INTERPRETATION OF CONCRETE MIX DESIGNS BY SURFACE HARDNESS METHOD

Structural health monitoring is an indispensable procedure that is to be carried out to evaluate the serviceability of existing structures. Non-destructive testing methods are attaining increasing popularity for the assessment of concrete strength due to the ease of operation and reliability of the results. The application of machine learning in the field of engineering has increased rampantly. In this study, the compressive strength of concrete has been forecasted using support vector regression which is a machine learning technique. Artificial intelligence is nothing but the potential to impersonate human intelligence. The advantage of artificial intelligence over human intelligence is the absence of human errors that can arise due to various factors which might decrease the accuracy of results. Rebound hammer (RBH), Windsor probe penetration (WPP) and ultra-sonic pulse velocity(USV) are the non-destructive testing that has been employed to assess the concrete compressive strength. The use of multiple non-destructive testing methods than single testing methods has improved the accuracy of prediction model. Further the results of different combination models were compared using coefficient of determination which is a commonly used statistical parameter for prediction accuracy comparison. The prediction model is in accordance with the experimental results obtained. And the accuracy of prediction indicates that support vector regression can be successfully used for predicting the compressive strength of concrete.

Rebound hammer is the most frequently used non-destructive method for estimation of concrete strength. Measurement results are influenced by various factors (components and composition of concrete, humidity or age), which are well known for normal-weight concrete. High-strength concrete (HSC) differs in quality and the question is to which extent these factors influence the test results. Knowledge about influence of aggregate and results of measurements with Schmidt rebound hammer is stated. Test results and evaluation of compressive strength are considerably influenced by the strength of parent material from which the coarse aggregate is made. Influence of granulometric curve is less important and for aggregate with strength over 90 MPa, it is negligible. To determine compressive strength of HSC with Schmidt rebound hammer, it is necessary to elaborate special calibration relationships for aggregate with various strength of parent rock. Use of calibration relationships elaborated for normal-weight concrete for determination of strength of HSC is problematic (different properties of aggregate and upper limit of calibration relationships is 70 MPa). Shifting the upper limit of calibration relations for normal-weight concrete up to the maximal rebound number measured at testing HSC is not recommendable; values of compressive strength calculated in accordance with adjusted formulas are smaller than real strength of HSC.

Study of the Physico-Mechanical Properties of a Recycled Concrete Incorporating Admixtures by the Means of NDT Methods

Compressive strength of solid clay brickwork of masonry bridges: Estimate through Schmidt Hammer tests

In the design of new structures, EN 1992‐1‐1 normatively regulates the correlation between the compressive strength and tensile strength of concrete, in accordance with EN 206‐1. However, as there are many time‐ and load‐dependent impacts on existing structures, the relationship described by EN 1992‐1‐1 cannot be assumed without any further observations for existing structures. At the moment, there is no comprehensive investigation on the influences of, for example, subsequent hardening and carbonation on the concrete compressive or tensile strength. The tensile strength of concrete defined in EN 1992‐1‐1 is based on a water storage and a testing age of 28 days. Furthermore, the in‐situ tensile strength is affected by micro‐crack formation cause of internal stresses due to outflow of hydration heat, for example. Generally, there is a different influence of micro‐crack formation on the tensile strength of concrete than on the compressive strength of concrete. The actual value of the in‐situ tensile strength of concrete affects the shear force resistance and the load‐bearing capacity of anchors with concrete cone failure as decisive failure mode. Within the framework of a research project (SWD‐10.08.18.7‐13.01), financed by “Forschungsinitiative Zukunft Bau”, an initiative of the Federal Institute for Research on Building, Urban Affairs and Spatial Development (BBR) structural investigations on the correlation between the compressive and tensile strength of old concretes have been performed. Below, the results of the research project are presented.

