Effect of curing methods on strength and durability of concrete under hot weather conditions

This paper reports the results of a research study conducted to evaluate the effect of curing methods on the mechanical properties of ordinary Portland cement (OPC) and Silica Fume Cement (SFC) concretes. Slab and beam specimens were prepared and cured by covering them with wet burlap or applying a curing compound under field conditions. Four types of curing compounds, namely water-, acrylic-, and bitumen-based and coal tar epoxy, were applied on the concrete specimens. The curing compounds were applied immediately after casting or after an initial period of burlap curing. The effect of the selected curing regime on the properties of OPC and SFC concrete specimens was evaluated by measuring compressive strength, water-absorption and chloride permeability. The strength and durability characteristics of both OPC and SFC concrete specimens cured by applying the selected curing compounds were similar or better than that of concrete specimens cured by covering with wet burlap. Though no significant change in strength could be noted due to the curing methodology; however, its effect was noticeable on the durability. The best performance was shown by concrete specimens cured by applying the bitumen-based curing compound followed by those cured by applying coal tar epoxy, acrylic-based or water-based curing compound. The initial period of water curing, prior to the application of the curing compound, was also noted to be beneficial in increasing the durability of concrete.     

A study on durability properties of high-performance concretes incorporating high replacement levels of slag

This paper presents an experimental study of combined effects of curing method and high replacement levels of blast furnace slag on the mechanical and durability properties of high performance concrete. Two different curing methods were simulated as follows: wet cured (in water) and air cured (at 20°C and 65% RH). The concretes with slag were produced by partial substitution of cement with slag at varying amounts of 50–80%. The water to cementitious material ratio was maintained at 0.40 for all mixes. Properties that include compressive and splitting tensile strengths, water absorption by total immersion and by capillary rise, chloride penetration, and resistance of concrete against damage due to corrosion of the embedded reinforcement were measured at different ages up to 90 days. It was found that the incorporation of slag at 50% and above-replacement levels caused a reduction in strength, especially for the early age of air cured specimens. However, the strength increases with the presence of slag up to 60% replacement for the 90 day wet cured specimens. Test results also indicated that curing condition and replacement level had significant effects on the durability characteristics; in particular the most prominent effects were observed on slag blended cement concrete, which performed extremely well when the amount of slag used in the mixture increased up to 80%.     

Curing and the durability of OPC,fly ash and blast-furnace slag concretes

This paper presents preliminary results of a research project into the influence of moist curing on the potential durability of concrete. Durability is characterized by measuring the oxygen permeability and water absorption at various depths in the covercrete. Concretes containing plain OPC, an OPC-FA blend and an OPC-GGBS blend were used. A range of strength grades was tested for each of these binder types. Concretes were exposed to moist curing conditions for 1, 3, 7 and 28 days before being tested at 28 days after casting. The main conclusions are: (i) moist curing has a marked influence on the potential durability of concrete and (ii) a relatively greater influence on durability can be effected by extending the duration of early-age moist curing rather than decreasing the binder/water ratio.     

Shrinkage of plain and silica fume cement concrete under hot weather

Supplementary cementing materials (SCMs) are widely used these days to improve the durability of concrete. Silica fume has gained world wide acceptance due to its high pozzolanic reactivity compared to other SCMs. While silica fume cement concrete has several advantages over other blended cement concretes its main draw back is increased plastic and drying shrinkage, particularly under hot weather conditions. This paper reports results of a study conducted to assess these properties of plain and silica fume cement concrete specimens cast and cured in the field under hot weather conditions. The effect of specimen size and method of curing on plastic and drying shrinkage and some of the mechanical properties of silica fume and plain cement concrete specimens were evaluated. Results indicated that the type of cement significantly affected both the plastic and drying shrinkage of concrete in that these values in the silica fume cement concrete specimens were more than those in the plain cement concrete specimens. As expected, the shrinkage strains in both the plain and silica fume cement concrete specimens cured by continuous water-ponding were less than that in similar concrete specimens cured by covering them with wet burlap. The results point to the importance of selecting a good quality silica fume and good curing for avoiding cracking of concrete due to plastic and drying shrinkage, particularly under hot weather conditions.     

Durability of lightweight OPS concrete under different curing conditions

The use of waste materials and by products from different industries for building construction has been gaining increased attention due to the rapid depletion of natural resources. It has been found that oil palm shell (OPS), which is a waste from the agricultural sector, can be used as coarse aggregate for the manufacture of structural lightweight concrete. However, for OPS concrete to be used in practical applications, its durability needs to be investigated. Therefore, this paper presents the durability performance of OPS concrete under four curing regimes. The durability properties investigated include the volume of permeable voids (VPVs), sorptivity, water permeability, chloride diffusion coefficient and time to corrosion initiation from the 90-day salt ponding test, and Rapid Chloride Penetrability Test (RCPT). Results showed that the durability properties of OPS concrete were comparable to that of other conventional lightweight concretes and proper curing is essential for OPS concrete to achieve better durability especially at the later ages.     

