M.N. Haque

Effect of initial curing on early strength and physical properties of a lightweight concrete

A 50 MPa 28-day cube compressive strength structural lightweight concrete of a fresh concrete density of 1800 kg/m3 was produced using Lytag coarse and fine aggregate. The long-term strength development and the durability characteristics of this lightweight concrete are being monitored in both the severe hot and dry and hot-coastal and salt-laden exposure conditions prevalent in Kuwait. The early results of the investigation suggest that the compressive strength of this concrete is less sensitive to lack of initial curing. However, depth of water penetration, which is indicative of the concrete’s permeability and hence durability, has been found to be more sensitive to the duration of initial curing even for the specimens exposed to the high-humidity seaside ambient conditions. The drying shrinkage of this concrete has been found to be more than 600 microstrain in the first 3 months’ duration. Longer term durability data will be reported in due course.

PROPERTIES OF HIGH STRENGTH CONCRETE USING A FINE FLY ASH

A Class F fine fly ash (FFA) with a fineness of 99% passing a 45 μm sieve was used to produce workable high-strength concrete. Six mixtures were cast with total cementitious contents of 400 and 500 kg/m3. The replacement of cement by FFA, on equal mass basis, was 0, 10, and 15%. The mixtures were tested for workability and strength. Drying shrinkage and water absorption characteristics were determined as indicative of durability. The slump varied between 45 to 110 mm and fluid/cementitious ratio varied between 0.25 to 0.38. The optimum cement replacement for both 400 and 500 kg total cementitious material mixtures was 10%. The 28-day maximum strength for the two optimum mixtures was 94 and 111 MPa with a slump of 45 and 85 mm, respectively. The indirect tensile strength of the two concretes was only 5 and 6% of their compressive strength, respectively. The 2 h water penetration of the two concretes was comparatively low, 11 and 13mm, respectively. The drying shrinkage of all the six concretes were very similar with a maximum value of 470 microstrain after 56 days of standard exposure. The 28-day modulus of elasticity of all the concretes varied between 40–46 GPa.

Free and water soluble chloride in concrete

Abstract Two methods were used to determine the ‘free’ and ‘water soluble’ chloride ions concentration in four differing strength grade concretes. Each grade of concrete had varying concentration of Cl− admixed as NaCl. In all 24 different batches of concretes were made with admixed Cl− concentration varying from 0.2 to 2%, by weight of cement. The first method was the analysis of the pore solution expressed from specimens and the second by the analysis of the decanted solution of pulverised specimens. The results suggest that the concentration of Cl− and OH− ions as measured in the expressed pore solution represent their true free ion concentrations. Furthermore, the Cl− concentration of both the expressed and decanted solutions were found to depend on the (i) concentration of the admixed) Cl− ions; (ii) strength of the concrete; (iii) presence or absence of fly ash and (iv) presence or absence of a superplasticiser. Accordingly, there is no simple relationship between the Cl− concentration as found in decanted and pore solutions. Whereas the presence of fly ash improves the chloride binding capacity of a concrete, the presence of superplasticisers tends to lower it. The paper concludes that in order to limit the damage caused by Cl− in the concrete, an upper limit on the total Cl− contents of the admixed chloride rather than the percentage of Cl− by the weight of cement should be prescribed.

Effect of carbonation on the chloride concentration in pore solution of mortars with and without flyash

Abstract 1% of Cl− ion by weight of cementitious materials was added as CaCl2 to mortar mixes with and without flyash. Accelerated carbonation was applied on the mortars after short and long fog curing periods. The results showed significant increase of Cl− ions in the pore solution occurring as a result of carbonation. The increase was much more pronounced in flyash mortars. Prolonged initial curing helped to retain Cl− bound within the hydration compounds in the case of plain mortars while it resulted in the increase of Cl− content in the pore solution of flyash mortars.

CHLORIDE PENETRATION AND THE RATIO OF CL-/OH- IN THE PORES OF CEMENT PASTE

Abstract Hardened Portland cement paste samples were immersed in sodium chloride solution. Their pore solution was expressed and analysed for Cl − and OH − ions concentrations. The development of free Cl − and of Cl − /OH − ratio is discussed in relation to the limits previously established for the onset of depassivation of steel reinforcement. It was found that within a relatively short period penetration of the chloride ions could cause the Cl − /OH − ratio in the pores to exceed the depassivation limits.

Strength development of slag and ternary blend concrete

Abstract Ground granulated blast furnace (GGBF) slag is increasingly used in concrete construction due to its technical and economic benefits. This paper describes the strength development of 3 grades of plain cement concretes, portland blast furnace slag (slagment) concretes and concretes in which 15 and 35% of slagment was replaced by flyash (ternary blends). These concretes were cured in standard and nonstandard curing conditions using standard and nonstandard cylindrical specimens. The straight replacement of portland cement by slagment (65% cement and 35% slag) gave higher strength for low to medium strength concretes (20 and 35 MPa). The results also suggest that the strength of the concretes made with slagment was less affected under inadequate curing conditions as compared to the strength of the plain cement concretes. It was also found that the indicated strength of the smaller cylinders (75×150 mm) was adversely affected in the drying curing regimes whereas it was only marginally different from the strength of 150×300 mm cylinders under continuous fog curing. For all the concretes tested, initial curing for 7 days seems to be the most appropriate as it is both practicable and gives adequate strength.

Estimation of insitu strength of concrete

Abstract Twelve concretes of low, medium and high strength, with and without a fly ash and with and without a proprietary superplasticizer were designed. From each concrete 375 × 375 × 150 mm slabs and 100 × 200 mm cylinders were cast. The slabs were stored in out-door exposure conditions and 75 × 150 mm cores were extracted after 28, 91 days and 6 months duration. The moulded cylinders were cured in a fog room, control room maintained at 45% R.H. and placed alongside the slabs. The concrete strength is known to be highly variable and depends on a great many factors. Nonetheless, the results suggest that the insitu strength can be estimated by dividing the strength of the fog cured cylinders by a factor of 1.25. Similarly, for most concretes it can also be assumed that the core strength is equal to the strength of the field-cured cylinders. In spring/summer-like ambient conditions, the core strength is 80% of the strength of the standard cured cylinders instead of 85% required by some codes of practice.

The effect of practical curing methods used in Saudi Arabia on compressive strength of plain concrete — A discussion

This article discusses the paper by taryal, chowdhury and matala (see IRRD 293269). The following points are made: (a) the effectiveness of proprietary curing compounds have yet to be evaluated in the severe climate conditions described in the paper; (b) the practibility of the curing regime of covering the concrete surface by a plastic sheet followed by water spray is challenged; (c) the effect of moisture on the indicated strength of concrete is well established, and when strength developed under differing curing regimes is being compared, it is good practice to bring the specimens to the same degree of surface wetness. It was not stated whether this was done in this programme; (d) the very marginal advantage in strength gain of plain concrete specimens observed by the authors by curing with saline water should be mentioned with great reservation and caution, as curing with salt water without also washing the coarse and fine aggregate used to make it would augment the salt ion concentration in the concrete. Durability of reinforced concrete might also be decreased by this method. (TRRL)