We studied the effects of superplasticizer (SP) (PCE1 and PCE2), air‐entraining admixtures (AEA), and supplementary cementitious material (silica fume and fly ash) on the mechanical and durability properties of concrete samples. Eight concrete mix designs were prepared. The first six concrete mix design contained similar aggregates, PCE2 SP, AEA, 350 kg/m3cement, and water to cement ratio equal to 0.38, and one of the mentioned mix design was selected as control samples with water to cement ratio equal to 0.57, without PCE2 SP and AEA. We used a different quantity of PCE2 SP and AEA and replaced the fly ash or silica fume as part of cement in two of the mentioned mix designs. The last two concrete mix designs studied the effect of PCE1 SP and AEA on freezing and thawing of concrete mix design. Adding PCE2 SP in concrete mix design increased compressive strength at age 11, 42, and 90 days sharply and reduced the depth of water penetration at the age of 28 and 90 days compared to the control sample. Using simultaneous PCE2 and AEA in concrete mix design did not improve compressive strength significantly and increased slightly depth of water penetration compared to only using PCE2. However, increasing the quantity of AEA to 5% improved both compressive strength and reduced depth of water penetration. In the second group of concrete mix design, adding both PCE1 and AEA increased the number of resistible freezing and thawing cycles of concrete. Side chain length and molecular weight of PCE1 and PCE2 SPs had no important effect on the compressive strength and performance properties of concrete.

Compression strength is the most important factor in concrete structures, and there are two methods to evaluate that compressive strength, destructive and non-destructive method. This work presents a study on the deference and comparison between Destructive (compressive strength test) and a Non-Destructive Method (Rebound Hammer and Ultrasonic test) for testing the compressive strength of concrete.The investigations aimed to develop the method of assessment the strength of concrete of both non-destructive tests with greater accuracy. Destructive method, to determine the strength of the in-situ concrete, and also destructive testing (DT), includes methods where the material is broken down in order to determine its mechanical properties. From the obtained results it is observed that the Rebound Hammer readings increased with the compressive strength of concrete. And the Ultrasonic pulse velocity were greatly influenced by the cements and aggregate, extent of moist curing and presence of flaws and voids in concrete, more than their influence on the measured strengths. This demonstrates the limitation of using ultrasonic pulse velocity tests for estimating compressive strength of concrete. Combined use of Ultrasonic pulse velocity (UPV) and Rebound Hammer tests for assessment of concrete strength in structures with greater reliability.

Correlation between rebound hammer number and mechanical properties of steel fibre reinforced pavement quality concrete

The aim of this research is the study of using Non–destructive tests to evaluate the serviceability and the safety of structures. Schmidt hammer and ultrasonic pulse velocity tests are very common in evaluating the concrete strength. In this research, 150 concrete cubes were prepared with different mixture proportions, their strengths varied between 3MPa–65MPa. Rebound number and ultrasonic pulse velocity tests were carried out prior to the crushing strength of specimens. An equation has derived (multi-variation equation) correlate compressive strength of concrete to both a rebound number and ultrasonic pulse velocity values. The adopted equation was applied to estimate the compressive strength of cores which were extracted from the components of previously built structures constructed with different types of concrete. A statistical analysis is performed for the comparison with previous published models.

Durability of reinforced concrete structures depends on the maintenance conditions, surveillance, and well-timed repair of structures or reconstructions. Usually, the main attention falls on the durability determination based on the evaluation of change of main physical –mechanical properties, especially, on the compression strength of concrete. In this study, tests with the rebound hammer and concrete cores extracted from the existing reinforced concrete elements in hydraulic structures are presented. The comparison of strength values obtained with the rebound hammer and the concrete core specimens of reinforced concrete in hydraulic structures is carried out. The research was performed during the scientific expedition in the period of 2010–2014. The investigated objects are allocated in hydroschemes of Druskininkai, Marijampolė, Klaipėda districts. It was established that the results obtained using the non-destructive method were by 17 % higher than the ones obtained by performing the destructive test. However, it can be said that despite this fact, the non-destructive method offers simplicity and rapidity in use: test results are readily available on site and there is a possibility to test concrete strength of those structures where cores cannot be drilled due to thin-walled or densely reinforced structures.

