Strength Of Cement Concrete Research Articles

Validation of HIPERPAV for Prediction of Early-Age Jointed Concrete Pavement Behavior

HIPERPAV is a computer program that was developed to predict the time growth of early-age portland cement concrete strength and stresses in jointed concrete pavements for the prediction of cracking. Each individual environmental, construction, design, and mix design input that influences early-age pavement behavior can be adjusted to find the optimum paving solution. A design that minimizes stresses and maximizes concrete strength will increase the potential for an extended service life. FHWA, aware of the benefits that HIPERPAV can provide in the field, provided funding for experimental validation. The results of this validation are presented. The experimental field instrumentation efforts are described and the analyses performed are discussed. The end result of validating HIPERPAV is increased confidence in the accurate prediction of early-age pavement behavior.

Concrete Maturity Progress: Survey of Departments of Transportation

Results are provided from a concrete maturity survey that was distributed to representatives of 50 departments of transportation (DOTs) in the summer of 2000. The purpose was to display the state of national advancements and practices relating to concrete applications for predicting in situ portland cement concrete strength. Information such as DOT project and research details, method of maturity determination, state applications, and general attitudes toward the concept was requested. Representatives from 44 states replied to the 12-question survey. Results revealed that about 73 percent of the states that responded either have conducted or are currently involved in at least minor research with the concept. Furthermore, approximately 30 percent of the represented states have protocol or specifications governing the use of this developing technology. Representatives throughout the United States have reported that maturity is being used to predict critical strengths for such actions as pavement opening to the public; pavement opening to construction traffic; structural acceptance; and formwork removal for bridges, pavements, and other highway structures.

REINFORCEMENT INFLUENCE ON NON-AUTOCLAVE FOAM CEMENT CONCRETE STRENGTH CHARACTERISTICS/ARMAVIMO ĮTAKA PUTŲ CEMENTBETONIO STIPRUMO CHARAKTERISTIKOMS

The volume and surface reinforcement treatment influence on 900–1000 kg/m3 density non-autoclave foam cement concrete strength characteristics are discussed in this paper. Carbon fibres of a length up to 5 mm and diameter of 4, 6-7.7 mkm were used for volume reinforcement. An alkali- resistant glass fabric net with a 4,5 or 9 mm mesh satisfying the requirements of Austrian standard SSA-5433 was used for surface reinforcement. The chemical composition of raw materials used are given in Table 1. One glass fabric net was put on the form bottom before pouring the mixture, while the other net was put on top, pushed into the formation mixture and trowelled with a roller. The material contracts when the Portland cement hardens and microcracks appear. These microcracks have a negative influence on the materials structure, it decreases the materials strength characteristics, frost-resistance and durability. Small amounts of synthetic carbon fibres (0.2–0.4% based on solids) increase the 900 kg/m3 density foam cement concrete flexural strength from 6 to 24.5% (Fig 2). The carbon fibre adhesion to foam cement concrete is insufficient. The fibres are dragged out of the material during bending or extension (Fig 3). Besides volume reinforcement, surface reinforcement was tested by introducing a glass fabric net 2 mm below the formation mixtures surface. All the specimens were additionally reinforced with 0,2% carbon fibre. The obtained results show that samples reinforced with a 4.5 mesh net have a twice better flexural resistance than control samples. Samples reinforced with a 9 mm mesh glass fabric net showed a 1.8 time increase (Fig 4). The dependency of reinforced foam cement concrete flexural strength on material thickness and samples size were also investigated. Compressive strength measurement experiments showed that standard prisms (40×40×160 mm) ruptured regardless whether they were reinforced with carbon fibres or additionally by a glass fabric net (Fig 5, samples 1, 2). Thin-wall specimens under load bent at first, later ruptured, however the net was not destroyed. A gypsum board was also tested for comparison. The obtained data is given in Fig 5. Thin-wall (10 mm) specimens under a load bent and returned to initial position when the load was removed during testing. The boards behaved springly, but did not rupture. It was found that thin-wall foam cement shock-resistance does not depend on the article thickness. It was compared with “Bison” firm boards. The obtained results are given in Fie 6. As we can see, reinforced foam cement concrete specimen shock-resistance reaches a (7,0 ÷ 8,0) · 10−3 MPa · m value, while specimens without a reinforcing net give a shock-resistance value of only 1,35 · 10−3 MPa · m, which is substantially lower than for control samples. It is well known, that steaming increases the cement concrete flexural strength [10] and the steaming duration depends on its temperature [11]. It was determined, that with the aim of ensuring the glass fibre surface polymer layer stability, the steaming should be conducted at 45–50 °C temperature. In this case, its flexural strength increases by 2,6 times (Fig 7a, sample 3) in comparison with the interval of temperatures 55–85 °C. The steamed and hardened in natural conditions foam cement concrete microstructure is similar. This was confirmed by X-raygrams and thermograms (Fig 8 and 9). So we came to the conclusion, that clinker mineral hydratation takes place during the hardening of foam cement concrete and tobermorite groups calcium hydrosilicates are formed. The main conclusion: we can increase the flexural strength up to 2 times and the shock-resistance by 5.5 times by reinforcing non-autoclave foam cement concrete with a glass fibre.

