Strategic reinforcement for controlling volume-change cracking in base-restrained concrete walls

In this work the control of volume-change cracking in concrete walls with continuous base restrains was considered. Investigators and design codes have proposed several different procedures for the calculation of the amount and distribution of steel reinforcement required for the control of this type of cracking. All these methods provide amounts of reinforcement which are in excess to that really required. This is either due to simplifying the design procedure or neglecting the effect of variable restraint in the wall. Since crack formation and its width depends on the amount of total restrained movement in th concrete member, steel reinforcement is necessary only in positions in which wide cracks are expected to form. Using finite-element analysis to obtain the distribution of restraint in the wall, degree-of-restraint contour diagrams in walls with different length/height ratios of 1, 2, 3, 4, 6 and 8 were prepared. These diagrams can be used to determine the amount and distribution of steel reinsforcement in positions in which it is really effective in controlling cracking. At positions of low restraint in the walls in which no or only narrow cracks will form, only minimum reinforcement will be provided.     

Crack development in concrete structures due to imposed strains—Part II: Parametric study of a wall fully restrained at the base

Crack development due to imposed strains in concrete walls fully restrained at the base is studied in order to improve control of cracking. In this second paper the influence of reinforcement and concrete properties as well as geometry on crack width and crack spacing is studied. In a first paper a two-dimensional FE-method is described, with closing forces in cracks concentrated to spring elements. Temperatures changes are used as load and the calculations are performed stepwise with opening of nodes and implementation of spring elements. It is shown that the two-dimensional behaviour of the wall only gives about half the crack widths compared to a one-dimensional bar with the same percentage of reinforcement. The reason is that the restraint along the base will effectively facilitate distribution of cracking along the wall. The two-dimensional analysis shows that the crack widths are limited also by low reinforcement ratios. For the same reinforcement ratio the crack width will increase with tensile strength of the concrete. The geometry of the wall has very little influence on the cracking behaviour unless the wall is very short. Bond stiffness and bar diameter have a limited effect on the crack width in the wall.     

Effect of reinforcement on plastic shrinkage and settlement of self-consolidating concrete as repair material

The experimental investigation on plastic shrinkage and plastic settlement for different self-consolidating concrete (SCC) mixtures as repairing materials is presented. The concrete mixtures were placed on the surface of the concrete substrate slabs at different restraint degrees. Four different types of repairing materials such as plain SCC, SCC with silica fume (SF), SCC with latex and SF, and SCC with latex, SF and fiber were evaluated. The slabs included both reinforcement and without reinforcing bars. The tests involved measurement of concrete bleeding and evaporation rates, steel bar strains and crack characteristics. The results indicated that bleeding rate is not the only controlling factor, but restraint condition, configuration of steel bars and the concrete strain capacity are also affect plastic shrinkage and settlement behaviors significantly. Latex and fiber were found to be effective in reduction of cracks and concrete strains. Cracks did not develop on the surface of concrete containing latex and fiber. The results showed that if a slab is reinforced by one single bar (in each direction of the slab), at relatively large diameter as used in this study, it would not improve the plastic shrinkage behavior, although it causes plastic settlement cracking.     

Digital Imaging Methodology for Measuring Early Shrinkage Cracking in Concrete

The measurement of early shrinkage cracking in concrete is important to prevent aesthetic issues and avoid surface cracking that could lead to reinforcement corrosion and reduce the durability, long‐term service life and integrity of a structure. Moreover, the lack of standards and subjectivity of the very few methodologies proposed so far complicate its estimation. This research presents a new imaging methodology for evaluating and quantifying early shrinkage cracking patterns. The methodology was developed testing highly restrained square concrete slabs subjected to severe conditions of restraint and moisture loss. Its quantification consisted of photographing, processing the pictures and highlighting the cracks. For the first time, early shrinkage cracking in concrete can be measured through an experimental technique and quantified by means of geometric figures. In this way, more precise and automatic results are achieved, as flat figures adapt to the shape of cracks and store their properties. Therefore, parameters such as the total cracked area, total crack length, maximum crack width or average crack width were easily calculated. The results demonstrated the suitability of the wind tunnel test to produce significant cracking patterns, as well as the great capacity of the imaging methodology to identify and characterize the cracking pattern.     