Mechanism of early age shrinkage of concretes

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. The work presented in this paper was carried out when Tarek Aly was a research student at the Department of Civil Engineering, Monash University.     

The effect of realistic curing temperature on the strength and E-modulus of concrete

The strength and E-modulus of concrete are decisive parameters when it comes to ultimate limit state design, serviceability limit state design, and early age crack assessment. The properties of concrete are generally determined in the laboratory under 20 °C isothermal conditions and then used as the basis for calculations under realistic temperature conditions. It is well-known, however, that the curing temperature affects both the rate of property development in concrete and the “final value” of a given property. The current study investigated the effect of a realistic temperature history on the compressive cube strength, the tensile strength, and the tensile E-modulus for two concretes, a reference concrete and a fly ash concrete. Concrete specimens were subjected to either (1) 20 °C isothermal curing conditions, or (2) realistic temperature curing conditions for 14 days and then 20 °C isothermal conditions, until they were tested after 28 and 91 days. Parallel tests performed in a Temperature-Stress Testing Machine were also used to evaluate the results. The reference concrete showed a general reduction in strength and E-modulus when subjected to a realistic curing temperature, whereas the fly ash concrete showed an 11% increase in the 28-day E-modulus when cured under realistic temperature conditions. Furthermore, in both isothermal and realistic curing temperature conditions, the fly ash concrete showed a pronounced property development beyond 28 days, which could not be described by the material model currently used.     

Mechanical and durability performance of sustainable structural concretes: An experimental study

This study reports the results of a wide experimental campaign intended at investigating the mechanical and durability performance of structural concretes made with Recycled Concrete Aggregates (RCAs) and coal Fly Ash (FA). To this end, twelve mixtures were designed by replacing part of the ordinary constituents (i.e. cement, sand and coarse aggregates) of a reference one with RCAs and FA. Samples of these mixtures were subjected to various tests aimed at assessing both their structural properties and durability performance. As for the former, time evolution of compressive strength was monitored at various curing times up to 365 days, and the splitting strength was determined at 28 days. Moreover, the expected durability performance of the aforementioned concrete mixtures was scrutinised by measuring some relevant physical quantities, such as water permeability, carbonation depth and chloride-ions ingress at various curing ages.The results obtained from these tests are often not self-evident, as they unveil the synergistic effect of combining both RCAs and FA on the resulting physical and mechanical properties of “green” concrete. Moreover, they demonstrate that the current code restrictions on the use of both RCAs and FA for structural concrete might be significantly relaxed, especially if the delayed binder effect, induced by the latter, is duly taken into account and, hence, concrete properties are measured at curing times longer than the conventional 28 days.     

Accelerated strength and testing of concrete in Jordan

Accelerated strength testing using the boiling water procedure of ASTM C 684 was performed to evaluate this test method for use in the routine quality control of concrete made of local materials, and in the prediction of potential quality and strength of concrete at later ages. 190 groups of standard concrete specimens were sampled: in each group two cylinders represented the accelerated strength and were tested at 28.5 h, while the other two were normally cured and tested at 28 days. Test results were recorded and statistically evaluated. A computer program was developed to carry out the numerical statistical computations and regression analysis. Correlations between the 28-day compressive strength and the corresponding accelerated strength were established, taking account of the utilization of local materials and practices. The outcome of this study confirms that accelerated strength testing can do everything the 28-day test can do but 27 days sooner. The conclusions derived provide experimental evidence in favour of implementing the standard boiling water method for use in relation to quality control and prediction of concrete strength at later ages.     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

Influence of curing regimes on compressive strength of ultra high performance concrete

The present paper is aimed to identify an efficient curing regime for ultra high performance concrete (UHPC), to achieve a target compressive strength more than 150 MPa, using indigenous materials. The thermal regime plays a vital role due to the limited fineness of ingredients and low water/binder ratio. By activation of the reaction kinetics, the effectiveness of the binder is enhanced which leads to improvements in mechanical as well as durability properties. The curing cycle employed are ambient air curing, water curing and hot air curing. The specimens were exposed to thermal regime at (90°C/150°C/200°C) for duration of 24, 48 or 72 hours at the age of 3rd and 7th day followed with air curing or water curing till 28 days. The results showed a marked difference in compressive strength ranging from 217 to 142 MPa with change in curing regimes. The samples when thermally cured at the age of 3rd and 7th day produced an average ultimate strength of 217–152 MPa and 196–150 MPa, respectively.     