REINFORCEMENT INFLUENCE ON NON-AUTOCLAVE FOAM CEMENT CONCRETE STRENGTH CHARACTERISTICS

Summary The volume and surface reinforcement treatment influence on 900–1000 kg/m3 density non-autoclave foam cement concrete strength characteristics are discussed in this paper. Carbon fibres of a length up to 5 mm and diameter of 4, 6-7.7 mkm were used for volume reinforcement. An alkali- resistant glass fabric net with a 4,5 or 9 mm mesh satisfying the requirements of Austrian standard SSA-5433 was used for surface reinforcement. The chemical composition of raw materials used are given in Table 1. One glass fabric net was put on the form bottom before pouring the mixture, while the other net was put on top, pushed into the formation mixture and trowelled with a roller. The material contracts when the Portland cement hardens and microcracks appear. These microcracks have a negative influence on the materials structure, it decreases the materials strength characteristics, frost-resistance and durability. Small amounts of synthetic carbon fibres (0.2–0.4% based on solids) increase the 900 kg/m3 density foam cement concrete flexural strength from 6 to 24.5% (Fig 2). The carbon fibre adhesion to foam cement concrete is insufficient. The fibres are dragged out of the material during bending or extension (Fig 3). Besides volume reinforcement, surface reinforcement was tested by introducing a glass fabric net 2 mm below the formation mixtures surface. All the specimens were additionally reinforced with 0,2% carbon fibre. The obtained results show that samples reinforced with a 4.5 mesh net have a twice better flexural resistance than control samples. Samples reinforced with a 9 mm mesh glass fabric net showed a 1.8 time increase (Fig 4). The dependency of reinforced foam cement concrete flexural strength on material thickness and samples size were also investigated. Compressive strength measurement experiments showed that standard prisms (40×40×160 mm) ruptured regardless whether they were reinforced with carbon fibres or additionally by a glass fabric net (Fig 5, samples 1, 2). Thin-wall specimens under load bent at first, later ruptured, however the net was not destroyed. A gypsum board was also tested for comparison. The obtained data is given in Fig 5. Thin-wall (10 mm) specimens under a load bent and returned to initial position when the load was removed during testing. The boards behaved springly, but did not rupture. It was found that thin-wall foam cement shock-resistance does not depend on the article thickness. It was compared with “Bison” firm boards. The obtained results are given in Fie 6. As we can see, reinforced foam cement concrete specimen shock-resistance reaches a (7,0 ÷ 8,0) · 10−3 MPa · m value, while specimens without a reinforcing net give a shock-resistance value of only 1,35 · 10−3 MPa · m, which is substantially lower than for control samples. It is well known, that steaming increases the cement concrete flexural strength [10] and the steaming duration depends on its temperature [11]. It was determined, that with the aim of ensuring the glass fibre surface polymer layer stability, the steaming should be conducted at 45–50 °C temperature. In this case, its flexural strength increases by 2,6 times (Fig 7a, sample 3) in comparison with the interval of temperatures 55–85 °C. The steamed and hardened in natural conditions foam cement concrete microstructure is similar. This was confirmed by X-raygrams and thermograms (Fig 8 and 9). So we came to the conclusion, that clinker mineral hydratation takes place during the hardening of foam cement concrete and tobermorite groups calcium hydrosilicates are formed. The main conclusion: we can increase the flexural strength up to 2 times and the shock-resistance by 5.5 times by reinforcing non-autoclave foam cement concrete with a glass fibre.

Efficiency, skin strength and sorptivity of fly ash concretes

Four grades of concrete with and without fly ash were devised and tested for compressive strength. The concretes were cured in three different curing regimes. The skin strength of concretes under inadequate curing was calculated by assuming a linear model for the variation of strength, and the strength difference between cement and fly ash concretes has been worked out. The skin strength of cement concretes was found to be higher than that of fly ash concretes. The test results were found to be affected by the size of the test specimen, when proper curing was not provided. The difference in sorptivity of fly ash and cement concretes cured for four days and not provided with any initial curing has been included. For all grades of concrete, the sorptivity of fly ash concrete was found to be marginally higher. The difference in sorptivity between fly ash and cement concretes was observed to increase as the strength of the mix decreased. The effect of initial curing was found to be highly significant. The sorptivity of samples with no curing was twice as much as those with four days initial curing. Besides the material properties, the age and strength of a fly ash concrete were also found to be important factors in determining the cementing efficiency of the fly ash.

EXPERIMENTS ON THE STRENGTH OF CEMENT CONCRETE. (ABRIDGED).

EXPERIMENTS ON THE STRENGTH OF CEMENT CONCRETE. (ABRIDGED).