Prediction of Interfacial Cracking due to Differential Drying Shrinkage of Concrete in Precast Shell Pier Cap

In a precast shell pier cap, cracking at the interface between the precast shell and the cast-in-place concrete may happen due to differences between the drying shrinkage of the inner and the outer concrete. The objective of this study is to establish a prediction method for interfacial cracking that will consider the real mechanism of differential drying shrinkage and creep. The main parameters used in the analysis were determined from experiments for a concrete mix that is applied to the manufacturing of pier caps. The variation of internal relative humidity over time was first calculated based on the nonlinear moisture diffusion; cracking analysis then followed. Prediction of the initiation of interface cracks and the increase of their width over time was performed. It was found that additional reinforcement across the interface is very effective at reducing crack width.     

Crack development in concrete structures due to imposed strains—Part I: Modelling

In the present paper, a technique for the modelling of crack development in reinforced concrete structures exposed to imposed deformations is described. In a second paper, parametric studies are performed for a wall fully restrained at the base. The objective of this research is to improve the control of cracking in engineering design. A two-dimensional Finite Element model with fournode elements is used to simulate concrete. Closing forces in cracks are modelled with spring elements. The spring stiffness is estimated from bond stress—slip relations for reinforcement and tension softening of concrete. Yield of reinforcement is also included in the model. Temperature change is used as load and the calculations are performed stepwise with opening of nodes and implementation of spring elements.     

Influence of basalt fibres on free and restrained plastic shrinkage

Early-age cracking due to plastic shrinkage is often attributed to reducing the durability of concrete structures. The objective of this paper is to evaluate the potential use of chopped basalt fibres in preventing these cracks. Testing was undertaken to measure the magnitude of shrinkage strain that develops in unrestrained specimens, and the severity of cracking that occurs when shrinkage is restrained. Results indicate basalt fibres are effective in preventing cracks by reducing the magnitude of free shrinkage, and by restricting the growth of cracks if they do occur. The latter mechanism is more prominent when the w/c ratio is decreased.     

Thermally induced softening and cracking in concrete with minimal reinforcement

This paper deals with modeling of softening and cracking induced by non-uniform cooling or shrinkage of reinforced concrete. Concrete and reinforcement are described by a set of constitutive equations and cooling is assumed to be a linear thermal diffusion process. The investigated structure is a medium-thick wall with external restraint modeled by springs. The main purpose of the paper is to study the role and the amount of the socalled minimum reinforcement necessary to control cracking. Another purpose is to find the force actually arising in the wall during cooling and also to compute the range of softening and cracking of the concrete. Sensitivity of the results is studied in a parametric investigation.     

Plastic shrinkage cracking in cementitious repairs and overlays

A study was undertaken to investigate the influence of mixture proportions on plastic shrinkage cracking in cementitious repairs and overlays. The following variables were studied: water-cement ratio (w/c), sand-cement ratio (s/c) aggregate-cement ratio (a/c), fly-ash content, and the presence of a shrinkage reducing admixture (SRA). A recently developed bonded overlay method was used. In this method, the cementitious material to be investigated is cast on a substrate base with protuberances and the entire assembly is subjected to drying in an environmental chamber. With a high degree of restraint, cracking develops in the overlay over time which is then characterized with the help of magnification device and image analysis software. Results indicate that a lower water-cement ratio significantly reduces crack widths and crack areas and also reduces the ‘time to first crack’. The exact influence of s/c on cracking could not be established. The study further demonstrated that increasing the a/c provided internal restraint and significantly reduced early-age cracking. The addition of Class C fly-ash increased the amount of cracking but also increased the ‘time to first crack.’ This was however true only to a certain threshold value of fly-ash content beyond which fly-ash can be effective in reducing early age shrinkage cracking. Finally, the addition of a shrinkage reducing admixture was effective in reducing cracking but the cracks occurred sooner.     

