability. Many studies have shown that the total prevention of concrete from undergoing shrinkage is impractical. However, different practices have been used to control various types of shrinkage in concrete and limit its magnitude. This paper provides a detailed review of the major and latest findings rega...

Cement and Concrete Research, 2017

This experimental study imposes limited relative humidity (RH) gradients to small mature concrete samples, at a constant temperature T = 20, 50 or 80°C. Mass loss and shrinkage are recorded until stabilization at each RH and T, for up to 1991 days. Firstly, our mass loss data are consistent with those presented in former research (on different samples of the same batch). After presenting and analyzing shrinkage kinetics, experimental data are fitted with usual models for shrinkage prediction, at each temperature of 20, 50 and 80°C. An adequate match is obtained by combining capillarity (i.e. Vlahinic's model coupling poro-elastic constants and water saturation level) and desorption (Bangham's equation). Subsequently, relative mass variation (RMV) is plotted against shrinkage ε sh dry data. Three distinct phases are obtained at 20 or 50°C and down to 30%RH; up to four distinct phases are observed at T = 80°C and down to 12%RH. The latter are confirmed by experiments on (60°C; 7%RH) dried concrete. The four phases in the (RMV ε , sh dry) diagram are interpreted against shrinkage data on mature cement paste dried at 60°C; 7%RH and against the literature.

Cement and Concrete Research, 2001

This paper presents the results of an experimental study on the influence of curing temperature and type of cement [Portland cement and blast-furnace slag (BFS) cement] on the autogenous deformations and self-induced stresses in early-age concrete. It was found that higher temperatures do not lead to higher deformations in the observed period, but generally cause a faster shrinkage and a faster development of self-induced stresses. Another experimental finding is that, at the temperatures tested, concrete made with BFS cement shows higher shrinkage in the first days than concrete made with Portland cement.

Maǧallaẗ al-handasaẗ wa-al-tiknūlūǧiyā, 2021

 In UHPC, autogenous shrinkage dominates over the drying shrinkage.  High pouring temperature and coarse sand increase the number and size of pores.  Increasing silica fume over 25% of cement content reduces compressive strength.  Adding steel or basalt fibers minimize the inverse effect of autogenous shrinkage. Ultra-High Performance Concrete (UHPC) is a new generation of concrete characterized by its high strength, high durability, and high stiffness. Autogenously shrinkage represents one of the issues of UHPC that occurred at early ages. It occurs particularly during the first 48 hours after casting. This paper focuses on the ways that can be depended on to mitigate the autogenously shrinkage and obtain the outstanding mechanical properties of UHPC. The results showed that the use of coarse sand and high dose of high range water reduced the admixture above 5% of cementations of materials weight, and high ambient temperature at the time of mixing and casting led to increasing the autogenously shrinkage. While using fine sand, silica fume at 25% of cement weight, and crushed ice at 50% of mixing water to control the mixing temperature can reduce autogenously shrinkage significantly.

IRJET, 2022

Through its lifespan, concrete undergoes several physical and chemical changes, which normally led to shrinkage of concrete, especially at an early age, when the initial hydration processes take place. The shrinkage of concrete at an early stage of hardening may lead to the initial formation of cracks that vary in shape and size and depends on the concrete constituents and surrounding conditions, including temperature and/or the moisture state that may lead to volumetric deformation Many studies have shown that the total prevention of concrete from undergoing shrinkage is impractical. However, different practices have been used to control various types of shrinkage in concrete and limit its magnitude. This paper provides the shrinkage behavior of cement concrete incorporated with different mineral admixtures and fibers.