# Concrete Armor Units for Breakwaters

Document Sample

Concrete Armor

Units for

Breakwaters

calculations show importance of limiting residual stresses arising during hardening

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R ubble mound breakwaters are constructed worldwide

to protect harbors and habitats against hydraulic

forces induced by high waves. These revetments usually

mixture’s adiabatic temperature development during the

initial 7 days of curing (Fig. 1(a)). Ten cubes were used

for compression and tension tests at ages of 1, 2, 3, 7, and

comprise rock or concrete armor over a core of rock, 28 days. Tensile strength as a function of time is provided

gravel, or sand. Many are protected against wave action in Fig. 1(b). With the exception of Mixture 3, which had

by an armor layer of large concrete elements. Mostly high fly ash content, the mixtures reached tensile

unreinforced, these elements exist in different sizes and strengths of about 4 MPa (580 psi) at 28 days.

shapes, varying from simple massive cubes to complicated

shapes like Accropodes® or Xblocs® (see Reference 1). nUMeriCAl Model

Resistance against wave action has been the main design During concrete hardening, chemical-physical reaction

criterion for armor units, but several cases of failure have processes generate exothermic heat, leading to increases

highlighted the importance of structural integrity. Our in concrete temperature. A temperature increment ∆T

research team set out to answer the question: Can can lead to deformations that, when restrained, cause an

internal mechanisms occurring during hardening of the incremental stress ∆σ, given by

concrete contribute to cracking or breakage of concrete

armor units? ∆σ = (∆T ⋅ αc + ∆εa) ⋅ Ε ⋅ y ⋅ R (1)

ConCrete properties where αc = the coefficient of thermal expansion in units of

Mixture proportions strain per degree temperature change; ∆εa = the incremental

To study the influence of the concrete composition strain resulting from autogenous shrinkage (a negative

on the concrete properties, six different mixtures were value); E = the elastic modulus; y = the relaxation

studied (Table 1). For each mixture, the cementitious coefficient; and R = the degree of restraint (y and R range

material content was 420 kg/m3 (760 lb/yd3) and the from 0 to 1). Of course, consistent units must be applied.

water-cementitious material ratio (w/cm) was 0.45. The properties, αc, E, and y change during hardening, so

Water reducers were not used to reduce the number of the stress σ consists of the summation of the ∆σ values

mixture ingredients. found for each time increment.

To calculate these stresses for the concrete armor

experimental testing blocks, numerical tools must also account for nonlinear

For each mixture, properties were determined using material behaviors. We used a hardening model called

150 mm (6 in.) cubes. One cube was used to measure the Finite element Concrete Curing Control System, or

34 october 2009 / Concrete international

Table 1:

O verview Of The cOncreTe mixTures used in This sTudy

Concrete Mixture Cement

type no. type Binder Aggregate Comments

1 cem iii/b blended cement sand-gravel 30% portland cement; 70% slag cement

Normal

2 cem i Na sand-gravel reference mixture

3 cem i Fly ash (

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