Stress distribution and crack formation in full-scaled ultra-high strength concrete columns

Full-scaled model columns were placed both in summer and winter with an ultra-high strength concrete with a compressive strength more than 150?MPa, and stress distribution and cracking were experimentally evaluated. Specimens placed in summer exhibited cracks around the steel reinforcements which sometimes joined together. Specimens placed in winter showed, in addition to the cracks around the reinforcement, an internal crack perpendicular to the column axis as well as at the specimen surface. These cracks were found to be dependent on the temperature history due to hydration heat liberation and associated autogenous shrinkage strains. It was shown that the autogenous shrinkage of concrete increased when temperature after mixing was low and the maximum temperature during temperature history was high. This accounts for the numerous cracks found in the specimen placed in winter. Strain perpendicular to the axial direction was smaller than that of the axial direction implying the tensile stress due to autogenous shrinkage acting perpendicular to the axial direction as far as the autogenous shrinkage is isotropic. Finite element analysis confirmed the lateral stress due to autogenous shrinkage. Possible influences of autogenous shrinkage of ultra-high strength concrete on the structural performance include (1) early spalling of cover concrete and degradation of flexural strength, (2) degradation of bond and shear strength, and (3) prospect of longitudinal crack in center of the column and degradation of flexural strength.     

Experimental investigation of drying shrinkage cracking of composite mortars incorporating crushed tile fine aggregate

Drying shrinkage is generally classified as an important hardened concrete property. It expresses the strain occurring in hardened concrete due to the loss of water. During the drying process, free and absorbed water is lost from the concrete. When the drying shrinkage is restrained, cracks can occur, depending on the internal stresses in the concrete. The ingress of deleterious materials through these cracks can cause decrease in the compressive strength and the durability of concrete. In this study, being as a fine aggregate in mortars, crushed tile (CT) effect on drying shrinkage and drying shrinkage cracking is investigated. Thus, compressive and flexural strength, modulus of elasticity, and free and restrained drying shrinkage tests are conducted on mortar specimens produced with and without crushed tile fine aggregate. The ring test has been used in order to investigate the cracks induced by restrained drying shrinkage. In this way, free drying shrinkage strain, along with the number and development of drying shrinkage cracks, of the crushed tile fine aggregate mortar composites are quantified and observed.     

水工混凝土弥散型裂缝数值模型中开裂判据的研究

重大水利工程中,混凝土结构多处于高水压环境,坝体或多或少存在着裂缝,然而大部分开裂的混凝土坝仍能够安全运行,从而引申出是否可以允许混凝土结构出现裂缝,多大的裂缝才不至于引起破坏,以及用怎样一个合理的指标来对混凝土开裂程度进行描述评判的问题。该文以等效塑性应变表征混凝土弥散型裂缝模型数值模拟中的开裂破坏,通过混凝土单轴拉伸数值模拟,并根据试验资料及相关文献,建立了等效塑性应变与主拉应变、裂缝宽度之间的关系,得到对应于“有害裂缝”的等效塑性应变值。将方法和成果运用到混凝土重力坝的开裂研究中,具有较好的实际工程意义。     

A study of the behaviour of volume change cracking in base restrained concrete walls

The present work studies the problem of cracking due to volume change of base restrained reinforced concrete walls. The cracking behaviour of some 61 full size walls and 14 experimental walls was investigated. The observed primary and secondary crack spacings and widths were compared with the values obtained using recently developed formulas and with formulas developed previously. Good agreement was found between the observed values and those predicted using the developed formulas. On the practical side, the results showed clearly that crack spacing and, consequently, the crack width increased with increase of wall height and, therefore, a higher percentage of reinforcement or closer joints are required for their control. Furthermore, crack width was not uniform with the wall height, but varied according to the change of restraint associated with cracking and, therefore, the percentage of reinforcement may be varied with the wall height to obtain approximately uniform crack widths. This may lead to savings in reinforcement cost.     