Failure of concrete structures leading to collapse of buildings has initiated various researches on the quality of construction materials. Collapse of buildings resulting to injuries, loss of lives and investments has been largely attributed to use of poor quality concrete ingredients. Information on the effect of silt and clay content and organic impurities present in building sand being supplied in Nairobi County and its environs as well as their effect to the compressive strength of concrete was lacking. The objective of this research was to establish level of silt, clay and organic impurities present in building sand and its effect on compressive strength of concrete. This paper presents the findings on the quality of building sand as sourced from eight supply points in Nairobi County and its environs and the effects of these sand impurities to the compressive strength of concrete. 27 sand samples were tested for silt and clay contents and organic impurities in accordance with BS 882 and ASTM C40 respectively after which 13 sand samples with varying level of impurities were selected for casting of concrete cubes. 150 mm × 150 mm × 150 mm concrete cubes were cast using concrete mix of 1:1.5:3:0.57 (cement:sand:coarse aggregates:water) and were tested for compressive strength at the age of 7, 14 and 28 days. The investigation used cement, coarse aggregates (crushed stones) and water of similar characteristics while sand used had varying levels of impurities and particle shapes and texture. The results of the investigations showed that 86.2% of the sand samples tested exceeded the allowable limit of silt and clay content while 77% exceeded the organic content limit. The level of silt and clay content ranged from 42% to 3.3% for while organic impurities ranged from 0.029 to 0.738 photometric ohms for the unwashed sand samples. With regard to compressive strength, 38% of the concrete cubes made from sand with varying sand impurities failed to meet the design strength of 25 Mpa at the age of 28 days. A combined regression equation of with R2 = 0.444 was generated predicting compressive strength varying levels of silt and clay impurities (SCI), and organic impurities (ORG) in sand. This implies that 44% of concrete’s compressive strength is contributed by combination of silt and clay content and organic impurities in sand. Other factors such as particle shapes, texture, workability and mode of sand formation also play a key role in determination of concrete strength. It is concluded that sand found in Nairobi County and its environs contain silt and clay content and organic impurities that exceed the allowable limits and these impurities result in significant reduction in concrete’s compressive strength. It is recommended that the concrete design mix should always consider the strength reduction due to presence of these impurities to ensure that target strength of the resultant concrete is achieved. Formulation of policies governing monitoring of quality of building sand in Kenya and other developed countries is recommended.

Currently, there is a need to assess the structural integrity of concrete structures situated near or within seawater in a faster manner using a rebound hammer test.However, this test is limited to its reliability, particularly if the rebound hammer device has not been calibrated according to its specific tested environment.Hence, this study assessed the reliability of the rebound hammer test in the compressive strength estimation of concrete cured in a specific environmental condition.As such-the seawater environment, the seawater in a controlled area, and the potable water stored in a normal room condition were the environmental conditions that were considered in the study.Results showed that the rebound hammer test consistently underestimated the direct compression test in three (3) environmental conditions.It was found that the underestimated compressive strength errors ranged from 15.22% to 59% in seawater environment, 33.33% to 58.33% in seawater in a controlled area, and 37.70% to 57.57% in potable water stored in a normal room condition, respectively.Furthermore, this study also established a rebound correlation model, both graphical curve and empirical equation, which can be the basis for concrete compressive strength estimation cured in three (3) different environments.

RETRACTED: The impact of GGBS and ferrous on the flow of electrical current and compressive strength of concrete

Investigation of greywater and zeolite usage in different cement contents on concrete compressive strength and their interactions

This paper examined the comparison between the mechanical properties of mortar infiltrated fibers concrete with two types of fibers (End hooked steel fiber and Synthetic fiber) with varying fibre volume. The difference in properties of the concrete with steel fiber, concrete without fiber and concrete with Synthetic fiber was determined. Compressive strength and flexural strength (Modulus of rupture) of concrete were determined. The concrete mixing containing [0% control, (2,4 and 6%)] fibre volume of (End Hooked) and Synthetic fiber were used for compressive and flexural strengths tests. Steel fiber was (30 mm) in length and (0.5mm) diameter, while Synthetic fiber was (60mm) in length and (0.84mm) diameter. the concrete cube used was (100×100mm) for compressive strength test, also beam used for flexural strength test with dimensions (400×100×100mm). The results showed that with increasing volumetric ratio of fiber, the compressive strength and flexural strength increase. The compressive strength and flexural strength of hooked fiber have a much higher extent increasing than that of synthetic fiber.