Effect of cement type and limestone particle size on the durability of steam cured self-consolidating concrete

This paper describes a laboratory program to investigate the influence of cement and limestone filler (LF) particle size on the hardened properties and durability performance of steam cured self-consolidating concrete. In addition, the interplay between cement type and LF particle size was investigated. CSA (Canadian Standards Association) Type GU (General Use) and HE (High Early-strength) cements were used with 5% silica fume (SF) [1]. The water-to-cement ratio was 0.34. LF with two nominal particle sizes of 17 μm and 3 μm, which correspond to Blaine fineness of 475 and 1125 m²/kg, respectively, were used. In addition to fresh concrete properties, hardened properties including compressive strength, elastic modulus, ultrasonic pulse velocity and density were measured at 12 h and 16 h, and at 3, 7 and 28 days. Indicators of durability performance including rapid chloride permeability testing (RCPT), sulfate resistance, linear shrinkage, salt scaling resistance and freeze-thaw resistance were evaluated. The results showed that LF improved the 12 and 16-h strength with no influence on later age strength (i.e., 3–28 days). The linear shrinkage and RCPT decreased with the addition of LF. This reduction was linked to the production of calcium mono-carboaluminate. LF did not impact the sulfate resistance, salt scaling resistance or freeze-thaw resistance of concrete.     

Fly ash concrete-potential without misuse

This paper presents a comprehensive review of the engineering properties of fly ash concrete in its fresh and hardened form. It is shown that two major factors influence the properties of fly ash concrete—its intrinsic variation in composition, and the philosophy of mix proportioning adopted to produce the concrete. The effects of ash variability can be covered by three basic criteria of mix proportioning—namely, low water-cementitious ratio, inclusion of a water-reducing plasticizer, and an early and long curing period. It is then shown that these criteria can be utilized to produce concrete containing 50% ash replacement and having 40 to 60 MPa 28-day strength, with high workability, low water/cementitious ratios of 0.30 to 0.45, and one-day strengths of 10 to 20 MPa. Properties of strength, elasticity, shrinkage and creep are reported, and durability aspects briefly discussed. The paper unfolds the secret of utilizing the full pozzolanic potential of fly ash in practice.     

Accelerated strength and testing of concrete using blended cement

Accelerated strength testing using the boiling water procedure of ASTM C 684 was performed to evaluate this test method for use in the routine quality control of concrete made of local materials with particular emphasis on the use of blended cements, and in the prediction of potential quality and strength of concrete at later ages. Large number of groups of standard concrete specimens are sampled; in each group one cube represented the accelerated strength and is tested at 28.5 h while the other one is normally cured and tested at 28 days. Test results were recorded and statistically evaluated. A computer program was developed to carry out the numerical statistical computations and regression analysis. Correlation between the 28-day compressive strength and the corresponding accelerated strength was established considering the utilization of local materials and practices. The outcome of this study in the form of prediction models confirm that accelerated strength testing could be accepted in lieu of the standard 28-day testing. The conclusions derived may provide experimental evidence in favor of implementing the standard boiling water method for use in relation to quality control and prediction of concrete strength at later ages. Advanced Cement Based Materials 1997, 5, 49–56.     

Influence of industrially produced recycled aggregates on flow properties of concrete

This research aims at evaluating the main risks for the durability of concrete made of industrially produced recycled aggregates called Recycled Aggregate Concrete (RAC). A characterisation of recycled aggregates is performed and their peculiarities are highlighted. A comparison between the behaviour of RAC and that of ordinary natural aggregate concrete is carried out. The influence of both the composition and the curing conditions is discussed. The durability study is focused on the assessment of parameters representing the porous structure and concrete characteristics. Because of the high total water/cement ratio of RAC, their flow properties control their durability. It is established that RAC are characterised by significantly higher water absorption and air permeability. The diffusion of the carbon dioxide is faster, too. That leads to a weaker resistance of RAC to environmental attacks. Since the main durability problems are caused by the fine recycled fraction, its use needs to be restricted. Another way to increase RAC durability seems to be the extended curing in wet environment.     