A structural experimental technique to characterize the viscoelastic behavior of concrete under restrained deformations

An innovative variable restraint frame is proposed to characterize the viscoelastic behavior of concrete under tensile stresses induced by restraints to shrinkage deformations (mainly due to drying). Two concrete specimens with the same cross section are used, subjected to equal thermal and moisture conditions: one is made of plain concrete, to assess the “free” deformations due to shrinkage and temperature; the other is reinforced with two steel threaded rods, which induce a manually controlled axial restraint to shrinkage. The restrained specimen is installed on a reaction frame, being stretched in force control mode. The concrete and the rods are instrumented with strain gauges and temperature sensors, which allow separation of the different components of concrete strains with the aid of equations based on equilibrium and compatibility conditions. This permits identifying the elastic and tensile creep concrete strains, as well as the concrete tensile stresses induced by the restrained shrinkage. The device also allows assessing the concrete modulus of elasticity during the test and remains operational even upon concrete cracking, features of great interest for the intended material characterization.     

克裂速纤维增强混凝土抗裂性能

用聚丙烯纤维来防止混凝土的早期塑性收缩裂缝是近年来为解决混凝土裂缝难题而采取的新措施。研究了两种聚丙烯纤维(Cemfiber和DF)的掺量、纤维种类等参数对塑性收缩裂缝的影响规律;分别采用圆形、平板状试件来研究砂浆、普通混凝土和高性能混凝土的抗裂性能。研究结果表明:(1) 聚丙烯纤维可以显著提高混凝土抗裂能力,纤维掺量越高,抗裂能力越强;(2) 为防止裂缝,应该尽可能降低水泥用量和提高骨料用量;(3) 聚丙烯纤维提高混凝土抗裂能力的主要原因是纤维提高混凝土的早期应变能力、减小收缩应变、提高塑性抗拉强度和减小毛细管的表面张力。     

Modeling damage in concrete caused by corrosion of reinforcement: coupled 3D FE model

Reinforced concrete structures, which are exposed to aggressive environmental conditions, such as structures close to the sea or highway bridges and garages exposed to de-icing salts, often exhibit damage due to corrosion. Damage is usually manifested in the form of cracking and spalling of concrete cover caused by expansion of corrosion products around reinforcement. The reparation of corroded structure is related with relatively high direct and indirect costs. Therefore, it is of great importance to have a model, which is able to realistically predict influence of corrosion on the safety and durability of reinforced concrete structures. In the present contribution a 3D chemo-hygro-thermo-mechanical model for concrete is presented. In the model the interaction between non-mechanical influences (distribution of temperature, humidity, oxygen, chloride and rust) and mechanical properties of concrete (damage), is accounted for. The mechanical part of the model is based on the microplane model. It has recently been shown that the model is able to realistically describe the processes before and after depassivation of reinforcement and that it correctly accounts for the interaction between mechanical (damage) and non-mechanical processes in concrete. In the present paper application of the model is illustrated on two numerical examples. The first demonstrates the influence of expansion of corrosion products on damage of the beam specimen in cases with and without accounting for the transport of rust through cracks. It is shown that the transport of corrosion products through cracks can significantly influence the corrosion induced damage. In the second example the numerically predicted crack patterns due to corrosion of reinforcement in a beam are compared with experimental results. The influence of the anode?Ccathode regions on the corrosion induced damage is investigated. The comparison between numerical results and experimental evidence shows that the model is able to realistically predict experimentally observed crack pattern and that the position of anode and cathode strongly influences the crack pattern and corrosion rate.     

The effects of fibres on the plastic shrinkage cracking of high strength concrete

Plastic shrinkage cracking in concrete usually occurs during the first 5 hours after placing and therefore the mechanics of fibre reinforcement were studied during this period. Two types of polypropylene fibres were mixed at 0.1% by volume. The development of bond strengths and the stresses which the fibres could sustain across cracks were measured by uniaxial tensile tests during the first 5 hours after mixing. Fibre stresses up to 130 MPa at 5 hours were achieved which were equivalent to a composite post-crack strength of 65 kPa. Restrained ring tests were used to assess the amount of cracking which occurred during the first 24 hours in a different high strength mix and, although the results were very variable, the fibres were found to reduce the crack area by between 40% and 85% compared with plain concrete, depending on fibre type.     

超高性能混凝土圆环约束收缩试验研究

以约束水平、环境条件（密闭或干燥）和钢纤维等为参数,开展了超高性能混凝土（UHPC）圆环约束收缩试验。研究了钢环应变随龄期的发展规律;分析了各参数对圆环约束下的残余应力与各关键龄期的力学性能的影响;采用了拉应力水平和应力松弛率来评价UHPC的开裂性能。为配合圆环收缩试验,开展了自由收缩与基本力学性能试验。试验表明,未掺钢纤维的UHPC早期开裂风险大,在14 d前均发生开裂,裂缝平均宽度大于0.25 mm,含钢纤维试件均未开裂。不同约束程度对拉应力水平与应力松弛率的影响均显著,降低约束程度能有效降低开裂风险。与自由收缩测试结果不同,圆环约束UHPC在密闭条件下后期的开裂风险会高于环向干燥条件。建议以密闭条件下14 d的抗裂性能作为控制指标评价圆环约束下UHPC的开裂性能。     

Numerical simulation of reinforced concrete beams with different shear reinforcements under dynamic impact loads

The behavior of concrete/reinforced concrete structures is strongly influenced by the loading rate. Reinforced concrete structural members subjected to impact loads behave quite differently as compared to the same subjected to quasi-static loading. This difference is attributed to the strain-rate influence on strength, stiffness, and ductility as well as to the activation of inertia forces. These influences are clearly demonstrated in experiments. Moreover, for concrete structures, which exhibit damage and fracture phenomena, the failure mode and cracking pattern depend significantly on loading rate. In general, there is a tendency that with the increase of loading rate the failure mode changes from mode-I to mixed mode. Furthermore, theoretical and experimental investigations indicate that after the crack reaches critical speed of propagation there is crack branching. The present paper focuses on 3D finite-element study of reinforced concrete beams with different amount of shear reinforcement under impact. The experiments reported in literature are numerically simulated using the rate sensitive microplane model as constitutive law for concrete, while the strain-rate influence is captured by the activation energy theory. Inertia forces are implicitly accounted for through dynamic finite element analysis. However, the impact was modeled not by explicit modeling of two bodies but by incrementing the load point displacement till the maximum value and at the rate reported from the test. The results of the numerical study show that the numerical analysis using the procedure followed in this work can very well simulate the impact behavior of reinforced concrete beams. The static and dynamic reactions, crack patterns and failure modes as predicted in analysis are in close agreement with their experimentally observed counterparts. It was concluded that under impact loads, of the order as simulated in this work (blunt impact with velocity of around 1 m/s), the shear reinforcement does not get activated and therefore the dynamic reactions, unlike static reactions, are almost independent of the amount of shear reinforcement in the beams. However, the presence of shear reinforcement significantly affects the crack pattern and the cracks are well distributed in the presence of shear reinforcement, thus avoiding the formation of shear plugs.     

Analysis technique for restrained shrinkage of concrete containing chlorides

Cracks in concrete containing chlorides easily occur due to restraint conditions and they can be the main reasons of durability and safety issues. In this paper, analysis technique which can handle mixed chloride and its effect on restrained drying shrinkage is proposed. For the evaluation of stress development and cracking time due to restrained drying shrinkage, free and restrained drying shrinkage test are carried out for concrete specimens containing different sodium chloride (NaCl) content. The results show that mixed chloride content increases restraint stress but does not increase strength. Considering the effect of chloride on shrinkage based on the test results, effective restraint stress development and cracking of concrete specimens containing different level of chloride are evaluated through utilizing previously developed models for behaviors in early-age concrete like hydration and moisture transport. The results from this proposed technique are verified by comparison with test results.