A study on reinforcement corrosion and related properties of plain and blended cement concretes under different curing conditions

This paper presents the results of an experimental investigation on the steel reinforcement corrosion, electrical resistivity, and compressive strength of concretes. Concretes having two different water–cement ratios (0.65 and 0.45) and two different cement contents (300 and 400 kg/m³) were produced by using a plain and four different blended portland cements. Concrete specimens were subjected to three different curing procedures (uncontrolled, controlled, and wet curing). The effect of using plain or blended cements on the resistance of concrete against damage caused by corrosion of the embedded reinforcement has been investigated using an accelerated impressed voltage setup. The resistivity of the cover concrete has been measured non-destructively by placing electrodes on concrete surface. The compressive strength, electrical resistivity, and corrosion resistance of the concretes were determined at different ages up to 180 days. The results of the tests indicated that the wet curing was essential to achieve higher strength and durability characteristics for both plain and especially blended cement concretes. The concretes, which received inadequate (uncontrolled) curing, exhibited poor performance in terms of strength and corrosion resistance.     

Freeze/thaw durability of concrete with synthetic fibre additions

Concrete can be affected by freeze/thaw damage from the point of placing to being fully cured. The lack of available test data in the early life performance of concrete, with polypropylene fibre additions was a key factor justifying this research.This work examines the effects of freeze/thaw cycles starting at 5 days of curing where the concrete has reached about half of the design strength. The test methods used to evaluate durability were weight loss, final compressive strength and relative pulse velocity.A freeze/thaw test was carried to ASTM 666 B for 300 cycles and enhanced freeze/thaw protection was observed by the use of Type 1 polypropylene fibres in concrete when compared to plain and air entrained concrete.Fibres had the ability to entrain air and this is believed to be part of the reason for the demonstrated improvement in freeze/thaw durability.     

Artificial neural networks for prediction of percentage of water absorption of geopolymers produced by waste ashes

In the present work, percentage of water absorption of geopolymers made from seeded fly ash and rice husk bark ash has been predicted by artificial neural networks. Different specimens, made from a mixture of fly ash and rice husk bark ash in fine and coarse form together with alkali activator made of water glass and NaOH solution, were subjected to permeability tests at 7 and 28 days of curing. The curing regime was different: one set cured at room temperature until reaching to 7 and 28 days and the other sets were oven cured for 36 h at a range of 40–90 °C and then cured at room temperature for 7 and 28 days. To build the model, training and testing using experimental results from 120 specimens were conducted. According to these input parameters, in the neural networks model, the percentage of water absorption of each specimen was predicted. The training and testing results in the neural networks model have shown a strong potential for predicting the percentage of water absorption of the geopolymer specimens.     

Accelerated microwave curing of concrete: A design and performance-related experiments

Microwave (MW)-accelerated curing has emerged as an innovative and popular curing method for concrete materials. This paper reports the results of a study to model the horn antenna used for the MW irradiation of a workpiece with a mobile MW-accelerated concrete curing unit, based on a coupled thermal and electromagnetic analysis. The mathematical models were useful for evaluating the heat generation within a horn antenna and as a basis for constructing a mobile MW-accelerated curing unit with an operating frequency of 2.45 GHz and a MW power level of 800 W. Further, the early-age compressive strength development and volume stability of MW-cured concrete were investigated in terms of its shrinkage and compared to the properties of autoclave-cured concrete. The design results showed that under the concept of the allowable maximum temperature for the concrete workpiece, which was controlled to less than 80 °C, a horn antenna that was 216.70 mm wide, 333.68 mm long, and 273.0 mm high produced a uniform thermal distribution in a concrete workpiece. Moreover, experimental investigations showed that the application period for curing using a mobile MW-curing unit was considerably shorter than that in autoclave curing methods. The appropriate delay time (time after concrete mixing) was 30 min, and MW irradiation for 45 min could improve the maximum 8-h early-age compressive strength of MW-cured concrete, whereas an application time of 15 min produced the 28-day compressive strength. When a concrete workpiece was cured at high temperature using MW energy for more than 15 min at a temperature greater than 80 °C, the effect was a continuous increase in the early-age compressive strength, which was greater than that achieved by autoclave curing. In terms of volumetric stability, MW-curing for 30 and 45 min increased the ultimate shrinkage to a greater extent than that by autoclave curing and vice versa in the case of a MW application time of 15 min.     

The effect of curing period and curing delay on concrete in hot weather

The effect of curing period and of delay in carrying out curing by the wet burlap method on some properties of concrete with different mix proportions (cement content, water content) in hot weather was studied. The results show that a minimum of 3 days curing was sufficient for rich mixes, while a longer period was required for leaner mixes (min. 7 days). The delay of curing had a harmful effect on concrete and the first day of delay caused the largest effect. Curing after delaying increased the compressive strength of concrete, but it did not recover the reduction in strength caused by the curing delay.